Why not an eternal and infinite space?

[Chiclayo guy]

One of the most common questions on this forum over the years is, “What is the universe expanding into?” The common answer in one form or another is always ‘nothing.’ My question is, why does current thinking preclude an eternal and infinite space… a void populated by the physical stuff we know about?

 

[Garth]

One of the most common questions on this forum over the years is, “What is the universe expanding into?” The common answer in one form or another is always ‘nothing.’ My question is, why does current thinking preclude an eternal and infinite space… a void populated by the physical stuff we know about?

Hi Chiclayo guy!

Your post does not make it clear what you are asking about.

If you are suggesting the universe is expanding into an "eternal and infinite space" then you have not understood the cosmology of the expansion of the universe.

In General Relativity space-time forms a four dimensional continuum that suffers curvature because of the presence of mass within it.

A freely falling body experiences this curvature as a gravitational field and falls towards the massive body that is causing the curvature.

If you average out the masses within the universe into a thin 'gas' that is the same and looks the same everywhere (it is homogeneous and isotropic) then you can solve the GR field equation for the whole universe - this is the cosmological solution.

This covers the whole of space, there is no other "eternal and infinite space" into which it is expanding, it is all included.

The stable solutions demand that space, a slice or foliation of space-time, must be expanding or contracting.

Hubble's observations in the 1920's showed that our universe (the whole of it) is actually expanding. There is nothing outside.

This space described by the GR cosmological solution may well be infinite and eternal - with no end in the future. Some speculative cosmologies (eternal inflation) also want to go back into the past before the 'Big Bang' so there is no beginning either.

My own tongue in cheek answer to the question, “What is the universe expanding into?” is "It is expanding into its future!"

I hope this helps,
Garth

 

[phinds]

My own tongue in cheek answer to the question, “What is the universe expanding into?” is "It is expanding into its future!"

I like that !

 

[phinds]

As far as "eternal" goes, there ARE theories that posit an eternal universe, they just don't match up with experimental observation.

 

[marcus]

... they just don't match up with experimental observation.

Are you sure? This could be be misleading to some people. Loop cosmology matches up with observation equally with the standard classical ΛCDM model.
And it has no "beginning". So the natural thing to assume, since there is no evidence of or theoretical need for a beginning, is that the universe always was.

 

[phinds]

Are you sure?

Well, I was not aware that any such theories match up ... if I'm wrong, I'm wrong. Thanks.

 

[Torbjorn_L]

This could be be misleading to some people. Loop cosmology matches up with observation equally with the standard classical ΛCDM model.

Generally we inhabit the present and can extend our predictions into the past and the future. So to be precise about eternal systems, they can be past and/or future eternal.

Already Big Bang theory was future eternal in some cases (non-collapsing universe), and the LCDM cosmology likely is in our case. (Flat universe - the new Planck data release will say curvature is 0.000 +/- 0.005, according to pre-release conferences on the web the last few weeks.)

If we go in the other direction I hear it is hard, aka needs finetuning, to avoid [future] eternal inflation in an inflation theory like standard LCDM. That may or may not lead to past eternal cosmologies.

LCDM cosmology as such does not contain much constraint on the vacuum of quantum field theory it contains, and you can certainly rig up vacuum physics to make a finite time universe in any of those cases. Hawking has arxiv papers exploring that in his variant of inflation cosmologies.

Now I don't know if "loop cosmology" refers to bounce cosmologies that putatively can "loop back" if I understand one of Steinhardt's web seminars correctly, or loop quantum cosmologies as I know them.

Classically the problem with bounce cosmologies was that they _didn't_ "loop back" as entropy increased, which IIRC meant that they grew shorter and shorter in period between bounces. But Steinhardt has fixed that I think, with more ad hoc add ons. (You also need an ad hoc add on to predict the new Planck polarization data, which unlike inflation isn't a natural part of the physics.)

