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David H. Lyth

Publications and source records attributed to David H. Lyth.

At least 19 recordsLinked to original sources
arXiv2017

Thermal Inflation with a Thermal Waterfall Scalar Field Coupled to a Light Spectator Scalar Field

A new model of thermal inflation is introduced, in which the mass of the thermal waterfall field is dependent on a light spectator scalar field. Using the $δN$ formalism, the "end of inflation" scenario is investigated in order to ascertain whether this model is able to produce the dominant contribution to the primordial curvature perturbation. A multitude of constrains are considered so as to explore the parameter space, with particular emphasis to key observational signatures. For natural values of the parameters, the model is found to yield a sharp prediction for the scalar spectral index and its running, well within the current observational bounds.

hep-ph↗
arXiv2015

Generating $\fnl$ at $\ell\lsim 60$

The CMB anisotropy at $\ell\lsim 60$ seems to have some special features which include (i) a dipole modulation and (ii) a decrease in power. It is known that both of these effects can be generated if a curvaton-type field has a super-horizon perturbation. It is also known that this will generate non-gaussianity $\fnl$ in the same range of $\ell$, whose magnitude has a lower bound coming from the magnitude of the observed CMB quadrupole. I revisit that bound in the present paper, and point out that it may or may not be compatible with current data which should therefore be re-analysed.

astro-ph.CO↗
arXiv2014

BICEP2, the curvature perturbation and supersymmetry

The tensor fraction $r\simeq 0.16$ found by BICEP2 corresponds to a Hubble parameter $H\simeq 1.0\times 10^{14}\GeV$ during inflation. This has two implications for the (single-field) slow-roll inflation hypothesis. First, the inflaton perturbation must account for much more than $10\%$ of the curvature perturbation $ζ$, which barring fine-tuning means that it accounts for practically all of it. It follows that a curvaton-like mechanism for generating $ζ$ requires an alternative to slow roll such as k-inflation. Second, accepting slow-roll inflation, the excursion of the inflaton field is at least of order Planck scale. As a result, the flatness of the inflaton presumably requires a shift symmetry. I point out that if such is the case, the resulting potential is likely to have at least approximately the quadratic form suggested in 1983 by Linde, which is known to be compatible with the observed $r$ as well as the observed spectral index $\ns$. The shift symmetry does not require supersymmetry. Also, the big $H$ may rule out a GUT by restoring the symmetry and producing fatal cosmic strings. The absence of a GUT would correspond to the absence of superpartners for the Standard Model particles, which indeed have yet to be found at the LHC. It therefore seems quite possible that the quantum field theory chosen by Nature is not supersymmetric.

hep-ph↗
arXiv2013

The CMB asymmetry from inflation

Erickcek, Kamionkowski and Carroll proposed in 2008 that the dipole modulation of the CMB could be due to a very large scale perturbation of the field $ϕ$ causing the primordial curvature perturbation. We repeat their calculation using weaker assumptions and the current data. If $ϕ$ is the inflaton of {\em any} single-field inflation with the attractor behaviour, the asymmetry is almosy certainly too small. If instead $ϕ$ is {\em any} curvaton-type field (ie.\ one with the canonical kinetic term and a negligible effect during inflation) the asymmetry can agree with observation if $|\fnl|$ in the equilateral configuration is $\simeq 10$ for $k\mone=1\Gpc$ and $\lsim 3$ for $k\mone=1\Mpc$. An $\fnl$ with these properties can apparently be obtained from the curvaton with an axionic potential. Within any specific curvaton-type model, the function $\fnl(k_1,k_2,k_3)$ required to generate the asymmetry would be determined, and could perhaps already be confirmed or ruled out using existing Planck or WMAP data.

astro-ph.CO↗
arXiv2013

The statistically anisotropic curvature perturbation generated by f(ϕ)^2 F^2

The inflaton might be coupled to a gauge field through a term f^2(ϕ) F_μνF^μν. If f \propto a^{-2} where a(t) is the scale factor, the perturbation δW of the gauge field generates a potentially observable statistically anisotropic contribution to the primordial curvature perturbation during slow-roll inflation. The spectrum and bispectrum of this contribution have been calculated using the in-in formalism of quantum field theory. We give a simpler and more complete calculation using only the classical perturbations. The results suggest that either the entire curvature perturbation ζ(both the statistically isotropic and anisotropic parts) is generated during slow-roll inflation, or else it is generated afterwards.

