Prior to the Big Bang Question

[XanPaul]

What was the force holding the matter together before the Big Bang?
Gravity, electromagnetic forces, or something completely different?

 

[Dmitry67]

Big Bang is NOT an explosion of something into empty space
Before Big Bang there was neither time nor space, so question is meaningless

 

[marcus]

What was the force holding the matter together before the Big Bang?
Gravity, electromagnetic forces, or something completely different?

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.

The main line of research that is currently dealing with conditions before and at the BB time is called Loop Quantum Cosmology.

The way you should ask the question is "what caused the temporarily very high density at the Big Bang moment according to LQC?"

If you ask that question in the context of according to classical non-quantum cosmology it cannot be answered because classical cosmology does not say.

Here is a keyword search of professional articles since 2006, keyword "quantum cosmology".

http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29

The papers are listed with the most highly cited ones first. These tend to be the most important papers.

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.

In this context it is very easy to answer your question! The extremely high density only occurred briefly during the bounce, and was produced by the prior collapse. And the large but finite density that was achieved can be calculated within the context of the LQC bounce model.

Feel free to ask if you have more questions and I will see if I can answer---others may as well. I didn't give you that Stanford technical research link expecting that you would read the articles, but just so you can see what the professional literature on this subject looks like.

 

[Dmitry67]

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?

 

[marcus]

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?

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.
The Second Law is something that holds as long as you have a consistent classification of states. And it rules out various things, that no observer shall be able to see, and machines no observer shall be allowed to build

But isn't it naive to suppose that two observers, one Before the crunch and one After the bang, are really the same observer and share exactly the same map of state space?

I don't think the Second Law is even meaningful in this situation.
whose map of state space, whose micro/macro distinction? Mr Before's or Mr After's?
Who is going to build a perpetual machine to extract illegal work and violate the law?

In other words the operational meanings of the second law that we are used to do not apply when the whole universe collapses and then re-expands.

That's my guess anyway, Dima. I'd like to hear your opinion.

 

[Dmitry67]

Thank you. I understand your logic.

When I first read about the 'bounce' I had another explanation: imagine an infinite chess board. When you increase density (put more and more figures) the number of combinations increases but then it starts to decrase when density >50%. Finally, when all squares are filled there is only 1 possible state so all history is lost and entropy is 'reset'

However, that was before I read about the black hole informational paradox - so it appears that information and entropy are believed to be so fundamental that they are believed to survive even the singularity... No now I am not sure that our both explanations are valid.

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 can't talk about the entropy during the inflation era and soon after that, because there could not be any obserers and machines, in principle (empty space or too hot)

 

[Chronos]

Even logic fails prior to the big bang. Until we have observational evidence affirming or denying the propositions, such discussions are philosophy, not science. Not that I object, just wish to keep it in perspective.




w

 

[apeiron]

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.

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 big bounce story has to contend with a certain difficulty called dark energy. If there is an acceleration (as observation suggests) then how does gravity achieve the recollapse?

Of course, there is a frantic search going on to find a mechanism to change the sign of the acceleration. And perhaps they may even find it.

The other major reason to object to the big bounce is the second law. Again, what we actually observe is accelerative expansion and an asymptotic approach to a heat death. So any discussion of a collapse, and the thermodynamic consequences of a collapse are at the moment complete speculation.

 

[Dmitry67]

Yes, an yet another thing: fine tuning that our universe is almost flat. And that fine tuning had to be extremely precise!

 

[marcus]

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 can't 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)

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.

I am skeptical of the Second Law only within a few Planck times of the bounce, because no classical geometry exists there. I suspect there can be no continuity between the perspective of someone before and someone after.

BTW this is model dependent, certainly! We don't know conditions around the start of expansion, all we have is some models. I think you were asking about LQC, so we have to look at that model and try to guess how entropy would be calculated.

In that model the convergence to classical GR is very fast. A few Planck times after the bounce (and the definition of time is problematical, but very quickly) the LQC model closely approximates GR. It also gives a good approximation of classical spacetime up to shortly before the bounce. Perhaps i should say classical geometry rather than spacetime.

There is a brief hiatus---sometimes called the quantum regime---when it is suspected that there is no geometry, concepts like dimensionality would presumably not be defined. I don't see how a macro quantity like temperature could be defined either.

Obviously I am not an authority about the LQC bounce model. We could write email to someone knowledgeable and ask about this, or do a literature search and it might turn out that I'm wrong. But mainly I am just skeptical that thermodynamics can be well-defined during that brief quantum regime.

LQC is fairly new, there may be research on this very question that I just don't know about.

If there is an acceleration (as observation suggests) then how does gravity achieve the recollapse?
...

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.

 

[marcus]

Yes, an yet another thing: fine tuning that our universe is almost flat. And that fine tuning had to be extremely precise!

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.
The usual solution to that puzzle is inflation.

LQC accommodates inflation, they recently (2008) started putting that in and running computer models of the bounce with inflation. A recent review article would cover that, if you want I'll fetch a link.

