If the universe is seen to be expanding, what is the reason behind it?
The dynamical equations of geometry (discovered in 1915 and checked a lot since then) say that once geometry starts expanding it will tend to keep on (takes a lot to stop it or even slow it down much)
That's just how gravity works, and it works extremely well.
So why shouldn't distances be expanding? You tell me, if you can think of a reason, smart guy.
If you want links to some current research addressing the problem of how it got started in the first place, ask for links. I'd be really happy to supply some.
If it wasn't going to expand, what would be the alternative? It can't just sit still and not expand, because gravity would cause it to collapse.
Good! a really concise memorable explanation!
It's certainly an advantage to have pithy one liners that stick in the mind---to take care of really basic questions like this so everybody can move on.
What I was saying takes more words and is not as memorable: Once a geometry has started expanding then (according to the well-established 1915 law of gravity) it continues. You have to do something to it to slow or stop the expansion. You don't need any applied "force" merely to have it continue. It's hard to imagine what that would even look like.
If you want a law of gravity (i.e. a law of dynamically evolving geometry) where some weird "force" is required in order to cause expansion to continue, then you would have to throw out the Einstein law of gravity that we currently use and depend on, and find some other law of gravity where geometry behaves differently, and test it with lots of observations etc. etc. It probably wouldn't give the right numbers.
The upshot is as long as you accept the presentday law of gravity you don't have to explain geometric expansion continuing.
It's still a good question what got it started expanding in the first place. That is an active research area and people have various views and can reasonably differ on that. I wish the poster would have asked about that.
I suspect your reply is better, because more concise and memorable. We'll see how it goes, if McLachlan goes for another round and so on.
I think bcrowell wins for pithiness, but marcus has the upper hand when it comes to judicious use of bold facing
Hahah, the use of bold facing is much underrated skill.
I think i understand, thanks very much guys.
Marcus, if you have links to research i'd very much appreciate it.
Stuart, my answer will be a bit roundabout. Looking over the field from various viewpoints before picking one particular research paper. Or perhaps you will pick one for yourself.
With research papers, as you may know, even a fairly technical one can have stuff in ordinary English in the introduction section and in the conclusions paragraph at the end. Also just scanning titles and abstracts can give some idea of what's going on in a field. So if you like technical reading, great, and if not you may still get some clues from this.
A research area where among other things they model the start of expansion with models that don't break down and develop a singularity is called Quantum Gravity. The subfield is Quantum Cosmology. (When you quantize you can avoid the breakdown of the classical model.) These days I'm especially interested in watching QC phenomenology.
So I constructed this Spires search:
It is not perfect. It gets some stuff that I'm not interested in. But it's still pretty good. I'm only interested in recent QC pheno so I set "DATE>2008" which means all the papers are 2009 or later.
Phenomenologists look for ways to TEST and study the effectiveness of various observational tests. Early universe pheno people have gotten interested in Loop QC, which predicts a bounce, with certain characteristics. And they are asking what can we look for now in the CMB ancient light that would allow us to constrain or rule out or somehow test for the Loop QC bounce?
If you click on that Spires search you see a bunch of that kind of paper. Barrau, Grain, Wen Zhao, Mielczarek, Tsujikawa, Cailleteau are phenomenologists. They don't have a professional stake in Loop, their business is testing. Some of the papers are by pheno people, and some are by Loop people (like Ashtekar, Sloan, Corichi, Wilson-Ewing.)
Loop QC has a clear enough explanation for the start of expansion because the Loop people found a way to quantize the basic equation of cosmology where quantum effects cause gravity to be repellent at high density. So they put their model in the computer and run time backwards and find a prior collapsing phase and a bounce. The expansion gets kicked off in a very clear way. Then as soon as density goes back down the model converges to classical behavior and continues on the familiar track. The regime where quantum effects dominate is where the density is around 1 percent of Planck density or greater. The bounce happens at around 40% of Planck density.
The key thing now is to test. It may require putting some new intruments in orbit, to map the polarization of the CMB in finer detail than is currently possible. It may be possible to constrain LQC or even rule it out. Or it may pass the first observational tests.
So that's one thing.
Another thing is to look at the whole field of Quantum Cosmology. Here is a Spires search with the same date restriction of DATE>2008. This is a very broad search and does not pick out papers with a Phenomenology focus. It gets 245 papers of all QC kinds (not just bounce-type and not only Loop).
You might look down the first 20 or 30 (or 50 why not?) on the list of titles to get an impression of what the main topics are and who the main authors are of the more highly cited papers. The more highly cited ones are listed first.
I'd rather not point to one specific paper until you have had a chance to get a general impression of what research is going on. Also do you have any questions about what ypu see in these Spires searches?
Although I don't understand the math, I like the idea of a bounce proposed by LQG. It does seem to beg the question of why black holes don't bounce, given that relativity predicts a singularity of infinite density (which we know to be wrong). I was wondering if the math of LQG might predict a volume at the core of a black hole that is greater than zero, thus limiting the density to less than 40% of Planck density? Forgive me if this is a naive question or one that has been covered.
We don't know whether black holes bounce or not. The collapse could conceivably lead to a new expanding tract of spacetime, or it could stall (as you may be suggesting.)
Various Loop researchers have studied black hole collapse and attempted to get an answer out of the theory. My imprression is that they came to different conclusions. The results so far are not satisfactory.
It is harder than studying the collapse of an entire universe, because in that case there is no outside. the whole thing collapses. It is comparatively simple to study, and you get a bounce. It transitions to an expanding phase of the same universe. The whole thing has been repeatedly run in computer, exploring the range of input parameters and various assumptions (inflation, cosmological constant, different kinds of sample matter)
With a black hole the original space continues to exist, you have a horizon, you have whatever happens at the pit----and a separate expanding region if a bounce occurs.
People's modeling attempts have tended to involve patchwork.
Studying BH collapse is also harder than static black holes. Most of the Loop black hole literature has been about BH entropy, assuming a static black hole, taken as a given. You don't worry about how it came into existence or what might have occurred down at the very pit.
Intuitively you are right. Loop gravity OUGHT to be able to say something about the process of BH collapse, and a possible bounce. But so far AFAIK this has proven intractable.
Different researchers say different things and no conclusive answer. Oddly enough the cosmological singularity turned out much easier for Loop gravity to resolve and develop into something potentially testable.
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