|May4-06, 02:42 AM||#1|
May i ask how work on this theory is progressing, there was a lot of talk
about it at one time, i think some one has a new/ or improved theory, and
maybe a paper soon, i may be wrong.
|May4-06, 09:54 AM||#2|
Or it could be discussed in Cosmology forum (where it would necessarily draw on beyond-standard work).
In the Loop Quantum Cosmology (LQC) model there are quantum corrections to how gravity behaves which dominate the situation at very high density and make gravity REPULSIVE at very high density.
this means that you can't get singularities. As you extrapolate back in time using the LQC model (reversing the observed expansion) you get to a regime of very high---Planck scale---pressure temperature and density but before the model goes completely haywire and tries to compute infinities it TURNS AROUND.
At a high but finite concentration the repulsive effect takes over and you enter a mode where the universe expands as you work back in time.
the conclusion in LQC is that the expansion of the past 13 billion years was preceded by a contraction phase.
The LQC reproduces classical results to high accuracy after you get clear of the singularity, on either side.
So on either side of the turnaround, before or after the ex-singularity, you get a standard-looking Friedman Robertson Walker universe. FRW universes come in two styles, contracting and expanding, and you get one of each----LQC closely approximates the contracting FRW before, and the expanding FRW after.
Your question which applies especially to this "Beyond" forum, is what sort of PROGRESS is being made currently with this LQC model. I will try to answer.
|May4-06, 10:09 AM||#3|
What have been the recent developments in LQC?
There are a dozen or so active researchers (maybe more but that's how many come to mind) and the main ones are Bojowald and Ashtekar. So you can get an idea of developments in LQC by looking up what they are doing. There are two sorts of progress evident.
1. They keep weakening the assumptions to make the results more robust. that is, they extend the results of LQC to cover more and more cases.
A recent paper of this type was Bojowald's http://arxiv.org/gr-qc/0604105
The point is that cosmologists often simplify by assuming a universe that is uniform or symmetric in the sense of being "homogeneous and isotropic". The first LQC paper that removed the bang singularity assumed that----back in 2001.
To remove the singularity WITHOUT assuming homog, or without assuming iso, is more difficult---you don't have such a simple model. So gradually they have been dealing with more and more complicated cases.
===to give the flavor I quote the abstract of gr-qc/0604105===
Singularities in Isotropic Non-Minimal Scalar Field Models
M. Bojowald, M. Kagan
Non-minimally coupling a scalar field to gravity introduces an additional curvature term into the action which can change the general behavior in strong curvature regimes, in particular close to classical singularities. ... This paper addresses the classical singularity issue in isotropic models and extends singularity removal in quantum gravity to non-minimal models.
2. The other kind of progress being made by LQC is that it is gaining acceptance. There are signs that it is gaining precedence over other QC and becoming the benchmark QC. I will list some of these signs.
|May4-06, 10:30 AM||#4|
Here are some signs that LQC is moving into position of being the benchmark type of QC.
Major science publisher Ensevelier has an Encyclopedia of Mathematical Physics. They invited Bojowald to write the article on Quantum Cosmology for their encyclopedia. It is a 10-pager and the preprint is online.
The Planck Institute has a kind of online encyclopedic collection called LIVING REVIEWS IN RELATIVITY. They got Bojowald to contribute a 100-page article on Loop Quantum Cosmology.
KITP Santa Barbara is a prestigeous theoretical physics institute headed by Laureate David Gross with many famous names on board. KITP has been mainly associated with STRING. It holds elite workshops.
KITP (pronounced "kit pee") has scheduled a workshop for January 2007 on Quantum Cosmology and they have invited Bojowald. the others are mostly into stringy cosmology IIRC. So the workshop could be called "String meets LQC and Martin Bojowald". At this point May 2006---according to the KITP webpage about it---registration for the workshop is full and closed.
I will get some stuff about Ashtekar's doings. He is a world-class scientific figure and the more senior. Bojowald was a postdoc at Ashtekar's QG institute at one time----around 2001-2003 IIRC.
The roles are something like that Bojo goes out ahead, with Ashtekar encouragement, and gets results which Ashtekar then thinks about long and hard, and elaborates and checks out with other people. then maybe several years later Ashtekar puts his official stamp of approval on.
On the acceptance front, here are some links
Bojo encyclopedia article
Bojo Living Review article
Ashtekar invited talk at Paris 2005 Einstein Century celebration conference
Ashtekar and others The Quantum Nature of the Big Bang
KITP Quantum Cosmology workshop webpage
|May4-06, 01:03 PM||#5|
(QUOTE) this means that you can't get singularities. As you extrapolate back in time using the LQC model (reversing the observed expansion) you get to a regime of very high---Planck scale---pressure temperature and density but before the model goes completely haywire and tries to compute infinities it TURNS AROUND. (/QUOTE)
Ah, this is what i am looking for, so i guess one can look before the BB
now, i will try to keep up Marcus.
|May4-06, 01:11 PM||#6|
it can be and will be tested.
but so far WE CAN ONLY GET LIGHT FROM a time when the U was about 300,000 years old, maybe eventually we will get other signals like ancient neutrinos, from earlier than that, but that is still from after the start of expansion.