Again, I am skeptical of the need for inflation. There could be some other reason for the amazing flatness of the early universe. But I can't offer one so if you think you need inflation to get flatness you can certainly have inflation in the LQC context.

Keep in mind that LQC is quite new. A revolution occurred in 2006 where a new dynamics was introduced, it has no name except "the improved dynamics". The picture of the bounce changed quite a bit. They are still busy filling in the picture.

I should really get a link to the latest Ashtekar review paper, something 2008 or 2009, and we should look it over to see where they are at the moment.
Ashtekar is the representative author, when one is talking about LQC. To make sure we are all talking about the same thing here are links to some of his recent papers.

In looking back, I see that one of the recent papers happens to involve entropy. So I took the liberty of including the abstract. It shows that Ashtekar has been thinking about that as well, in the LQC context.

http://arXiv.org/abs/0903.3397
Loop quantum cosmology of Bianchi I models
Abhay Ashtekar, Edward Wilson-Ewing

[Dima, notice that they are extending LQC to cover less symmetric cases. The isotropy condition is being relaxed.]

http://arXiv.org/abs/0901.0933
Quantum field theory on a cosmological, quantum space-time
Abhay Ashtekar, Wojciech Kaminski, Jerzy Lewandowski

http://arXiv.org/abs/0812.4703
Singularity Resolution in Loop Quantum Cosmology: A Brief Overview
Abhay Ashtekar

http://arXiv.org/abs/0805.3511
The covariant entropy bound and loop quantum cosmology
Abhay Ashtekar, Edward Wilson-Ewing
15 pages, 3 figures; Phys.Rev.D78:064047,2008
(Submitted on 22 May 2008)
"We examine Bousso's covariant entropy bound conjecture in the context of radiation filled, spatially flat, Friedmann-Robertson-Walker models. The bound is violated near the big bang. However, the hope has been that quantum gravity effects would intervene and protect it. Loop quantum cosmology provides a near ideal setting for investigating this issue. For, on the one hand, quantum geometry effects resolve the singularity and, on the other hand, the wave function is sharply peaked at a quantum corrected but smooth geometry which can supply the structure needed to test the bound. We find that the bound is respected. We suggest that the bound need not be an essential ingredient for a quantum gravity theory but may emerge from it under suitable circumstances."

 

[marcus]

Since Dima's initial question to me was about entropy and the Second Law in LQC context, I will quote what Ashtekar said on page 24 of this paper:

http://arXiv.org/abs/0812.0177
Loop Quantum Cosmology: An Overview
Abhay Ashtekar
To appear in the Proceedings of the Bad Honef Workshop entitled Quantum Gravity: Challenges and Perspectives, dedicated to the memory of John A. Wheeler
(Submitted on 30 Nov 2008)
"A brief overview of loop quantum cosmology of homogeneous isotropic models is presented with emphasis on the origin of and subtleties associated with the resolution of big bang and big crunch singularities. These results bear out the remarkable intuition that John Wheeler had. Discussion is organized at two levels. The the main text provides a bird's eye view of the subject that should be accessible to non-experts. Appendices address conceptual and technical issues that are often raised by experts in loop quantum gravity and string theory."

Dmitry, this passage starting on page 23 has some relevance to your question since it deals with the concepts, although I don't think it provides a conclusive answer. The highlighting for emphasis here is mine, not Ashtekar's

"...This result suggests the following overall viewpoint on the entropy bound. Recall that the bound is strongly motivated by the generalized second law of thermodynamics (which also does not have a sharp, definitive formulation). Now, already the standard second law of thermodynamics is a deep fact of Nature but it has a ‘fuzziness’ which is not shared by other deep laws such as energy-momentum conservation. In particular, the second law requires a coarse graining in an essential way. [Dima, my point was "whose coarse-graining?" Does God have a favorite one? or do we use Mr Before and Mr After's coarse-graining in discontinuous alternation?]

It is not a statement about the evolution of micro-states; in a fundamental theory their dynamics is always time reversible (leaving aside, for simplicity, quantum measurements). Rather, it is a statement about how the number of micro-states compatible with a pre-specified coarse graining changes in time. For specific processes, the increase of entropy can be calculated using statistical mechanics. But this entropy has little relevance to the fundamental dynamics of micro-states and is not an input in the construction of statistical mechanics. In the same vein, it seems unlikely that covariant entropy bounds would be essential ingredients in the construction of a quantum theory of gravity. It seems more natural to expect the covariant entropy bound should emerge from a fundamental quantum gravity theory under suitable conditions. Returning to the LQC calculation..."

My intuitive feeling is that for a few Planck times around the bounce, for that brief interval of quantum regime, there are only microstates. No meaningful coarse-graining exists. Classical geometry has ceased to exist and will re-emerge momentarily (a voice on the loudspeaker says "we apologize for the interruption of service and hope that it causes you no inconvenience" )

 

[Dmitry67]

Marcus, but are you feeling comfortable about the whole idea of the bounce?