astro-ph.CO↗
arXiv2012

Modulation of the waterfall by a gauge field

We present the first complete calculation of the curvature perturbation generated during the hybrid inflation waterfall, caused by the coupling of the waterfall field to a gauge field A whose kinetic function f^2 depends on the inflaton field. We impose an upper bound on the field W=f*A which ensures that it has a negligible effect before the waterfall. We confirm the claim of Soda and Yokoyama, that the perturbation δW generates a statistically anisotropic spectrum and bispectrum, which could easily be observable. We also discover a new phenomenon, whereby the time-dependent `varyon' field W causes the inflaton contribution to vary during the waterfall. The varyon mechanism might be implemented also with a scalar field and might not involve the waterfall.

astro-ph.CO↗
arXiv2012

Statistically anisotropic curvature perturbation generated during the waterfall

If the waterfall field of hybrid inflation couples to a U(1) gauge field, the waterfall can generate a statistically anisotropic contribution to the curvature perturbation. We investigate this possibility, generalising in several directions the seminal work of Yokoyama and Soda. The statistical anisotropy of the bispectrum could be detectable by PLANCK even if the statistical anisotropy of the spectrum is too small to detect.

astro-ph.CO↗
arXiv2012

The hybrid inflation waterfall and the primordial curvature perturbation

Without demanding a specific form for the inflaton potential, we obtain an estimate of the contribution to the curvature perturbation generated during the linear era of the hybrid inflation waterfall. The spectrum of this contribution peaks at some wavenumber $k=k_*$, and goes like $k^3$ for $k\ll k_*$, making it typically negligible on cosmological scales. The scale $k_*$ can be outside the horizon at the end of inflation, in which case $ζ=- (g^2 - \vev{g^2})$ with $g$ gaussian. Taking this into account, the cosmological bound on the abundance of black holes is likely to be satisfied if the curvaton mass $m$ much bigger than the Hubble parameter $H$, but is likely to be violated if $m\lsim H$. Coming to the contribution to $ζ$ from the rest of the waterfall, we are led to consider the use of the `end-of-inflation' formula, giving the contribution to $ζ$ generated during a sufficiently sharp transition from nearly-exponential inflation to non-inflation, and we state for the first time the criterion for the transition to be sufficiently sharp. Our formulas are applied to supersymmetric GUT inflation and to supernatural/running-mass inflation

astro-ph.CO↗
arXiv2011

The inflating curvaton

The primordial curvature perturbation ζmay be generated by some curvaton field σ, which is negligible during inflation and has more or less negligible interactions until it decays. In the current scenario, the curvaton starts to oscillate while its energy density ρ_σis negligible. We explore the opposite scenario, in which ρ_σdrives a few e-folds of inflation before the oscillation begins. In this scenario for generating ζit is exceptionally easy to solve the ηproblem; one just has to make the curvaton a string axion, with anomaly-mediated susy breaking which may soon be tested at the LHC. The observed spectral index n can be obtained with a potential V\propto ϕ^p for the first inflation; p=1 or 2 is allowed by the current uncertainty in n but the improvement in accuracy promised by Planck may rule out p=1. The predictions include (i) running n'\simeq 0.0026 (0.0013) for p=1 (2) that will probably be observed, (ii) non-gaussianity parameter f_NL \sim -1 that may be observed, (iii) tensor fraction r is probably too small to ever observed.

astro-ph.CO↗
arXiv2011

Primordial black hole formation and hybrid inflation

We calculate the contribution to the curvature perturbation, that is generated while the waterfall field grows exponentially (which can occur only if the waterfall field mass is $m\gg H$). We consider the upper bound on its spectrum coming from constraints on the abundance of primordial black holes, taking account of possible non-gaussianity. The constraint is satisfied, but extrapolation of our result to the regime $m\sim H$ suggests that the constraint might not be satisfied there.

