## [astro-ph/0507573] Post-inflation increase of the cosmological tensor-to-scalar perturbation ratio

 Authors: N. Bartolo, Edward W. Kolb, A. Riotto Abstract: We show that the amplitude of scalar density perturbations may be damped after inflation. This implies that CMB anisotropies do not uniquely fix the amplitude of the perturbations generated during inflation. One consequence is that the present tensor-to-scalar ratio may be larger than produced in inflation, increasing the prospects of detection of primordial gravitational radiation. [PDF]  [PS]  [BibTex]  [Bookmark]

Discussion related to specific recent arXiv papers
Antony Lewis
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### [astro-ph/0507573] Post-inflation increase of the cosmologic

This paper essentially emphasises the (well-known) fact that single-field slow-roll inflation results do not hold in general if there is more than one scalar field, and discusses a particular setup. They argue that this makes gravity waves more likely to be detectable because T/S after inflation can be larger than you'd estimate for a single field model. However it's not clear to me what T/S are actually plausible? (e.g. if natural two field models give exponentially small tensor ratios anyway then their argument wouldn't really help)

The paper repeats an argument that having \Delta\phi ~ M_p (needed for easily detectable gravity waves in single-field simple cases) is difficult for renormalizable particle theories. However I thought (e.g. from Linde hep-th/0402051) loop corrections depended on the effective mass coming from the potential V(\phi), and hence only required V << M_p^4 rather than a constraint on \Delta\phi?

Note: the quoted requirement for detectable CMB B-modes of r > 10^{-3} may be unduly pessimistic: more optimal delensing can in principle do orders of magnitude better as shown in astro-ph/0310163.
Last edited by Antony Lewis on July 28 2005, edited 1 time in total.

David Seery
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### Re: [astro-ph/0507573] Post-inflation increase of the cosmol

Antony Lewis wrote:
The paper repeats an arugment that having \Delta\phi ~ M_p (needed for easily detectable gravity waves in single-field simple cases) is difficult for renormalizable particle theories. However I thought (e.g. from Linde hep-th/0402051) loop corrections depended on the effective mass coming from the potential V(\phi), and hence only required V << M_p^4 rather than a constraint on \Delta\phi?
I believe there is still something of a split in opinion about whether the requirement is $V \ll M_p^4$ or $\phi \ll M_p$. So I suppose the argument here is just that $\Delta\phi$ is rolling over a Planck distance and therefore under the more restrictive choice the effective field theory describing inflation won't be valid over the whole range, because before we get to $\phi \sim M_p$ we ought to have integrated in new degrees of freedom belonging to new physics.

I think the issue of loop corrections isn't settled; there is a claim (gr-qc/9609026) that loop corrections lead to an effective screening of the cosmological constant and therefore that such effects are important (astro-ph/0505236) but perhaps the authors didn't intend to invoke loop effects explicitly.

David Seery
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### [astro-ph/0507573] Post-inflation increase of the cosmologic

In the "N-flation" paper out last week (hep-th/0507205), the authors give a specific string-theoretic example of this objection (Sec. 2.1, p. 4) based on the idea that as the field gets a Planck distance away from any given minimum of the potential, string scale modes can become light. These light degrees of freedom control the low-energy physics, so the effective field theory changes.

Unfortunately they don't give a reference for this idea. Does anyone have more details? I suppose the scalar is supposed to be interpreted as an inter-brane distance and as one pushes it away from the potential minimum, the branes separate, so at large distances there is a tower of winding modes which are becoming light.

Antony Lewis
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### Re: [astro-ph/0507573] Post-inflation increase of the cosmol

The new paper astro-ph/0509015 gives a nice fairly detailed critique of this paper. They argue that getting any damping of the adiabatic modes is very hard, and give some additional discussion of the \Delta\psi issue.