The only Big Bang and our Universe - I am ok with it.
Multiverse with the colliding branes, creating new universes with different parameters again and again - no problem.

But the bounce... Our expansion is accelerating. Hence, our expansion is the last one. What was before?

An infinite or finite sequences of smaller bounces? Or even a stranger solution where before the rebounce there was a contraction from t= - infinity?

I would say, God does not like the rebounce :)

 

[marcus]

Or even a stranger solution where before the rebounce there was a contraction from t= - infinity?
...

Dima, I enjoy your comment very much! Your perspective is different from mine. My bias is toward the empirical observational as opposed to aesthetic.

I would like to see formal predictions about CMB and gravitation waves derived from Lqc bounce models and compared with data. There have been some papers moving in that direction. The Lqc bounce must have a signature.

Right now I do not feel the urge to ask "what came before" the bounce. At least not long before. The origin of the universe is a remote question that doesn't concern me at the moment.
I only think "what replaces the singularity?"

The singularity must be wrong, it is just a failure of theory. The bounce is the simplest of all what can replace it. It requires no extra dimensions and elaborate machinery (like branes which moreover do not work very well).

With the bounce you do not have to "make up" or fantasize anything. You essentially just quantize Friedman (the equation already in use by cosmologists) and run it back in time and you find it does not break down but instead evolves back to a phase of contraction.

It seems to me this is obviously the thing to look at and test FIRST. Because by far the simplest and least exotic. So I am happy to hear from Apeiron about postdocs flocking to help study it!

I don't know about postdocs specifically but I see papers by established senior people, relativists/cosmologists, who already have a publication record outside Lqc. I see a general movement of people into the field---including of course postdocs. I would call this rational focus on the proper order of business, not mindless enthusiasm. Maybe some of them can find a way to empirically disprove the Lqc bounce. That would also be progress.

So you asked if I was comfortable. Yes very much. And it is from a pragmatic empirical point of view rather than one of aesthetic and cosmogony.

We have to see what observations confirm. Only then do we ask what came before the bounce and how did it happen.

 

[Dmitry67]

Well, as I understand it is very difficult to find any observational evidence because, as you say, loop gravity converges into GR after few plank times.

I have some questions about how loop gravity explains isothropy ofthe universe without an inflation, but I need to read the articles first. 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?

P.S.
How do you know that Dmitry=Dima? :)

P.P.S
Cant stop thinking about the bounce...
Sorry for the pure speculation, just trying to find something goodon the intuitive level.

Say, there is a contraction era from t=-infinity to t=0, and our era from 0 to infinity.
During the contraction era entropy decreases, so for the observers it is not a contraction, but an expansion! If solution is symmetric for the sign of t, then there are not 2 worlds, but just one...

 

[marcus]

... 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?

...

I know. It is always a fruitful time for me when the modem breaks or the ISP interrupts service. Reading things on actual paper.

I think I gave only two links to general survey papers by Ashtekar. I would look at the shorter of the two. And then possibly the longer---following up references if any excite curiosity.

The longer overview paper is this:

http://arXiv.org/abs/0812.0177
Loop Quantum Cosmology: An Overview
Abhay Ashtekar
To appear in the Proceedings of the Bad Honef Workshop entitled Quantum Gravity: Challenges and Perspectives, dedicated to the memory of John A. Wheeler
(Submitted on 30 Nov 2008)
"A brief overview of loop quantum cosmology of homogeneous isotropic models is presented with emphasis on the origin of and subtleties associated with the resolution of big bang and big crunch singularities. These results bear out the remarkable intuition that John Wheeler had. Discussion is organized at two levels. The the main text provides a bird's eye view of the subject that should be accessible to non-experts. Appendices address conceptual and technical issues that are often raised by experts in loop quantum gravity and string theory."

As I recall the first few pages are historical/philosophical and I would be inclined to skip over them and go to where he summarizes recent research results.

The shorter essay is this:
http://arXiv.org/abs/0812.4703
Singularity Resolution in Loop Quantum Cosmology: A Brief Overview
Abhay Ashtekar
12 pages

I can't think of a better place to start than with a recent short review paper written for nonspecialists (e.g. classic relativists unfamiliar with any Lqg or Lqc specifics.)

Let me know if this doesn't work for you. This would teach me something, I think.

 

[apeiron]

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 elephant in the room, it is always a relevant fact to mention.

If LQC can only give a big crunch followed by a big bang, then all you have done is held up a mirror and doubled the original mystery. You may have got rid of the singularity in between, but quantum foam thinking would do that for you. The aim of LQC is of course to move to a cyclical bounce model (as the admirably lucid Ashtekar brief overview outlines). So it would be relevant unless you really are content with something so ugly as a single origin-less crunch being followed by a single destiny-less bang.

And the dark energy/cosmological constant issue would be relevant even to this horribly limited project. If dark energy exists in the big bang era, it must have some analog in the big crunch era (or else, why not?).

