Creationism, Science, and the Age of the Universe
Cartwheel galaxy in the sculptor constellation, about 500,000,000 light-years away. From the the NASA Space Telescope Science Institute.
Young-earth creationism holds that the entire universe was created less than 10,000 years ago, and that the earth, the large-scale universe, and the biosphere sprang into existence fully-formed within a single 96 hour period (three days of the creation week were spent either creating light, dividing the upper waters from the lower waters, or resting, leaving 4 days, or 96 hours). In contrast to this view, there is excellent physical evidence that the universe is much older than 10,000 years, and that galaxies and stars existed billions of years before our earth formed.
The Age of the Oldest Observed Stars
One way to demonstrate that the universe is older than 10,000 years, and much older than the earth, is by estimating distances from the most distant objects in the observable universe, and dividing that distance by the speed of light. In order for this method to work, two variables must be solved. First, the speed of light must be known. This variable is known, and 300,000km per sec is a good enough estimation for our purposes. Secondly, we must know the distance of a given stellar object from earth. There are a variety of ways to do this.
The simplest way to determine stellar distances is through parallax measurements. When a light source is viewed from different angles, it appears to "move" a small amount. This apparent movement is called parallax. For example, As the earth completes its yearly cycle around the sun, it essentially gives astronomers a means of seeing distant objects from two different angles. The sky can be divided up into 360 degrees. Each degree can be divided into 60 arcminutes, and each arcminute can be divided into 60 arcseconds. Every 6 months, the earth will have rotated 1/2 way around the sun. If a star, for example, appears to move 10 arcseconds over this six-month period, it is said to have a parallax of 10 seconds, and would be 10 parsecs, or 33 light-years away. Unfortunately, parallax measurements are only useful with relatively close objects, mainly stars in our own galaxy. On the other hand, parallax measurements are useful in that they allow distance scales to calibrated with these close stars.
Another way of determining distances is based on the relationship between apparent and intrinsic brightness. For example, if one knows how intrinsically bright a given star is, independently of assumptions about its distance from you, then its distance can be determined by comparing its apparent brightness, or luminosity, with its intrinsic brightness. This is possible because the apparent brightness of a light source decreases as the square of its distance from the observer. One class of "standard candles" whose intrinsic brightness can be accurately determined are called Cepheid variables, so called because both the size and luminosity of these stars vary periodically. Since the luminosity of these stars is directly related to its period (the amount of time it takes to complete one 'cycle'), and since its period can be observed directly, the intrinsic brightness can be determined. Thus, Cepheids can be used to measure stellar distances. Using the Hubble telescope, individual Cepheids can now be detected as far away as 60 million light years (Ferris, p 56).
Another tool for measuring distance is the standard ruler, a light source of known size rather than known luminosity. As with luminosity, there is a straightforward relationship between apparent size and distance. One type of "standard ruler" is the ionized spheres which surround new-born stars. Since the size of these ionized spheres is thought to be constant for a given class of stars, it provides a structure of known intrinsic size which, when compared with apparent size, yields a measure of the structure's distance from earth.
Using these standard candles and standard rulers, one can determine that many galaxies are over 10 billion light years away from earth. If current interpretation of the Hubble Deep-Field images is correct, starlight is reaching earth from at least 10 billion light years away. Even if the speed of light were twice its currently measured speed, these stellar objects would still have existed longer than the 4.55 billion years or so that the earth has existed, not to mention the 6-10,000 year creationist timescale.