Speaking of potentially misleading, classically the problem with LCG 'cosmologies' is that they aren't such. LQG is a mathematical theory. But unlike the same status string theory it can't predict harmonic oscillators so doesn't have dynamics as I understand it. That is because LQG doesn't have a lowest energy, and I think they haven't licked that since last I looked at it a few years ago. (Layman here, but even I can see their handwaving and/or failures, depending on which paper had the proposal.)

The way they do cosmology is to have simulations making a movie of "allowed moves" of their triangulations, neglecting that there isn't any physics driving this. So it fails to predict simpler physics, which must be considered when they do match up on some parameters.

It is perhaps best considered as fringe, even if it has status as being part of some conferences? [I don't know how to distinguish it from pseudoscience to be frank, except that from the viewpoint of outside looking in, physicists seem hesitant to do so.]

 

[marcus]

...
Now I don't know if "loop cosmology" refers to bounce cosmologies that putatively can "loop back" if I understand one of Steinhardt's web seminars correctly, or loop quantum cosmologies as I know them.

Classically the problem with bounce cosmologies was that they _didn't_ "loop back" as entropy increased, which IIRC meant that they grew shorter and shorter in period between bounces. But Steinhardt has fixed that I think, with more ad hoc add ons. (You also need an ad hoc add on to predict the new Planck polarization data, which unlike inflation isn't a natural part of the physics.)

Speaking of potentially misleading, classically the problem with LCG 'cosmologies' is that they aren't such. LQG is a mathematical theory. But unlike the same status string theory it can't predict harmonic oscillators so doesn't have dynamics as I understand it. That is because LQG doesn't have a lowest energy, and I think they haven't licked that since last I looked at it a few years ago. (Layman here, but even I can see their handwaving and/or failures, depending on which paper had the proposal.)

The way they do cosmology is to have simulations making a movie of "allowed moves" of their triangulations, neglecting that there isn't any physics driving this. So it fails to predict simpler physics, which must be considered when they do match up on some parameters.

It is perhaps best considered as fringe, even if it has status as being part of some conferences? [I don't know how to distinguish it from pseudoscience to be frank, except that from the viewpoint of outside looking in, physicists seem hesitant to do so.]

Interesting collection of opinions! For contrast, here is a 52-page 2013 review of LQC invited chapter in The Handbook of Spacetime (edited by Abhay Ashtekar ) to be published by Springer. It's as if the authors and T_L are describing two entirely different formalisms and two entirely different research programs! :w
http://arxiv.org/pdf/1302.3833v1.pdf

 

[Bystander]

why does current thinking preclude an eternal and infinite space

Or, "What does 'eternal and infinite space' lack that drives other investigations and hypotheses?" Start with "Olber's Paradox."

 

[marcus]

Or, "What does 'eternal and infinite space' lack that drives other investigations and hypotheses?" Start with "Olber's Paradox."

But one should add that Olber's Paradox doesn't apply except in special cases. It is not an obstacle to this version of eternal infinite spatial-volume-cosmology
http://arxiv.org/abs/1412.2914
ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing
(Submitted on 9 Dec 2014)
We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
14 pages, 8 figures

It's an interesting collaboration. Yi-Fu Cai is at McGill, in Robert Brandenberger's group and a frequent co-author with Brandenberger. Wilson-Ewing is an Ashtekar PhD and co-author. Now has a postdoc position at Max Planck Institute (AEI) near Berlin.

In this picture space can be infinite (if you choose) but there is no "Olber's problem" essentially because of Hubble law distance expansion.

What they are doing here is seeing how this particular bounce model fits the observed spectrum of primordial fluctuations and the low tensor ratio that Planck (not BICEP!) seems also to have observed. They seem able to fit observation without invoking inflation. This is something that has become common with Brandenberger's group at McGill. His research focus used to be on string theory. Shifted towards non-string cosmology. He and his group have a type of bounce cosmology called "matter bounce" that they work on.
In this case "matter bounce" Yi-Fu Cai is getting together with "loop" Wilson-Ewing and finding they can collaborate and match real-world observations.

