arXiv subjects

Juan Garcia-Bellido

Publications and source records attributed to Juan Garcia-Bellido.

At least 145 records · Page 8Linked to original sources

arXiv2006

Fluctuations of the Gravitational Constant in the Inflationary Brans-Dicke Cosmology

According to the Brans--Dicke theory, the value of the gravitational constant G which we measure at present is determined by the value of the Brans--Dicke scalar field ϕat the end of inflation. However, due to quantum fluctuations of the scalar fields produced during inflation, the gravitational constant G(ϕ) may take different values in different exponentially large parts of the Universe. We investigate the probability distribution P_p to find a domain of a given volume with a given value of the gravitational constant G at a given time. The investigation is performed for a wide class of effective potentials of the scalar field σwhich drives inflation, and with two different time parametrizations. Our work is based on the analytical study of the diffusion equations for P_p, as well as on the computer simulation of stochastic processes in the inflationary Universe. We have found that in some inflationary models the probability distribution P_p rapidly approaches a stationary regime. The shape of the distribution depends, however, on the choice of the time parametrization. In some other models the distribution P_p is not stationary. An interpretation of our results and of all ambiguities involved is outlined, and a possible role of anthropic considerations in determination of the gravitational constant is discussed.

astro-ph↗

arXiv2006

Gauge invariant MSSM inflaton

We argue that all the necessary ingredients for successful inflation are present in the flat directions of the Minimally Supersymmetric Standard Model. We show that out of many gauge invariant combinations of squarks, sleptons and Higgses, there are two directions, ${\bf LLe}$, and ${\bf udd}$, which are promising candidates for the inflaton. The model predicts more than $10^3$ e-foldings with an inflationary scale of $H_{\rm inf}\sim {\cal O}(1-10)$ GeV, provides a tilted spectrum with an amplitude of $δ_H\sim 10^{-5}$ and a negligible tensor perturbation. The temperature of the thermalized plasma could be as low as $T_{rh}\sim {\cal O}(1-10)$~TeV. Parts of the inflaton potential can be determined independently of cosmology by future particle physics experiments.

hep-ph↗

arXiv2005

Squeezing the window on isocurvature modes with the Lyman-alpha forest

Various recent studies proved that cosmological models with a significant contribution from cold dark matter isocurvature perturbations are still compatible with most recent data on cosmic microwave background anisotropies and on the shape of the galaxy power spectrum, provided that one allows for a very blue spectrum of primordial entropy fluctuations (n_iso > 2). However, such models predict an excess of matter fluctuations on small scales, typically below 40 Mpc/h. We show that the proper inclusion of high-resolution high signal-to-noise Lyman-alpha forest data excludes most of these models. The upper bound on the isocurvature fraction alpha=f_iso^2/(1+f_iso^2), defined at the pivot scale k_0=0.05/Mpc, is pushed down to alpha<0.4, while n_iso=1.9+-1.0 (95% confidence limits). We also study the bounds on curvaton models characterized by maximal correlation between curvature and isocurvature modes, and a unique spectral tilt for both. We find that f_iso<0.05 (95% c.l.) in that case. For double inflation models with two massive inflatons coupled only gravitationally, the mass ratio should obey R < 3 (95% c.l.).

astro-ph↗

arXiv2005

Cosmology and Astrophysics

In these lectures I review the present status of the so-called Standard Cosmological Model, based on the hot Big Bang Theory and the Inflationary Paradigm. I will make special emphasis on the recent developments in observational cosmology, mainly the acceleration of the universe, the precise measurements of the microwave background anisotropies, and the formation of structure like galaxies and clusters of galaxies from tiny primodial fluctuations generated during inflation.

astro-ph↗

arXiv2005

Bayesian model selection and isocurvature perturbations

Present cosmological data are well explained assuming purely adiabatic perturbations, but an admixture of isocurvature perturbations is also permitted. We use a Bayesian framework to compare the performance of cosmological models including isocurvature modes with the purely adiabatic case; this framework automatically and consistently penalizes models which use more parameters to fit the data. We compute the Bayesian evidence for fits to a dataset comprised of WMAP and other microwave anisotropy data, the galaxy power spectrum from 2dFGRS and SDSS, and Type Ia supernovae luminosity distances. We find that Bayesian model selection favours the purely adiabatic models, but so far only at low significance.

