PRESS RELEASE Date Released: Sunday, October 31, 2004 Source: University of Chicago Scientists zero in on why time flows in one direction The big bang could be a normal event in the natural evolution of the universe that will happen repeatedly over incredibly vast time scales as the universe expands, empties out and cools off, according to two University of Chicago physicists. "We like to say that the big bang is nothing special in the history of our universe," said Sean Carroll, an Assistant Professor in Physics at the University of Chicago. Carroll and University of Chicago graduate student Jennifer Chen are scheduled to post a paper describing their ideas at http://arxiv.org/ Thursday evening. Carroll and Chen's research addresses two ambitious questions: why does time flow in only one direction, and could the big bang have arisen from an energy fluctuation in empty space that conforms to the known laws of physics? The question about the arrow of time has vexed physicists for a century because "for the most part the fundamental laws of physics don't distinguish between past and future. They're time-symmetric," Carroll said. And closely bound to the issue of time is the concept of entropy, a measure of disorder in the universe. As physicist Ludwig Boltzmann showed a century ago, entropy naturally increases with time. "You can turn an egg into an omelet, but not an omelet into an egg," Carroll said. But the mystery remains as to why entropy was low in the universe to begin with. The difficulty of that question has long bothered scientists, who most often simply leave it as a puzzle to answer in the future. Carroll and Chen have made an attempt to answer it now. Previous researchers have approached questions about the big bang with the assumption that entropy in the universe is finite. Carroll and Chen take the opposite approach. "We're postulating that the entropy of the universe is infinite. It could always increase," Chen said. To successfully explain why the universe looks as it does today, both approaches must accommodate a process called inflation, which is an extension of the big bang theory. Astrophysicists invented inflation theory so that they could explain the universe as it appears today. According to inflation, the universe underwent a period of massive expansion in a fraction of a second after the big bang. But there's a problem with that scenario: a "skeleton in the closet," Carroll said. To begin inflation, the universe would have encompassed a microscopically tiny patch in an extremely unlikely configuration, not what scientists would expect from a randomly chosen initial condition. Carroll and Chen argue that a generic initial condition is actually likely to resemble cold, empty space-not an obviously favorable starting point for the onset of inflation. In a universe of finite entropy, some scientists have proposed that a random fluctuation could trigger inflation. This, however, would require the molecules of the universe to fluctuate from a high-entropy state into one of low entropy-a statistical longshot. "The conditions necessary for inflation are not that easy to start," Carroll said. "There's an argument that it's easier just to have our universe appear from a random fluctuation than to have inflation begin from a random fluctuation." Carroll and Chen's scenario of infinite entropy is inspired by the finding in 1998 that the universe will expand forever because of a mysterious force called "dark energy." Under these conditions, the natural configuration of the universe is one that is almost empty. "In our current universe, the entropy is growing and the universe is expanding and becoming emptier," Carroll said. But even empty space has faint traces of energy that fluctuate on the subatomic scale. As suggested previously by Jaume Garriga of Universitat Autonoma de Barcelona and Alexander Vilenkin of Tufts University, these flucuations can generate their own big bangs in tiny areas of the universe, widely separated in time and space. Carroll and Chen extend this idea in dramatic fashion, suggesting that inflation could start "in reverse" in the distant past of our universe, so that time could appear to run backwards (from our perspective) to observers far in our past. Regardless of the direction they run in, the new universes created in these big bangs will continue the process of increasing entropy. In this never-ending cycle, the universe never achieves equilibrium. If it did achieve equilibrium, nothing would ever happen. There would be no arrow of time. "There's no state you can go to that is maximal entropy. You can always increase the entropy more by creating a new universe and allowing it to expand and cool off," Carroll explained. http://www.spaceref.com/news/viewpr.html?pid=15407

ASTROFÍSICA Supernova inminente Las supernovas son fenómenos astronómicos muy interesantes pero hasta ahora casi impredecibles. La detección de tres poderosas explosiones en tres regiones totalmente diferentes, sin embargo, ha cambiado las cosas. Estos estallidos, que duraron sólo unos segundos, podrían actuar como sistemas de alerta temprana avisándonos de la inminencia de varias supernovas. 08:02 - 21/10/2004 | Fuente: AMAZING Los primeros dos estallidos, llamados destellos de rayos-X, ocurrieron el 12 y el 16 de septiembre. Fueron seguidos por uno más potente el día 24, que parecía estar en la frontera entre un destello de rayos-X y un estallido de rayos gamma. Si estas señales llevan de algún modo hasta las supernovas, como se espera, los científicos tendrían una manera de predecir estas explosiones y entonces observarlas desde que empiezan hasta que acaban. Los destellos se detectaron con el satélite HETE-2 (High-Energy Transient Explorer), y la información fue analizada por el equipo del Dr. George Ricker, del Massachusetts Institute of Technology. Ahora, diversos grupos de todo el mundo están trabajando en los datos para priorizar qué región de las tres debería ser vigilada más de cerca. Los estallidos gamma son las explosiones más potentes en el Universo, dejando aparte el Big Bang original. Muchos podrían ser causados por la muerte de una estrella masiva convirtiéndose en agujero negro. Otros podrían surgir por la fusión entre dos agujeros negros o estrellas de neutrones. En cualquier caso, el suceso produce dos chorros muy estrechos y potentes de radiación que se alejan en direcciones opuestas. Si uno de los chorros apunta hacia la Tierra, vemos esta energía como un estallido de rayos gamma. Los destellos de rayos-X, menos potentes, podrían ser estallidos de rayos gamma vistos mediante un ángulo ligeramente desviado respecto a la dirección del chorro de energía, de la misma manera que la luz de un faro es menos cegadora si es contemplada de manera menos directa. Los destellos de rayos-X son detectados por el HETE-2, el cual los analiza e informa de sus coordenadas. A continuación, otros observatorios pueden estudiar el "resplandor", que es todo lo que queda tras el destello, ya que éste dura sólo unos pocos segundos. Los destellos de rayos-X suelen estar más cerca de la Tierra que los estallidos de rayos gamma. Por tanto, si están relacionados con las supernovas, algo que sabremos pronto, entonces podremos estudiarlas con más detalle. Las detecciones serán más fáciles en breve, con el lanzamiento del observatorio Swift, que transportará tres telescopios (uno para los rayos gamma, otro para los rayos-X y otro para el espectro óptico/ultravioleta). Información adicional en: GSFC http://www.laflecha.net/canales/ciencia/200410192/