Paul, if you want to ask questions about conditions before the Big Bang then you have to go to quantum gravity models that deal with that.What was the force holding the matter together before the Big Bang?
Gravity, electromagnetic forces, or something completely different?
Thermodynamic entropy requires the viewpoint of an observer who decides what is microstates and what is macrostates----so how many microstates are subsumed in one macro.Marcus, I remember I red when I was young (in 80x :) ) that as(if) entropy is preserved during the bounce, then the entropy after the Big Bang should be ery high, and it is not cosistent with the obserations. So, what is different now?
Yes, but. The flocking behaviour of post-docs is a subject in itself. How many times in the past 30 years have such examples of enthusiasm born concrete fruit?The main line of research that is currently dealing with conditions before and at the BB time is called Loop Quantum Cosmology.
The top 20 papers are almost all about explaining the Big Bang as a bounce, or rebound from a prior collapse. A mechanism has been conjectured that makes this happen and there has been a surge of interest in studying this.
Heh. Mr After has pretty thick skin, Dima. He tolerates high temperature in any regime mild enough for temperature to be meaningfully defined. But seriously, we ourselves are representative of the observers after the big bang, and we can talk about entropy during inflation. Our thermodynamic perspective can extend continuously all the way back through the presumed inflation episode. All the stuff that happened since the start of expansion can be pictured as a continuous trajectory in our state space. We define the macro and micro states from our ("After") perspective. Remember that inflation is described within a classical geometry framework.For example, I can critisize your "Who is going to build a perpetual machine to extract illegal work and violate the law?" by saying that in your case we cant talk about the entropy during the inflation era and soon after that, because there could not be any observers and machines, in principle (empty space or too hot)
Why is that a relevant question, Apeiron? LQC does not predict a recollapse. It describes a prior collapse, which could depending on which model, be a unique one-time event. There are some interesting questions connected with collapse in the case of positive cosmological constant and Ashtekar has a paper in preparation about this, co-authored with Tomasz Pawlowski. It has been cited in something I saw but I haven't seen the paper yet. I hope if you are interested that we can discuss it when it comes out.If there is an acceleration (as observation suggests) then how does gravity achieve the recollapse?
Are we still talking about the LQC model (or models)? I hope so. I don't fully understand how your comment applies. Certainly it is true.Yes, an yet another thing: fine tuning that our universe is almost flat. And that fine tuning had to be extremely precise!
Dima, I enjoy your comment very much! Your perspective is different from mine. My bias is toward the empirical observational as opposed to aesthetic.Or even a stranger solution where before the rebounce there was a contraction from t= - infinity?
I know. It is always a fruitful time for me when the modem breaks or the ISP interrupts service. Reading things on actual paper.... You know, there is time to post in the forum and time to read longer articles, when nobody interrupts...
Thank you for so many links...What is the best to begin with?
If there is an elephant in the room, it is always a relevant fact to mention.Why is that a relevant question, Apeiron? LQC does not predict a recollapse. It describes a prior collapse, which could depending on which model, be a unique one-time event. There are some interesting questions connected with collapse in the case of positive cosmological constant and Ashtekar has a paper in preparation about this, co-authored with Tomasz Pawlowski. It has been cited in something I saw but I haven't seen the paper yet. I hope if you are interested that we can discuss it when it comes out.
Hey thanks for that. Shame its in Italian but I will follow that up. I've of course studied Anaximander intensively and he is a remarkably misunderstood and underappreciated philosopher.Hi Apeiron,
(BTW since apeiron was the favorite idea of Anaximander of Miletus, you might be interested to know that Carlo Rovelli has competed a book about Anaximander---Rovelli has history of science as a sideline. A link to the draft copy is at http://www.cpt.univ-mrs.fr/~rovelli/ )
I understand your point but from my perspective, there are multiple avenues to explore, LQC is not the only game in town for me. So I want to look down an avenue and gauge how likely it is to reach the destination.I don't need to have all the questions I can think of answered immediately. Like, were there a lot of prior bounces, or just one? That's speculative and aesthetic, not practical.
I can't say about the specifics but I do know that Rovelli is an Anaximander fan.. I've of course studied Anaximander intensively and he is a remarkably misunderstood and underappreciated philosopher.
...If Rovelli is into Anaximander, perhaps he too has this thought in mind?
Isn't this "quantum foam" simply a proposed "the singularity explained"?Personally, I see the story as the universe dissolving to a "quantum foam" rather than arriving at a singularity. So quantum gravity approaches are a good avenue in that regard. But it seems "obvious" to me that a quantum foam is now a vague state, an apeiron indeed, and not a crisp state. Therefore if you can indeed project the correct calculations beyond the planckscale first moment, you would have to get "something even foamier" rather than instead the foam for some reason turning crisply into a mirror universe the other side.
Loop Quantum Cosmology (the LQG bounce model) is compatible with inflation scenarios (which provide the correct CMB power spectrum and BAO). However it would be more satisfying if one could get some specific results without having to invoke inflation....
BTW, does LQG bounce gives the correct CMB power spectrum?And acoustic peaks?thank you.
Today's researchers typically do not assume that a cosmological singularity actually existed. You should read "A Tale of Two Singularities" at the Einstein Online website.It seems to me the the "singularity" is just a concise version of "there be dragons". It is the most efficient manner of referring to that which we simply cannot, as yet, explain...