[astroph/0507573] Postinflation increase of the cosmological tensortoscalar 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 tensortoscalar ratio may be larger than produced in inflation, increasing the prospects of detection of primordial gravitational radiation. 
[PDF] [PS] [BibTex] [Bookmark] 

 Posts: 1618
 Joined: September 23 2004
 Affiliation: University of Sussex
 Contact:
[astroph/0507573] Postinflation increase of the cosmologic
This paper essentially emphasises the (wellknown) fact that singlefield slowroll 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 singlefield simple cases) is difficult for renormalizable particle theories. However I thought (e.g. from Linde hepth/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 Bmodes of r > 10^{3} may be unduly pessimistic: more optimal delensing can in principle do orders of magnitude better as shown in astroph/0310163.
The paper repeats an argument that having \Delta\phi ~ M_p (needed for easily detectable gravity waves in singlefield simple cases) is difficult for renormalizable particle theories. However I thought (e.g. from Linde hepth/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 Bmodes of r > 10^{3} may be unduly pessimistic: more optimal delensing can in principle do orders of magnitude better as shown in astroph/0310163.
Last edited by Antony Lewis on July 28 2005, edited 1 time in total.

 Posts: 6
 Joined: January 25 2005
 Affiliation: University of Sussex
 Contact:
Re: [astroph/0507573] Postinflation increase of the cosmol
I believe there is still something of a split in opinion about whether the requirement is [tex]V \ll M_p^4[/tex] or [tex]\phi \ll M_p[/tex]. So I suppose the argument here is just that [tex]\Delta\phi[/tex] 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 [tex]\phi \sim M_p[/tex] we ought to have integrated in new degrees of freedom belonging to new physics.Antony Lewis wrote:
The paper repeats an arugment that having \Delta\phi ~ M_p (needed for easily detectable gravity waves in singlefield simple cases) is difficult for renormalizable particle theories. However I thought (e.g. from Linde hepth/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 think the issue of loop corrections isn't settled; there is a claim (grqc/9609026) that loop corrections lead to an effective screening of the cosmological constant and therefore that such effects are important (astroph/0505236) but perhaps the authors didn't intend to invoke loop effects explicitly.

 Posts: 6
 Joined: January 25 2005
 Affiliation: University of Sussex
 Contact:
[astroph/0507573] Postinflation increase of the cosmologic
In the "Nflation" paper out last week (hepth/0507205), the authors give a specific stringtheoretic 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 lowenergy 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 interbrane 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.
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 interbrane 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.

 Posts: 1618
 Joined: September 23 2004
 Affiliation: University of Sussex
 Contact:
Re: [astroph/0507573] Postinflation increase of the cosmol
The new paper astroph/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.