# Cosmology and gravity

1. Nov 21, 2006

### Chronos

I was intrigued by this paper and the approach:

http://arxiv.org/abs/gr-qc/0611090
Cosmology as a search for overall equilibrium
Authors: Carlos Barcelo
Comments: 9 pages, 1 figure

"In this letter we will revise the steps followed by A. Einstein when he first wrote on cosmology from the point of view of the general theory of relativity. We will argue that his insightful line of thought leading to the introduction of the cosmological constant in the equations of motion has only one weakness: The constancy of the cosmological term, or what is the same, its independence of the matter content of the universe. Eliminating this feature, I will propose what I see as a simple and reasonable modification of the cosmological equations of motion. The solutions of the new cosmological equations give place to a cosmological model that tries to approach the Einstein static solution. This model shows very appealing features in terms of fitting current observations."

This builds upon earlier works in collaboration with Matt Visser on the concept of Analogue Gravity - an alternative to string and LQG approaches to quantum gravity. It is an interesting idea that, by all appearances, is more touchy feely [testable] than these approaches. It has some warts, mainly the inability to produce the expected quantum backreactions, but not yet mortally wounded so far as I can see.

The idea is substantially based on this also fascinating paper from 2001:

Einstein Gravity as an emergent phenomenon?
Authors: Carlos Barcelo, Matt Visser, Stefano Liberati
Comments: 8 pages, Essay awarded an honorable mention in the year 2001 Gravity Research Foundation essay competition
Journal-ref: Int.J.Mod.Phys. D10 (2001) 799-806

"In this essay we marshal evidence suggesting that Einstein gravity may be an emergent phenomenon, one that is not fundamental'' but rather is an almost automatic low-energy long-distance consequence of a wide class of theories. Specifically, the emergence of a curved spacetime effective Lorentzian geometry'' is a common generic result of linearizing a classical scalar field theory around some non-trivial background. This explains why so many different analog models'' of general relativity have recently been developed based on condensed matter physics; there is something more fundamental going on. Upon quantizing the linearized fluctuations around this background geometry, the one-loop effective action is guaranteed to contain a term proportional to the Einstein--Hilbert action of general relativity, suggesting that while classical physics is responsible for generating an effective geometry'', quantum physics can be argued to induce an effective dynamics''. This physical picture suggests that Einstein gravity is an emergent low-energy long-distance phenomenon that is insensitive to the details of the high-energy short-distance physics."

Some tantalizing related material has arisen in condensed matter physics along these lines. Perhaps there is an underlying 'apples to oranges' problem with existing approaches to quantum qravity.

2. Nov 21, 2006

### Garth

Do these alternative approaches that asyptotically approach GR in the present low density limit also predict concordant BBN and CMB observables from the high density epoch?

Garth

3. Nov 24, 2006

### Chronos

I don't think those questions are relevant to the model, Garth. It does not forbid or require BBN, or CMB anisotropy, so far as I perceive.

4. Nov 24, 2006

### Garth

Thank you Chronos.

The model is an oscillating universe one.

From the OP paper:
The author seems to be saying that the SN Ia observations already contradict the model:
However, the recent HST observations of SN Ia at high z may reverse this conclusion.

In the BBN high energy regime:

So the model does affect BBN, but the author sees ways of compensating for this.

Garth

Last edited: Nov 24, 2006
5. Nov 24, 2006

### Chaos' lil bro Order

Contradicting type 1a observations seems a recipe for disaster. I thought type 1a's are our best standard candles.

6. Nov 24, 2006

### Garth

The best maybe, but that does not necessarily mean they are good!

There could be cosmological or systematic effects that result in their apparent magnitudes being fainter than expected.

The paper, as I read it, is simply claiming we cannot be sure about such a falsification.

Garth

7. Nov 25, 2006

### Chaos' lil bro Order

What about Cepheid Variables? Does the author criticize their use as standard candles too? (I haven't read the paper)

8. Nov 26, 2006

### Garth

They cannot be seen (yet) at such cosmological distances.

Garth

9. Nov 26, 2006

### Mike2

As I understand it, with the expansion of the universe comes an horizon, and horizons produce a bath of thermal particles. I take it that these particles can be permanent and can have mass. So it would seem that the matter/photon content of the universe is not constant as previously suspected. I wonder if this has been taken into account in the cosmological models.

10. Nov 27, 2006

### Chronos

The universe has always had a horizon. Particles always emerged inside that horizon. Photons had already established the boundary conditions. But spacetime had the jump. Photons [and baryons] were trapped inside the surface of last scattering giving spacetime a huge, inflationary headstart.

