arXiv subjects

Julien Lesgourgues

Publications and source records attributed to Julien Lesgourgues.

143 records · Page 8Linked to original sources

arXiv2003

Does the small CMB quadrupole moment suggest new physics?

Motivated by WMAP's confirmation of an anomalously low value of the quadrupole moment of the CMB temperature fluctuations, we investigate the effects on the CMB of cutting off the primordial power spectrum P(k) at low wave numbers. This could arise, for example, from a break in the inflaton potential, a prior period of matter or radiation domination, or an oscillating scalar field which couples to the inflaton. We reanalyze the full WMAP parameter space supplemented by a low-k cutoff for P(k). The temperature correlations by themselves are better fit by a cutoff spectrum, but including the TE temperature-polarization spectrum reduces this preference to a 1.4 sigma effect. Inclusion of large scale structure data does not change the conclusion. If taken seriously, the low-k cutoff is correlated with optical depth so that reionization occurs even earlier than indicated by the WMAP analysis.

astro-ph↗

arXiv2003

Galactic Halos of Fluid Dark Matter

Dwarf spiral galaxies - and in particular the prototypical DDO 154 - are known to be completely dominated by an unseen component. The putative neutralinos - so far the favored explanation for the astronomical dark matter - fail to reproduce the well measured rotation curves of those systems because these species tend to form a central cusp whose presence is not supported by observation. We have considered here a self-coupled charged scalar field as an alternative to neutralinos and investigated whether a Bose condensate of that field could account for the dark matter inside DDO 154 and more generally inside dwarf spirals. The size of the condensate turns out to be precisely determined by the scalar mass m and self-coupling lambda of the field. We find actually that for m^4 / lambda = 50 - 75 eV^4, the agreement with the measurements of the circular speed of DDO 154 is impressive whereas it lessens for larger systems. The cosmological behavior of the field is also found to be consistent - yet marginally - with the limits set by BBN on the effective number of neutrino families. We conclude that classical configurations of a scalar and self-coupled field provide a possible solution to the astronomical dark matter problem and we suggest further directions of research.

astro-ph↗

arXiv2003

Measuring the cosmological background of relativistic particles with WMAP

We show that the first year results of the Wilkinson Microwave Anisotropy Probe (WMAP) constrain very efficiently the energy density in relativistic particles in the universe. We derive new bounds on additional relativistic degrees of freedom expressed in terms of an excess in the effective number of light neutrinos Delta N_eff. Within the flat LambdaCDM scenario, the allowed range is Delta N_eff < 6 (95% CL) using WMAP data only, or -2.6 < Delta N_eff < 4 with the prior H_0= 72 \pm 8 km/s/Mpc. When other cosmic microwave background and large scale structure experiments are taken into account, the window shrinks to -1.6 < Delta N_eff < 3.8. These results are in perfect agreement with the bounds from primordial nucleosynthesis. Non-minimal cosmological models with extra relativistic degrees of freedom are now severely restricted.

astro-ph↗

arXiv2002

Cosmological constraints on quintessential halos

A complex scalar field has recently been suggested to bind galaxies and flatten the rotation curves of spirals. Its cosmological behavior is thoroughly investigated here. Such a field is shown to be a potential candidate for the cosmological dark matter that fills up a fraction Omega_cdm = 0.3 of the Universe. However, problems arise when the limits from galactic dynamics and some cosmological constraints are taken simultaneously into account. A free complex field, associated to a very small mass m = 10^{-23} eV, has a correct cosmological behavior in the early Universe, but behaves today mostly as a real axion, with a problematic value of its conserved quantum number. On the other hand, an interacting field with quartic coupling lambda = 0.1 has a more realistic mass m = 1 eV and carries a quantum number close to the photon number density. Unlike a free field, it would be spinning today in the complex plane - like the so-called ``spintessence''. Unfortunately, the cosmological evolution of such field in the early Universe is hardly compatible with constraints from nucleosynthesis and structure formation.

astro-ph↗

arXiv2002

Constraining the window on sterile neutrinos as warm dark matter

Sterile neutrinos may be one of the best Warm Dark Matter candidates we have today. Both lower and upper bounds on the mass of the sterile neutrino come from astronomical observations. We show that the proper inclusion of the neutrino momentum distribution reduces the allowed region to be 2.6 keV < m < 5 keV for the simplest models. A search for a spectral line with E=m/2 is thus more interesting than ever before.

