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[Contents = I. The Solar System, II. The Universe ]
I. The Solar System
1. Early Models of the Solar System:
p32 The Geocentric Universe ... The earliest models of the solar system followed the teaching of the Greek philosopher Aristotle (384-322 B.C.) and were geocentric in nature, meaning that Earth lay at the center of the universe and that all other bodies moved around it. ...
p33 ... Around A.D. 140, a Greek astronomer named Ptolemy constructed perhaps the best geocentric model of all time ... it explained remarkably well the observed paths of the five planets then known, as well as the paths of the Sun & the Moon. However, to achieve its explanatory and predictive power, the full Ptolemaic model required a series of no fewer than 80 distinct circles ...
p34 provided the intellectual framework for all discussion of the universe for well over 1000 years.
... some ancient Greek astronomers reasoned differently about the motions of heavenly bodies. ... Aristarchus of Samos (310-230 B.C.) who proposed that all the planets, including Earth, revolve around the Sun and, ... that Earth rotates on its axis once each day ...
... The geocentric model went largely unchallenged until the 16th century A.D.
... Nicholas Copernicus rediscovered Aristarchus’s heliocentric (Sun-centered) model and showed how, in its harmony and organization, it provided a more natural explanation of the observed facts than did the tangled geocentric cosmology. Copernicus asserted that Earth spins on its axis and, like the other planets, orbits the Sun. Only the Moon, he said, orbits Earth. ...
p35 ... The critical realization that Earth is not at the center of the universe is now known as the Copernican revolution.
p40 ... Galileo Galilei (1564-1642) built a telescope for himself in 1609 and aimed it at the sky. What he saw conflicted greatly with the philosophy of Aristotle and provided much new data to support the ideas of Copernicus ... In studying the planet Jupiter, Galileo saw 4 small points of light, invisible to the naked eye, orbiting it, and realized that they were moons. To Galileo, the fact that another planet had moons provided the strongest support for the Copernican model; clearly, Earth was not the center of all things. ...
Galileo published his findings, and his controversial conclusions supporting the Copernican theory, in 1610 ... Galileo was directly challenging the scientific establishment and religious dogma of the time. ... he must certainly have been aware that only a few years earlier, in 1600, the astronomer Giordano Bruno had been burned at the stake in Rome for his heretical teaching that Earth orbited the Sun. However, by all accounts, Galileo delighted in publicly ridiculing and irritating his Aristotelian colleagues. In 1616 his ideas were judged heretical, Copernicus’s works were banned by the Roman Church, and Galileo was instructed to abandon his cosmological pursuits.
But Galileo would not desist. In 1632 he raised the stakes by publishing Dialogue Concerning the Two Chief World Systems, which compared the Ptolemaic and Copernican models.
p41 These actions brought Galileo into direct conflict with the Church. Eventually, the Inquisition forced him, under threat of torture, to retract his claim that Earth orbits the Sun, and he was placed under house arrest in 1633; he remained imprisoned for the rest of his life. Not until 1992 were Galileo’s "crimes" publicly forgiven by the Church.
2. Origin of Earth’s Atmosphere:
p153 When Earth first formed, any primary atmosphere it might have had would have consisted of the gases most common in the early solar system. These were light gases, such as hydrogen, helium, methane, ammonia, and water vapor. ... Almost all this light material, and especially any hydrogen or helium, escaped into space during the first half-billion or so years after Earth was formed. ...
... Subsequently, Earth developed a secondary atmosphere, which was outgassed from the planet’s interior as a result of volcanic activity. Volcanic gases are rich in water vapor, methane, carbon dioxide, sulfur dioxide, and compounds containing nitrogen (such as nitrogen gas, ammonia, and nitric oxide.). Solar ultraviolet radiation decomposed the lighter, hydrogen-rich gases, allowing the H to escape, and liberated much of the nitrogen from its bonds with other elements. As Earth’s surface temperature fell, the water vapor condensed and oceans formed. Much of the carbon dioxide and sulfur dioxide became dissolved in the oceans or combined with surface rocks. Oxygen is such a reactive gas that any free oxygen that appeared at early times was removed as quickly as it formed. An atmosphere consisting largely of nitrogen slowly appeared.
The final major development in the story of our planet’s atmosphere is known so far to have occurred only on Earth. LIFE appeared in the oceans more than 3.5 billion years ago, and organisms eventually began to produce atmospheric oxygen. The ozone layer formed, shielding the surface from the Sun’s harmful radiation. Eventually, life spread to the land and flourished. The fact that oxygen is a major constituent of the present-day atmosphere is a direct consequence of the EVOLUTION OF LIFE on Earth.
3. Heat of the Earth:
p156 ... much of Earth was molten at some time in the past. As a result, the higher density matter sank to the core, and the lower-density material was displaced toward the surface. A remnant of this ancient heating exists today: Earth’s central temperature is nearly equal to the surface temperature of the Sun. What processes were responsible for heating the entire planet to this extent? ... we must try to VISUALIZE the past.
P157 ... when Earth formed, it did so by capturing material from its surroundings, growing in mass as it swept up "preplanetary" chunks of matter in its vicinity. As the young planet grew, its gravitation field strengthened and the speed with which newly captured matter struck its surface increased. This process generated a lot of heat -- so much, in fact, that Earth may alrready have been partially or wholly molten by the time it reached its present size. As Earth began to differentiate and heavy material sank to the center, even more gravitational energy was released, and the interior temperature must have increased still further.
Later, Earth continued to be bombarded with debris left lover from the formation process. At its peak some 4 billion years ago, this secondary bombardment was probably intense enough to keep the surface molten, but only down to a depth of a few tens of kilometers. Erosion by wind and water has long since removed all trace of this early period from the surface of Earth, but the Moon still bears visible scars of the onslaught.
A second important process for heating Earth soon after its formation was RADIOACTIVITY -- the release of energy by certain rarre, heavy elements, such as uranium, thorium, and plutonium. These elements emit energy as their complex, heavy nuclei decay into simpler, lighter ones. ... the heat built up in the interior, adding to the energy left there by Earth’s formation.
Provided that enough radioactive elements were originally spread throughout the primitive Earth ... the entire planet -- from crust to core -- could have mellted and remained molten for about a billion years.
... Measurements of the ages of some surface rocks indicate that Earth finally began to solidify roughly a billion years after it originally formed. Radioactive heating did not stop after the first billion years, ... It continued even after Earth’s surface cooled and solidified.
...The early source of heat diminished with time, allowing the planet to cool over the past 3.5 billion years. .. it cooled from the outside in ... the surface developed a solid crust ...
4. RADIOACTIVE DATING:
p158 ... Not all nuclei are stable ... Many nuclei -- for example, carbon-14, thorium-232, uranium-235, uranium-238, and plutonium-241 are inherently unstable. Left alone, they will eventually break up into lighter "daughter" nuclei, in the process emitting some elementary particles and releasing some energy. The change happens spontaneously, without any external influence. This instability is known as RADIOACTIVITY.
... Unstable heavy nuclei achieve greater stability by disintegrating into lighter nuclei, but they do not do so immediately. Each type of "parent" nucleus takes a characteristic amount of time to decay. The HALF-LIFE is the name given to the time required for half of a sample of parent nuclei to disintegrate. ... this is really a statement of probability. We cannot say which nuclei will decay in any given half-life interval, only that half of them are expected to do so.
... Every radioactive isotope has its own half-life, and most of them are now well known from studies conducted since the 1950s. ... the half-life of uranium-235 is 713 million years, and that of uranium-238 is 4.5 billion years. Some radioactive elements decay much more rapidly, others much more slowly, but these two types of uranium are particularly important to geologists because their half-lives are comparable to the age of the solar system. ...
The decay of unstable radioactive nuclei into more stable daughter nuclei provides us with a useful tool for measuring the ages of any rocks we can get our hands on. The first step is to measure the amount of stable nuclei of a given kind (for example, lead-206, which results from the decay of uranium-238). This amount is then compared with the amount of remaining unstable parent nuclei (in this case, uranium-238) from which the daughter nuclei descended. Knowing the rate (or half-life) at which the disintegration occurs, the age of the rock then follows directly [mathematically]. ... In practice, ages can be determined by these means to within an accuracy of a few percentage points. The most ancient rocks on Earth are dated at 3.9 billion years old. rare specimens have been found in Greenland and Labrador.
p159 The radioactive-dating technique rests
on the assumption that the rock has remained solid while the radioactive
decays have been going on. If the rock melts, there is no reason to expect
the daughter nuclei to remain in the same locations their parents had occupied,
and the whole method fails. Thus, radioactive dating indicates the time
that has elapsed since the last time the rock in question solidified. Hence
this 3.9 billion year value represents only a portion -- a lower limit
-- of the true age of our planet. It dooes not measure the duration of Earth’s
molten existence.
5. Reversals in the Earth’s Magnetic Field:
p165 ... Discovered after WW II ... giant fault called the Mid-Atlantic Ridge. It extends, like a seam on a giant baseball, all the way from Scandinavia in the North Atlantic to the latitude of Cape horn at the southern tip of South america. The entire ridge is a region of seismic and volcanic activity ...
p166 ... the ocean floor closest to the ridge is relatively young, whereas material farther away, on either side, is older -- exactly as we would expect if hot molten matter is upwelling and solidifying as the plates on either side drift apart. The Atlantic Ocean has apparently been growing in this way for the past 200 million years, the oldest age found on any part of the Atlantic seafloor.