If the dark energy is positive, then it would seem there could be no crunch. It would overwhelm gravitational collapse. And if dark energy is for some reason negative, an accelerating contraction, then we need a reason for its sign to be switched. And a reason why gravity is not also switched around to become a repulsive factor. Etc.

Ashtekar does discuss the ramifications of positive dark energy but - tellingly - becomes uncharacteristally opaque at this point of the paper.

But perhaps you can unravel the argument being sketched here...http://arxiv.org/abs/0812.4703

• The case of a positive cosmological constant is more subtle [33]. Now, as in the _ = 0
case, the classical theory admits two types of trajectories. One starts with a big bang and
expands to infinity while the other starts out with infinite volume and contracts into a big
crunch. But, in contrast to the _ = 0 case, they attain an infinite volume at a finite value
φmax of φ. The energy density ρ|φ at the ‘internal time’ φ goes to zero at φmax. Because
the φ ‘evolution’ is unitary in LQC, it yields a natural extension of the classical solution
beyond φmax. States which are semi-classical in the low ρ|φ regime again follow an effective
trajectory. Since ρ|φ remains bounded, it is convenient to draw these trajectories in the
ρφ-φ plane (rather than v-φ plane). They agree with the classical trajectories in the low ρφ
regime and analytically continue the classical trajectories beyond ρφ = 0.

 

[marcus]

If the dark energy is positive, then it would seem there could be no crunch. ...

But perhaps you can unravel the argument being sketched here...http://arxiv.org/abs/0812.4703

• The case of a positive cosmological constant is more subtle [33]...

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/ )

For me the most meaningful thing in that paragraph of Ashtekar you quote is "...[33]..."
It refers to a paper "in preparation" by Ashtekar and Tom Pawlowski. They are addressing the positive cosmo constant case.

I think of the human enterprise of understanding nature as a process of successive approximation.
The classical cosmo singularity represents (imho) zero understanding. Replacing it with a simple mechanism like the Lqc bounce is extremely interesting and could be the correct next step.

I'm confident that predictions will be derived and checked, and we will proceed from there.
The bounce could pass its first tests or it could be rejected, either way we learn.

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.

Since 1917 we have the deSitter universe which has a positive cosmological constant and something resembling a bounce. It has the "hour-glass" shape of contraction-rebound-expansion. That is certainly not the same thing as a Lqc bounce, but it is one route to thinking about the case with positive Lambda.

More significant, i think, is the question what is Lambda? We don't know what dark energy is yet. And of course we don't know if it is really constant or if it changes. It may be a cumulative quantum geometry correction and not an energy (Bojowald has explored that in the Lqg context.) Martin Bojowald has been invited to edit a special issue of some journal that will gather together many experts' ideas about what dark energy could be.

Basically I think it is simply not a fruitful time to be thinking about the entire history of the universe (which for simplicity sake I consider eternal, there is no scientific reason to introduce the extra complication of a beginning or an end.)

What seems to be fruitful is to focus on the one thing we know is wrong---the classical singularity. When that has been replaced with something that survives empirical tests, then we may profitably look further back. Meanwhile, fundamental concepts will have changed...

 

[apeiron]

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/ )
...

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.

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 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.

You yourself are enthusiastic about no go theorems. And dark energy would be a red flag for bounce cosmology at the moment. I am much less bothered by second law issues as the second law has only really been formulated for "within worlds" use, not in a wider form.

For those making a career in a particular field like LQC, of course they will have their heads down and plough through, having already made their choice.

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.

If Rovelli is into Anaximander, perhaps he too has this thought in mind?

 

[marcus]

. 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?

I can't say about the specifics but I do know that Rovelli is an Anaximander fan.

He's kind of a hero. He was at the beginning of Greek science (was it circa 600 BC) where they struggled to find some mechanistic explanation rather than "god did it" or "the gods want it that way" and "it sits on the back of a giant turtle".

I think Rovelli believes we have a lot to thank Ax'der for. The idea of physical law. The idea of dynamics, and that you could explain in an unfanciful way, or at least try. A critical standard of rational explanation.

I know Rovelli also thinks Anaximander is underappreciated, as you also say.

 

[apeiron]

Rovelli tells me an English translation is in the pipe-line for next year.

From what I can tell from a preliminary conversation, Rovelli is celebrating Anaximander as mainly the first true metaphysician, the first to seek deep logical principles. (But then he was improved upon)

I would go rather further and say Anaximander got it right as to the shape of those deep logical principles, then later philosophers - particularly the atomists - lurched off in a second direction.

Aristotle mounted a last ditch campaign to unite the two approaches and failed (or at least failed to be understood). Anaximander's vision has sputtered fitfully ever since. So anyone who brings it back to the public gaze for consideration is to be welcomed.

 

[Chronos]

Agreedl. In the mean time, science as usual remains a viable approach.

 

[aristurtle]

I think singularity may be wrong physically , but singularity in open or flat space is wrong logically. What is an infinite space with no volume? A closed space with singlarity is ok if physics admit it. But an open space with 0 scale factor? That's logically impossible I think.
BTW, does LQG bounce gives the correct CMB power spectrum?And acoustic peaks?thank you.

