
#1
Nov2907, 03:43 AM

P: 3

This is a lecture by Sir Roger Penrose that I was lucky enough to attend. Here is the flash presentation of it:
http://www.newton.cam.ac.uk/websemin.../1107/penrose/ From what I can understand his theory is that the big bang happens after the end of the universe, starting a new one. The universe can end because eventually all matter falls into blackholes and blackholes evaporate into energy. So the universe ends up being nothing but energy at very high entropy. He says that when there is no longer mass in the universe time no longer applies. This is because there is only energy left and energy perceives infinite time as no time at all. So the end of universe is infinitely in the future but with only energy left that infinite time is reached. At that limit of infinity only energy and the gravitational waves left by the blackholes are left and the waves translate to density variations in the next big bang. In the beginning of the next Big Bang (which is also a limit that mass cant approach but energy can) only energy is there with density variations based on the gravitational waves from our current universe. What happened before the Big Bang is happening now! So that's what i got from the presentation. Its a very elegant theory because of its simplicity and seems intuitive to me. Only thin i don't completely understand is why is it that with only energy left you can approach infinite time where as with mass you cant. And how does the infinite limit of end of time translate to the beginning limit of time? ("limit" being a mathematical point at which you can approach but may never reach. 



#2
Nov2907, 04:39 AM

P: 3

Attached is a .ram file that streams the interview with Roger Penrose where he talks about his theory amongst other topics. Very interesting and complements his lecture.




#3
Nov2907, 08:14 AM

P: 131

He seems to be saying that the decay of the last remaining particle into energy results in infinite entropy which he says is synonymous with zero entropy which is a new singularity. Since both space and time arise from the singularity, the event of the the decay of that last particle is the finite event that translates into the begining of time.




#4
Nov2907, 01:16 PM

Astronomy
Sci Advisor
PF Gold
P: 22,809

What happened before the BIG BANG, Roger Penrose's Theory
Great talk.
I heard him give the same talksame slides etcin person at the Math Institute at UCB. I'm glad you made a thread about it. We have used that link to the 2005 online version before in some PF threads, but it has been some time ago and i don't remember which. No use delving back in time to find them. For me, the whole crux of his talkthe linchpin or fulcrum that it all depends onis the picture of phase space: the map with the small and large regionswith the system doing a kind of random walk squiggle as it goes along, getting into larger and larger regions. that is his picture of the Second Law. and everything he says in the talkthe whole motivation for his speculative cosmology ideadepends on that picture. The trouble is THAT PICTURE REQUIRES SPECIFYING AN OBSERVER. to define entropy you need to say where the observer is whose map of phase space is being usedin whose eyes are some regions large and some smallwho defines what the macrostates are. the second law basically says no one should expect to observe a decrease in entropy. but if the universe collapses, and there's a bounce (as in quite a bit of recent quantum cosmology work) and a new expand phase, then where is the observer who sees a sudden decrease in entropy? Neither Mr Before nor Mr After sees. So Penrose can not rule out a simple bounce scenario (kind of thing that is much more offtheshelf than what he eventually starts describing) 



#5
Nov3007, 12:04 AM

P: 3

His theory does not really center around the phase space chart . The diagram makes no new claims its just a visualization of 2nd law thermodynamics. His theory centers around how the end of the universe in its infinity is the same as (or leads to) the infinity of the beginning. From what I can understand certain conditions remain at the end at are in common with the beginning. 1) only energy is left. 2) density variations are present as gravitational waves. 3) timespace is lost or redefined. Maybe its the same in the math. I am still a little fussy as to why timespace is lost. Time is lost some how because with no mass there is nothing to keep track of time or nothing that is influenced by it. So if one second passes or an eternity passes it would not make a difference to the energy that is left in the universe. As for space, I'm not too sure. But somehow the infinite space would be the zero space that is the singularity of the big bang. Any thoughts? 