Other studies of the Mid-Atlantic Ridge have yielded important information about Earth’s magnetic field. As hot mantle material (carrying traces of iron) emerges from cracks in the oceanic ridges and solidifies, it becomes slightly magnetized, retaining an imprint of Earth’s magnetic field at the time of cooling. Thus, the ocean floor has preserved within it a record of Earth’s magnetism during past times, ...
Earth’s current magnetism is oriented in the familiar north-south fashion, and when samples of ocean floor close to the ridge are examined, the iron deposits are oriented just as expected -- north-south. This is the "young" bassalt that upwelled and cooled fairly recently. However, samples retrieved farther from the ridge, corresponding to older material that upwelled long ago, are often magnetized with the opposite orientation. As we move away from the ridge, the imprinted magnetic field flips back and forth, more or less regularly and symmetrically on either side of the ridge.
Scientists BELIEVE that these different
magnetic orientations were caused by reversals in
Earth’s magnetic field that occurred as the plates drifted away
from the central ridge. Working backward, we can use the fossil magnetic
field to infer the past positions of the plates, as well as the orientation
of Earth’s magnetism. In addition to providing strong support for the idea
of seafloor spreading, these measurements, when taken in conjunction with
the data on seafloor age, allow us to time our planet’s magnetic reversals.
On average, Earth’s magnetic field reverses itself
roughly every half-million years. Dynamo theory suggests that such reversals
are part of the way in which all planetary magnetic fields are generated.
... a similar phenomenon (with a reversal time of approximately 11 years)
is also observed on the Sun.
6. Effect of Tides on Earth’s Rotation:
p169 ... Earth rotates once on its axis (relative to the stars) in 23h 56m - 1 sidereal day. ... We know from fossil measurements that Earth’s rotation is gradually slowing down, causing the length of the day to increase by about 1.5 milliseconds every century -- not much on the scale of a human lifetime, but over millions of years, this steady slowing of Earth’s spin adds up. At this rate, half a billion years ago, the day was just over 22 hours long and the year contained 397 days. ...
p170 Why is Earth’s spin slowing? The main reason is the tidal effect of the Moon. In reality, the tidal bulge raised in Earth by the Moon does not point directly at the Moon ... Instead, because of the effects of friction, both between the crust and the oceans and within Earth itself, Earth’s rotation tends to drag the tidal bulge around with it, causing the bulge to be displaced by a small angle from the Earth-Moon line, in the same direction as Earth’s spin. The net effect of the moon’s gravitational pull on this slightly offset bulge is to reduce our planet’s rotation rate.
At the same time, the Moon is spiraling
slowly away from Earth, increasing its average distance from our planet
by about 4 cm per year. This process will continue until Earth rotates
on its axis at exactly the same rate as the Moon orbits Earth. At that
time the Moon will always be above the same point on Earth and will no
longer lag behind the bulge it raises. Earth’s rotation period will be
47 of our present days, and the distance to the Moon will be 550,000 km
(about 43 percent greater than at present) However, this will take a very
long time -- many billions of years -- to occur.
7. Lunar Dust:
p191 Meteoroid collisions with the Moon are the main cause of the layer of pulverized ejecta -- also called lunar dust, or regolith (meaning "fine rocky layer") -- that covers the lunar landscape to an average depth of about 20 m. This microscopic dust has a typical particle size of about .01 mm. In consistency, it is rather like talcum powder or ready-mix dry mortar. ... Owing to the very low rate of lunar erosion, even these shallow bootprints will remain intact for million of years. The regolith is thinnest on the maria (10 m) and thickest on the highlands (over 100 m deep in places).
In contrast to Earth’s soil, the lunar regolith contains no organic matter like that produced by biological organisms. No life whatsoever exists on the Moon. ... Photograph of an Apollo astronaut’s bootprint in the lunar dust. The astronaut’s weight has compacted the regolith to a depth of a few centimeters. ...
p192 Nor were any fossils found in Apollo samples. Lunar rocks are barren of life and apparently always have been.
... The measured
ages for rock samples returned from the Moon are all greater than 3 billion
years. ... Apparently, the maria solidified
over 3 billion years ago and the Moon has been dormant ever since.
8. The Origin of the Moon:
p196 The origin of
the Moon is uncertain, although several theories have been advanced
to account for it. However, both the similarities and the differences between
the Moon and Earth conspire to confound many promising attempts to explain
the moon’s existence. ... Today, many astronomers favor a hybrid of the
capture and daughter themes. This idea -- often called the impact theory
-- postulates a collision by a large, MMars-sized object with a youthful
and molten Earth. Such collisions may have been quite frequent in the early
solar system. The collision presumed by the impact theory would have been
more a glancing blow than a direct impact. The matter dislodged from our
planet than reassembled to form the Moon.
9. Evolutionary History of the Moon:
p.197 ... The Moon apparently formed about 4.6 billion years ago. The approximate age of the oldest rocks discovered in the lunar highlands is 4.4 billion years, so we know that at least part of the crust must already have solidified by that time and survived to the present. ...
... About 3.9 billion years ago, around the time that Earth’s crust solidified, the heaviest phase of the meteoritic bombardment ceased. The moon was left with a solid crust, which would ultimately become the highlands, dented with numerous large basins, soon to flood with lava and become the maria.
Between 3.9 and 3.2 billion years ago, lunar volcanism filled the maria with the basaltic material we see today. The age of the youngest maria -- 3.2 billion years -- indicates the time when the volcanic activity subsided. The maria are the sites of the last extensive lava flows on the Moon, over 3 billion years ago. ...
p198 ... the lunar landscape has remained
more or less structurally frozen for the past 3 billion years. The Moon
is dead now, and it has been dead for a long time.
10. Comet Orbits
p321 ... The majority of comets take hundreds of thousands, some even millions, of years to complete a single orbit around the Sun. However, a few SHORT-PERIOD comets (conventionally defined as those having orbital periods of less than 200 years) return for another encounter within a relatively short time. According to Kepler’s third law, the short-period comets do not venture far beyond the distance of Pluto at aphelion.
Unlike the orbits of the other solar system objects ... the orbits of comets are not necessarily confined to within a few degrees of the ecliptic plane. Short-period comets do tend to have pro grade orbits lying close to the ecliptic, but LONG-PERIOD comets exhibit all inclinations and all orientations, both pro grade and retrograde, roughly uniformly distributed in all directions from the Sun.
Astronomers BELIEVE that the short-period comets originate beyond the orbit of Neptune, in a region of the solar system called the Kuiper belt. ... A little like the asteroids in the inner solar system, most Kuiper belt comets move in roughly circular orbits between about 30 & 100 A.U. from the Sun, never venturing inside the orbits of the jovian planets. Occasionally, however, a close encounter between two comets, or (more likely) the cumulative gravitational influence of one of the outer planets "kicks" a Kuiper belt comet into an eccentric orbit that brings it into the inner solar system, and into our view. The observed orbits of these comets reflect the flattened structure of the Kuiper belt.
What of the long-period comets? How do we account for their apparently random orbital orientations? Only a tiny portion of a typical long-period cometary orbit lies within the inner solar system, so it follows that for every comet we see, there must be many more similar objects at great distances from the Sun. On these general grounds, many astronomers reason that there must be a huge "cloud" of comets far beyond the orbit of Pluto, completely surrounding the Sun. This region, which may contain trillions of comets, of total mass comparable to the mass of the inner planets, is named the OORT CLOUD, after the Dutch astronomer Jan Oort, who first wrote (in the 1950s) of the possibility of such a vast and distant reservoir of inactive, frozen comets.
The observed orbital properties of long-period comets, have led researchers to BELIEVE that the Oort cloud may be up to 100,000 A.U. in diameter. Like those of the Kuiper belt, however, most of the comets of the Oort cloud never come anywhere near the Sun. ... Oort cloud comets rarely approach even the orbit of Pluto, let alone that of Earth. Only when the gravitational field of a passing star happens to deflect a comet into an extremely eccentric orbit that passes through the inner solar system do we actually get to see one of these objects. Because the Oort cloud surrounds the Sun in all directions, instead of being confined near the ecliptic plane like the Kuiper belt, the long-period comets we see can come from any direction in the sky. Despite their great distances and long orbital periods, ... the Oort cloud comets are still gravitationally bound to the Sun. Their orbits are governed by precisely the same laws of motion that control the planets.
p348 Interlude 15-2: No one has ever observed any comets in the faraway Oort cloud -- they are just too small and dim for us to see from Earth. But in the 1990s such faint objects began to be inventoried in the relatively nearby Kuiper belt, just beyond Keptune’s orbit. Ground-based telescopes have led the way in the painstaking work to capture the meager amounts of sunlight reflected from dozens of such dark objects orbiting in the outer Solar System. The largest of these objects is only a few hundred kilometers across, yet there must be many much smaller icy fragments in cold storage. In many ways, the Kuiper belt must resemble the asteroid belt, ...
As of mid-1998, the current count of trans-Neptunian, Kuiper belt objects was 62. They range in diameter from 100 to 400 km, which is considerably smaller than either Pluto (2300 km) or its moon Charon (1100 km). Estimates of the total number of such objects larger than 100 km run into the tens of thousands, so the combined mass of all the debris in the Kuiper belt could be hundreds of times larger than the inner asteroid belt (though still less than the mass of Earth.
... it is unlikely that objects much smaller than 100 km ... will be found in the near future. (Astronomers using the Hubble Space Telescope have reported finding numerous such smaller objects, but none has been confirmed after repeated tries.)
11. Meteorites
p331 ... Almost all meteorites are OLD.
Direct radioactive dating shows most of them to be
between 4.4 and 4.6 billion years old -- roughly the age of the
oldest lunar rocks. Meteorites, along with some lunar rocks, comets, and
perhaps the planet Pluto, provide essential clues to the original state
of matter in the solar neighborhood.