astro-ph↗

arXiv2001

Quintessential Haloes around Galaxies

The nature of the dark matter that binds galaxies remains an open question. The favored candidate has been so far the neutralino. This massive species with evanescent interactions is now in difficulty. It would actually collapse in dense clumps and would therefore play havoc with the matter it is supposed to shepherd. We focus here on a massive and non-interacting charged scalar field as an alternate option to the astronomical missing mass. We investigate the classical solutions that describe the Bose condensate of such a field in gravitational interaction with matter. This simplistic model accounts quite well for the dark matter inside low-luminosity spirals whereas the agreement lessens for the brightest objects where baryons dominate. A scalar mass m=0.4 to 1.6 10^{-23} eV is derived when both high and low-luminosity spirals are fitted at the same time. Comparison with astronomical observations is made quantitative through a chi-squared analysis. We conclude that scalar fields offer a promising direction worth being explored.

astro-ph↗

arXiv2001

The lepton asymmetry: the last chance for a critical-density cosmology?

We use a wide range of observations to constrain cosmological models possessing a significant asymmetry in the lepton sector, which offer perhaps the best chance of reconciling a critical-density Universe with current observations. The simplest case, with massless neutrinos, fails to fit many experimental data and does not lead to an acceptable model. If the neutrinos have mass of order one electron-volt (which is favoured by some neutrino observations), then models can be implemented which prove a good fit to microwave anisotropies and large-scale structure data. However, taking into account the latest microwave anisotropy results, especially those from Boomerang, we show that the model can no longer accommodate the observed baryon fraction in clusters. Together with the observed acceleration of the present Universe, this puts considerable pressure on such critical-density models.

astro-ph↗

arXiv2000

Remarks on the Boomerang results, the baryon density and the leptonic asymmetry

The very recent Boomerang results give an estimate of unprecedented precision of the Cosmic Microwave Background anisotropies on sub--degree scales. A puzzling feature for theoretical cosmology is the low amplitude of the second acoustic peak. Through a qualitative discussion, we argue that a scarcely considered category of flat models, with a leptonic asymmetry, a high baryon density and a low cosmological constant seems to be in very good agreement with the data, while still being compatible with big bang nucleosynthesis and some other observational constraints. Although this is certainly not the only way to explain the data, we believe that these models deserve to be included in forthcoming likelihood analyses.

astro-ph↗

arXiv2000

Features in the primordial power spectrum of double D-term inflation

Recently, there has been some interest for building supersymmetric models of double inflation. These models, realistic from a particle physics point of view, predict a broken-scale-invariant power spectrum of primordial cosmological perturbations, that may explain eventual non-trivial features in the present matter power spectrum. In previous works, the primordial spectrum was calculated using analytic slow-roll approximations. However, these models involve a fast second-order phase transition during inflation, with a stage of spinodal instability, and an interruption of slow-roll. For our previous model of double D-term inflation, we simulate numerically the evolution of quantum fluctuations, taking into account the spinodal modes, and we show that the semi-classical approximation can be employed even during the transition, due to the presence of a second inflaton field. The primordial power spectrum possesses a rich structure, and possibly, a non-Gaussian spike on observable scales.

hep-ph↗

arXiv2000

Cosmology of the Randall-Sundrum model after dilaton stabilization

We provide the first complete analysis of cosmological evolution in the Randall-Sundrum model with stabilized dilaton. We give the exact expansion law for matter densities on the two branes with arbitrary equations of state. The effective four-dimensional theory leads to standard cosmology at low energy. The limit of validity of the low energy theory and possible deviations from the ordinary expansion law in the high energy regime are finally discussed.

hep-ph↗

arXiv1999

Relic neutrino asymmetry, CMB and large-scale structure

We consider some consequences of the presence of a cosmological lepton asymmetry in the form of neutrinos. A relic neutrino degeneracy enhances the contribution of massive neutrinos to the present energy density of the Universe, and modifies the power spectrum of radiation and matter. Comparing with current observations of cosmic microwave background anisotropies and large scale structure, we derive some constraints on the relic neutrino degeneracy and on the spectral index in the case of a flat Universe with a cosmological constant.

