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Big Bangs Common To Our Universe?
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| occrider |
| quote: |
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
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Yea the title speaks of time but I thought the big bang info far more interesting. Basically a different variation of big bang to big crunch but with the same model of infinite time with no starting point. |
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| biznology |
| quote: | Originally posted by occrider
Yea the title speaks of time but I thought the big bang info far more interesting. Basically a different variation of big bang to big crunch but with the same model of infinite time with no starting point. |
whaddya mean! this is a political forum, ima flip flop on the issue!
interesting post tho| |
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| Streakfury |
An interesting read.
From what I understand though, String Theory says that there are in fact 10 dimensions in our universe, and the dimension in which our universe (and countless others) exists is called the 11th. It's the collision of these universes (rather like bubbles floating about in the air) that caues a big bang, and creates another universe.
| quote: | | 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. |
So what they're saying here kind of contradicts that, saying that the universe has always existed, but occupying an infinately small space, where everything was dark, cold and had little energy. Hmm, I wish I'd done Astrophysics now.
:conf: |
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| trancaholic |
Interesting read. However, the reasoning presented here seems to be fairly heavy on "yours infinity is smaller than mine", and "your impossible event is more likely than mine". Take
| quote: | | 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. |
for instance: They say that current explanations are flawed because the universe is unlikely to have an initial condition suitable for inflation theory to work. But that presupposes that the universe's initial condition would somehow be drawn from some sort of a uniform distribution. Why should it? And even if it was (the article is not that deep, so it's hard to judge their theory) some of the assertions they make seem to be at least as unlikely. |
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| tathi |
interesting read
have you read listened to Stephen Hawking - A Brief History of Time, Occ? |
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| cristalclair |
| okay- so scientifically i'm not very knowledgeable, and my brain cannot really grasp the entirety of string theory :conf: - but it seems that the ekpyrotic universe model makes more sense than the big bang theory with inflation. i read this article in scientific american that said in a few years there would be an experiment that would "prove" which one of these is correct... so maybe the battle will be settled soon? but that still doesn't resolve the nature of time... what IS it??? |
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| Q5echo |
well, which universe are we gonna pressume?
| quote: | The Entropy of a Closed Universe
If the universe contracts, no amount of technology will avert the impending doom, because, although life may continue until about a billion years before the end, all matter will then vapourise. Fortunately for civilisation the universe is unlikely to contract; quite apart from the fact that astronomers can only find 1% of the matter required to make the universe a 'closed' one, the laws of thermodynamics give a number of reason why a closed universe cannot exist.
The first problem arises when asking why the big bang happened at all, indeed why the universe is expanding. It has been shown that it is favourable in terms of entropy that matter falls together to form stars and galaxies. So why did the largest concentration of matter ever decrease its entropy massively by exploding outwards? Furthermore, how did it do it? Not even light can escape a singularity, and even if all the particles, by some stroke of luck, escaped, then gravity would pull them back into a singularity instantly.
The second contradiction arises when considering what happens between the big bang and the big crunch (the singularity the universe will become). As both singularities have the same entropy, any decrease in entropy (like the sun giving off light) must be matched by an increase in entropy at some other time - a clear violation of the second law. There have been a few attempts to get round this problem.
1. As the universe contracts, the second law of thermodynamics reverses, so then entropy always decreases. With a little thought, this is clearly wrong. As the law of thermodynamics reverses, it no longer becomes favourable for objects to fall together, so the law of gravity reverses. Thus the universe no longer recontracts. The only way to resolve this paradox is to say that entropy does not reverse.
2. The second singularity is more disordered than the first, so there is no entropy contradiction. Unfortunately, a property of black holes is that all black holes of a given mass, rotation etc. are indistinguishable. So to be more disordered, the second singularity needs to have gained mass or kinetic energy, all of which is forbidden by the first law of thermodynamics. |
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| quote: | The Entropy of Open and Flat Universes
The models of the open and the flat universe (which is a special case of an open universe) seem most likely to be correct, both in terms of observation and thermodynamics. I will first consider implications for the future and then examine the problems with the theory.
As the universe ages, all stars will use up their nuclear fuel and die, left as white dwarfs or black holes. The black holes will coalesce (join), becoming larger and eventually dominating the universe. It may be hundreds of billions of years, though, before life becomes impossible. We might have to move to a different star or galaxy, as ours would lose available energy, but we could survive. Problems with the theory start to emerge, though, when looking back into the past.
If the universe is finite, an open universe has the same problem of escaping from a singularity in the big bang as a closed universe. If the universe is infinite, however, there are even more problems, as all of space would not have fitted into a singularity. In other words, space would already have been in existence when the big bang happened, and the big bang caused all the particles to move away from each other.
However, the big bang is supposed to account for the entire creation of the universe. Yet a fledgling universe must exist first to be expanded - so where does this universe come from? Furthermore, for the big bang to occur simultaneously in all of the (infinite) universe, a signal must be transmitted instantaneously, violating the laws of physics governing the speed of information flow.
On the whole, the theory of the big bang creating an open universe is a successful one. It even explains one of the conundrums of thermodynamics - why the universe is ordered enough to degenerate into disorder. The universe was highly ordered at the beginning, because of the large amounts of gravitational energy it had following the initial explosion. Since then, this energy has been converted into higher entropy forms of energy, and entropy has consequentially increased.
In the next section, I will explore the variations of the big bang theory, to see whether they explain entropy better than the standard model. |
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