 

[Chiclayo guy]

Hi Chiclayo guy!

Your post does not make it clear what you are asking about.

If you are suggesting the universe is expanding into an "eternal and infinite space" then you have not understood the cosmology of the expansion of the universe.

In General Relativity space-time forms a four dimensional continuum that suffers curvature because of the presence of mass within it.

A freely falling body experiences this curvature as a gravitational field and falls towards the massive body that is causing the curvature.

If you average out the masses within the universe into a thin 'gas' that is the same and looks the same everywhere (it is homogeneous and isotropic) then you can solve the GR field equation for the whole universe - this is the cosmological solution.

This covers the whole of space, there is no other "eternal and infinite space" into which it is expanding, it is all included.

The stable solutions demand that space, a slice or foliation of space-time, must be expanding or contracting.

Hubble's observations in the 1920's showed that our universe (the whole of it) is actually expanding. There is nothing outside.

This space described by the GR cosmological solution may well be infinite and eternal - with no end in the future. Some speculative cosmologies (eternal inflation) also want to go back into the past before the 'Big Bang' so there is no beginning either.

My own tongue in cheek answer to the question, “What is the universe expanding into?” is "It is expanding into its future!"

I hope this helps,
Garth

Hello Garth------ Thanks for the reply and attempt to educate me. I do appreciate it.

 

[Garth]

Hello Garth------ Thanks for the reply and attempt to educate me. I do appreciate it.

You are welcome.

Garth

 

[Torbjorn_L]

nteresting collection of opinions! For contrast, here is a 52-page 2013 review of LQC invited chapter in The Handbook of Spacetime (edited by Abhay Ashtekar ) to be published by Springer. It's as if the authors and T_L are describing two entirely different formalisms and two entirely different research programs! :w
http://arxiv.org/pdf/1302.3833v1.pdf

If this is supposed to allay my concerns, it has three problems.

First, it quotes my entire comment as if it was an opinion.

Besides the lengthy description of facts and a question, my opinion amounts to this: " [I don't know how to distinguish it from pseudoscience to be frank, except that from the viewpoint of outside looking in, physicists seem hesitant to do so.]"

I don't see any specific response to my opinion.

And that is elevated by the second problem, which is that Ashtekar is the founder of loop quantum physics as per the transformation that it is based on. Seeing how my opinion is that it is pseudoscience, you tend to confirm that. By not having outside references that can reject my opinion, and by implicitly showing us that the subject is shielded from outside criticism.

Finally, what were you thinking? I don't think you trusted me to know or find out about Ashtekar, ego inflating as such an idea is. So naively it looks like you wanted to slip something by. That is another kind of problem, of course.

So perhaps we should split the difference? I take it that it is a fringe subject, but that for some reason that can't be described (perhaps some fuzzy societal) it isn't considered pseudoscience.

 

[RandyD123]

My own tongue in cheek answer to the question, “What is the universe expanding into?” is "It is expanding into its future!"

I like that as well...very nice!

 

[Priyank]

#ChiclayoGuy
Great Question indeed!
and nice answers by you all... :D

 

[skydivephil]

What i think most cosmologists agree with is that one needs a quantum thoeyr of gravity to understand what happened at the big bang.
Does anyone here disagree with that?
Attempts to apply existing candidate theories for quantum gravity such as strings, loop and Horava gravity all seem to point to a pre big bang pahse, often a contracting phase before the bang. These theories have not been tested so we cannot say they are right. But neither has GR in these extreme conditions. So I think its bets to say we don't know if the universe is eternal into the past or not.
the observations only tell us the universe was hotter and denser in the past not finite in age.