 

[c12_z]

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.

Isn't this "quantum foam" simply a proposed "the singularity explained"?

I'm not debating the merits, or otherwise, of this "quantum foam" idea, for I am just a layman perpetually wandering in a fascinating world of endless intrigue, but rather just questioning how it is distinct from the singularity of the standard model (per say)?

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. So your "quantum foam" is just an example of trying to get a good whack closer to defining where, and what, the "dragons" are.

As I say, though, I am just a perpetually ignorant layman, so would very much love for an attempted explanation as to what makes the "foam" something more than "the singularity explained".

 

[marcus]

...
BTW, does LQG bounce gives the correct CMB power spectrum?And acoustic peaks?thank you.

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.

There has been some speculation about this---a paper by Maguiejo and Singh about LQC structure formation, a recent paper by Brandenberger exploring CMB power spectrum resulting from another bounce cosmology (akin to but not identical with LQC.)

A lot of research interest on this now. A parallel session of the Marcel Grossmann conference in Paris this summer will be devoted to non-singular cosmology. Getting to be a hot topic.

But as yet the answer is no. LQC cannot by itself address all the puzzles solved by inflation.

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...

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.
The singularity (where the classic 1915 model breaks down) is used as a convenient time-marker. But one expects improvements in the theory to produce models which do not break down there.

I have a link to the Cosmo pages of Einstein Online in my signature (small print at end of post.) E-O is the public outreach website of one of the world's top research institutes for gen. rel. gravitation cosmology unified theories quantum geometry/gravity etc. Their stuff is up-to-date, accessible, written by physicists. Check it out.

A lot of modern research is going in the direction of a bounce--but whatever, no singularity in any case. Models must be testable by empirical observation as well as consistent with Gen Rel at low densities.

Most cosmologists would be quite surprised if it actually turned out that there was a singularity---infinite density, infinite curvature and all that. The term, as I say, is merely a time marker to designate a point in time so you can run your model by it and say "1/10 of a second after t=0" or "5/10 of a second before t=0". t=0 is where vintage 1915 Gen Rel blows up, not where Nature herself does

Maybe this agrees with what you are already thinking, so you will like reading the E-O web pages.

 

[squidsoft]

What was the force holding the matter together before the Big Bang?
Gravity, electromagnetic forces, or something completely different?

Why would you think there was matter prior to the Big Bang or even forces for that matter? How about something qualitatively different than existence today? No doubt it's speculation but perhaps there are hints all around us. We just have to know how to look.

 

[apeiron]

And to organise thinking, we could reduce to a few clear general alternatives. I see three general approaches people take.

1) something out of nothing (eg: cosmic QM fluctuations)
2) something out of something (eg: cyclic universe stories)
3) something out of everything (eg: phase transition approaches)

 

[Redbelly98]

Even logic fails prior to the big bang. Until we have observational evidence affirming or denying the propositions, such discussions are philosophy, not science. Not that I object, just wish to keep it in perspective.

This points to my question: in any theory that addresses the universe before the Big Bang, are there any tests or observations being proposed that would either refute or confirm the theory. Or put another way, what (if any) information from that time would survive in some fashion afterwards?

 

[marcus]

This points to my question: in any theory that addresses the universe before the Big Bang, are there any tests or observations being proposed that would either refute or confirm the theory. Or put another way, what (if any) information from that time would survive in some fashion afterwards?

I believe the answer is no, a follow up is to try and assess how close various theories are to testability. To get an idea about LQC/LQG (loop cosmology/gravity) here are abstracts of recent papers. The idea here is not to study the papers in detail but to look over the abstracts and get an impression of where the field is.

Here is an 80 page review article about this that came out in June 2008.
http://arxiv.org/abs/0806.0339
Quantum Gravity Phenomenology
Giovanni Amelino-Camelia
82 pages
(Submitted on 2 Jun 2008)
"I review the present status of the development of Quantum Gravity Phenomenology. Among the accomplishments of this young research area I stress in particular the significance of studies which established that some appropriate data analyses provide sensitivity to effects introduced genuinely at the Planck scale. The objective of testing/falsifying theories that provide comprehensive solutions to the quantum-gravity problem appears to be still rather far, but we might soon be in a position to investigate some 'falsifiable quantum-gravity theories of not everything'."