11. Nov 27, 2006

### Mike2

I have to wonder how there could be an horizon before there were photons. If photons (and baryons) were formed because of the contraction of the horizon, how was there ever a horizon to begin with before photons? How can you establish the speed of photons without photons?

As R.M. Wald says, perhaps particles are not feasible in tightly curved and changing spacetimes. So I suspect that the reason Inflation was so fast was because it was so tightly curled that no particles could exist to prevent its expansion through gravity? In other words, all there was before particle was vacuum energy. All the particles came from this vacuum energy. But before this vacuum energy decayed into particle it caused a negative pressure of expansion on the universe causing it to inflate.

12. Nov 28, 2006

### hellfire

It is not necessary to have any particle to define a horizon. A horizon is determined by the behaviour of geodesics is space-time.

The question whether there existed particles or fields seams not relevent to me here. What is relevant is the energy density that determines the behaviour of the scale factor according to the Friedmann equations. During inflation the energy density of the inflaton with an equation of state $p = - \rho$ was the dominant one. This lead to an accelerated expansion of space.

Moreover, the curvature of space-time seams also to be irrelevant. Consider a spatially flat universe (the universe became nearly flat soon after the start of inflation), with $k = 0$. The evolution of the scale factor during inflation is:

$$a = e^{Ht}$$

with $H$ the constant Hubble parameter during inflation. The most simple quantity that tells you something about curvature of space-time is the Ricci scalar:

$$R = \frac{6}{a^2}(a \ddot a + \dot a^2 + k)$$

Inserting for the scale factor:

$$R = 6 H^2$$

As you can see, the curvature of space-time is the same before than after inflation (H remains constant in a de-Sitter space-time).

Last edited: Nov 28, 2006
13. Nov 29, 2006

### Chronos

Excellent post hellfire. I merely wish to add that if gravity was emergent in the early universe, inflation had no brakes until gravity emerged from the chaos. I'm not comfortable with an infinite rate of expansion [a quantum fluctuation thing], but, I can see how it might easily have run wild in the first few planck ticks of time. I feel free to take poetic license here because causality has little meaning before matter freezes out of the primordial soup.

14. Nov 29, 2006

### Chaos' lil bro Order

I completely agree Chronos. Until space had grown sufficiently to thin out its Energy density, gravity was much to weak to influence any structural influence on the emerging universe.

15. Nov 30, 2006

### Mike2

You seem to be assuming that other fields coexisted with the Inflaton field. But as I understand it, other fields developed from the Inflaton field. So if the ONLY field is the Inflaton field causing Inflation, then there is nothing to stop Inflation.

Hold on there. I though one reason that motivated Inflation to begin with is to explain how the universe has become flat when it was curved to begin with. What are you talking about?

16. Nov 30, 2006

### hellfire

Other fields existed already, however, their energy density was negligible. After inflation the energy density of the inflaton was transferred to the other fields.

You are right that the theory of inflation was motivated to explain the flatness problem, providing a plausible mechanism to make the universe spatially flat. Flatness in cosmology means zero curvature of space, not space-time. However, the question whether particles may or not exist in very curved space-times relates to the curvature of space-time.

Last edited: Nov 30, 2006
17. Nov 30, 2006

### Mike2

Can you give me an idea of what epoc this was? Was this during the time when the universe consisted of a quark soup? Or was it before that when particles received their mass from the Inflaton field? I'm thinking it would have to be before the Standard Model since those fields are of particles which already have mass, etc. And if it is before the SM, then we really don't have a good idea what's going on in that mass receiving interation, do we?

So what are examples of curved spacetime if not near singularity such as black holes and big bangs? I thought Wald was trying to explain the problems with the particle picture associated with these "singularities". But if these situations are not what he is talking about, then I don't know what situations he is refering to.

18. Dec 1, 2006

### hellfire

The start and end of inflation is not known. I assume it can start at any time after the Planck time. The quark soup existed after inflation. See this for more information.

I am sorry but I do not understand what you mean here. Particles or fields is not an issue here. Fields have also mass.

You are right, these are the situations: when the length scale at which space-time curvature is noticeable is comparable to the de-Broglie wavelength of the particles. My intention was just to point out that the fact that energy was transferred to other fields from the inflaton has nothing to do with curvature of space-time.

19. Dec 1, 2006

### Mike2

Can you give me a little story of what's going on in the Lagrangian or Action Integral, or path integral that causes energy transfer from one field to the next? Would it be some way in which coupling constants are used in flat spacetime verses curved spacetime. Any references would be appeciated too. Thanks.

Last edited: Dec 1, 2006
20. Dec 1, 2006

### Chaos' lil bro Order

Hellfire.
The Inflationary Period is 10-37 to 10-32 seconds.

I'm not saying that by any means known, but that's what the Standard Model says.

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