 

[phinds]

What i think most cosmologists agree with is that one needs a quantum thoeyr of gravity to understand what happened at the big bang.
Does anyone here disagree with that?
Attempts to apply existing candidate theories for quantum gravity such as strings, loop and Horava gravity all seem to point to a pre big bang pahse, often a contracting phase before the bang. These theories have not been tested so we cannot say they are right. But neither has GR in these extreme conditions. So I think its bets to say we don't know if the universe is eternal into the past or not.
the observations only tell us the universe was hotter and denser in the past not finite in age.

While I agree w/ what you are saying, I don't think it's definitive that a solid theory of quantum gravity will necessarily be ENOUGH to understand what happened at t=0 ... since we don't know what happened, we can't know that there won't be some other effects that would not be covered by quantum gravity alone.

 

[marcus]

...such as strings, loop and Horava gravity all seem to point to a pre big bang phase, often a contracting phase before the bang...

And add the McGill group to the list, with their "matter bounce" investigations. Googling "matter bounce" gets a bunch of hits, as I recall. The senior person, the ringleader, is Robert Brandenberger . But I'm particularly impressed by Yi-Fu Cai.

Their scenarios are different from string and Horava---also distinct from Loop, but with just enough similarity to permit some crossover.

One reason I like Yi-Fu Cai is that (whereas Ashtekar Loop cosmology has always allowed inflation, never argued that it wasn't needed) Cai (and Brandenberger as well) typically find that when there is a bounce inflation is not needed. The McGill people seem inclined to pointedly omit inflation from their scenarios. Here is how the point is made in the Cai&Wilson-Ewing paper I summarized earlier.
http://arxiv.org/abs/1412.2914 (google search "LambdaCDM bounce")

==quote Cai&Wilson-Ewing page 1==
I. INTRODUCTION

Observations of the cosmic microwave background (CMB) —most recently [1, 2]— have clearly established that scalar perturbations in the early universe were nearly scale-invariant. It is thus necessary for any realistic cosmological model to generate, in some fashion, scale-invariant perturbations.

To achieve this, many cosmological models rely on the presence of matter fields (typically scalar fields) that have not yet been observed in nature. The new matter fields are necessary in these models as they play an essential role in the generation of scale-invariant perturbations. While it is of course a requirement for any cosmological scenario to predict near scale-invariance in order to be potentially viable, there are some cosmological scenarios where it is possible to avoid the weakness of postulating the existence of unknown matter fields and nonetheless obtain scale-invariance.
===endquote===

It's a strong opening statement, I think. Here are two postdocs. They are saying that they will DISPENSE WITH INFLATION and still get the good results that serve, in many people's minds, to necessitate and justify assuming inflation. And they explain that they want to dispense with inflation because it requires mythology---assuming the existence of exotic matter fields which have never been seen in nature.

===Cai&Wilson-Ewing page 1, continued===
We shall study one such model in this paper. This cosmological model consists of a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe, with a positive cosmological constant, cold dark matter, and radiation. These are three ingredients known to be present in our universe, and we will not assume the existence of any other matter fields. We also assume that the initial conditions are such that the space-time curvature is small and the universe is large and contracting.

As the universe contracts, the space-time curvature will increase, and quantum gravity effects are expected to become important at some point, likely when the space- time curvature nears the Planck scale. In this work, we will assume that loop quantum cosmology (LQC) captures the salient non-perturbative quantum gravity effects in the very early universe...predicts that a bounce occurs near the Planck scale and that, once these quantum gravity effects are included, the space-time is free of the singularities that appear in classical general relativity [3–5].

Thus, this model will be that of a bouncing universe, with a matter content of radiation and cold dark matter and a positive cosmological constant. ...if the pressure is slightly negative, for example due to the presence of a positive cosmological constant, then the long wavelength perturbation modes will be almost scale-invariant with a slight red tilt. Therefore, in the model considered here, we expect the modes that become large during the epoch of the universe that is dominated by cold dark matter to be almost scale-invariant, and those that become large when the effective equation of state is slightly negative to have a small red tilt.

