I read an article in the magazine Scientific American about how some believe that instead of a Big Bang, there was a Big Bounce. Now, I'm sort of new to this kind of "advanced" science, and therefore a little confused about some of the wording they used. Can anyone explain this in more lay mens terms?

--L.T.

Hi and welcome to Baut.

I can't explain it, but the following link might help. http://en.wikipedia.org/wiki/Big_Bounce

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Yukon Cornelius

Thanks! I get the basis of the theory, but, a kind of ironic thing is that, the other day I was watching a show on the Big Bang. They talked about how aroung 400,000 years ago, the dispersion of matter throughout the universe was so uniform that the temperature varied no more than 1/5,000 of a degree. This got me thinking that maybe the universe expanded at an un precidented rate during those first 400,000 years that the universe's mass was too far away to be detected today and then suddenly pulled back at an equally incredible rate, like when you strech a rubber band out, then let go of it. The next week, I read that article in Scientific American which further caused my belief in the subject, and a want for more information. Does anyone know about Quantum Loop Gravity. And I kind of feel stupid for this but, other than what the symbols mean and that time slows down as you reach light speed, I don't really know anything about General Reletivity. Can anyone explain either of these to me?

--L.T.

Hi and another welcome! If I understand you correctly here I think you are describing inflation, a theory which states that the Universe grew exponentially in size for a very brief moment right after the Big Bang.

I can't offer any concise info on QLG but I can point out that Einstein's General theory of Relativity describes the shape and interactions of space-time whereas his Special theory of Relativity is all about the nature light.
Happy researching!

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All,
Neither can I be a guide to Loop Quantum Gravity but you may want to read the latest New Scientist (13th December 2008). It includes an article (From Big Bang to Big Bounce, p.32) that describes recent work using Loop Quantum Gravity to produce a Loop Quantum Cosmology (LQC) theory. This is the work of Abhay Ashtekar of Pennsylvania State U.

The full article is available for a while at http://www.newscientist.com/article/...-big-bang.html

The implication of applying quantum mechanics in this way is to find that the Plank density is a limit. If the Universe contracts under gravity to that extent, then repulsive forces "bounce" it back out again. LQC has recently been refined so that it matches conventional, relativistic cosmology, in particular to allow smooth space-time at the beginning of inflation, and inflation itself, without presuming 'inflatons'. It also offers predictions that may be tested experimentally.

I found the article fascinating, although I can't criticise these theories. If you feel you can, you can read a recent article by Carlo Rovelli, one of Ashtekar's associates by downloading it from: http://arxiv.org/abs/0805.4585v1

Does this melding of Relativity and quantum theory bring us closer to a Unified Theory?

JOhn

Thank you John, that was a good article.

--L.T.

Yes, pretty good... for a news article.

That's a pretty good trick, considering the Heisenberg Uncertainty Principle....

That may be tested... in principle, but far from practice. Such test involves detecting the gravitational wave background. Such a test may not be doable for decades if not 100 years in the future. We have yet to detect the gravitational wave foreground... or any gravitational wave whatsoever.

The gravitational wave background is also called upon as a test of Alex Vilenkin's multiple universe theory, as described in his recent book Many Worlds in One [2006].

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And just to clarify, it's not just the existence of the GWB, but that it should have particular characteristics.

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Everyone is entitled to his own opinion, but not his own facts.

I can give a pointer in less layman's terms: Dark Energy and the Return of the Phoenix Universe by Lehners and Steinhardt. Read a paper, get brane, to paraphrase Akademiks. To be honest is more than my little brane can bear.

One thing that confuses me about the article is if it is millions of years that the universe is expanding and the crunch would be faster that the acceslleration how long would the crunch take respect to the time the universe has expanded.

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From Living Reviews in Relativity: Loop Quantum Gravity by Carlo Rovelli and Loop Quantum Cosmology by Martin Bojowald. Loop quantum gravity (LQG) is the leading competitor with superstring theory (SST), but not as ambitious. LQG is a theory only for quantizing gravity, but does not attempt to combine gravity with any other force in a single theoretical framework. SST, on the other hand, is necessarily a quantum theory that unites all of the 4 classical "forces" (strong, weak, electromagnetic & gravity, despite gravity not being a force at all in the framework of general relativity) into a single theoretical framework. Neither has actually achieved its goal as of yet, nor do we actually know that either can achieve its goal.

In a purely general relativistic cosmology the initial state of the universe is a singularity, meaning that it is completely undefined (and undefinable in the strictest possible sense). But a quantum theory of gravity (either LQG or SST) could eliminate the singularity and make it possible, maybe even reasonable, to talk about what happened "before" the big bang, or what might happen in the event of a big crunch. If I understand the LQG cosmological bounce business correctly, in the event of a big crunch the energy density of the quantum foam of spacetime becomes too great, and at some maximum value it results in a sudden transition to a strongly repulsive regime, and the universe bounces back from its maximum density at the crunch, to begin expanding once again (i.e., Bojowald, 2008; Mielczarek, Stachowiak & Szydłowski, 2008; Singh, Vandersloot & Vereshchagin, 2007 & etc.). But there are also scenarios for a cosmological bounce in SST (i.e., Greene, Kabat & Marnerides, 2008; Brandenberger, Firouzjahi & Saremi, 2007 & etc.) as well as other ways to generate cyclic cosmologies from SST (i.e., Lehners, 2008).

One important thing we see about cosmology at the dawn of the 21st century is that it is no longer reasonable to criticize as "meaningless", the question "What happened before the big bang?".

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

The Big Bounce is an intriguing theory. I especially like the speculation about the potential for leaving messages for the inhabitants of the next bounce in the cycle.

Could circumstantial evidence be found in our universe today which might be consistent with there having been intelligent biology in the last universe? I would not go as far as suggesting that the inhabitants of one universe can make the rules for the following one, but perhaps it can change the odds to make a certain outcome in the next universe more probable.

I realise this is complete unadulterated speculation :-)

Nevertheless, a good setup for a science fiction story. Lots of directions you could go with this!

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Everyone is entitled to his own opinion, but not his own facts.

As one of the contributors to the Big Bounce wiki articles, I'll have to stay out of this as i'm a bit on the biased side. And freely admit it.

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I just saw Hayley's comet, she waved, Said "why you always running in place? Even the man in the moon disappeared, Somewhere in the stratosphere" - Shinedown

http://worldsofothersuns.home.comcast.net/

Another possibility might be that of the ekpyrotic universe theory


source:http://news.softpedia.com/news/Which...se-62703.shtml

Oh, come on. Who doesn't have a little bias?

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Everyone is entitled to his own opinion, but not his own facts.

Well as a former biologist, I cant help but be a cycles guy, so it makes sense to me that the universe would either go crunch, bounce or boing! The big freeze just doesn't seem to be as elegant of solution.

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