It looks like we can only get PRIMARY DATA from the expanding phase we are in and from times after the start of expansion.
We can use the primary data to check the model. But what was before is still behind a veil in the sense that we only know what the model tells us. We do not see direct. We have EVIDENCE and we can reconstruct and make models of the past but it is not direct seeing.
have to go. be back later
|May4-06, 06:10 PM||#7|
just out of interest: I can't imagine LQG is the only scenario that manages bounces. I.e. I think I remember some string theory arguments relying on T-duality that say there is no Big Bang singularity or something like that. Anyone knows more?
Also, I would say that you don't really nead QG to make the point - it just takes some negative energy density that probably attempts to reach equilibrium with the usual density at Planckian temperatures. So you can avoid the singularity theorems and have a bounce. The question is just, where do you get the negative energy/repulsive gravity from.
|May4-06, 07:20 PM||#8|
|May4-06, 07:54 PM||#9|
thanks. Have you any VERY BASIC introduction at hand? I mean, it seems to me the argument ought to be fairly simple (like R ~ 1/R, therefore no singularity), but there is probably more to say about the gas-i-ness?
BTW, what do you think about the arguments from Karch+Randall
Relaxing to Three Dimensions
about why we live in 3-dimensions? I fail to see a) why it is natural for 4-dimensional gravity to be "at the intersection of three 7-branes where the interesection has spacetime dimension four" and b) how it can be that the dynamics of the whole FRW thing does not influence the gravity localized on the branes?
|May9-06, 10:12 AM||#10|
The idea of a bouncing universe gives me a headache. What are we to imagine, a tennis ball on concrete, a beach ball blown along by the wind on the sand, a super-ball thrown into a steel box?
Well if there is a bounce, there must be two things: an elastic collision, and an unforgiving surface to bounce off of. So where is this surface, exactly?
Surely it would be better to think of an inflection, as in calculus, or a horizon? A bounce just seems too severe. Every time we try to pin down the location of the concrete wall or whatever, we run into a problem.
Gas is a little better, since it introduces density, and we can imagine a nice cushiony shock absorbing small scale anti-gravity or dark energy kind of force or something.
Personally I prefer to eliminate gravity as a force and just think of the unified three-in-one.
Gravity seems to be accountable as simple expansion. Adding new forces, like adding more dimensions, just makes trouble in the long run.
Imagine empty space (infinite dimensions, no definitions) filled with some density of random, instantaneous events. Each event a tiny flash of light, no reflections. Is each flash event a thing as such? Or is there some object or objects moving around in the darkness, flashing as they go? Such objects would connect the dots, and have substance which would then be definable. How would you know?
Suppose you tried to analyse the flashes. Say you see a single flash. Can that be defined as an object? Maybe but it is not enough. Two flashes, then. Say two flashes occur side by side. Is that enough to support the idea that the flashes are connected by a definable object? Not. Even if the two flashes occur over and over again, an object cannot be inferred. In a random collection, there will always be an incidence of pairs. Not enough to establish a pattern.
But if there are three flashes, a sequence can be established, comparisons can be made, and the randomness is broken. We can say there must be some connection if we see that same pattern of three repeated again.
Why three? Why does it take three points to define a line? Why must every universe have an observor, an object to be observed, and a connection between them?
Call it poetry.
Call it a rainy day in the woods.
Call it graffitti, I hope you will not think it vandalism, or, forbid it three times, sabotage.
Wishing everyone a pleasent summer....
|May9-06, 12:44 PM||#11|
And a pleasant summer to you as well!
Next year in January there will be this
A workshop at SantaBarbara organized by or featuring
Martin Bojowald, Robert H. Brandenberger, Gary T. Horowitz, Hong Liu
on "spacetime singularities"
and essentially what they mean to say in the title is not singularities but bounces. these days the singularities are getting repaired and bounces are what replaces them.
a singularity is a fault in the model and if you remove the singularity you get a bounce. "bounce" is just their word for a removed spacetime singularity---I don't know of any other word that is being used much in this context.
they have to have some word for it, and even tho it is not much like a bouncing tennis ball we might as well let them say bounce. or what else do you propose?
Bojowald has accumulated a lot of OCCAMY EVIDENCE (clean economical evidence that doesnt need to assume a lot of extra structure that you could cut away with occam razor) that gravity is repulsive at high density.
the occam point about it is that to get that he just quantizes gravity in a fairly straightforward (Loop) way and gets hi-density repulsion out as a surprise---something he didnt put in by hand.
naturally the other bigname guys like Gary Horowitz and Robert Brandenberger are interested to hear about this because maybe they can use it in telling their various stories. so they set up this workshop where they can question Bojowald and see what they can do with it.
the others mostly have more complicated stringy structures they are thinking about, like a "gas" of branes drifting around and colliding. Wagner would have loved it and pictured it on the stage in one of his operas. People sing while branes collide.