In this paper we calculate the spectrum of the cosmological perturbations for this model.
==endquote==

So they are going to do without exotic matter so far never observed and its associated quaintly shaped "plateau" potentials as well, with no particular reason to support them either. :)
They are going to study a contracting ΛCDM universe starting from its matter&Lambda dominated phase, proceeding thru the radiation-dominated era, to the stage where quantum effects become important, and then back out, as it re-expands, thru radiation era to where matter and the cosmological constant again predominate. And they are going to get the good results without inflation.
It's a bold initiative. And they still have more to do (e.g. in ordinary baryonic matter in addition to DM). They discuss future work to be done on this project in the final section of the paper--also worth checking out.

At the moment I think this December 2014 "ΛCDM bounce" paper could be the most consequential one on the Combined 4th quarter MIP poll. Perhaps even the most important to have appeared all year.
https://www.physicsforums.com/threa...mip-poll-for-most-important-qg-papers.789671/

 

[skydivephil]

And add the McGill group to the list, with their "matter bounce" investigations. Googling "matter bounce" gets a bunch of hits, as I recall. The senior person, the ringleader, is Robert Brandenberger . But I'm particularly impressed by Yi-Fu Cai.

Their scenarios are different from string and Horava---also distinct from Loop, but with just enough similarity to permit some crossover.

One reason I like Yi-Fu Cai is that (whereas Ashtekar Loop cosmology has always allowed inflation, never argued that it wasn't needed) Cai (and Brandenberger as well) typically find that when there is a bounce inflation is not needed. The McGill people seem inclined to pointedly omit inflation from their scenarios. Here is how the point is made in the Cai&Wilson-Ewing paper I summarized earlier.
http://arxiv.org/abs/1412.2914 (google search "LambdaCDM bounce")

==quote Cai&Wilson-Ewing page 1==
I. INTRODUCTION

Observations of the cosmic microwave background (CMB) —most recently [1, 2]— have clearly established that scalar perturbations in the early universe were nearly scale-invariant. It is thus necessary for any realistic cosmological model to generate, in some fashion, scale-invariant perturbations.

To achieve this, many cosmological models rely on the presence of matter fields (typically scalar fields) that have not yet been observed in nature. The new matter fields are necessary in these models as they play an essential role in the generation of scale-invariant perturbations. While it is of course a requirement for any cosmological scenario to predict near scale-invariance in order to be potentially viable, there are some cosmological scenarios where it is possible to avoid the weakness of postulating the existence of unknown matter fields and nonetheless obtain scale-invariance.
===endquote===

It's a strong opening statement, I think. Here are two postdocs. They are saying that they will DISPENSE WITH INFLATION and still get the good results that serve, in many people's minds, to necessitate and justify assuming inflation. And they explain that they want to dispense with inflation because it requires mythology---assuming the existence of exotic matter fields which have never been seen in nature.

===Cai&Wilson-Ewing page 1, continued===
We shall study one such model in this paper. This cosmological model consists of a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe, with a positive cosmological constant, cold dark matter, and radiation. These are three ingredients known to be present in our universe, and we will not assume the existence of any other matter fields. We also assume that the initial conditions are such that the space-time curvature is small and the universe is large and contracting.

As the universe contracts, the space-time curvature will increase, and quantum gravity effects are expected to become important at some point, likely when the space- time curvature nears the Planck scale. In this work, we will assume that loop quantum cosmology (LQC) captures the salient non-perturbative quantum gravity effects in the very early universe...predicts that a bounce occurs near the Planck scale and that, once these quantum gravity effects are included, the space-time is free of the singularities that appear in classical general relativity [3–5].

Thus, this model will be that of a bouncing universe, with a matter content of radiation and cold dark matter and a positive cosmological constant. ...if the pressure is slightly negative, for example due to the presence of a positive cosmological constant, then the long wavelength perturbation modes will be almost scale-invariant with a slight red tilt. Therefore, in the model considered here, we expect the modes that become large during the epoch of the universe that is dominated by cold dark matter to be almost scale-invariant, and those that become large when the effective equation of state is slightly negative to have a small red tilt.