astro-ph.CO↗
arXiv2011

Contribution of the hybrid inflation waterfall to the primordial curvature perturbation

A contribution $ζ_χ$ to the curvature perturbation will be generated during the waterfall that ends hybrid inflation, that may be significant on small scales. In particular, it may lead to excessive black hole formation. We here consider standard hybrid inflation, where the tachyonic mass of the waterfall field is much bigger than the Hubble parameter. We calculate $ζ_χ$ in the simplest case, and see why earlier calculations of $ζ_χ$ are incorrect.

astro-ph.CO↗
arXiv2011

Issues concerning the waterfall of hybrid inflation

We discuss the waterfall that ends hybrid inflation. Making some simplifying assumptions, that may be satisfied by GUT inflation models, two issues are addressed. First, the procedure of keeping the quantum fluctuation of the waterfall field only in the regime where it can be regarded as classical. Second, the contribution to the primordial curvature perturbation that is generated during the waterfall. Because the waterfall field is heavy during inflation, the spectrum of this contribution is strongly blue and hence is negligible on cosmological scales.

astro-ph.CO↗
arXiv2010

Preheating and the non-gaussianity of the curvature perturbation

The perturbation of a light field might affect preheating and hence generate a contribution to the spectrum and non-gaussianity of the curvature perturbation ζ. The field might appear directly in the preheating model (curvaton-type preheating) or indirectly through its effect on a mass or coupling (modulated preheating). We give general expressions for ζbased on the δN formula, and apply them to the cases of quadratic and quartic chaotic inflation. For the quadratic case, curvaton-type preheating is ineffective in contributing to ζ, but modulated preheating can be effective. For quartic inflation, curvaton-type preheating may be effective but the usual δN formalism has to be modified. We see under what circumstances the recent numerical simulation of Bond et al. [0903.3407] may be enough to provide a rough estimate for this case. This paper is dedicated to the memory of Lev Kofman who died on 12th November 2009

hep-ph↗
arXiv2009

Non-gaussianity at tree and one-loop levels from vector field perturbations

We study the spectrum P_ζand bispectrum B_ζof the primordial curvature perturbation ζwhen the latter is generated by scalar and vector field perturbations. The tree-level and one-loop contributions from vector field perturbations are worked out considering the possibility that the one-loop contributions may be dominant over the tree level terms (both (either) in P_ζand (or) in B_ζ) and viceversa. The level of non-gaussianity in the bispectrum, f_{NL}, is calculated and related to the level of statistical anisotropy in the power spectrum, g_ζ. For very small amounts of statistical anisotropy in the power spectrum, the level of non-gaussianity may be very high, in some cases exceeding the current observational limit.

astro-ph.CO↗
arXiv2009

Statistical anisotropy of the curvature perturbation from vector field perturbations

The δN formula for the primordial curvature perturbation ζis extended to include vector as well as scalar fields. Formulas for the tree-level contributions to the spectrum and bispectrum of ζare given, exhibiting statistical anisotropy. The one-loop contribution to the spectrum of ζis also worked out. We then consider the generation of vector field perturbations from the vacuum, including the longitudinal component that will be present if there is no gauge invariance. Finally, the δN formula is applied to the vector curvaton and vector inflation models with the tensor perturbation also evaluated in the latter case.

astro-ph↗
arXiv2009

Anisotropic non-Gaussianity from vector field perturbations

We suppose that a vector field perturbation causes part of the primordial curvature perturbation. The non-Gaussianity parameter fNL is then, in general, statistically anisotropic. We calculate its form and magnitude in the curvaton scenario and in the end-of-inflation scenario. We show that this anisotropy could easily be observable.

astro-ph↗
arXiv2008

Non-gaussianity of inflationary field perturbations from the field equation

We calculate the tree-level bispectrum of the inflaton field perturbation directly from the field equations, and construct the corresponding f_NL parameter. Our results agree with previous ones derived from the Lagrangian. We argue that quantum theory should only be used to calculate the correlators when they first become classical a few Hubble times after horizon exit, the classical evolution taking over thereafter.

astro-ph↗