12. The Formation of the Solar System:
p338 ... Any theory of the origin and architecture of our planetary system must adhere to the known facts. We know of nine outstanding properties of our solar system as a whole:
1. Each planet is relatively isolated in space.
2. The orbits of the planets are nearly circular.
3. The orbits of the planets all lie in nearly the same plane.
4. The direction in which the planets orbit the Sun (counterclockwise as viewed from above Earth’s North Pole) is the same as the direction in which the Sun’s rotates on its axis.
5. The direction in which MOST planets rotate on their axis is roughly the same as the direction in which the Sun rotates on its axis. This property is less general than the one just described for revolution, as three planets -- Venus, Uranus, and Pluto -- do not share it. [6 of 9 = 67% sharee it, 33% do not share the property]
6. MOST of the known moons revolve about their parent planets in the same direction that the planets rotate on their axes. [of 63 known moons, 40 = 63.5% revolve about their parent planets in the same direction that the Sun rotates (pro grade) while 23 = 36.5% revolve about their parent planets in the opposite direction that the Sun rotates (retrograde).
7. Our planetary system is highly differentiated. The inner terrestrial planets are characterized by high densities, moderate atmospheres, slow rotation rates, and few or no moons. By contrast, the jovian planets, farther from the Sun, have low densities, thick atmospheres, rapid rotation rates, and many moons.
8. The asteroids are very old and exhibit a range of properties not characteristic of either the inner or the outer planets or their moons. The asteroid belt ... appears to be made of primitive, unevolved material, and the meteorites that strike Earth are the oldest rocks known.
9. The comets are primitive, icy fragments that do not orbit in the ecliptic plane and reside primarily at large distances from the Sun.
All these observed facts, taken together, strongly suggest a high degree of order within our solar system. The whole system is not a random assortment of objects spinning or orbiting this way or that.
The overall organization points toward a single formation, an ancient but one-time event, 4.6 billion years ago.
p339 It is equally important to recognize what our theory of the solar system does not have to explain. There is plenty of scope for planets to evolve after their formation, so circumstances that have developed since the initial state of the solar system was established need not be included in our list. Examples are Mercur’s 3:2 spin-orbit coupling. Venus’s runaway greenhouse effect, the Moon’s synchronous rotation, the emergence of life on Earth and its absence on Mars, the Kirkwood gaps in the asteroid belt, and the rings and atmospheric appearance of the jovian planets. There are many more. ... all the properties of the planets for which we have already provided an evolutionary explanation need not be included as items that our theory must account for at the outset.
In addition to its many regularities, our solar system also has many notable irregularities ...
Far from threatening our theory, however, these irregularities are important facts for us to consider in shaping our explanations. For example, it is necessary that the explanation for the solar system not insist that all planets rotate in the same sense or have only pro grade moons, because that is not what we observe. Instead, the theory of the solar system should provide strong reasons for the observed planetary characteristics yet be flexible enough to allow for and explain the deviations, too.
p341 The Role of Dust:
... The model currently favored by most astronomers is ... known
as the condensation theory ... The key new ingredient in the modern picture
is the presence of interstellar dust in the solar nebula. Astronomers
now recognize that the space between the stars is strewn with microscopic
dust grains, an accumulation of the ejected matter of many long-dead stars.
These dust particles probably formed in the cool atmospheres of old stars,
then grew by accumulating more atoms and molecules from the interstellar
gas within the Milky Way Galaxy. ... [note - by this model
you need old stars to form a new one, so how did the first star form ?]
Dust grains play an
important role in the evolution of any gas. Dust helps to cool warm
matter by efficiently radiating its heat away in the form of infrared radiation,
reducing the pressure (which is just proportional to the gas temperature)
and allowing the gas to collapse more easily under the influence of gravity.
Furthermore, the dust grains greatly speed up the process of collecting
enough atoms to form a planet. They act as condensation nuclei ...
Planet Formation: Modern models trace
the formative stages of our solar system ...
IMAGINE a dusty interstellar cloud fragment ... intermingled
with the preponderance of hydrogen and helium atoms in the cloud is some
heavy-element gas and dust. Some external influence, such as the
passage of another interstellar cloud or perhaps the explosion of a nearby
star [to form a new star an old star is required so how did the first star
form?], starts the fragment contracting, down to a size of about 100 A.U.
As the cloud collapses, it rotates faster and begins to flatten ...
According to the condensation
theory, the planets formed in three stages ...
1) Early on, dust grains
in the solar nebula formed condensation nuclei around which matter began
to accumulate ... forming the first small clumps of matter ... grew rapidly
by sticking to other clumps ... gradually grew into objects of pebble size,
baseball size, basketball size and larger. ... this process
of accretion ... created objects a few hundred kilometers across.
... At the end of this first stage, the solar system was made up
of hydrogen and helium gas and millions of planetesimals - objects the
size of small moons ...
2) ... gravitational
forces between the planetesimals caused them to collide and merge, forming
larger and larger objects ... eventually almost all the planestesimal
material was swept up into a few large protoplanets
p342 Mathematical modeling
... indicates that after about 100 million years, the primitive solar system
had evolved into nine protoplanets
3) ... The four largest
protoplanets became large enough to enter a third phase of planetary development:
sweeping up large amounts of gas from the solar nebula to form what would
ultimately become the jovian planets ...
p346 ... A key prediction of this model is that some of the original planetesimals should have remained behind, forming the broad band known as the Kuiper belt, lying beyond the orbit of Neptune. In 1993 several such asteroid-sized objects were discovered, lying between 30 & 35 A.U. from the Sun and lending strong support to the condensation theory. Over 60 Kuiper belt objects, having diameters ranging from 100 to 400 km, are now known.
p348 ... A possible weak link in the condensation theory is sometimes known as the angular momentum problem. Although our Sun contains about 1000 times more mass than all the planets combined, it possesses a mere .3 percent of the total angular momentum of the solar system.
p348 ... The problem here is that all mathematical models predict that the Sun should have been spinning very rapidly during the earliest epochs of the solar system and should command most of the solar system's angular momentum, basically because it contains most of the mass. However, ... the reverse is true
p349 ... Despite some minor controversy as to how this angular momentum quandary can best be resolved, nearly all astronomers agree that some version of the condensation theory is correct. The details have yet to be fully worked out ...
p354 The Sun: ... is the sole source of light and heat for the maintenance of life on Earth. It is a STAR, a glowing ball of gas held together by its own gravity and powered by nuclear fusion at its center. ... our Sun appears to be a rather "typical" star, lying right in the middle of the observed ranges of stellar mass, radius, brightness, and composition
p374 Observations of
Solar Neutrinos: Theorists are quite sure that the proton-proton
chain operates in the Sun. ... neutrinos that arise as by-products
... escape from the Sun. Interacting with virtually nothing, they
leave at or near the speed of light, escaping into space a few seconds
after being created in the core.
Earth-based neutrino-detection devices
... Given the size of the detector and the physical conditions in
the Sun's core implied by the Standard Solar Model, about one solar neutrino
of the roughly E16 that streamed through the tank each day should have
been detected ... but the numbers were not as large as predicted ...
This disagreement between theory and observation is known as the SOLAR
NEUTRINO PROBLEM
P375 ... there is a real
discrepancy between the Sun's theoretical neutrino output and the neutrinos
we detect on Earth. How can we explain this contradiction?
If, as we think, the detectors are working correctly, there are really
only two possibilities, Either neutrinos are not produced as frequently
as we think, or not all of them make it to Earth. ... Do we
really know what processes are at work deep in the hearts of stars?
For now, the mystery of the solar neutrinos remains
unsolved.
II. The Universe:
1.
Globular Clusters
p401
... Apparently, globular clusters formed long ago ... their spectra show
few heavy elements, implying that these stars formed in the distant past
when heavy elements were much less abundant than they are today.
... astronomers estimate that
all globular clusters are at least 10 billion years old.
They contain the oldest known stars in the Milky Way Galaxy. As such
globular clusters are considered to be remnants of the earliest stages
of our Galaxy's existence.
... We will never be able to watch a single star move through all its evolutionary phases. ... Instead, we must observe stars as they presently exist - through "snapshots" taken at specificc moments in their life cycles. ... we can patch together an understanding of a star's "life story" without having to follow a few individuals from birth to death. ... evolutionary studies ...
p463 ... All the globular clusters in our Galaxy appear to have formed between about 10 & 12 billion years ago.
2.
Star Formation: ...
p443 For a
star like the Sun, the whole formation process
takes about 50 million years
...
Many of the objects predicted by the theory of star formation have been
observed in real astronomical objects.
...
Shock waves can compress other interstellar clouds and trigger star formation.
Star birth and the production of shock waves are thought to produce a chain
reaction of star formation in molecular cloud complexes.
3.
Stellar Evolution:
p464
... Stellar evolution is one of the great success stories of astrophysics
... Today, the theory of stellar evolution is a cornerstone of modern
astronomy.
p467
...The theory of stellar evolution can be tested by observing star clusters,
all of whose stars formed at the same time. As time goes by; the
most massive stars evolve off the main sequence first, then the intermediate-mass
stars, and so on. At any instant, no stars with masses above the
cluster's main-sequence turnoff mass remain on the main sequence.
Stars below this mass have not yet evolved into giants and so still lie
on the main sequence. By comparing a particular cluster's main-sequence
turnoff mass with theoretical predictions, astronomers can measure the
age of the cluster.
4.