Now here are some papers that appeared after this review:

http://arxiv.org/abs/0807.0160
Loop Quantum Cosmology corrections to inflationary models
Michal Artymowski, Zygmunt Lalak, Lukasz Szulc
16 pages, 1 figure
(Submitted on 1 Jul 2008)
"In the recent years the quantization methods of Loop Quantum Gravity have been successfully applied to the homogeneous and isotropic Friedmann-Robertson-Walker space-times. The resulting theory, called Loop Quantum Cosmology (LQC), resolves the Big Bang singularity by replacing it with the Big Bounce. We argue that LQC generates also certain corrections to field theoretical inflationary scenarios. These corrections imply that in the LQC the effective sonic horizon becomes infinite at some point after the bounce and that the scale of the inflationary potential implied by the COBE normalisation increases. The evolution of scalar fields immediately after the Bounce becomes modified in an interesting way. We point out that one can use COBE normalisation to establish an upper bound on the quantum of length of LQG."

http://arxiv.org/abs/0807.0712
Gravitational waves from the Big Bounce
Jakub Mielczarek
19 pages, 9 figures
(Submitted on 4 Jul 2008)
"In this paper we investigate gravitational waves production during the Big Bounce phase inspired by the Loop Quantum Cosmology. We consider the influence of the holonomy corrections to the equation for tensor modes. We show that they act like additional effective graviton mass, suppressing gravitational waves creation. However, this effects can be treated perturbatively. We investigate the simplified model without these corrections and find its exact analytical solution. For this model we calculate a spectrum of the gravitational waves from the Big Bounce phase. The obtained spectrum decreases to zero for the low energy modes. Based on this observation we indicate that this effect can lead to the low CMB multipoles suppression and gives a potential way to test Loop Quantum Cosmology models. We also consider a scenario with a post-bounce inflationary phase. The obtained power spectrum gives qualitative explanation of the CMB spectra, including low multipoles suppression. This result is a challenge to construct a consistent bounce+inflation model in the Loop Quantum Cosmology."

http://arxiv.org/abs/0810.0104
The gravitational wave background from super-inflation in Loop Quantum Cosmology
E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
8 pages, 3 figures
(Submitted on 1 Oct 2008)
"We investigate the behaviour of tensor fluctuations in Loop Quantum Cosmology, focusing on a class of scaling solutions which admit a near scale-invariant scalar field power spectrum. We obtain the spectral index of the gravitational field perturbations, and find a strong blue tilt in the power spectrum with $$n_t \approx 2$$. The amplitude of tensor modes are, therefore, suppressed by many orders of magnitude on large scales compared to those predicted by the standard inflationary scenario where $$n_t \approx 0$$."

http://arxiv.org/abs/0810.4330
Loop quantum cosmology and tensor perturbations in the early universe
Gianluca Calcagni, Golam Mortuza Hossain
12 pages. Invited contribution to the special issue of Advanced Science Letters on "Quantum gravity, Cosmology and Black Holes"
(Submitted on 23 Oct 2008)
"We study the tensor modes of linear metric perturbations within an effective framework of loop quantum cosmology. After a review of inverse-volume and holonomy corrections in the background equations of motion, we solve the linearized tensor modes equations and extract their spectrum. Ignoring holonomy corrections, the tensor spectrum is blue tilted in the near-Planckian superinflationary regime and may be observationally disfavoured. However, in this case background dynamics is highly nonperturbative, hence the use of standard perturbative techniques may not be very reliable. On the other hand, in the quasi-classical regime the tensor index receives a small negative quantum correction, slightly enhancing the standard red tilt in slow-roll inflation. We discuss possible interpretations of this correction, which depends on the choice of semiclassical state."

http://arxiv.org/abs/0902.0928
Chaplygin inflation in loop quantum cosmology
Xin Zhang, Jingfei Zhang, Jinglei Cui, Li Zhang
6 pages, accepted for publication in Mod. Phys. Lett. A
(Submitted on 5 Feb 2009)
"In this paper we discuss the inflationary universe in the context of a Chaplygin gas equation of state within the framework of the effective theory of loop quantum cosmology. Under the slow-roll approximation, we calculate the primordial perturbations for this model. We give the general expressions of the scalar spectral index, its running, and the tensor-to-scalar ratio, etc. For the chaotic inflation with a quadratic potential, using the WMAP 5-year results, we determine the parameters of the Chaplygin inflation model in loop quantum cosmology. The results are consistent with the WMAP observations."

http://arxiv.org/abs/0902.2490
Tensor power spectrum with holonomy corrections in LQC
Jakub Mielczarek
13 pages, 13 figures
(Submitted on 14 Feb 2009)
"In this paper we consider tensor perturbations produced at a bounce phase in presence of the holonomy corrections. Here bounce phase and holonomy corrections originate from Loop Quantum Cosmology. We re-derive formulas for the of the corrections for the model with a scalar field content. Background dynamics with a free scalar field and multi-fluid potential are considered. Both analytical approximations as well as numerical investigations were performed. We have found analytical solutions on super-horizontal and sub-horizontal regimes and derived corresponding power spectra. Also occupation number $$n_{\bf k}$$ and parameter $$\Omega_{\text{gw}}$$ were derived in sub-horizontal limit, leading to its extremely low present value. Final results are numerical power spectra of the gravitational waves produced in the presence of quantum holonomy corrections. In the super-horizontal limit the obtained spectrum behaves like $$\mathcal{P}_T \propto k^3(C_1+\log^2(k))$$ while on sub-horizontal scales it exhibits oscillations around $$\mathcal{P}_T \propto k^2$$. These results can be directly applied as initial conditions for the inflationary modes. We mention possible resulting observational features of the CMB in particular BB spectrum of polarization."

