Fermionic current-carrying cosmic strings: zero-temperature limit and equation of state

The equation of state for a superconducting cosmic string whose current is due to fermionic zero modes is derived analytically in the case where the back-reaction of the fermions to the background is neglected. It is first shown that the zero mode fermions follow a zero temperature distribution because of their interactions (or lack thereof) with the string-forming Higgs and gauge fields. It is then found that the energy per unit length U and the tension T are related to the background string mass m through the simple relation U+T=2m*m. Cosmological consequences are briefly discussed.Comment: 8 pages, 1 figure, uses ReVTe

Dynamical stability for the gravitational evolution of a homogeneous polytrope

URL: http://www-spht.cea.fr/articles/s00/008 Stabilité dynamique de l'évolution gravitationnelle d'un polytrope homogèneThe dynamic stability of the spherical gravitational evolution (collapse or expansion) for a homogeneous polytropic gas with any exponent $\gamma ,$ is studied using the lagrangian formalism. We obtain the analytical expression for density perturbations at the first order. In the case $\gamma =~4/3,$ the Jeans'criterion is easily generalized to a self-similar expanding background. The collapsing case is found to be always unstable. The stability of density modes obtained for $\gamma \not = 4/3$ does not introduce any conditions on the wavelength perturbation, but only a criterion on the polytropic index. As a result, stability is obtained for an expanding gas provided $\gamma 5/3.

Microlensing by Cosmic Strings

We consider the signature and detectability of gravitational microlensing of distant quasars by cosmic strings. Because of the simple image configuration such events will have a characteristic light curve, in which a source would appear to brighten by exactly a factor of two, before reverting to its original apparent brightness. We calculate the optical depth and event rate, and conclude that current predictions and limits on the total length of strings on the sky imply optical depths of \la 10^{-8} and event rates of fewer than one event per $10^9$ sources per year. Disregarding those predictions but replacing them with limits on the density of cosmic strings from the CMB fluctuation spectrum, leaves only a small region of parameter space (in which the sky contains about $3\times10^5$ strings with deficit angle of order 0.3 milli-arcseconds) for which a microlensing survey of exposure $10^7$ source-years, spanning a 20--40-year period, might reveal the presence of cosmic strings.Comment: 4 pages, accepted for publication in MNRA

The bispectrum of matter perturbations from cosmic strings

We present the first calculation of the bispectrum of the matter perturbations induced by cosmic strings. The calculation is performed in two different ways: the first uses the unequal time correlators (UETCs) of the string network - computed using a Gaussian model previously employed for cosmic string power spectra. The second approach uses the wake model, where string density perturbations are concentrated in sheet-like structures whose surface density grows with time. The qualitative and quantitative agreement of the two gives confidence to the results. An essential ingredient in the UETC approach is the inclusion of compensation factors in the integration with the Green's function of the matter and radiation fluids, and we show that these compensation factors must be included in the wake model also. We also present a comparison of the UETCs computed in the Gaussian model, and those computed in the unconnected segment model (USM) used by the standard cosmic string perturbation package CMBACT. We compare numerical estimates for the bispectrum of cosmic strings to those produced by perturbations from an inflationary era, and discover that, despite the intrinsically non-Gaussian nature of string-induced perturbations, the matter bispectrum is unlikely to produce competitive constraints on a population of cosmic strings

Graviton confinement inside hypermonopoles of any dimension

We show the generic existence of metastable massive gravitons in the four-dimensional core of self-gravitating hypermonopoles in any number of infinite-volume extra-dimensions. Confinement is observed for Higgs and gauge bosons couplings of the order unity. Provided these resonances are light enough, they realise the Dvali-Gabadadze-Porrati mechanism by inducing a four-dimensional gravity law on some intermediate length scales. The effective four-dimensional Planck mass is shown to be proportional to a negative power of the graviton mass. As a result, requiring gravity to be four-dimensional on cosmological length scales may solve the mass hierarchy problem.Comment: 23 pages, 6 figures, uses iopart. Misprints corrected, references added, matches published versio

From heaviness to lightness during inflation

We study the quantum fluctuations of scalar fields with a variable effective mass during an inflationary phase. We consider the situation where the effective mass depends on a background scalar field, which evolves during inflation from being frozen into a damped oscillatory phase when the Hubble parameter decreases below its mass. We find power spectra with suppressed amplitude on large scales, similar to the standard massless spectrum on small scales, and affected by modulations on intermediate scales. We stress the analogies and differences with the parametric resonance in the preheating scenario. We also discuss some potentially observable consequences when the scalar field behaves like a curvaton.Comment: 23 pages; 8 figures; published versio

Localisation of massive fermions on the brane

We construct an explicit model to describe fermions confined on a four dimensional brane embedded in a five dimensional anti-de Sitter spacetime. We extend previous works to accommodate massive bound states on the brane and exhibit the transverse structure of the fermionic fields. We estimate analytically and calculate numerically the fermion mass spectrum on the brane, which we show to be discrete. The confinement life-time of the bound states is evaluated, and it is shown that existing constraints can be made compatible with the existence of massive fermions trapped on the brane for durations much longer than the age of the Universe.Comment: 20 pages, LaTeX-RevTex, 15 figures, submitted to PR