astro-ph↗

arXiv2004

Bounds on CDM and neutrino isocurvature perturbations from CMB and LSS data

Generic models for the origin of structure predict a spectrum of initial fluctuations with a mixture of adiabatic and isocurvature perturbations. Using the observed anisotropies of the cosmic microwave backgound, the matter power spectra from large scale structure surveys and the luminosity distance vs redshift relation from supernovae of type Ia, we obtain strong bounds on the possible cold dark matter/baryon as well as neutrino isocurvature contributions to the primordial fluctations in the Universe. Neglecting the possible effects of spatial curvature and tensor perturbations, we perform a Bayesian likelihood analysis with thirteen free parameters, including independent spectral indexes for each of the modes and for their cross-correlation angle. We find that around a pivot wavenumber of k=0.05 h/Mpc the amplitude of the correlated isocurvature component cannot be larger than about 60% for the cold dark matter mode, 40% for the neutrino density mode, and 30% for the neutrino velocity mode, at 2 sigma. In the first case, our bound is larger than the WMAP first-year result, presumably because we prefer not to include any data from Lyman-alpha forests, but then obtain large blue spectral indexes for the non-adiabatic contributions. We also translate our bounds in terms of constraints on double inflation models with two uncoupled massive fields.

astro-ph↗

arXiv2004

Modern Cosmology

I review in these lectures our present understanding of the origin and evolution of the universe, making emphasis on the most recent observations of the acceleration of the universe, the precise measurements of the microwave background anisotropies, and the formation of structure like galaxies and clusters of galaxies from tiny primodial fluctuations generated during inflation.

hep-ph↗

arXiv2004

Bounds on Isocurvature Perturbations from CMB and LSS

Using both cosmic microwave background and large scale structure data, we put stringent bounds on the possible cold dark matter, baryon and neutrino isocurvature contributions to primordial fluctuations in the Universe. Neglecting the possible effects of spatial curvature, tensor perturbations and reionization, we perform a Bayesian likelihood analysis with nine free parameters, and find that the relative amplitude of the isocurvature component cannot be larger than about a few percent for uncorrelated models. On the other hand, for correlated adiabatic and isocurvature components, the ratio could be slightly larger. However, the cross-correlation coefficient is strongly constrained, and maximally correlated/anticorrelated models are disfavored.

astro-ph↗

arXiv2004

The Paradigm of Inflation

The standard model of cosmology is based on the hot Big Bang theory and the inflationary paradigm. Recent precise observations of the temperature and polarization anisotropies in the cosmic microwave background and the matter distribution in large scale structures like galaxies and clusters confirm the general paradigm and put severe constrains on variations of this simple idea. In this essay I will discuss the epistemological foundations of such a paradigm and speculate on its possible realisation within a more fundamental theory.

hep-ph↗

arXiv2003

Chern-Simons production during preheating in hybrid inflation models

We study the onset of symmetry breaking after hybrid inflation in a model having the field content of the SU(2) gauge-scalar sector of the standard model, coupled to a singlet inflaton. This process is studied in (3+1)-dimensions in a fully non-perturbative way with the help of lattice techniques within the classical approximation. We focus on the role played by gauge fields and, in particular, on the generation of Chern-Simons number. Our results are shown to be insensitive to the various cut-offs introduced in our numerical approach. The spectra preserves a large hierarchy between long and short-wavelength modes during the whole period of symmetry breaking and Chern-Simons generation, confirming that the dynamics is driven by the low momentum sector of the theory. We establish that the Chern-Simons production mechanism is associated with local sphaleron-like structures. The corresponding sphaleron rates are of order 10^{-5} m^4, which, within certain scenarios of electroweak baryogenesis and a (not unnaturally large) additional source of CP violation, could explain the present baryon asymmetry of the universe.

hep-ph↗

arXiv2003

Inflation from branes at angles

In this lecture I will review the present status of inflation within string-based brane-world scenarios. The idea is to start from a supersymmetric configuration with two parallel static Dp-branes, and slightly break the supersymmetry conditions to produce a very flat potential for the field that parametrises the distance between the branes, i.e. the inflaton field. This breaking can be achieved in various ways. I will describe one of the simplest examples described in Ref. [1]: two almost parallel D4-branes in a flat compactified space, with a small relative angle between the branes, softly breaking supersymmetry. If the breaking parameter is sufficiently small, a large number of e-folds can be produced within the D-brane, for small changes of the configuration in the compactified directions. Such a process is local, i.e. it does not depend very strongly on the compactification space nor on the initial conditions. Moreover, the breaking induces a very small velocity and acceleration, which ensures very small slow-roll parameters and thus an almost scale invariant spectrum of metric fluctuations, responsible for the observed temperature and polarization anisotropies in the microwave background. Another prediction of the model is the small amplitude of the gravitational wave spectrum, which may not be detectable in the near future. Inflation ends as in hybrid inflation, triggered by the negative curvature of the string tachyon potential.