This is not a complete list, just sample. I haven't been updating it regularly. Nothing here especially impressive, but worth keeping track of.
Giovanni A-C who provided the June 2008 review paper, recently gave an overview talk at Perimeter. This discusses some observational results and brings updates his survey of quantum gravity phenomenology. I'll get the link. Probably one can just google "camelia perimeter". Yes:
http://pirsa.org/09030039/
this gives video of the lecture and pdf of the slides.
Still not very impressive. Some interesting observational data from the Fermi satellite but meaningless until and if repeated.
===================
Another place progress is needed is in connecting LQG and LQC. (weakening the homogeneity assumptions in LQC).
http://arxiv.org/abs/gr-qc/0701132
Relating loop quantum cosmology to loop quantum gravity: Symmetric sectors and embeddings.
Jonathan Engle (Marseille, CPT & Provence U. & Marseille U., Luminy) . 20pp.
Published in Class.Quant.Grav.24:5777-5802,2007.
http://arxiv.org/abs/0805.4585
Stepping out of Homogeneity in Loop Quantum Cosmology
Carlo Rovelli, Francesca Vidotto
16 pages
(Submitted on 29 May 2008)
There have been some recent papers in this area. Extending LQC bounce to Bianchi I models (non-isotropic). I will see if I can find the links.
Yes! Here's one:
http://arxiv.org/abs/0903.3397
Loop quantum cosmology of Bianchi I models
Abhay Ashtekar, Edward Wilson-Ewing
(Submitted on 19 Mar 2009)
"The 'improved dynamics' of loop quantum cosmology is extended to include anisotropies of the Bianchi I model. As in the isotropic case, a massless scalar field serves as a relational time parameter. However, the extension is non-trivial because one has to face several conceptual subtleties as well as technical difficulties. These include: a better understanding of the relation between loop quantum gravity (LQG) and loop quantum cosmology (LQC); handling novel features associated with the non-local field strength operator in presence of anisotropies; and finding dynamical variables that make the action of the Hamiltonian constraint manageable. Our analysis provides a conceptually complete description that overcomes limitations of earlier works. We again find that the big bang singularity is resolved by quantum geometry effects but, because of the presence of Weyl curvature, Planck scale physics is now much richer than in the isotropic case. Since the Bianchi I models play a key role in the Belinskii, Khalatnikov, Lifgarbagez (BKL) conjecture on the nature of generic space-like singularities in general relativity, the quantum dynamics of Bianchi I cosmologies is likely to provide considerable intuition about the fate of generic space-like singularities in quantum gravity. Finally, we show that the quantum dynamics of Bianchi I cosmologies projects down exactly to that of the Friedmann model. This opens a new avenue to relate more complicated models to simpler ones, thereby providing a new tool to relate the quantum dynamics of LQG to that of LQC."
__________________

 

[marcus]

I should mention that there are other non-singular or bounce cosmology models that also stand a fair chance of being predictive. I don't know a lot about them but I can report on one that has gotten a lot of attention lately. Here is a sample link:

http://arxiv.org/abs/0904.2835
Matter Bounce in Horava-Lifgarbagez Cosmology
Robert Brandenberger (McGill University and CERN)
6 pages, 1 figure
(Submitted on 18 Apr 2009)
"Horava-Lifgarbagez gravity, a recent proposal for a UV-complete renormalizable gravity theory, may lead to a bouncing cosmology. In this note we argue that Horava-LIfgarbagez cosmology may yield a concrete realization of the matter bounce scenario, and thus give rise to an alternative to inflation for producing a scale-invariant spectrum of cosmological perturbations. In this scenario, quantum vacuum fluctuations exit the Hubble radius in the pre-bounce phase and the spectrum is transformed into a scale-invariant one on super-Hubble scales before the bounce because the long wavelength modes undergo squeezing of their wave-functions for a longer period of time than shorter wavelength modes. The scale-invariance of the spectrum of curvature fluctuations is preserved during and after the bounce. A distinctive prediction of this scenario is the amplitude and shape of the bispectrum."

What one could hope for is that two or more bounce cosmology models appear and they make different predictions about details of the CMB and early universe structure formation. So observations can distinguish between them and one or both can be shot down.
I have to go. But I think this is a good topic to build up some background material on. I'll get back to this as time permits.
Thanks for raising the question!

 

[Redbelly98]

Thanks Marcus, I skimmed through the abstracts you quoted. I don't fully understand everything, but it sounds like information in the form of gravitational waves is theorized to survive from before the Big Bang. If so, then it will be a long time indeed before any observational tests are realized (Given LIGO has been 20 years in the making and still no detection).

I am unfamiliar with "tensor perturbations", and Chaplygin's equation of state. Is there a layman's explanation? (Even though I completed physics grad school, I never took a course in general relativity or cosmology, instead specializing in experimental work with atoms and lasers.)

