Big Bang or Big Bounce: Which Theory Explains Our Universe Better?

[Chimps]

I've been trying to understand the big bang theory recently and would appreciate some expert opinions.

I was talking to a physicist who is convinced that there was no big bang and that in fact we are living in a 'big bounce' Universe because apparently that is the implications from string theory.

Can anyone help me to understand the difference and what this implies?

 

[twofish-quant]

I was talking to a physicist who is convinced that there was no big bang and that in fact we are living in a 'big bounce' Universe because apparently that is the implications from string theory.

There is a very speculative idea called loop quantum cosmology in which it turns out that when you compress things to big bang like densities gravity becomes repulsive. So one of the ideas that people are working on is that there was a universe pre-big bang that collapsed, when things got hyper-dense, gravity became repulsive, and then the collapse became an expansion.

People are really just guessing about this, but the nice thing about this idea is that you can maybe show that it happened, when the collapse becomes as expansion, the theory predicts that there will be some fluctuations, and you may be able to calculate what those vibrations look like and maybe see it in the cosmic microwave background.

Again, this is all at the "interesting idea that people are working on" stage. There is a lot of theoretical and observational work around this idea, and we should be in a better position in a few years to know if this idea will work out or not.

 

[marcus]

...
I was talking to a physicist who is convinced that there was no big bang and that in fact we are living in a 'big bounce' ...

Twofish provided a clear carefully worded response. I couldn't say it better.
The bounce idea is getting a lot of attention. Computer modeling. Analytic modeling by equations. Different models and assumptions leading often to the same conclusion.

But the models have to be tested---they have to derive details from them that the models predict will be observed and then go look for them in the data. That's going to be hard.
Read twofish's post carefully---he doesn't say we have answers yet.

Most of the bounce work is not coming out of string theory. Nowadays you scarcely ever hear of string in this context. But who knows what your physicist was really talking about?
There may be a new bounce scenario brewing in the string community as well. The old string bounce scenario associated with the name Veneziano and "pre-big-bang" is something you don't hear about much. The papers were from before 2002 or 2003. It pretty much dropped out of sight. There is however a string theorist named Horava who has developed a non-string quantum gravity approach people call "Horava gravity". It has no strings and no extra dimensions---just 4D. Horava gravity is one of several non-string approaches that predict a bounce.

So it's hard to say what the connection with string could be. But the research topic is hot. If you want an overview here is a listing of some 400 "quantum cosmology" papers that have appeared in 2006-2010, in order of the number of citations. The most frequently cited papers listed first. Almost all the first 100 or so papers are about early universe models where there is a bounce.
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=dk+quantum+cosmology+and+date%3E2005&FORMAT=WWW&SEQUENCE=citecount%28d%29
These are not papers I'm suggesting you read! Just scan the list of titles to get an idea of what's going on in early universe cosmology. If you see the word "Loop" it's probably a paper using the Loop Quantum Cosmology approach that twofish mentioned in his post, one of those where the singularity is replaced by a bounce.

 

[twofish-quant]

Most of the bounce work is not coming out of string theory. Nowadays you scarcely ever hear of string in this context. But who knows what your physicist was really talking about?

LQC takes a fundamentally different approach to string theory. The idea of string theory is that we take all of the forces of nature and assume that they are different "vibrations" of some higher dimensional object. The idea is that we look at all of the forces of nature and try to come up with something that looks like them by different vibrations. Nice idea. People have been trying for 20 years and haven't gotten very far.

The idea behind LQC is different. Instead of looking at all of the forces of nature, let's forget about all of them except for gravity. We aren't going to try to explain EM, strong forces, weak forces, different types of particles. All we are interested in is gravity. If you take the equations of gravity and you *assume* that space is "chunky" then you start coming up with some predictions. They might be *totally wrong* predictions, but the reason that people are looking at this seriously is that *totally wrong* predictions is more than what people have come up with in string theory.

Why do you assume space is "chunky". Well if you assume space is smooth, you end up with divide by zeros all over the place, so if you assume space is chunk you don't get divide by zero problems. Well aren't you making a semi-bogus and random assumption just to get some numbers to work out? Well yes. Got a problem with that?

The most frequently cited papers listed first. Almost all the first 100 or so papers are about early universe models where there is a bounce.

At this point the heavy duty mathematicians come in. One obvious question is if you look at two different models are they really different?

One thing that mathematicians like to do is to put things into categories. You can put spheres and cube in one category. Donuts and inner tubes into another category. (things with holes, things without holes) A lot of heavy duty math comes in trying to put different theories into categories (theories with bounces, theories with no bounce, theories that do something weird) And then there is also the heavy duty math that you need to get some number out.

 

[Chronos]

Bounces are difficult to model, as twofish noted, the math is frightfully complicated and requires unproven, and possibly unprovable, assumptions. It is, however, fascinating. The right model, if one exists, could provide valuable insights on the nature and workings of this universe.