Stellar Explosions:
p480
Supernova 1987A ... the theory of stellar evolution described in
the text has held up very well. ...
p481
... About 20 hours before the supernova was detected optically, a brief
(13 second) burst of neutrinos was simultaneously recorded by underground
detectors in Japan and the United States. ... the neutrinos are predicted
to arise when electrons and protons in the star's collapsing core merge
to form neutrons. The neutrinos preceded the light because they escaped
during the collapse ... theoretical models consistent with these observations
suggest that vastly more energy was emitted in the form of neutrinos than
in any other form.
Despite some unresolved details in SN1987A's behavior, detection of this
neutrino pulse is considered to be a brilliant confirmation of theory.
5.
The Cycle of Stellar Evolution:
p488
The theory of stellar nucleosynthesis can naturally account for the observed
differences in heavy-element abundance between the old globular cluster
stars and stars now forming in our galaxy. ...
...
Only at the end of the star's life are its newly created elements released
and scattered into space. Thus the spectra of the youngest stars
show the most heavy elements, because each new generation of stars increases
the concentration of these elements in the interstellar clouds from which
the next generation forms. ... Knowledge of stellar evolution allows
astronomers to estimate the ages of stars from spectroscopic studies ...
... Complete
cycle of star formation and evolution in our Galaxy = summary:
1. Stars form when part of an interstellar cloud is compressed beyond
the point at which it can support itself against its own gravity.
The cloud collapses and fragments, forming a cluster of stars. The
hottest stars heat and ionize the surrounding gas, sending shock waves
through the surrounding cloud, possibly triggering new rounds of star formation.
2. Within the cluster, stars evolve. The most massive stars
evolve fastest, creating heavy elements in their cores and spewing them
forth into the interstellar medium in supernova explosions. ...
3. The creation and dispersal of new heavy elements are accompanied
by further shock waves. Their passage simultaneously enriches the
interstellar medium and compresses it into further star formation.
... although some material is used up in each cycle -- turned into energy or locked up in low-mass stars -- the Galaxy continuously recycles its matter. Each new round of formation creates stars with more heavy elements than the preceding generation had. From the old, metal-poor globular clusters to the young, metal-rich open clusters, we observe this enrichment process in action. Our Sun is the product of many such cycles. We ourselves are another. Without the heavy elements synthesized in the hearts of stars, life on Earth would not exist.
6.
Einstein's Theories of Relativity:
p503
... two key facts from relativity 1) nothing can travel faster than the
speed of light 2) all things, including light, are attracted by gravity
p504
1. The Special Theory
of Relativity proposed by Einstein
in 1905, deals with the preferred status of the speed of light. ...
the speed of light "c" is the maximum speed attainable in the universe
... a further important and unique aspect of light -- that the measured
speed of a beam of light is independent of the motion of the observer or
the source. No matter what our speed may be relative to the source
of the light, we always measure precisely the same value for c ...
... The rules
that apply to particles moving at or near the speed of light are different
from those we are used to in everyday life.
Special relativity is the mathematical framework that allows us to extend
the familiar laws of physics from low speeds (speeds much less than c)
to very high relativistic speeds, comparable to c.
p505
Relativity is equivalent to Newtonian mechanics when objects move much
more slowly than the speed of light, but it differs greatly in its predictions
at relativistic velocities. For example, special relativity predicts
that a rapidly moving spacecraft will appear to contract in the direction
of its motion, its clocks will appear to run slow, and its mass will appear
to increase. Despite their somewhat nonintuitive nature, all the
theory's predictions have been repeatedly verified to very high accuracy.
Today special relativity is at the heart of all physical science.
2. General Relativity
is what results when gravity is included in
the framework of special relativity. ... Einstein reasoned,
there is no way to tell the difference between a gravitational field and
an accelerated frame of reference (such as a rising elevator). Gravity
can therefore be incorporated into special relativity as a general acceleration
of all particles. However, another major modification to the theory
of special relativity must be made. Central to relativity is the
notion that space and time are not separate quantities but instead must
be treated as a single entity = spacetime. To incorporate the effects
of gravity, the mathematics forces us to the conclusion that spacetime
has to be curved.
In general relativity, then, gravity is a manifestation of curved spacetime.
There is no such thing as a "gravitational field" in the Newtonian sense.
Instead, objects move as they do because they follow the curvature of spacetime,
and this curvature of spacetime is determined by the amount of matter present.
p506
... Modern notions about black holes rest squarely
on the theory of relativity. ... only the modern Einsteinian
theory of relativity can properly account for the bizarre physical properties
of black holes.
A central concept of general relativity is this: matter --all matter
-- tends to "warp" or curve space in itts vicinity. ... In Einsteinian
relativity ... particles move on curved trajectories because they are following
the curvature of space produced by some nearby massive object. The
more the mass, the greater the warping. Close to a black hole, the
gravitational field becomes overwhelming and the curvature of space extreme.
p508 Special relativity is the most thoroughly tested and most accurately verified theory in the history of science. General relativity, however, is on somewhat less firm experimental ground. The problem with verifying general relativity is that its effects on Earth and in the solar system ... are very small. Just as special relativity produces major departures from Newtonian mechanics only when velocities approach the speed of light, general relativity predicts large departures from Newtonian gravity only when extremely strong gravitational fields are involved -- in effect, when orbit speeds and escape velocities become relativistic.
Two classical tests
of General Relativity: At the heart of general relativity
is the premise that everything, including light, is affected by gravity
because of the curvature of spacetime.
1) Einstein noted that light from a star should be deflected by a measurable
amount as it passes the Sun. The closer to the Su;n the light comes,
the more it is deflected. Thus, the maximum deflection should occur
for a ray that just grazes the solar surface. Einstein calculated
that the deflection angle should be 1.75" - a small, but detectable, amount.
... In 1919 a team of observers, ... succeeded in measuring the deflection
of starlight during an eclipse. The results were in excellent agreement
with the prediction of general relativity. ... Recently, the
high-precision Hipparcos satellite has observed shifts in the apparent
positions of many stars, even those whose line of sight is far from the
Sun. The shifts are exactly as predicted by Einstein's theory.
2) Another prediction of general relativity is that planetary orbits should
deviate slightly from the perfect ellipses of Kepler's laws. ...
Relativity predicts that Mercury's orbit is not a closed ellipse.
Instead, its orbit should rotate slowly ... The amount of rotation is very
small - only 43" per century ... when other (nonrelativistic)
gravitational influences, primarily the perturbations due to the other
planets, are taken into account, the rotation is in complete agreement
with the foregoing prediction.
p510 ... a redshift induced by the black hole's gravitational field, clearly predicted by Einstein's general theory of relativity and known as Gravitational Redshift. ... photons are attracted by gravity. As a result, in order to escape from a source of gravity, photons must expend some energy ... They don't slow down at all - photons always move at the speed of light - they just lose energy. Because a photon's energy is proportional to the frequency of its radiation, light that loses energy must have its frequency reduced. ... radiation coming from the vicinity of a gravitating object will be redshifted by an amount depending on the strength of the gravitational field.
p511 ... Time Dilation is another clear prediction of general relativity, and in fact it is closely related to the gravitational redshift. To see this connection, imagine that we use our light source as a clock, with the passage of a wave crest constituting a "tick". The clock thus ticks at the frequency of the radiation. As the wave is redshifted, the frequency drops, and fewer wave crests pass the distant observer each second - the clock appears to slow down. This thought experiment demonstrates that the redshift of the radiation and the slowing of the clock are one and the same.
p512
... Can an entire star simply shrink to a point and vanish? General
relativity predicts that without some agent to compete with gravity, the
core remnant of a high-mass star will collapse all the way to a point at
which both its density and its gravitational field become infinite -- a
so-called singularity. ... Singularities always signal the
breakdown of the theory producing them. ... the present laws of physics
are simply inadequate to describe the final moments of a star's collapse.
...
Singularities are places where the rules break down, and some very strange
things may occur near them. ... Because these regions are places
where science fails, their presence causes serious problems for many of
our cherished laws of physics, from causality (the idea that cause should
precede effect, which runs into immediate problems if time travel is possible),
to energy conservation (which is violated if material can hop from one
universe to another through a black hole).
...
Although general relativity is not proven, there is presently no reason
to disbelieve it, and black holes are one of its most striking predictions.
7.
Our Milky Way Galaxy:
p527
Shapley's bold interpretation of the globular clusters as defining the
overall distribution of stars in our Galaxy was an enormous step forward
in human understanding of our place in the universe. Five hundred
years ago, Earth was considered the center of all things. Copernicus
argued otherwise, demoting our planet to an undistinguished place removed
from the center of the solar system. In Shapley's time, ... the prevailing
view was that our Sun was the center not only of the Galaxy but also of
the universe. Shapley showed otherwise. With his observations
of globular clusters he simultaneously increased the size of our Galaxy
by almost a factor of 10 over earlier estimates and banished our parent
Sun to its periphery, virtually overnight.
p528 Astronomers often refer to young disk stars as Population I stars and to old halo stars as Population II stars.
p529 ... The entire Galactic disk is rotating about the Galactic center. In the vicinity of the Sun, the orbital speed is about 220 km/s. At the Sun's distance of 8 kpc from the Galactic center, material takes about 225 million years -- an interval of time sometimes known as 1 Galactic year -- to complete one circuit.
p530 ... Is there some evolutionary scenario that can naturally account for the Galactic structure we see today? The answer is that there is, and it takes us all the way back to the birth of our Galaxy, 10-15 billion years ago. ... the early stages of the Milky Way are still very poorly understood.
p531 ... The Milky Way Galaxy possibly formed via the merger of several smaller systems. Astronomers reason that, early on, our Galaxy was irregularly shaped, with gas distributed throughout its volume. When stars formed during this stage, there was no preferred direction in which they moved and no preferred location in which they were found. In time, rotation caused the gas and dust to fall to the Galactic plane and form a spinning disk. The stars that had already formed were left behind, forming the halo. New stars forming in the disk inherit its overall rotation and so orbit the Galactic center on ordered, circular orbits.
p532
... A central problem facing astronomers trying to understand spiral structure
is how that structure persists over long periods of time. The basic
issue is simple: we know that the inner parts of the Galactic disk
rotate more rapidly than do the outer regions. ... this differential
rotation makes it impossible for any large-scale structure "tied" to the
disk material to survive. ... a spiral pattern consisting always
of the same group of stars and gas clouds would necessarily "wind up" and
disappear within a few hundred million years. Yet spiral arms clearly
do exist in our own galaxy, and their prevalence in other disk galaxies
suggests that they last for considerably longer than this. ... How
then do the Galaxy's spiral arms retain their structure over long periods
of time in spite of differential rotation?