#6
Nov3007, 03:57 AM

Astronomy
Sci Advisor
PF Gold
P: 22,809

Please read what I said
I've seen the talk live and several versions of it online. he always begins by emphasizing and expounding the 2nd law because it plays an important role in what he's going to say. what he does is, he starts by REJECTING bounce scenarios on 2nd law grounds. but bounce scenarios are the natural thing to get into when you discuss prebig bang. his own theory starts by rejecting bounce and would be poorly motivated otherwise (because his is very far fetched, he himself refers to it as "crazy" or "outrageous") In fact I think that Penrose nonbounce idea has failed to get much attention, and is fading from sight, primarily for the reason i mentioned. His 2nd law argument for rejecting bounce is flawed. Therefore people increasingly are studying bounce scenarios, and not studying Penrose idea. Bounce cosmology papers get way more citations than his (check the SPIRES index if you want) In effect, for him, the motivation depends on a 2nd law argument which is basically unconvincing. however I love the talk. he is brilliant and his handdrawn slides communicate vividly. One can learn a lot from the talk even if the "crazy idea" proposal may not be destined to catch on. 



#7
Nov3007, 01:57 PM

Astronomy
Sci Advisor
PF Gold
P: 22,809

The question came up what are bounce cosmology scenarios
and how do they compare with what penrose proposed in his 2006 Before the Big Bang paper? The thing about Penrose idea is it has not gotten any citations. He published the paper in early 2006 and it has zero cites so far. Despite the fact that he has given his wonderful talk promoting it at many places (Berkeley, Perimeter, Cambridge...) I have my own ideas about why his approach is flawed but dont need to go into that. The leading quantum cosmologists, if you go by citations in the literature, are Bojowald, Ashtekar, and Param Singh. http://physicsforums.com/showthread....10#post1523310 They all work on the BOUNCE picture of before the big bang. Run computer models, calculate critical density and other stuff. they study the universe evolving across what used to be called the "singularity". So one can reasonably ask, and someone was asking just this morning, what are the various bounce scenarios? It just happened that this morning someone brought up an older thread about that called "Bounce replaces bang". The first few posts give some links and discuss some issues related to bounce cosmology. Here is a link to that thread: http://physicsforums.com/showthread.php?t=184869 If somebody wanted to do a survey of bounce cosmology research it could get lengthy. Many many people, many ideas. But one way is just to skim the cream, and just look at the most cited bounce work. that is basically papers by those three Bojowald Ashtekar Singh with their many collaborators. Roy Maartens, observational cosmologist involved in testing theories, has gotten in on it too. For people like him it is a good thing to TEST and maybe shoot down. Should lead to visible signatures in the CMB and early universe structure formation. the basic reason you get bounce is because when you quantize gravity (by quantizing GEOMETRY dynamics) then quantum corrections tend to show up which take over at very high density and make gravity repell instead of attract. this happens in various ways in various models but it is pretty robust, always seems to happen. so when you quantize gravity you seem not to encounter a singularity at the big bang and you can keep on evolving the model back in timeit is one of the main things that quantum cosmologists do Hope this helps answer whatever basic questions people have about cosmological bounce scenarios 



#8
Dec107, 12:05 AM

P: 131

Hopefully this question is not too far out of line.......
But in the Bounce Scenarios, when gravity reverses at high densities and repells, does the time arrow turn around too? 



