arXiv subjects

David H Lyth

Publications and source records attributed to David H Lyth.

24 records · Page 2Linked to original sources

arXiv1994

False Vacuum Inflation with Einstein Gravity

We investigate chaotic inflation models with two scalar fields, such that one field (the inflaton) rolls while the other is trapped in a false vacuum state. The false vacuum becomes unstable when the inflaton field falls below some critical value, and a first or second order transition to the true vacuum ensues. Particular attention is paid to Linde's second-order `Hybrid Inflation'; with the false vacuum dominating, inflation differs from the usual true vacuum case both in its cosmology and in its relation to particle physics. The spectral index of the adiabatic density perturbation can be very close to 1, or it can be around ten percent higher. The energy scale at the end of inflation can be anywhere between $10^{16}$\,GeV and $10^{11}$\,GeV, though reheating is prompt so the reheat temperature can't be far below $10^{11}\,$GeV. Topological defects are almost inevitably produced at the end of inflation, and if the inflationary energy scale is near its upper limit they can have significant effects. Because false vacuum inflation occurs with the inflaton field far below the Planck scale, it is easier to implement in the context of supergravity than standard chaotic inflation. That the inflaton mass is small compared with the inflationary Hubble parameter is still a problem for generic supergravity theories, but remarkably this can be avoided in a natural way for a class of supergravity models which follow from orbifold compactification of superstrings. This opens up the prospect of a truly realistic, superstring

astro-ph↗

arXiv1993

Introduction to Cosmology

These notes form an introduction to cosmology with special emphasis on large scale structure, the cmb anisotropy and inflation. In some places a basic familiarity with particle physics is assumed, but otherwise no special knowledge is needed. Most of the material in the first two sections can be found in several texts, except that the discussion of dark matter and the cosmological constant is more up to date. Most of that in the remaining sections can be found in a review of structure formation and inflation done with Andrew Liddle, which describes original work by various authors including ourselves and Ewan Stewart. The reader is referred to these works for more detail, and a very complete list of references.

astro-ph↗

arXiv1993

Inflation and Mixed Dark Matter Models

Recent large scale structure observations, including COBE, have prompted many authors to discuss modifications of the standard Cold Dark Matter model. Two of these, a tilted spectrum and a gravitational wave contribution to COBE, are at some level demanded by theory under the usual assumption that inflation generates the primeval perturbations. The third, whose motivation comes by contrast from observation, is the introduction of a component of hot dark matter to give the Mixed Dark Matter model. We discuss the implication of taking these modifications together. Should Mixed Dark Matter prove necessary, very strong constraints on inflationary models will ensue.

astro-ph↗

arXiv1993

The Cold Dark Matter Density Perturbation

This is a review of the Cold Dark Matter model of structure formation, and its variants. The approach is largely from first principles, the main aim being to impart a basic understanding of the relevant theory with an eye to the likely intense activity of the next few years, but the current observational status of the model is also critically assessed. The evolution of adiabatic and isocurvature density perturbations is described, and their effect on the large scale cmb anisotropy calculated as well as that of any gravitational waves. The generation of all three types of perturbation during inflation is described, and the normalisation and spectral indices are calculated in terms of the inflationary potential and its first and second derivatives. The comparison of the theory with each type of observation is described, starting with the COBE data and moving down in scale to the non-linear regime. Constraints on the spectrum of the adiabatic density perturbation are exhibited, the spectrum being parametrised by its normalisation and its degree of tilt. Finally extensions of the CDM model are considered, which replace some of the cold dark matter by hot dark matter or a cosmological constant.

astro-ph↗

arXiv1992

The Spectral Index in the CDM Cosmogony

In a recent paper, we suggested that the density fluctuation spectra arising from power-law (or extended) inflation, which are tilted with respect to the Harrison--Zel'dovich spectrum, may provide an explanation for the excess large scale clustering seen in galaxy surveys such as the APM survey. In the light of the new results from COBE, we examine in detail here cold dark matter cosmogonies based on inflationary models predicting power-law spectra. Along with power-law and extended inflation, this class includes natural inflation. The latter is of interest because, unlike the first two, it produces a power-law spectrum without significant gravitational wave production. We examine a range of phenomena, including large angle microwave background fluctuations, clustering in the galaxy distribution, bulk peculiar velocity flows, the formation of high redshift quasars and the epoch of structure formation. Of the three models, only natural inflation seems capable of explaining the large scale clustering of optical galaxies. Such a model, though at best marginal even at present, has some advantages over standard CDM and on most grounds appears to perform at least as well. Power-law inflation's primary interest may ultimately only be in permitting a larger bias parameter than standard CDM; it appears unable to explain excess clustering. Most models of extended inflation are ruled out at a high confidence level.

astro-ph↗

arXiv1992

COBE, Gravitational Waves, Inflation and Extended Inflation

We analyse the implications for inflationary models of the cosmic microwave background (cmb) anisotropy measured by COBE. Vacuum fluctuations during inflation generate an adiabatic density perturbation, and also gravitational waves. The ratio of these two contributions to the cmb anisotropy is given for an arbitrary slow-roll inflaton potential. Results from the IRAS/QDOT and POTENT galaxy surveys are used to normalise the spectrum of the density perturbation on the scale $20h^{-1}\Mpc$, so that the COBE measurement on the scale $10^3h^{-1}\Mpc$ provides a lower bound on the spectral index $n$. For `power law' and `extended' inflation, gravitational waves are significant and the bound is $n>0.84$ at the $2$-sigma level. For `natural' inflation, gravitational waves are negligible and the constraint is weakened to $n>0.70$, at best marginally consistent with a recent proposal for explaining the excess clustering observed in the APM galaxy survey. Many versions of extended inflation, including those based on the Brans--Dicke theory, are ruled out, because they require $n\lsim 0.75$ in order that bubbles formed at the end of inflation should not be observed now in the cmb.

astro-ph↗