A Brief History of Cosmology
The Static Universe
Until the first half of the twentieth century, nearly all cosmologies posited an essentially unchanging large-scale universe. While the sun and planets "revolved" around earth with clockwork precision, the stars seemed to be motionless over long periods of time. In Aristotle's cosmology, these stars were like luminous gems embedded on the inside of a "celestial sphere," the sky being much like the interior of a planetarium. Given the very limited amount of information available to Aristotle and to other ancient cosmologists, such views of the cosmos would have seemed imminently plausible. It would be many centuries later before a crucial technological invention, the telescope, would make the scientific study of the large-scale universe possible. In short order, the cosmologies of Aristotle, Ptolomy, and the Roman Church were shown to be wrong. The earth was shown to be neither immobile nor the center of the universe, and stars, it turned out, were not gems on the inside of a celestial sphere. The earth was in fact a relatively small planet, orbiting a medium size sun; the stars, on the other hand, are just massive numbers of other suns, at greatly vary distances from us. The simple, attractive univese known to poets, philosophers, and the church for thousand of years, died a sudden death at the hands of Galileo and his telescope observations.
The Expanding Universe
In the 1920's, cosmology underwent another revolution. In 1905 and 1916, Albert Einstein published his Special and General Theories of Relativity respectively. At this time, Einstein supposed like everyone else that the large-scale universe was static. This led to a theoretical problem with relativity -- why, in a static universe, had gravitational forces not caused galaxies to collapse into one another under mutual attraction? To avoid this "problem," Einstein posited the existence of an antigravitational force which he called the "cosmological constant." In the early 20's, a young Russian polymath named Alexander Friedmann convinced Einstein that a relativistic universe must be either expanding or contracting, and cannot be static. Even with the addition of the cosmological constant, the slightest change would cause the universe to begin to expand or collapse. Einstein later referred to the constant as the greatest error of his career.
At about the same time that the theorists were agreeing that Einstein's theory predicted a dynamic large-scale universe, astronomers were accumulating evidence that the universe is indeed dynamic, specifically, that the universe is expanding.
Redshifted Light
Using Cepheid variable stars to chart the distance to several galaxies, Edwin Hubble discovered a startling correlation -- the further away a galaxy is, the redder the light from it appears. What's more, this relationship is invariant, and has been repeatedly verified by observations of thousands of galaxies. Hubble proposed an elegant explanation for these observations based on the Doppler effect.
In the classical Doppler effect, sounds waves are compressed or stretched depending on the motion of the sound source. As a jet approaches an observer, the sound waves will be compressed in the direction of motion, resulting in a higher frequency sound. As the jet passes by, however, the sound source will now be moving away from you. Hence, sound waves will be stretched, resulting in a lower frequency sound.
Hubble proposed that the reddening of distant galaxies is the result of the the Doppler effect acting on electromagnetic waves emitted from a receding source. If a given galaxy is moving away from our galaxy, its light spectra will be stretched slightly into longer, and hence redder wavelengths. Likewise, for a galaxy approaching ours at high speed, its light waves will be compressed, resulting in a blueshifting of its spectra.
Judged by the increasing degree of redshift with distance, all galaxy clusters are moving away from our galaxy galaxy at a rate proportional to the clusters distance. Those galaxies furthest away are moving faster than those which are closer. This relationship between the distance and recession velocity of galaxies, known as Hubble's Law, has been consistently verified, and its mathematical value is referred to as the Hubble constant. Hubble's law states that V = HD, where V is the galaxies recession velocity in km per sec, H is Hubble's constant, and D is a galaxy's distance from our own in megaparsecs. While it might seem from this discovery that the earth is the center of universal expansion, there is a much simpler explanation -- all galaxy clusters are moving away from other galaxy clusters. A common way to visualize this is by thinking of dots marked on a deflated balloon. As the balloon is inflated, the marks on the balloon will recede from each other.
The Big Bang
Several conclusions followed from Hubble's observations. First, the universe has a finite past. If we could travel backwards in time, we would see the universe becoming more dense as galaxies rush together. Unless the laws of gravity cease to apply, we will arrive at a singularity point, where all matter is compressed into a superdense state. This led to the conclusion that the universe as we see it today is the product of a single inflationary event, wherein all of the matter in the universe began expanding from a singular, superdense point.
Second, by determining the value of the Hubble constant, and using its corresponding rate to extrapolate backwards in time, the length of time since the beginning of universal expansion can be determined (assuming that the rate of expansion has remained linear). This works because Hubble's constant "is in units of kilometers per second per megaparsec, and since both kilometers and parsecs are measures of distance, 1/Ho gives the time, known as the Hubble time or Hubble age, since the inflation of the universe began" (Dalrymple, p359).