In this paper we calculate the spectrum of the cosmological perturbations for this model.
==endquote==

So they are going to do without exotic matter so far never observed and its associated quaintly shaped "plateau" potentials as well, with no particular reason to support them either. :)
They are going to study a contracting ΛCDM universe starting from its matter&Lambda dominated phase, proceeding thru the radiation-dominated era, to the stage where quantum effects become important, and then back out, as it re-expands, thru radiation era to where matter and the cosmological constant again predominate. And they are going to get the good results without inflation.
It's a bold initiative. And they still have more to do (e.g. in ordinary baryonic matter in addition to DM). They discuss future work to be done on this project in the final section of the paper--also worth checking out.

At the moment I think this December 2014 "ΛCDM bounce" paper could be the most consequential one on the Combined 4th quarter MIP poll. Perhaps even the most important to have appeared all year.
https://www.physicsforums.com/threa...mip-poll-for-most-important-qg-papers.789671/

And add the McGill group to the list, with their "matter bounce" investigations. Googling "matter bounce" gets a bunch of hits, as I recall. The senior person, the ringleader, is Robert Brandenberger . But I'm particularly impressed by Yi-Fu Cai.

Their scenarios are different from string and Horava---also distinct from Loop, but with just enough similarity to permit some crossover.

One reason I like Yi-Fu Cai is that (whereas Ashtekar Loop cosmology has always allowed inflation, never argued that it wasn't needed) Cai (and Brandenberger as well) typically find that when there is a bounce inflation is not needed. The McGill people seem inclined to pointedly omit inflation from their scenarios. Here is how the point is made in the Cai&Wilson-Ewing paper I summarized earlier.
http://arxiv.org/abs/1412.2914 (google search "LambdaCDM bounce")

==quote Cai&Wilson-Ewing page 1==
I. INTRODUCTION

Observations of the cosmic microwave background (CMB) —most recently [1, 2]— have clearly established that scalar perturbations in the early universe were nearly scale-invariant. It is thus necessary for any realistic cosmological model to generate, in some fashion, scale-invariant perturbations.

To achieve this, many cosmological models rely on the presence of matter fields (typically scalar fields) that have not yet been observed in nature. The new matter fields are necessary in these models as they play an essential role in the generation of scale-invariant perturbations. While it is of course a requirement for any cosmological scenario to predict near scale-invariance in order to be potentially viable, there are some cosmological scenarios where it is possible to avoid the weakness of postulating the existence of unknown matter fields and nonetheless obtain scale-invariance.
===endquote===

It's a strong opening statement, I think. Here are two postdocs. They are saying that they will DISPENSE WITH INFLATION and still get the good results that serve, in many people's minds, to necessitate and justify assuming inflation. And they explain that they want to dispense with inflation because it requires mythology---assuming the existence of exotic matter fields which have never been seen in nature.

===Cai&Wilson-Ewing page 1, continued===
We shall study one such model in this paper. This cosmological model consists of a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe, with a positive cosmological constant, cold dark matter, and radiation. These are three ingredients known to be present in our universe, and we will not assume the existence of any other matter fields. We also assume that the initial conditions are such that the space-time curvature is small and the universe is large and contracting.

As the universe contracts, the space-time curvature will increase, and quantum gravity effects are expected to become important at some point, likely when the space- time curvature nears the Planck scale. In this work, we will assume that loop quantum cosmology (LQC) captures the salient non-perturbative quantum gravity effects in the very early universe...predicts that a bounce occurs near the Planck scale and that, once these quantum gravity effects are included, the space-time is free of the singularities that appear in classical general relativity [3–5].

Thus, this model will be that of a bouncing universe, with a matter content of radiation and cold dark matter and a positive cosmological constant. ...if the pressure is slightly negative, for example due to the presence of a positive cosmological constant, then the long wavelength perturbation modes will be almost scale-invariant with a slight red tilt. Therefore, in the model considered here, we expect the modes that become large during the epoch of the universe that is dominated by cold dark matter to be almost scale-invariant, and those that become large when the effective equation of state is slightly negative to have a small red tilt.