A leading explanation for the existence of spiral arms holds that they
are spiral density waves -- coiled waves of gas compression that move through
the Galactic Disk, squeezing clouds on interstellar gas and triggering
the process of star formation as they go.
8.
The Large-Scale Structure of the Universe:
p554
... no one has ever succeeded in explaining the observed properties
of normal galaxies in evolutionary terms. Isolated normal galaxies
do not evolve from one type to another. ... astronomers know
of no parent-child relationship among normal galaxies. ... the subject
of galaxy evolution is still poorly understood.
... Astronomers estimate that some 40 billion galaxies exist in the observable universe.
p557 ... a group of galaxies held together by their mutual gravitational attraction is called a galaxy cluster.
... Most astronomers believe that the galaxy clusters themselves are clustered, forming titanic agglomerations of matter known as superclusters.
p559 ... Radio observations indicate that the rotation curves of many spiral galaxies, ... remain flat (do not decline and may even rise slightly) far beyond the visible image of the galaxy. ... may have invisible dark halos ... a lot more mass than we can see is needed to bind galaxy clusters ... upward of 90 percent of the universe is composed of dark matter
p571
... Distant galaxies are observed to be receding from the Milky Way at
rates that increase proportional to their distances from us. This
relationship between recessional speed and distance is called HUBBLE'S
LAW. Its value is believed to lie between 45 and 90 km/s/Mpc.
...
...
The redshift associated with the Hubble expansion is called the cosmological
redshift.
9. Quasi-stellar Objects =
QUASARS:
p587 The most striking characteristic
of the several hundred quasars now known is that their spectra all show
large redshifts, ranging from .06 up to the current maximum of about 5.
(see Relativistic Redshifts below)
p589 ... Thus all quasars lie
at large distances from us -- the closest is 240 Mpc away, the farthest
nearly 5500 Mpc. ... We therefore see most quasars as they existed
long ago -- they represent the universe as it was in the distant past.
... Thus, despite their unimpressive optical appearance ... the large
distances implied by quasar redshifts mean that these faint "stars" are
in fact the brightest known objects in the universe! ... quasars
range in luminosity from around E38 Watts ... up to nearly E42 watts.
A value of E40 watt, comparable to 20 trillion Suns or 1000 Milky Way Galaxies,
is fairly typical. Thus quasars outshine the brightest normal and
active galaxies by about a factor of 1000.
10. Relativistic Redshifts
& Look-Back Time:
p590 When discussing very distant objects such as
quasars, astronomers usually talk about their redshifts rather than their
distances. ... it is very common for researchers to speak of an event
occurring "at" a certain redshift -- meaning that the light received today
from that event is redshifted by the specified amount. ... because
of Hubble's law, redshift and distance are equivalent to each other.
However, redshift is the "preferred" quantity, because it is a directly
observable property of an object, whereas distance is derived from redshift
using Hubble's constant, whose value is not accurately known.
The redshift of a beam of light is, by definition,
the fractional increase in its wavelength resulting from the recessional
motion of the source. Thus, a redshift of 1 corresponds to a doubling
of the wavelength. ... From the formula for the Doppler shift,
the redshift of radiation received from a source moving away from us with
speed v is given by
redshift = (observed wavelength - true wavelength) / true wavelength&nbbsp; = (recessional velocity v) / (speed of light c)
... while the foregoing equation is quite correct for low velocities,
it does not take into account the effects of relativity. ... radiation
received from an object moving away from us at nearly the speed of light
would be red shifted to almost infinite wavelength.
... many quasars have redshifts greater than 1. This does not
mean that they are receding faster than light! It simply means that
their recessional velocities are relativistic -- comparable to the speed
of light -- and the preceding simple formula is not applicable.
... Because the universe is expanding, the "distance"
to a galaxy is not very well defined -- do we mean the distance
when the galaxy emitted the light we see today, or the present distance,
even though we do not see the galaxy as it is today? ... Largely
because of this ambiguity, astronomers prefer to work in terms of a quantity
known as the LOOK-BACK TIME, which is simply how long ago an object emitted
the radiation we see today. Researchers talk frequently about redshifts
and sometimes about look-back times, but hardly ever of the distances to
high-redshift objects.
p591 For nearby sources the look-back time is numerically equal to the distance in light years -- the light we receive tonight from a galaxy at a distance of 100 million light years was emitted 100 million years ago. However, for more distant objects, the look-back time and the present distance in light years differ because of the expansion of the universe, and the divergence increases dramatically with increasing redshift. For example, a galaxy now located 15 billion light years from Earth was much closer to us when it emitted the light we now see. Consequently, its light has taken considerably less than 15 billion years -- in fact, only about 8.8 billion yearrs -- to reach us. ... the present distance to a galaxy with a given redshift depends in detail on how the universe expanded in the past. ... the details of the past expansion are not well known, so the distance is not well determined. The distances ... are subject to considerable uncertainty. ... the recessional velocity equals the speed of light -- and the redshift becomes infinite -- for objects that emitted their radiation about 10 billion years ago.
REDSHIFT v/c
d in ly x million
LOOK-BACK TIME milion y
.010
.010 149
149
.500
.386 5523
4570
1.00
.600 8815
6483
5.00
.946 17809
9347
100
1.000 27101
10019
infinite
1.000 30096
10029 (10.029 billion y)
11. ALTERNATIVE EXPLANATION OF COSMOLOGICAL REDSHIFT?
P594 ... the
respected observer Halton Arp and the equally reputable theorist Geoffrey
Burbidge, sought an alternative explanation for quasars. Instead
of believing that these objects were at cosmological distances and so very
luminous, these researchers argued that perhaps there was an alternative
, noncosmological, explanation for their great redshifts. Quasars
could then be relatively nearby and hence much less bright.
12.
THE BIG BANG - The Ultimate Origin of All Things (?)
p605
On the basis of these rather sketchy data, theoretical insight, and not
a little philosophical preference, cosmologists,
astronomers who study the large-scale structure and dynamics of the universe,
ASSUME
that the universe is HOMOGENEOUS on scales greater than a few hundred megaparsecs.
... the universe looks smooth on the largest scales.
THE COSMOLOGICAL
PRINCIPLE:
THE BIRTH OF THE
UNIVERSE
Hubble's Law: recession velocity = Ho x
distance where Ho = 65
km/s/Mpc [an average of Ho taken from a range of 45 to 90 km/s/Mpc]
We used this relation
as a convenient means of determining the distances to galaxies and quasars,
but it is much more than that.
time = distance / velocity = distance / ( Ho x
distance ) = 1 / Ho
For Ho = 65 km/s/Mpc
[average] Time = 1 / 65 = 15
billion years
For Ho = 45
Time = 1 / 45 = 22
billion years
For Ho = 90
Time = 1 / 90 = 11
billion years
[thus age estimate ranges from 11 to 22 billion
years] or [10 to 20 Billion
yrs]
Hubble's law therefore implies that at some time in the past - [11 to 22
Billion years ago] - all the galaxies in the universe lay right on
top of one another. In fact, astronomers believe
that
everything in the universe -- matter and radiation alike -- was confined
to a single point at that instant [note - a point has a radius = 0
so the volume is zero so we are talking about the
entire Universe coming from nothing].
Then the point exploded, flying apart at high speeds. The present
locations and velocities of the galaxies are a direct consequence of that
primordial blast. This gargantuan explosion, involving everything
in the universe, is known as the BIG
BANG. As best we can tell, it
marked the beginning of the
universe.
...
The range of possible error in this age [ 15 billion years ] is considerable,
both because Hubble's constant is not known precisely and because the assumption
that galaxies moved at constant speed in the past is not a very good one
... the critical fact here is that the age
of the universe is finite.
...
Whether the universe is actually finite or infinite in extent is irrelevant
... We see only a finite part of it -- the region lying within roughly
15 billion light years of us. What lies beyond is unknown -- its
light has not yet had time to reach us.
WHERE WAS THE BIG
BANG?
p610
... Such is the cosmological principle: no observer anywhere
in the universe has a privileged position. There is no center to
the expansion and no position that can be identified as the location from
which the universal expansion began.
THE COSMOLOGICAL
REDSHIFT
RELATIVITY AND THE
UNIVERSE
BEFORE THE BIG BANG
THE FATE OF THE UNIVERSE
p614
... In 1998, astronomers announced the first results from a long-term,
systematic survey of distant Type-I (carbon detonation) supernovae.