 

[marcus]

... I skimmed through the abstracts you quoted...

I'm glad you skimmed the abstracts. It's good to have someone else to talk to who might be interested in keeping track of this part of the literature. To me, the most revealing paper on the list was this one:

http://arxiv.org/abs/0904.2835
Matter Bounce in Horava-Lifgarbagez Cosmology
Robert Brandenberger (McGill University and CERN)
6 pages, 1 figure
(Submitted on 18 Apr 2009)

I'll quote some excerpts to give an idea of content.

"...Since the curvature perturbation R grows on super-Hubble scales, a matter bounce leads to a larger amplitude of non-Gaussianities than slow-roll single-field inflation. Since it is a different mode of R which dominates, the shape of the non-Gaussianities is also different from what is obtained in slow-roll single-field inflation models.

The specific predictions for the amplitude and shape of the three-point function (the “bispectrum”) were worked out in [21]. In particular, the predicted amplitude of the bispectrum is very close to the level which could be detected using the Planck satellite experiment.

...

...Thus, one of the main messages of this Note is that it is not necessary to force a period of inflationary expansion into Horava-Lifgarbagez cosmology. The alternative matter bounce scenario predicts an amplitude of the normalized bi-spectrum is the order of 1, and a specific shape of this three-point function, as studied in detail in [21].

These specific predictions are potentially within the reach of upcoming CMB missions such as PLANCK.

To obtain a successful late-time cosmology, the model presented here must be supplemented with a mechanism to transfer the energy at late times to Standard Model matter and radiation..."

In case anyone is unfamiliar with Brandenberger's standing, he was co-organizer of a 3week workshop at Kavli ITP Santa Barbara which brought LQG and String people together to study resolution of the cosmo singularity, and spacetime singularities in general (The Quantum Nature of Spacetime Singularities).
This year at the big Paris meeting (Marcel Grossmann 12) he is the one chosen to chair the parallel session on bounce cosmologies.
To put it crassly, he is a respected old boy of the inner circle.
=====================

I was unfamiliar with the Chaplygin equation of state.
Just took a look at the introduction to this paper, which has something about it:
http://arXiv.org/pdf/gr-qc/0202064v2
p = - A/rho
where A is some positive constant.
If anyone has some better sources please share them. This seems to be just another type of inflation, with a different kind of exotic matter for the inflaton field. This time the "exotic fluid" postulated to drive the inflation has an equation of state of that particular form.
"...
thereby avoiding the above mentioned fine-tuning problems. This is achieved via the introduction, within the frame work of FRW cosmology, of an exotic background fluid, the Chaplygin gas, described by the equation of state... "
Fortunately only one of the papers listed depends on the universe tanking up on Chaplygin gas so I'm going to focus on some of the others.

I like Brandenberger's because among other things it obviates inflation. This puts Willy the Razor in high good humor, grinning from ear to ear.

 

[marcus]

I guess the general lesson is that when you test an early universe model you test the model against what we can see in the present.

This is apt to be of stuff like the CMB.

You don't test the idea of a bounce. You test a model of how the universe works, and if it has a bounce...well, then it has a bounce.

=================

At some level it doesn't matter very much whether it is a HoravaLifgarbagez model or a Loop Cosmology model. The important thing is to have some signature in the CMB that the Planck spacecraft can look for. Or else say how much finer resolution than Planck's you want to get.

Planck is supposed to launch May 6. Just a couple of weeks!
http://www.esa.int/SPECIALS/Operations/SEM45HZTIVE_0.html
http://www.esa.int/esaSC/120398_index_0_m.html
=================

Everybody seems to be talking about Horava-Lifgarbagez quantum gravity---not just Robert Brandenberger, a dozen or so papers by others have appeared recently. But none have pointed out the obvious shortened terminology---taking the initial and final syllables.

 

[WaveJumper]

Why are cosmologists putting so much efforts into trying to find ways around the singularity at t=0? Isn't it a better idea to first make sure we know for certain what kind of a universe we find ourselves in - is it a 2D hologram, 11-dimensional, N-dimensional or 3-dimensional?

 

[marcus]

I guess the general lesson is that when you test an early universe model you test the model against what we can see in the present.

This is apt to be of stuff like the CMB.

You don't test the idea of a bounce. You test a model of how the universe works, and if it has a bounce...well, then it has a bounce.

=================

At some level it doesn't matter very much whether it is a HoravaLifgarbagez model or a Loop Cosmology model. The important thing is to have some signature in the CMB that the Planck spacecraft can look for. Or else say how much finer resolution than Planck's you want to get.

Planck is supposed to launch May 6. Just a couple of weeks!
http://www.esa.int/SPECIALS/Operations/SEM45HZTIVE_0.html
http://www.esa.int/esaSC/120398_index_0_m.html
=================
...

Planck launch date has been set back to May 14.

Wavejumper, the standard model cosmologists work with is spatial 3D. You seem to want everybody to put off doing ordinary cosmology until extra dimensions (for which there is no experimental evidence) have been conclusively ruled out.