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David H. Lyth

Publications and source records attributed to David H. Lyth.

At least 37 records · Page 2Linked to original sources
arXiv2008

Black hole formation and slow-roll inflation

Black hole formation may occur if the spectrum of the curvature perturbation ζincreases strongly as the scale decreases. As no such increase is observed on cosmological scales, black hole formation requires strongly positive running n' of the spectral index n, though the running might only kick in below the `cosmological scales' probed by the CMB anisotropy and galaxy surveys. A concrete and well-motivated way of producing this running is through the running mass model of slow roll inflation. We obtain a new observational bound n' < 0.026 on the running provided by this model, improving an earlier result by a factor two. We also discuss black hole production in more general scenarios. We show that the usual conditions ε<< 1 and |η| << 1 are enough to derive the spectrum {\cal P}_ζ(k), the introduction of higher order parameters ξ^{2} etc. being optional.

hep-ph↗
arXiv2008

Flaxino dark matter and stau decay

If the spontaneous breaking of Peccei-Quinn symmetry comes from soft supersymmetry breaking, the fermionic partners of the symmetry-breaking fields have mass of order the gravitino mass, and are called flatinos. The lightest flatino, called here the flaxino, is a CDM candidate if it is the lightest supersymmetric particle. We here explore flaxino dark matter assuming that the lightest ordinary supersymmetric particle is the stau, with gravity-mediated supersymmetry breaking. The decay of the stau to the flaxino is fast enough not to spoil the standard predictions of Big Bang Nucleosynthesis, and its track and decay can be seen in future colliders.

hep-ph↗
arXiv2008

The curvature perturbation in a box

The stochastic properties of cosmological perturbations are best defined through the Fourier expansion in a finite box. I discuss the reasons for that with reference the curvature perturbation, and explore some issues arising from it.

astro-ph↗
arXiv2007

More hilltop inflation models

Using analytic expressions, we explore the parameter space for hilltop inflation models with a potential of the form $V_0\pm m^2ϕ^2 -aϕ^p$. With the positive sign and p>2 this converts the original hybrid inflation model into a hilltop model, allowing the spectral index to agree with the observed value n=0.95. In some cases the observed value is theoretically favored, while in others there is only the generic prediction $|n-1|\lsim 1$.

hep-ph↗
arXiv2006

MSSM inflation

Variants of the $A$-term model of hep-ph/0605035 are considered. They are equally successful, indicating that the model is quite robust once the relation between $A$ and the soft mass is regarded as tunable. Alternatively a flat direction might support modular inflation.

hep-ph↗
arXiv2006

A-term inflation and the MSSM

The parameter space for A-term inflation is explored with $W=λ_p ϕ^p/(p M_P^{p-3})$. With p=6 and λ_p~1, the observed spectrum and spectral tilt can be obtained with soft mass of order 10^2 GeV but not with a much higher mass. The case p=3 requires λ_p~10^{-9} to 10^{-12}. The ratio m/A requires fine-tuning, which may be justified on environmental grounds. An extension of the MSSM to include non-renormalizable terms and/or Dirac neutrino masses might support either A-term inflation or modular inflation.

hep-ph↗
arXiv2006

A numerical study of non-gaussianity in the curvaton scenario

We study the curvaton scenario using gauge-invariant second order perturbation theory and solving the governing equations numerically. Focusing on large scales we calculate the non-linearity parameter f_nl in the two-fluid curvaton model and compare our results with previous analytical studies employing the sudden decay approximation. We find good agreement of the two approaches for large curvaton energy densities at curvaton decay, Omega_dec, but significant differences of up to 10 percent for small Omega_dec.

astro-ph↗
arXiv2006

Higher order contributions to the primordial non-gaussianity

In this paper we calculate additional contributions to that part of the non-Gaussianity of the curvature perturbation, which come from the three-point correlator of the field perturbations. We estimate this contribution in the following models for its origin: single-component inflation, multi-component chaotic inflation, a two-component "hybrid" inflationary model, and the curvaton scenario. In all of these models, the additional contributions to the primordial non-gaussianity considered here, are too small to be ever detected.

astro-ph↗
arXiv2005

Generating the curvature perturbation at the end of inflation

The dominant contribution to the primordial curvature perturbation may be generated at the end of inflation. Taking the end of inflation to be sudden, formulas are presented for the spectrum, spectral tilt and non-gaussianity. They are evaluated for a minimal extension of the original hybrid inflation model.

astro-ph↗
arXiv2005

A Bound Concerning Primordial Non-Gaussianity

Seery and Lidsey have calculated the three-point correlator of the light scalar fields, a few Hubble times after horizon exit during inflation. Lyth and Rodriguez have calculated the contribution of this correlator to the three-point correlator of the primordial curvature perturbation. We calculate an upper bound on that contribution, showing that it is too small ever to be observable.