... these findings seem to indicate that the expansion of the universe
is not slowing, but actually accelerating. According to the supernova
data, galaxies at large distances are receding less rapidly than Hubble's
law would predict. These observations, if confirmed, are inconsistent
with the standard Big Bang model ... and may necessitate a major revision
of our view of the cosmos. ...
13.
THE AGE OF THE UNIVERSE
p615
... The assumption of a constant expansion rate ... leads to
an overestimate of the universe's age -- the universe is actually younger
than the 15 billion years we calculated earlier [using an average Ho of
65 km/s/Mpc]. ... In the special case of critical density,
the age of the universe happens to be ... 2/3 of the forgoing value, or
10 billion years.
... A low-density, unbound universe
is older than 10 billion years (but still less than 15 billion years old);
a high-density, bound universe is younger.
p616
The Age Controversy Revisited
...
in the 1980s, some observers measured a value for Ho of around 90 km/s/Mpc,
implying 1/Ho = 11 billion years. At the same time stellar evolution
theories maintained that the oldest globular clusters had to be at least
14 billion years old.
14.
THE COSMIC MICROWAVE BACKGROUND
p621
When we observe the microwave background, we are looking almost all the
way to the very beginning of the universe.
15.
THE EARLY UNIVERSE
PAIR
PRODUCTION
p628
... Pair production
in the very early universe was directly responsible for all the matter
that exists in the universe today. Everything we see around us was
created out of radiation as the cosmos expanded and cooled. ...
we know that some matter must have survived these early moments.
For some reason there was a
slight excess of matter over antimatter at early times.
[note: this slight excess
of matter just happens to consist of the entire universe]
A small residue of particles that outnumbered their antiparticles was left
behind as the temperature dropped below the threshold for creating them.
With no antiparticles left to annihilate them, the number of particles
has remained constant ever since. These survivors are said to have
frozen out of the radiation field as the universe cooled.
This "matter-creation" phase of the universe's evolution ended when the
electrons - the lightest known elementary particle - appeared out of the
cooling primordial fireball. From that point on, matter
has continued to evolve,
clumping together into more and more complex structures, eventually forming
the atoms, planets, stars, galaxies, and large-scale structure we see today,
but no new matter has been created since that early time.
HELIUM
FORMATION IN THE EARLY UNIVERSE
16.
THE INFLATIONARY UNIVERSE
p636
These observations constitute "problems" because cosmologists want to be
able to explain the present condition of the universe, not just accept
it "as is".
COSMIC
INFLATION
p637
The inflationary epoch provides a natural solution to the horizon and flatness
problems ... The horizon problem is solved because inflation took
regions of the universe that had already had time to communicate with one
another - and so had established similar physical properties - and then
dragged them far apart ...
p638
inflation solves the flatness problem ... Any curvature the
universe may have had before inflation has been expanded so much that space
is now virtually flat, at least on the scale of the observable universe
...
17.
The Formation of Structure in the Universe
THE GROWTH OF INHOMOGENEITIES
DARK
MATTER
Hot dark matter consists of lightweight particles - much less massive than
the electron. ... Simulations of a universe filled with hot
dark matter indicate that large structures, such as superclusters and voids,
form fairly naturally, but the computer models cannot account for the existence
of structure on smaller scales. ... p640 ...
concluded that models based purely on hot dark matter are unable to explain
the observed structure of the universe.
Cold dark matter consists of very massive particles, possibly formed during
the GUT era ... Computer simulations modeling the universe
with these particles as the dark matter easily produce small-scale structure.
With the understanding that galaxies form preferentially in the densest
regions, and with some fine-tuning, these models can also be made to produce
large-scale structure comparable to what is actually observed.
...
Perhaps the best results to date (the results that agree most closely with
observations) come from calculations in which a mixture of hot and cold
dark matter is assumed ...
...
Although calculations like this
cannot prove that dark-matter models are the correct description of the
universe, the similarities between
the models and reality are certainly very striking.
THE MICROWAVE BACKGROUND
18.
TOWARD CREATION
... If this view is correct, then our Galaxy, the Sun, Earth, even life
itself, are direct consequences of a series of random events that occurred
during an unimaginably short period of time some 10-15 billion years ago.
... these ideas are very
speculative. In the strict sense, they are not
really science at all, as they violate one of the central tenets of the
scientific method: they are practically impossible to test experimentally.
19.
LIFE IN THE UNIVERSE
LIFE IN THE UNIVERSE
B.
The opposing view maintains that intelligent life on Earth is the product
of a series of extremely fortunate accidents -- astronomical, geological,
chemical, and biological events unlikely
to have occurred anywhere else in the universe.
CHEMICAL EVOLUTION
...
a dissenting view has emerged. ... These researchers suggest that
much, if not all, of the organic material that combined to form the first
living cells was produced in interstellar space and subsequently arrived
on Earth in the form of interplanetary dust and meteors ...
p651
However the basic materials appeared on Earth, we know that life DID appear.
...
The fossil record leaves no doubt that biological organisms have changed
over time -- all scientists accept the reality of biological evolution.
...
...
A new kind of evolution had begun, namely, cultural
evolution, the changes in the ideas and behavior
of society. Our more recent ancestors have created, within only the
past 10,000 years or so, the entirety of human civilization.
LIFE IN THE SOLAR
SYSTEM
...
the possibility of liquid water below Europa's icy surface has refueled
speculation about the development of life there.
...
Comets contain many of the basic ingredients for life -- for instance,
ammonia, methane, and water vapor ...
...
a small fraction of the meteorites that survive the plunge to Earth's surface
do contain organic compounds. ...
...
The moderately large molecules found in meteorites and in interstellar
clouds are our only evidence that chemical evolution has occurred elsewhere
in the universe.
Most researchers regard this organic matter as pre biotic -- ... matter
that could eventually lead to life but that has not yet done so.
...
The planet most likely to harbor life (or to have harbored it in the past)
seems to be MARS ... the Viking lander carried a television camera to seek
fossilized remnants of large plants or animals. No fossils of any
kind were seen. Mars Pathfinder also surveyed part of the Martian
surface, again without finding any evidence of present or past life on
Mars. Viking landers scooped up Martian soil and tested for life
... but no unambiguous evidence of Martian life has emerged.
THE SEARCH FOR EXTRATERRESTRIAL
INTELLIGENCE
...
Arp has reported many examples of instances where galaxies and quasars
are found close together on the sky but have very different, conflicting
redshifts. ... He argues that there are simply too many of these
"coincidences" where a foreground galaxy lies in nearly the dame direction
as a supposed background quasar, for the distant-quasar hypothesis to be
correct. Instead, he claims, the quasars must be physically close
to the galaxies, and the redshifts have some other, noncosmological, explanation.
Arp and coworkers have gone further, citing instances where neighboring
galaxies have conflicting redshifts, such as ... where one
of the five galaxies (Stephen's Quintet) has a redshift very different
from those of the others. Similarly,
... a small galaxy appears to be connected
to a larger galaxy (NGC 7603) having a very different redshift by a faint
"bridge" of gas, reinforcing the view that the two really are close together
in space. Such
findings call into question both the cosmological interpretation of all
galactic redshifts and hubble's law itself.
Most astronomers would argue that the claims of conflicting redshifts are
not statistically significant. ... given the numbers of known galaxies
and quasars, accidental superposition's on the sky should be quite commonplace,
and the observed quasar-galaxy & galaxy-galaxy alignments are quite
consistent with pure chance. The apparent bridges may simply be photographic
or image-processing defects. ... Hubble's law is well established
for galaxies within a few hundred megaparsecs, and some quasars have been
found in galaxy clusters, sharing the redshift of their neighbors.
Thus, at least some quasar redshifts are known to be cosmological, so the
violations of Hubble's law that Arp claims exist appear only quite selectively
- if they really do exist at all.
If quasars are actually local, no convincing explanation has ever been
advanced for their redshifts. ... no matter where we try to put the
quasars, their redshifts pose problems. An alternative attempt to
explain the redshift as a gravitational redshift suffered by light as it
climbs out from the vicinity of a black hole also fails because it cannot
account for the observed widths of quasar spectral lines.
There is no clear observational evidence for conflicting quasar redshifts,
and no satisfactory mechanism for producing the redshifts by noncosmological
means. Furthermore, ... there really is no "quasar luminosity problem"
anymore. Quasar luminosities can be explained by the same mechanism
that powers active galaxies, [black holes in their center] without
posing a serious challenge to the laws of physics.
Consequently, the overwhelming majority of astronomers hold that quasar
redshifts are cosmological in origin and that quasars really are the most
distant objects known in the universe. Of course, there
are many instances in the history of astronomy where the majority has later
been proved totally wrong!
Cosmic
homogeneity is the first of two
major assumptions
that cosmologists
make when studying the large-scale structure of the universe. Observations
suggest that it is true, but it is by no means proven.
The second assumption, also supported by observational evidence and theoretical
reasoning, is that the universe is ISOTROPIC
-
that is, it looks the same in any direction. Isotropy is on much
firmer observational ground than homogeneity. Apart from regions
of the sky that are obscured by our Galaxy, the universe does look much
the same in all directions, at any wavelength, provided we look far enough.
...
The assumptions of homogeneity and isotropy form the foundation of modern
cosmology. Together, these twin pillars of cosmology are known as
the COSMOLOGICAL PRINCIPLE. No
one really knows if this principle is absolutely correct.
All that we can say is that, so far, it seems consistent with observations.
... Should it turn out to be incorrect - for example, were a structure
a few thousand megaparsecs across to be discovered tomorrow - then some
of the discussion that follows would be on very shaky ground indeed!