astro-ph↗

arXiv2003

Symmetry Breaking and False Vacuum Decay after Hybrid Inflation

We discuss the onset of symmetry breaking from the false vacuum in generic scenarios in which the mass squared of the symmetry breaking (Higgs) field depends linearly with time, as it occurs, via the evolution of the inflaton, in models of hybrid inflation. We show that the Higgs fluctuations evolve from quantum to classical during the initial stages. This justifies the subsequent use of real-time lattice simulations to describe the fully non-perturbative and non-linear process of symmetry breaking. The early distribution of the Higgs field is that of a smooth classical gaussian random field, and consists of lumps whose shape and distribution is well understood analytically. The lumps grow with time and develop into ``bubbles'' which eventually collide among themselves, thus populating the high momentum modes, in their way towards thermalization at the true vacuum. With the help of some approximations we are able to provide a quasi-analytic understanding of this process.

hep-ph↗

arXiv2003

The evolution of the Universe

With the recent measurements of temperature and polarization anisotropies in the microwave background by WMAP, we have entered a new era of precision cosmology, with the cosmological parameters of a Standard Cosmological Model determined to 1%. This Standard Model is based on the Big Bang theory and the inflationary paradigm, a period of exponential expansion in the early universe responsible for the large-scale homogeneity and spatial flatness of our observable patch of the Universe. The spectrum of metric perturbations, seen in the microwave background as temperature anisotropies, were produced during inflation from quantum fluctuations that were stretched to cosmological size by the expansion, and later gave rise, via gravitational collapse, to the observed large-scale structure of clusters and superclusters of galaxies. Furthermore, the same theory predicts that all the matter and radiation in the universe today originated at the end of inflation from an explosive production of particles that could also have been the origin of the present baryon asymmetry, before the universe reached thermal equilibrium at a very large temperature. From there on, the universe cooled down as it expanded, in the way described by the standard hot Big Bang model.

hep-ph↗

arXiv2003

Black Hole Astrophysics in AdS Braneworlds

We consider astrophysics of large black holes localized on the brane in the infinite Randall-Sundrum model. Using their description in terms of a conformal field theory (CFT) coupled to gravity, deduced in Ref. [1], we show that they undergo a period of rapid decay via Hawking radiation of CFT modes. For example, a black hole of mass ${\rm few} \times M_\odot$ would shed most of its mass in $\sim 10^4 - 10^5$ years if the AdS radius is $L \sim 10^{-1}$ mm, currently the upper bound from table-top experiments. Since this is within the mass range of X-ray binary systems containing a black hole, the evaporation enhanced by the hidden sector CFT modes could cause the disappearance of X-ray sources on the sky. This would be a striking signature of RS2 with a large AdS radius. Alternatively, for shorter AdS radii, the evaporation would be slower. In such cases, the persistence of X-ray binaries with black holes already implies an upper bound on the AdS radius of $L \la 10^{-2}$ mm, an order of magnitude better than the bounds from table-top experiments. The observation of primordial black holes with a mass in the MACHO range $M \sim 0.1 - 0.5 M_\odot$ and an age comparable to the age of the universe would further strengthen the bound on the AdS radius to $L \la {\rm few} \times 10^{-6} $ mm.

hep-th↗

arXiv2002

Particle Physics and Cosmology

In this talk I review the present status of inflationary cosmology and its emergence as the basic paradigm behind the Standard Cosmological Model, with parameters determined today at better than 10% level from CMB and LSS observations.

hep-ph↗

arXiv2002

20+ years of Inflation

In this talk I will review the present status of inflationary cosmology and its emergence as the basic paradigm behind the Standard Cosmological Model, with parameters determined today at better than 10% level from CMB and LSS observations. I will also discuss the recent theoretical developments on the process of reheating after inflation and model building based on string theory and D-branes.

hep-ph↗

arXiv2002

Complex structure moduli stability in toroidal compactifications

In this paper we present a classification of possible dynamics of closed string moduli within specific toroidal compactifications of Type II string theories due to the NS-NS tadpole terms in the reduced action. They appear as potential terms for the moduli when supersymmetry is broken due to the presence of D-branes. We particularise to specific constructions with two, four and six-dimensional tori, and study the stabilisation of the complex structure moduli at the disk level. We find that, depending on the cycle on the compact space where the brane is wrapped, there are three possible cases: i) there is a solution inside the complex structure moduli space, and the configuration is stable at the critical point, ii) the moduli fields are driven towards the boundary of the moduli space, iii) there is no stable solution at the minimum of the potential and the system decays into a set of branes.

hep-th↗

arXiv2002

Particle production from symmetry breaking after inflation and leptogenesis

Recent studies suggest that the process of symmetry breaking after inflation typically occurs very fast, within a single oscillation of the symmetry-breaking field, due to the spinodal growth of its long-wave modes, otherwise known as `tachyonic preheating'. We show how this sudden transition from the false to the true vacuum can induce a significant production of particles, bosons and fermions, coupled to the symmetry-breaking field. We find that this new mechanism of particle production in the early Universe may have interesting consequences for the origin of supermassive dark matter and the generation of the observed baryon asymmetry through leptogenesis.

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