astro-ph↗
arXiv2005

The inflationary prediction for primordial non-gaussianity

We extend the δN formalism so that it gives all of the stochastic properties of the primordial curvature perturbation ζif the initial field perturbations are gaussian. The calculation requires only the knowledge of some family of unperturbed universes. A formula is given for the normalisation \fnl of the bispectrum of ζ, which is the main signal of non-gaussianity. Examples of the use of the formula are given, and its relation to cosmological perturbation theory is explained.

astro-ph↗
arXiv2005

The bound $r \leq 16ε$ on the primordial tensor perturbation

I recall the well-known sufficient conditions for the bound $r<16ε$ on the spectrum of the primordial tensor perturbation. Two recent papers claim a violation of this bound, without stating explicitely any violation of the sufficient conditions.

astro-ph↗
arXiv2005

Leptogenesis and tensor polarisation from a gravitational Chern-Simons term

Within an effective field theory derived from string theory, the universal axion has to be coupled to the the gravitational Chern-Simons (gCS) term. During any era when the axion field is varying, the vacuum fluctuation of the gravitational wave amplitude will then be circularly polarised, generating an expectation value for the gCS term. The polarisation may be observable through the Cosmic Microwave Background, and the vacuum expectation value of the gCS term may generate the baryon asymmetry of the Universe. We argue here that such effects cannot be computed without further input from string theory, since the `vacuum' in question is unlikely to be the field-theoretic one.

hep-th↗
arXiv2005

Non-gaussianity from the second-order cosmological perturbation

Several conserved and/or gauge invariant quantities described as the second-order curvature perturbation have been given in the literature. We revisit various scenarios for the generation of second-order non-gaussianity in the primordial curvature perturbation ζ, employing for the first time a unified notation and focusing on the normalisation f_{NL} of the bispectrum. When the classical curvature perturbation first appears a few Hubble times after horizon exit, |f_{NL}| is much less than 1 and is, therefore, negligible. Thereafter ζ(and hence f_{NL}) is conserved as long as the pressure is a unique function of energy density (adiabatic pressure). Non-adiabatic pressure comes presumably only from the effect of fields, other than the one pointing along the inflationary trajectory, which are light during inflation (`light non-inflaton fields'). During single-component inflation f_{NL} is constant, but multi-component inflation might generate |f_{NL}| \sim 1 or bigger. Preheating can affect f_{NL} only in atypical scenarios where it involves light non-inflaton fields. The curvaton scenario typically gives f_{NL} \ll -1 or f_{NL} = +5/4. The inhomogeneous reheating scenario can give a wide range of values for f_{NL}. Unless there is a detection, observation can eventually provide a limit |f_{NL}| \lsim 1, at which level it will be crucial to calculate the precise observational limit using second order theory.

astro-ph↗
arXiv2005

A general proof of the conservation of the curvature perturbation

Without invoking a perturbative expansion, we define the cosmological curvature perturbation, and consider its behaviour assuming that the universe is smooth over a sufficiently large comoving scale. The equations are simple, resembling closely the first-order equations, and they lead to results which generalise those already proven in linear perturbation theory and (in part) in second-order perturbation theory. In particular, the curvature perturbation is conserved provided that the pressure is a unique function of the energy density.

astro-ph↗
arXiv2005

Low scale inflation and the curvaton mechanism

The primordial curvature perturbation may be due to a `curvaton' field, which dominates (or almost dominates) the energy density before it decays. In the simplest version of the curvaton model the scale of inflation has to be quite high corresponding to a Hubble parameter H>10^7 GeV. We here explore two modifications of the curvaton model which can instead allow inflation at a low scale. (i) The curvaton is a Pseudo Nambu-Goldstone Boson (PNGB), with a symmetry-breaking phase transition during inflation. (ii) The curvaton mass increases suddenly at some moment after the end of inflation but before the onset of the curvaton oscillations. Both proposals can work but not in a completely natural way. Also, the lower bound on the scale of inflation depends somewhat on the details of the framework used. Nevertheless, we show that inflation with H as low as 1 TeV or lower is possible to be attained.

hep-ph↗
arXiv2004

The running-mass inflation model and WMAP

We consider the observational constraints on the running-mass inflationary model, and in particular on the scale-dependence of the spectral index, from the new Cosmic Microwave Background (CMB) anisotropy measurements performed by WMAP and from new clustering data from the SLOAN survey. We find that the data strongly constraints a significant positive scale-dependence of $n$, and we translate the analysis into bounds on the physical parameters of the inflaton potential. Looking deeper into specific types of interaction (gauge and Yukawa) we find that the parameter space is significantly constrained by the new data, but that the running mass model remains viable.

astro-ph↗