The cosmological principle has very far reaching implications. ...
it implies that there can be
no edge to the universe, because that
would violate the assumption of homogeneity. ... it implies
that there is no center, because that
would mean that the universe would not be the same in all directions from
any non central point, a violation of the assumption of isotropy.
Thus, this single principle strongly limits what the overall geometry of
the universe can be.
The cosmological principle is
the ultimate expression of the principle of mediocrity.
It states not only that are
we not central to the universe but
that no one can be central, because the universe has no center!
p606
... all the galaxies on the universe are rushing away from us in a manner
described by
Assuming that all velocities have remained constant in time, we can ask
a simple question: How long has it taken for any given galaxy to
reach its present distance from us? ... The time taken is simply
the distance traveled divided by the velocity, so
...
even though it appears to place us at the center of the expansion, Hubble's
law does not violate the cosmological principle in any way. ...
Each observer [at different positions in the Universe ] sees an overall
expansion described by Hubble's law, and the constant of proportionality
- Hubble's constant - is the same in alll cases. Far from singling
out any one observer as central, Hubble's law is in fact the only expansion
law consistent with the cosmological principle.
p607
To understand why there is no "center" to the expansion, we must make a
great leap in our perception of the universe. If we were to imagine
the Big Bang as simply an enormous explosion that spewed matter out into
space, ultimately to form the galaxies we see, than the foregoing reasoning
would be quite correct - there would be a center and an edge, and the cosmological
principle would not apply. But the Big Bang was NOT an explosion
in an otherwise featureless, empty universe. The only way that we
can have Hubble's law hold and retain the cosmological principle is to
realize that the Big Bang involved the entire universe - not just the matter
and radiation within it, but the universe itself. ... the galaxies
are not flying apart into the rest of the universe. The universe
itself is expanding. ... Hubble's law describes the expansion of
the universe itself. ... the portion of the galaxies' motion
that makes up the Hubble flow is really an expansion of space itself.
The expanding universe remains homogeneous at all times. There is
no "empty space" beyond the galaxies into which they rush.
At the time of the Big Bang, the galaxies did not reside at a point located
at some well-defined place within the universe. The
entire universe was a point.
That point was in no way different from the rest of the universe; that
point was the universe. Therefore, there was no one point where the
Big Bang "happened" -- because the Big Bang involved the entire universe,
it happened everywhere at once.
...
we have explained the redshift of galaxies as a Doppler shift, a consequence
of their motion relative to us. However, we have just argued that
the galaxies are not in fact moving with respect to the universe, in which
case the Doppler interpretation is incorrect. The true explanation
is that as a photon moves through space its wavelength is influenced by
the expansion of the universe. ... The
cosmological redshift is a consequence of the changing size of the universe
- it is not related to velocity at all..
The redshift of a photon measures the amount by which the universe has
expanded since that photon was emitted. ... In general, the larger
a photon's redshift, the smaller the universe was at the time the photon
was emitted, and so the longer ago that emission occurred. Because
the universe expands with time and redshift is related to that expansion,
cosmologists routinely use redshift as a convenient
means of expressing time.
p611
notion of the entire universe expanding from a point -- with nothing, not
even space and time, outside ... this picture of the universe
lies at the heart of modern cosmology.
...
the more powerful techniques of Einstein's general relativity, with its
built-in notions of warped space and dynamic spacetime, are needed.
... loosely summarize its description of the universe by saying that
the presence of matter or energy causes a curvature of space (more correctly,
spacetime) ... The amount of curvature depends on the amount of matter
present, ...
...
a surprising prediction of general relativity is that the Newtonian picture
gives the right result! ... Thus, we can discuss the expansion of the universe
in simple Newtonian terms, but we need general relativity to justify our
doing so.
p611
The Big Bang was a singularity in space and time -- an instant when the
present laws of physics say the universe had
zero size [came from
nothing] and infinite temperature and density. ... The presence
of these singularities signals that, under extreme conditions, the theory
- in this case, general relativity - maaking the predictions has broken
down. At present no theory exists to let us penetrate the singularity
at the start of the universe. We have no means of describing these
earliest of times, so we have no way of answering the question, What came
before the Big Bang? ... The Big Bang represented the beginning
of the entire universe -- mass, energy, space, and time came into being
at that instant. Without time, the notion of "before" does not exist.
...
Theorist estimate that the "known" physics of today is adequate to describe
the universe since about E-43 sec after the Big Bang.
p612
At present the universe is expanding. ... the universe has only two
options: it can continue to expand forever - an unbound universe
- or the present expansion will somedayy stop and turn around into a contraction
- a bound universe.
...
What determines which of these possibilities will actually occur?
The answer is the density of the universe. ... In a high-density
universe, there is enough mass to s top the expansion and cause a recollapse
- the universe is bound. A low-deensity universe, conversely, is unbound
and will expand forever. The dividing line between these two outcomes,
the density corresponding to a marginally bound universe, is called the
critical density. Its value depends on Hubble's constant
So, what is the ultimate fate of the universe? The answer is still
not known with absolute assurance. However, although there is a large
uncertainty in the value of the cosmic density parameter [ratio of the
actual density to the critical value], most astronomers would probably
agree that it lies between .1 and 1. As best we can tell, given the
current data, the universe is destined to expand forever.
...
these numbers highlight a continuing problem in astronomy. Unless
the universe is of quite low density, and hence closer to 15 billion years
old, the age that we obtain from cosmology is uncomfortably close to the
low end of the 10-12 billion
year age range implied by studies of globular clusters in our own Galaxy.
Because the Galaxy cannot be older than the universe, and because the density
of the universe appears to be at least relatively close to the critical
value, we are forced to conclude that there
may be a contradiction
between these two major areas of astronomy.
...
we note a potentially serious
discrepancy in the estimates of the age of the universe yielded by two
independent branches of astronomy.
For Ho = 65 km/s/Mpc, cosmologists calculate that the likely range of cosmic
ages is 10-15 billion years. ... In part because observations
seem to indicate a cosmic density of at least a few tenths the critical
value, and for theoretical reasons ... a value closer to the lower limit
is favored by many cosmologists. However, this estimate may be at
odds with the ages of the oldest star clusters in our galaxy, as determined
by analyses of the turn-off point for mainsequence stars on the H-R diagram.
For example, the age of the globular cluster 47 Tucanae, according to this
method, is at least 12 billion years. Hence the
paradox at hand: some stars seem to be older than the universe itself.
Again, improved observations have partly alleviated this problem, although
it has not yet been completely eliminated. Reanalysis of the abundance
of helium in globular clusters, together with a general increase in estimates
of their distances, suggest that the ages of these clusters may have been
their ages overestimated by almost 20 percent, reducing the 14 billion
year limit to the 12 billion ... At the same time, a recalibration
of the cosmic distance scale has resulted in an increase in galactic distance
estimates, a corresponding reduction in Ho, and hence an increase in the
"cosmological" age of the universe.
The value of Ho is still being vigorously
debated at observatories around the world.
... Some researchers continue to maintain that a high value, perhaps
as much as 80 km/s/Mpc, is correct, in which case the age problem persists.
Others, who have always favored a lower value -- 50 km/s/Mpc or less --
point out that in their universe, the latest "age problem" never existed
in the first place. The choice of Ho = 65 adopted in the text is
a compromise value, favored by many moderates, between these extremes.
p618
... in 1964 ... Arno Penzias & Robert Wilson ... were studying
the Milky Way's emission at microwave (radio) wavelength ...
In their data they noticed a bothersome background "hiss" ... Regardless
of where and when they pointed their antenna, the hiss persisted.
Never diminishing or intensifying, the weak signal was detectable at any
time of the day, any day of the year, apparently filling all space.
p619
... The radio hiss that Penzias & Wilson detected is now known as the
cosmic microwave background.
... researchers had predicted the existence and general
properties of the microwave background well before its discovery.
... they argued, this redshifted "fossil remnant" of the primeval
fireball should have a temperature of no more than a few tens of kelvins
- peaking in the microwave part of the spectrum. ... The Princeton
researchers confirmed the existence of the microwave background and estimated
its temperature at about 3 K. ...
...
In 1989 the Cosmic Background Explorer (COBE) satellite measured the intensity
of the microwave background at wavelengths straddling the peak of the curve,
... The near-perfect fit corresponds to a universal temperature of
about 2.7 K.
A striking aspect of the cosmic microwave background is its high degree
of isotropy. Its intensity is virtually constant (in fact, to about
1 part in E5) from one direction on the sky to another. This isotropy
provides strong support for the assumption of the cosmological principle
...
P626
... In the beginning the universe consisted of pure energy, at unimaginably
high temperatures. As it expanded and cooled, the ancient energy
gave rise to the particles that make up everything we see around us today.
...
at the present moment the density of matter in the universe far exceeds
the density of radiation. In cosmological terminology, we say that
we live in a matter-dominated
universe. ... even though
today the radiation density is much less than the matter density, there
must have been a time in the past when they were equal. Before that
time, radiation was the main constituent of the cosmos. The universe
is said to have been radiation dominated then. ... the time
at which the densities of matter and radiation were equal -- occurred a
few thousand years after the Big Bang, when the universe was 20,000 times
smaller than it is today. The temperature of the background radiation
at that time was about 60,000 K, ...
...
The key to understanding events at very early times lies in a process called
pair production, in
which two photons give rise to a particle-antiparticle pair ... [example
= electron & positron, proton & antiproton, etc.] In this
way matter can be created from radiation. The reverse process can
also occur -- a particle and its antiparticle can annihilate each other
to produce energy in the form of electromagnetic radiation ... energy in
the form of radiation can be freely converted into matter in the form of
particles and antiparticles, and particles and antiparticles can be freely
converted back into radiation, subject only to the law of conservation
of mass and energy.