One piece of evidence that convinced many scientific skeptics of the big bang theory was the discovery and subsequent mapping of what is now known as the Cosmic Microwave Background Radiation. If the universe has expanded from a superdense, superhot state, theorists reasoned in the 1940's, then there should remain evidence of this in the form of a remnant electromagnetic radiation. Several quantitative predictions were made about the this remnant radiation: it should be about 3° Kelvin, it should display a blackbody spectrum, and it should have almost exactly the same intensity in all directions of space (isotropic).
In 1965, the background radiation was serendipitously discovered by Arnio Penzias and Robert Wilson at Bell Labs. Subsequent attempts to map the CMB precisely were highly successful, especially Nasa's Cosmic Background Explorer (COBE) sattelite, launched in 1989. The CMB was found to be 2.7° K, and was found to display the predicted blackbody spectrum. John Barrow notes:
"It was the most perfect blackbody spectrum ever seen in nature, and a striking confirmaton that the universe was once hundreds of thousands of degrees hotter than it is today. For only under such extreme conditions could the radiation in the universe assume a blackbody form of such high precision" (p 12).
Three years later, in 1992, when COBE finished its all-sky map of the intensity of the CMB, it became clear that it was isotropic as predicted. The CMB is the same in intensity to at least 1 part in 1000 in every part of the sky.
The Age of the Universe
Astronomers have used a variety of methods to determine the value of the Hubble constant, hence determining an "age of the universe". Some attempts (Sandage et al.) use Cepheid variables in distant galaxies and other standard candles. Others use the standard rulers, such as the ionized spheres which surround new-born stars. Florentine-Nielson (1984) used a completely different method to estimate the Hubble constant, based on the effects of gravitationally lensing in the precense of quasars.
Estimates based on these methods vary by about a factor of 2, but most calculations of Hubble's constant yield results between 50 and 100 [km per sec per megaparsec], with corresponding Hubble ages of between 10 and 20 billion years. Dalrymple (1991) lists 21 estimates of Hubble's constant from between 1976 and 1986, using a range of techniques. All but one fall between 50 and 100. More recent estimates of Hubble's constant based on Hubble Telescope observations agree within the lower limits of this range. Sandage et al (1996), based on their analysis of twenty or so Cepheids in the galaxy NGC 4639, estimate the Hubble constant at 57.
The Elemental Composition of the Earth and Sun
The elemental composition of our earth and sun shows them to be younger than the first suns. Our sun, like all stars, is composed almost exclusively of hydrogen, which was produced by the big bang, and helium, which is a normal byproduct of hydrogen fusion in stars. Taken together, these two simple elements account for about 95% of the mass of the universe. All the other heavy elements, so far as can be determined, are only produced during the death sequence of stars, when they run out of hydrogen fuel and begin fusing heavier elements.
Some stars, like out own sun, are called population I stars and are relatively rich in heavy elements. Other stars, like those which exist in globular clusterson the outskirts of galaxies, are called population II stars and contain very little heavy elements. The most plausible explanation of these facts is that population I stars like our sun are newer than population II stars, and that the heavy elements in our sun and earth are the fusion products of stars which lived and died long before our solar system was formed. Unless the heavy elements which compose our earth were created by entirely different processes than those which currently produce heavy elements in our universe (stellar fusion, etc.), then our heavy-element-laden earth and sun could only have come into existence long after the first suns.
Works Cited
Barrow, John D. (1994). The Origin of the Universe. Basic Books
Dalrymple, Brent G. (1991). The Age of the Earth. Stanford University Press, Stanford, CA.
Ferris, Timothy. (1997). The Whole Shebang: A State of the Universe(s) Report. Simon and Schuster, New York.
Rowan-Robinson, Michael. (1985). The Cosmological Distance Ladder: Distance and Time in the Universe. Freeman Publishing, New York.