hep-ph↗

arXiv1999

Cosmological implications of a Relic Neutrino Asymmetry

We consider some consequences of the presence of a cosmological lepton asymmetry in the form of neutrinos. Relic neutrino degeneracy enhances the contribution of massive neutrinos to the present energy density of the Universe, and modifies the power spectrum of radiation and matter. Comparing with current observations of cosmic microwave background anisotropies and large scale structure, we derive some constraints on the neutrino degeneracy and on the spectral index in the case of a flat Universe with a cosmological constant.

hep-ph↗

arXiv1999

Double D-term inflation

Comparisons of cosmological models to current data show that the presence of a non-trivial feature in the primordial power spectrum of fluctuations, around the scale k=0.05 h/Mpc, is an open and exciting possibility, testable in a near future. This could set new constraints on inflationary models. In particular, current data favour a Lambda-CDM model with a steplike spectrum, and more power on small scales. So far, this possibility has been implemented only in toy models of inflation. In this work, we propose a supersymmetric model with two U(1) gauge symmetries, associated with two Fayet-Iliopoulos terms. Partial cancellation of the Fayet-Iliopoulos by one of the charged fields generates a step in the primordial power spectrum of adiabatic perturbations. We show that when this field if charged under both symmetries, the spectrum may have more power on small scales.

hep-ph↗

arXiv1998

On the Phase-Space Volume of Primordial Cosmological Perturbations

We show how to determine the typical phase space volume $Γ$ for primordial gravitational waves produced during an inflationary stage, which is invariant under squeezing. An expression for $Γ$ is found in the long wavelength regime. The quasi-classical entropy of a pure vacuum initial state defined as the logarithm of $Γ$ modulo a constant remains zero in spite of the generation of fluctuations (creation of real gravitons).

gr-qc↗

arXiv1998

The Coherence of Primordial Fluctuations Produced During Inflation

The behaviour of quantum metric perturbations produced during inflation is considered at the stage after the second Hubble radius crossing. It is shown that the classical correlation between amplitude and momentum of a perturbation mode, previously shown to emerge in the course of an effective quantum-to-classical transition, is maintained for a sufficiently long time, and we present the explicit form in which it takes place using the Wigner function. We further show with a simple diffraction experiment that quantum interference, non-expressible in terms of a classical stochastic description of the perturbations, is essentially suppressed. Rescattering of the perturbations leads to a comparatively slow decay of this correlation and to a complete stochastization of the system.

gr-qc↗

arXiv1997

CMB anisotropy predictions for a model of double inflation

We consider a double-inflationary model with two massive scalar fields interacting only gravitationally in the context of a flat cold dark matter (CDM) Universe. The cosmic microwave background (CMB) temperature anisotropies produced in this theory are investigated in great details for a window of parameters where the density fluctuation power spectrum P(k) is in good agreement with observations. The first Doppler (``acoustic'') peak is a crucial test for this model as well as for other models. For the ``standard'' values of the cosmological parameters of CDM, our model is excluded if the height of the Doppler peak is sensibly higher than about three times the Sachs-Wolfe plateau.

astro-ph↗

arXiv1996

Quantum-to-classical Transition of Cosmological Perturbations for Non-vacuum Initial States

Transition from quantum to semiclassical behaviour and loss of quantum coherence for inhomogeneous perturbations generated from a non-vacuum initial state in the early Universe is considered in the Heisenberg and the Schrödinger representations, as well as using the Wigner function. We show explicitly that these three approaches lead to the same prediction in the limit of large squeezing (i.e. when the squeezing parameter $|r_k|\to \infty$): each two-modes quantum state (k, -k) of these perturbations is equivalent to a classical perturbation that has a stochastic amplitude, obeying a non-gaussian statistics which depends on the initial state, and that belongs to the quasi-isotropic mode (i.e. it possesses a fixed phase). The Wigner function is not everywhere positive for any finite $r_k$, hence its interpretation as a classical distribution function in phase space is impossible without some coarse graining procedure. However, this does not affect the transition to semiclassical behaviour since the Wigner function becomes concentrated near a classical trajectory in phase space when $|r_k|\to \infty$ even without coarse graining. Deviations of the statistics of the perturbations in real space from a Gaussian one lie below the cosmic variance level for the N-particles initial states with N=N(|k|) but may be observable for other initial states without statistical isotropy or with correlations between different k modes. As a way to look for this effect, it is proposed to measure the kurtosis of the angular fluctuations of the cosmic microwave background temperature.

gr-qc↗