P632
... The theory of stellar nucleosynthesis accounts very well for
the observed abundances of heavy elements in the universe, but there are
discrepancies between theory and observations when it comes to the abundances
of the light elements, especially helium. ... there is far
more helium in the universe -- about 25 percent by mass -- than can be
explained by nuclear fusion in stars. The accepted explanation is
that this base level of helium is primordial -- that is, it was created
during the early, hot epochs of the universe, before any stars had formed.
The production of elements heavier than hydrogen by nuclear fusion shortly
after the Big Bang is called primordial nucleosynthesis.
... The universe had to wait until it became cool enough for the deuterium
to survive. ... once the universe passed the deuterium bottleneck,
fusion proceeded rapidly and large amounts of helium were formed.
In just a few minutes most of the free neutrons were consumed, leaving
a universe whose matter content was primarily hydrogen and helium.
... the expansion of the universe caused fusion to stop at helium.
... helium accounted for about one quarter of the total mass of matter
in the universe ... The remaining 75 percent of the matter in the universe
was hydrogen. It would be almost a billion years before nucleosynthesis
in stars would change these numbers.
p635
In the late 1970s, cosmologists trying to piece together the evolution
of the universe were confronted with two nagging problems
that had no easy explanation within the standard
Big Bang model. ...
The first problem is known as the horizon problem, and it concerns the
remarkable isotropy of the cosmic microwave background. ... The fact
that the background radiation is isotropic to high accuracy means that
regions A & B had very similar densities and temperatures at the time
the radiation we see left them. The problem is, within the Big Bang
theory [standard BB] , there is no particular reason why these regions
should be so similar to each other ... they are separated by many megaparsecs,
and there has not been time for information, which can travel no faster
than the speed of light, to travel from one to the other. In cosmological
parlance, the two regions are said to be outside each other's horizon.
.. with no possibility of communication between them, the only alternative
is that regions A & B simply started off looking alike -- an assumption
that cosmologists are very unwilling to make.
The second problem with the standard Big Bang model is called the flatness
problem. ... the density of the universe is fairly near the critical
value needed for the expansion barely to continue forever. ... again
there is no good reason why the universe should have formed with a density
very close to critical.
...
theory implies that the universe briefly entered a very odd, and unstable,
high-energy state that physicists call the "false vacuum."
... For a short while, empty space acquired an enormous pressure, which
temporarily overcame the pull of gravity and accelerated the expansion
of the universe at an enormous rate. The pressure remained constant
as the cosmos expanded, and the acceleration grew more and more rapid with
time ... this period of unchecked cosmic expansion ... is called the epoch
of inflation.
Eventually, the universe returned to the lower-energy "true vacuum" state.
... with the return of the true vacuum, inflation stopped. The whole
episode lasted a mere E-32 sec, but during that time the universe swelled
in size by the incredible factor of about E50. When the inflationary
phase ended, the grand unified force was gone forever.
...
the universe expanded much faster than the speed of light during the inflationary
epoch ...
... this resolution to the flatness problem -- the universe looks
pretty flat because it is flat -- has a very important consequence.
Because the universe is flat, the density of matter must be exactly critical.
That means that there must be a lot of invisible matter in the universe
beyond the clusters and the superclusters, filling the huge voids on the
largest scales. ... the density of normal matter is at most 3 percent
of the critical, it follows that the rest of the mass -- at least 97 percent
of all the matter in the universe -- must be in the form of dark
matter (whatever it may be).
p639
Just as stars form from inhomogeneities in interstellar clouds, galaxies,
galaxy clusters, and larger structures are believed to have grown from
small density fluctuations in
the matter of the expanding universe.
... Cosmologists calculate that regions of higher-than-average density
that contained more than about a million times the mass of the Sun would
have begun to contract. There was thus
a natural tendency for million-solar
mass "pregalactic" objects to form. ... these pregalactic fragments
might have interacted and merged to form galaxies.
By the early 1980s, cosmologists had come to realize that galaxies could
not have formed from the contraction of inhomogeneities involving only
NORMAL matter. ... if galaxies had grown from density
fluctuations in the normal-matter component of the early universe, then
the fluctuations would have had to be so large as to leave a clearly observable
imprint on the cosmic microwave background. That imprint is not observed.
P639
... The fact that most of the universe is made of dark matter ... provides
an alternative explanation for the large-scale structure we see today.
...
the nature of the dark matter is still unknown, theorists have considerable
freedom in choosing its properties when they attempt to simulate the formation
of structure in the universe. Cosmologists distinguish between two
basic types of dark matter on the basis of its temperature at the time
when galaxies began to form. These types are known as hot dark matter
and cold dark matter ... they lead to quite different kinds of structure
in the present-day universe.
P640
... dark matter models predict that there should be
tiny "ripples" in the microwave background -
temperature variations of only a few parts per million from place to place
on the sky. ... In 1992, ... the COBE team announced that the
expected ripples had indeed been detected. The temperature variations
are tiny - only 30-40 millionth of a kelvin from place to place in the
sky - but they are there. [note - other sources report that the reported
variations were below the sensitivity level of the instruments and that
the ripples were reported only after much statistical analysis with computers
etc. which could mean that they are not actually there
Initially, it seemed that the inhomogeneities in the microwave background
were not consistent with the "standard" dark-matter models that provided
the best agreement with actual observations of structure in the present-day
universe. The ripples seen by COBE ... appeared to imply too little
structure on large scales ...
However, with some
modifications to the details of the models, it now looks as though the
disagreement is not so serious as it first seemed, and a growing number
of cosmologists are coming to regard the COBE observations as confirmation
of a central prediction of dark-matter theory
P643
Many researchers feel that once we have in hand the proper description
of quantum gravity, our understanding may automatically include a natural
description of creation itself. It is even conceivable that the
primal energy that formed the universe originally emerged from literally
nothing.
Even in a perfect vacuum - a region of space containing neither matter
nor energy - virtual particle-antiparticle pairs are constantly appearing
and disappearing within a time span too short to observe, causing natural
quantum fluctuations to occur in empty space. We might be living
in a sort of "self-creating universe" that erupted into existence spontaneously
from just such a random quantum fluctuation! This sort of "statistical"
creation of the primal cosmic energy from absolutely nothing
has been dubbed "the ultimate free lunch."
P648
COSMIC EVOLUTIONARY
STAGES = PARTICULATE, GALACTIC, STELLAR,
PLANETARY, CHEMICAL, BIOLOGICAL, CULTURAL
= the continuous transformation of matter and energy that has led to the
appearance of life and civilization on Earth.
From the Big Bang, to the formation of galaxies, to the birth of the solar
system, to the emergence of life, to the evolution of intelligence and
culture, the universe has evolved from simplicity to complexity.
We are the result of an incredibly complex chain of events that spanned
billions of years.
A.
The general case in favor of extraterrestrial life is summed up in what
are sometimes called the assumptions of mediocrity:
1) because life on Earth depends on just a few basic molecules
2) because the elements that make up these molecules are common to all
stars
3) if the laws of science we know apply to the entire universe ... then
- given sufficient time - life must havve originated elsewhere in the cosmos.
P649
... idea that complex
molecules could have evolved naturally from simpler ingredients found on
the primitive Earth have been around
since the 1029s. The first experimental verification was provided
in 1953 when scientists Harold Urey & Stanley Miller, using laboratory
equipment ... took a mixture of the materials thought to be present on
Earth long ago - ... water, methane, carbon dioxide, & ammonia - and
energized it by passing an electrical discharge through the gas.
After a few days they analyzed their mixture and found that it contained
many of the same amino acids found today in all living things on Earth
... Although none of these experiments has ever produced a living organism,
or even a single strand of DNA, they do demonstrate conclusively that "biological"
molecules can be synthesized by strictly nonbiological means, using raw
materials available on the early Earth. ... while no
actual living cells have yet been created "from scratch" in any laboratory,
many biochemists feel that the chain of events leading from simple nonbiological
molecules almost to the point of life itself has been amply demonstrated.
...
Interstellar molecular clouds are known to contain very complex molecules,
and large amounts of organic material were detected on comet Halley by
space probes ... similarly complex molecules were observed on comet
Hale-Bopp.
Thus, the idea that organic matter is constantly
raining down on Earth from space in the form of interplanetary debris is
quite plausible. Whether or not this was the primary means by which
complex molecules first appeared in Earth's oceans remains unclear.
For now, the issue is unresolved.
...
The study of fossil remains shows the initial appearance about
3.5 billion years ago of simple one-celled organisms ...
followed about 2 billion years ago by more complex one-celled creatures
... Multicellular organisms such as sponges did not appear until about
1 billion years ago, after which there flourished a wide variety of increasingly
complex organisms -- insects, reptiles, mammals, & humans.
...
What led to these changes? Chance ...
...
The evolution of the rich variety of life on our planet, including human
beings, occurred as chance mutations -- changes in genetic structure --
led to changes in organisms over millions of years.
p652
... Life as we know it is generally taken to mean carbon-based life
that originated in a liquid water environment = life on earth....
...
Is there any reason to suppose that such life might exist elsewhere in
our solar system? The answer appears to be no. It seems that
no environment in the solar system besides Earth is particularly well suited
for sustaining Earth like life.
The consensus among
biologists and chemists today is that Mars does not house any life similar
to that on Earth ... It seems that a solid verdict regarding
life on Mars will not be reached until we have thoroughly explored our
intriguing neighbor.
p659
... A few radio searches are now in progress ... So far, ... nothing
resembling an extraterrestrial signal has been detected.