But Bojowald already went ahead and wrote a paper that SHOWS them how to use his hi-density repulsive gravity in the context of colliding branes! He made it easy. The repulsion cushions the collision so they dont actually even have to TOUCH! there is no sign I can see that Bojo believes in branes, he just tossed off the paper to show how you can use quantized gravity (Loop style) to improve other models.
I will get the link. Here it is:
Loop Quantum Gravity and the Cyclic Universe
Martin Bojowald, Roy Maartens, Parampreet Singh
6 pages, 4 figures
Phys.Rev. D70 (2004) 083517
"Loop quantum gravity introduces strong non-perturbative modifications to the dynamical equations in the semi-classical regime, which are responsible for various novel effects, including resolution of the classical singularity in a Friedman universe. Here we investigate the modifications for the case of a cyclic universe potential, assuming that we can apply the four-dimensional loop quantum formalism within the effective four-dimensional theory of the cyclic scenario. We find that loop quantum effects can dramatically alter the near-collision dynamics of the cyclic scenario. In the kinetic-dominated collapse era, the scalar field is effectively frozen by loop quantum friction, so that the branes approach collision and bounce back without actual collision."
Anyway the answer to Wolram question is that the business of repairing spacetime singularities is going GREAT! Progress in all sorts of ways on it. Lots of papers to read if anyone wants to follow the progress in technical detail.
|Mar14-11, 10:17 PM||#12|
Interesting developments and good discussion! I don't understand why the concept of bounce isn't the most naturally appealing of all. Just replaying the expansion in 3D (letting the arrow of time reverse) gives a nice symmetric vectorless mass point, perfect to "bounce off" - no "steel wall" needed. I have thought for years that this makes a perfect point to compress and re-expand off.
However, this still leaves the other end as a problem. If we have an increasingly expanding universe, how can we ever get it to reverse in our models? The infinitely bouncing universe needs both ends bound. Any developments on that end?
PS-On a purely humorous note, the bouncing universe unites the Buddhists (reincarnation), Hindus (the great mandala of the universe), and the Jewish/Christian/Muslim religions (in the beginning...let there be light). Grand unified theory indeed! One can only smile...
|Mar14-11, 11:31 PM||#13|
Their professional interest is in testing, so they are happy either way, if the tested theory passes or if it fails. Here is a link to 14 recent papers about testing bounce cosmology which appeared 2009-2011.
The proposed tests involve a spacecraft to measure the temperature and polarization of the Cosmic Background Radiation (the CBR), using the next level of techology beyond what is currently being used by the European Space Agency's "Planck" spacecraft which is now taking data. They also in some cases employ data already collected to constrain the bounce cosmology models.
The whole thing is now on more of an empirical/observational track, so it does not matter so much whether or not it appeals to one's imagination. It will eventually either get ruled out or not, by the observations of the CBR.
|Mar14-11, 11:50 PM||#14|
Thanks, Marcus! I look forward to reading the papers. Looking at polarization makes sense, and nothing wrong with empirical measurements. After all, it is only in the matching of theory and measurement that keeps physics away from metaphysics, science away from religion. Although the bouncing universe appeals to me, I don't wish for anything; let the data rule! I worship only one god, Veritas!
|Mar15-11, 12:20 AM||#15|
That's the spirit! The good old Baconian spirit I mean. But 14 is too many papers. I woud only look at the titles and some of the abstracts. What matters more, if you haven't done this yet, is to learn to use a search tool like Spires.
That link in post #13 is to the German mirror of Stanford-SLAC research literature database called Spires. The home version has gotten slow lately so the German mirror is better.
You can see how to put in things like author, or keyword.
Like try putting in "find dk quantum cosmology and date > 2008"
dk means a kind of keyword. Select ordering by citecount to get the most important papers first.
The help section has a list of keywords.
If you have any trouble getting a listing of quantum cosmo papers ordered by citecount, let me know. Once you do that you can do all kind of searches.
Ahz is a strange name. Reminds me of Oz, which means Australia. I currently imagine you as Australian.
|Mar22-11, 12:18 AM||#16|
may it happen that time doent reaches 0 seconed befor reaching t=0 seconed it starts again increasing than no law violates niether einstine law nor law of conservation of energy?
|Mar22-11, 11:47 AM||#17|
The Universe expands because the entropy increases. Is it possible to decrease the entropy of the whole Universe to get a Big Bounce again ?
According to Holographic Principle the information can't disappear from the closed volume and decrease the surface of the Event Horizon. Perhaps into an another Universe of a different dimensions ?
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