#9
Dec107, 08:05 AM

Astronomy
Sci Advisor
PF Gold
P: 22,809

By my standards a global time reversal would be kind of exotic. there are fringe theories like Peter Lynd where time reverses but I can't say I understand or know much about them. what we're talking about is more mundane and ordinary: when you quantize anything, you expect extra terms appearing in the equations called quantum corrections. they are always there but in the classical or low energy or macroscopic regime they are negligible. they amount to so little you can ignore them. then when you move into some more extreme situation, the correction terms ramp up and become importantand can eventually dominate. so that is pretty usual in whatever area of physicsand it also looks like it may apply in the case of gravity. when you quantize spacetime geometry (i.e. gravity) you get an equation for how geometry evolves and as you would expect from other areas of physics, it has quantum correction terms. these terms are small and unimportant when you evaluate at low density, so you get familiar behavior as density increases, the correction terms become larger and more important, until at near planck scale they actually dominate. I am talking about the models used by the more highly cited Quantum Cosmology people (Ashtekar, Bojowald, Singh,...) mentioned earlier. actually it is a mild oversimplification to say that gravity repels at high density (that's only how to say it in 5 words or less) in the quantum gravity models I'm talking about, gravity is just the same equation at all densities, but at low density the correction terms are small and they gradually get more important as density increaseseventually they dominate, so the net effect can be repellent. it is not something to picture as an abrupt reversal, like a sign flip or a mirror symmetry. it is more routine. like routine stuff in other areas of physics where the type of effects you see can depend on temperature, pressure or magnetic field strength ================= Kevin Vandersloot, who got his PhD with Ashtekar and is now postdoc at Portsmouth, just posted a paper about black holes in which he found that collapse did not bounce! the correction terms kicked in as expected at high density, but not fast enough to cause a bounce. what happened, in Kevin's model, was that the effects canceled in such a way that this very high density phase stabilized. He gave a seminar talk at ILQGS last weekthe audio and PDF slides are online. roughly the conclusion would be that BB and BH are different. It looks like BB singularity gets replaced by bounce, but BH singularity might very well not get replaced by a bounce but by something else. There have been papers suggesting this for 2 or 3 yearsthis is another theoretical nail in the coffin of black hole bounce. none of this work has been confirmed experimentally 



#10
Dec107, 01:36 PM

P: 131





#11
Dec107, 02:04 PM

P: 32





#12
Dec107, 02:21 PM

Astronomy
Sci Advisor
PF Gold
P: 22,809

I don't know what might happen in reality. You and I can try not to confuse the model with reality. In the LQC modelsa lot of variations have been tried and the bounce is robusttime looks to me like it becomes merely an ordering of events. One loses touch with the familiar idea of a clock measuring durations that you could compare with later durations in a replicable way. It looks to me like a lot is lost making it thru the former singularity, including some standardized clocktime notion of duration. But maybe there is a more reliable unambiguous time coordinate in the recent (20062007) work. We should probably go back and look over the latest Ashtekar paper to see what is becoming of time in the most recent work. It's a fastmoving field of research and I can't always manage to keep up. Maybe you could have a look: http://arxiv.org/abs/0710.3565 On the robustness of key features of loop quantum cosmology Abhay Ashtekar, Alejandro Corichi, Parampreet Singh 28 pages, 1 figure (Submitted on 18 Oct 2007) "A small simplification based on well motivated approximations is shown to make loop quantum cosmology of the k=0 FRW model (with a massless scalar field) exactly soluble. Analytical methods are then used i) to show that the quantum bounce is generic; ii) to establish that the matter density has an absolute upper bound which, furthermore, equals the critical density that first emerged in numerical simulations and effective equations; iii) to bring out the precise sense in which the Wheeler DeWitt theory approximates loop quantum cosmology and the sense in which this approximation fails; and iv) to show that discreteness underlying LQC is fundamental.... An effort has been made to make the underlying structure transparent also to those who are not familiar with details of loop quantum gravity." I guess that the new thing in 2007 is these EXACTLY SOLUBLE MODELS which DO have a coordinate time. But from 2001 thru 2006 they were using discrete step by step difference equation models, which they had to run using the computer because there was no slick formula analytical solution. I still think those earlier discrete numbercrunch models (which don't have as clearly defined time) are more basic, and the exactly soluble models are a kind of approximate substitute that is easier to play around with. I would suggest having a look at Ashtekar's October paper just to see where the field is at present. Parts of it (like at beginning and end, introduction and conclusion paragraphs) will be readable and can give you a taste. 



#13
Feb612, 11:46 PM

P: 90





#14
Feb712, 12:15 AM

PF Gold
P: 11,057

Yont1, this thread has been dead for over 4 years. Please don't bring up old threads. Plus your whole post borders on nonsense.




#15
Feb712, 12:18 AM

P: 90





#16
Feb712, 12:21 AM

PF Gold
P: 11,057





#17
Feb712, 12:27 AM

P: 90




Register to reply 
Related Discussions  
Roger Penrose's book  Science & Math Textbook Listings  23  
Penrose's new ideas on the big bang  General Physics  20  
Penrose's new ideas on the big bang  General Physics  20  
What happened before the Big Bang?  General Astronomy  34 