Inflaton Field and Boucne Models

[skydivephil]

Inflaton Field and Bounce Models

In traditional inflation, there is a field known as the inflaton which a boson of the same name. Hybrid models of inflation , if I've understood correrctly(?) have a field to start inflation and a field to stop it. However recent work on bounce mdoels such as in LQC has argued inflation arises as a consequence of the bounce. Do bounce cosmsologies have anything to say about the nature of the inflaton field. Do we still need to invoke such a field and its boson or is it superflouous in these mdoels?

 

[Chalnoth]

In traditional inflation, there is a field known as the inflaton which a boson of the same name. Hybrid models of inflation , if I've understood correrctly(?) have a field to start inflation and a field to stop it. However recent work on bounce mdoels such as in LQC has argued inflation arises as a consequence of the bounce. Do bounce cosmsologies have anything to say about the nature of the inflaton field. Do we still need to invoke such a field and its boson or is it superflouous in these mdoels?

No, bounce cosmologies don't have any inflaton field. They generate the perturbations in a different manner.

 

[marcus]

In traditional inflation, there is a field known as the inflaton which a boson of the same name. Hybrid models of inflation , if I've understood correrctly(?) have a field to start inflation and a field to stop it. However recent work on bounce mdoels such as in LQC has argued inflation arises as a consequence of the bounce. Do bounce cosmsologies have anything to say about the nature of the inflaton field. Do we still need to invoke such a field and its boson or is it superflouous in these mdoels?

No, bounce cosmologies don't have any inflaton field. They generate the perturbations in a different manner.

Actually it varies. There are various bounce cosmologies, including I believe string-inspired, and they may differ in how they handle things. But to get adequate inflation in LQC you normally include an inflaton field.

That is how it has been done, in LQC, so far. See the Ashtekar paper on LQC and inflation---he's a central figure in LQC.

 

[Chalnoth]

Actually it varies. There are various bounce cosmologies, including I believe string-inspired, and they may differ in how they handle things. But to get adequate inflation in LQC you normally include an inflaton field.

That is how it has been done, in LQC, so far. See the Ashtekar paper on LQC and inflation---he's a central figure in LQC.

Hmm, guess I was mistaken. But that's profoundly disappointing. Kinda removes the whole idea of getting inflation "automatically", doesn't it?

 

[marcus]

Hmm, guess I was mistaken. But that's profoundly disappointing. Kinda removes the whole idea of getting inflation "automatically", doesn't it?

You could certainly be right! But I wouldn't say that yet. There are some new developments.

 

[marcus]

The situation is complicated and in flux. I have gotten word of some recent work in LQG in which Lambda runs, as it does in "safe" QG. That is the cosmological constant (that tends to accelerate expansion) is higher at very high energy density.

I think there is a real possibility that one will get adequate inflation for free in the safe QG approach ("asymptotic safety" QG, recent paper by Yi-Fu Cai and Damien Easson) but also a remote chance this could happen in the LQC approach as well.

Then the fluctuations in density which provided the seeds of structure so that stuff could begin to coagulate and fall together would have to be provided by fluctuations in another field. Cai and Easson have proposed the Higgs field as a possible candidate.

But just to focus on LQC, and what has been published so far: it is true that in LQC you do get a natural period of inflation immediately after the bounce. But it is TOO BRIEF. It ends naturally and too soon, so it is not sufficient to provide the full 60-efolds that cosmologists require.

A recent paper on inflation in LQC would be this by Ashtekar and Sloan:
Google "ashtekar probability inflation" and get http://arxiv.org/abs/1103.2475

To get sufficient inflation (the factor of e⁶⁰ or so which cosmologists consider adequate to reproduce what we seem to observe) Ashtekar and Sloan have to include a single inflaton field. But because of circumstances special to LQC they find they don't have to worry too much about fine tuning.

The initial brief period of natural inflation helps. During this brief interval the Hubble parameter is actually increasing and expansion is faster than exponential, so it can be called "superinflation". The Hubble parameter experiences a natural spike and gets up to near Planck scale (the Hubble time gets down near Planck time, i.e. 10^-43 second). But as I say this superinflation spike ends too soon to provide the full amount of inflation.

This what you see in the recent published accounts. So an "inflaton field" is included. I have a personal hunch that it may be possible, in LQC, to eventually dispense with the inflaton field and still get enough inflation to satisfy the observational cosmologists.

The reason for my hunch is this recent paper:
Google "easson inflation higgs" and get http://arxiv.org/abs/1202.1285
In this paper inflation is for free as a natural result of the running of the cosmo constant Lambda. The fluctuations needed to seed structure arise from the higgs field. So there is in a sense no "made up" exotic physics. We already know about Lambda and about the higgs field. So this line of thought is in a sense conservative. At the moment I see no reason why LQC should not eventually be able to accommodate inflation in a similar way (i.e. without resorting to an inflaton.) But I could be wrong.
It could be that in LQC one only gets some inflation (as we know) for free and still requires an inflaton field.

 

[skydivephil]

Thanks for the reply Marcus. So do the fluctuation that seed galaxy formation still happen in inflation or during super inflation or both?

 

[marcus]

Phil I think you are asking about the Loop cosmology picture specifically.

The standard picture there uses an inflaton field to get what is considered adequate inflation. The natural "superinflation" phase of the Loop bounce is too brief. It depends on quantum effects which prevail only at very high energy density, so as soon as there has been a substantial amount of expansion (reducing the energy density) it shuts down. zzzzzzzzzzzzzzzzzzzzzzbbbbbbbbbbbbbbbb

I'm falling asleep, have to take an early bedtime. Google "ashtekar" and you'll fnd his latest survey of Loop cosmology, or his latest discustion of inflation. It's largely just the conventional inflatonfield.

I've come around to thinking I was wrong earlier when I speculated that in Loop cosmology they might be able to imitate the neat Cai Easson trick of getting adequate inflation without an inflaton field. there is too much chance it won't work in LQC for me to be speculating about it. But it is an interesting possibility that comes up in the Asym Safety context that which Cai and Easson were operating in. Worth knowing about.