In this paper we calculate the spectrum of the cosmological perturbations for this model.
==endquote==

So they are going to do without exotic matter so far never observed and its associated quaintly shaped "plateau" potentials as well, with no particular reason to support them either. :)
They are going to study a contracting ΛCDM universe starting from its matter&Lambda dominated phase, proceeding thru the radiation-dominated era, to the stage where quantum effects become important, and then back out, as it re-expands, thru radiation era to where matter and the cosmological constant again predominate. And they are going to get the good results without inflation.
It's a bold initiative. And they still have more to do (e.g. in ordinary baryonic matter in addition to DM). They discuss future work to be done on this project in the final section of the paper--also worth checking out.

At the moment I think this December 2014 "ΛCDM bounce" paper could be the most consequential one on the Combined 4th quarter MIP poll. Perhaps even the most important to have appeared all year.
https://www.physicsforums.com/threa...mip-poll-for-most-important-qg-papers.789671/

Very interesting as always Marcus, that's pretty impressive they get a scale invariant spectrum wihtout an inflaton field, but what about the problems that inflaiton is supposed to solve? monopole, horizon etc?

 

[marcus]

Phil, I think they'll deal with one question at a time, it's in progress and IIRC they have a followup paper in the works. Let's try to think what further questions need to be addressed.
1. uniformity question (not enough past causal contact)
In other words "Why does the universe appear statistically homogeneous and isotropic in accordance with the cosmological principle?"
My guess is that this is not a problem in bounce cosmologies, because different parts of the sky had plenty of chance to be in contact during the prior contracting phase.
Other people studying other types of bounce cosmology have pointed this out, as I recall---I suppose Cai&W-E will eventually get around to mentioning it.

2. flatness question (wrinkles get worse as long as gravity is attractive)
The whole point of quantum bounce cosmology is that at extreme energy density quantum corrections to GR reverse gravity, making it repel. Then geometry tends to smooth out instead of getting more wrinkly. Of course, the quantum correction terms become dominant at high density and that is what causes the bounce in the first place! I gather that would also necessarily wipe out "structure".
I don't know how they will address the flatness issue , but I suppose Cai&W-E will eventually point this out in connection with their model. As with other bounce cosmology research, they will then have to dispose of the "low entropy objection". For instance: the definition of entropy requires a coarse-graining map on phase space grouping points into regions which are equivalent to a particular class of observers. It's an observer-dependent concept. Second law assumes continuity of the class of observers and constancy of the coarse-grain map. (Suddenly no observers, every point in phase space is unique, no second law, discontinuity, entropy reboots :w)

3. magnetic monopole issue.
Sir Martin Rees suggested that inflation could be viewed as preventative medicine for a disease which does not exist. :) We have no indication monopoles ever form. If inflation is supposed to cure the problem then they have to form only at temperatures which occur before inflation and which do not occur in the very high density and temperature which follows inflation when the "inflaton" field "decays". Otherwise a new crop of monopoles would form after the imagined inflation episode.
Well, maybe the temperature and density at the BOUNCE is only comparable to that.

Of course I don't know how Cai&W-E intend to address these other issues. But these issues have confronted various bounce cosmology researchers all along. It is not unusual for them to dispense with inflation, because the bounce seems to take care of some of the same things. It gets rid of the singularity AND it seems possibly to address all-sky uniformity and flatness as well. So these things are not new with Cai&W-E.
What they did in this December 2014 paper was to focus exclusively on what seem to be the most impressive predictions of properly fine-tuned inflation---having to do with scale-invariant power spectrum, red tilt. And the low tensor-scalar ratio. I think compared with those things the issues of uniformity flatness monopoles are for various reasons comparatively minor. But I still hope they'll touch on those as well.