arXiv subjects

Juan Garcia-Bellido

Publications and source records attributed to Juan Garcia-Bellido.

At least 181 records · Page 10Linked to original sources

arXiv1997

Complete power spectrum for an induced gravity open inflation model

We study the phenomenological constraints on a recently proposed model of open inflation in the context of induced gravity. The main interest of this model is the relatively small number of parameters, which may be constrained by many different types of observation. We evaluate the complete spectrum of density perturbations, which contains continuum sub-curvature modes, a discrete super curvature mode, and a mode associated with fluctuations in the bubble wall. From these, we compute the angular power spectrum of temperature fluctuations in the microwave background, and derive bounds on the parameters of the model so that the predicted spectrum is compatible with the observed anisotropy of the microwave background and with large-scale structure observations. We analyze the matter era and the approach of the model to general relativity. The model passes all existing constraints.

astro-ph↗

arXiv1997

Open Hybrid Inflation

We propose an open hybrid inflation scenario that produces an open universe with a `tilted' n>1 spectrum of metric perturbations. The model contains a symmetry breaking field that tunnels to its true vacuum, producing a single bubble inside which hybrid inflation drives the universe to almost flatness. In order to obtain density perturbations with n > 1 we use the recently proposed new version of hybrid inflation scenario called tilted hybrid inflation. In this scenario, unlike in the previously known versions of hybrid inflation, a considerable tilt of the spectrum can be obtained without fine-tuning. The stage of inflation in this model is rather short, which allows us to obtain an inflationary universe with Omega < 1 in a more natural way. We study the separate contribution of scalar perturbations coming from the continuum subcurvature modes, the discrete supercurvature mode and the bubble wall mode to the angular power spectrum of temperature fluctuations in open inflation. We derive bounds on the parameters of the model so that the predicted spectrum is compatible with the observed anisotropy of the microwave background.

astro-ph↗

arXiv1996

Tilted Hybrid Inflation

We propose a new version of the hybrid inflation scenario that produces a significantly tilted n>1 spectrum of curvature perturbations. This may happen in supersymmetric models where the inflaton field acquires a mass proportional to the Hubble constant, and in the models where this field non-minimally couples to gravity. A large tilt of the spectrum in this scenario is often accompanied by a considerable contribution of tensor perturbations to the temperature anisotropies. We also show that under certain conditions the spectrum of density perturbations in this scenario may have a minimum on an intermediate length scale.

astro-ph↗

arXiv1996

Normalization of modes in an open universe

We discuss the appropriate normalization of modes required to generate a homogeneous random field in an open Friedmann-Robertson-Walker universe. We consider scalar random fields and certain tensor random fields that can be obtained by covariantly differentiating a scalar. Modes of interest fall into three categories: the familiar sub-curvature modes, the more recently discussed super-curvature modes, and a set of discrete modes with positive eigenvalues which can be used to generate homogeneous tensor random fields even though the underlying scalar field is not homogeneous. A particular example of the last case which has been discussed in the literature is the bubble wall fluctuation in open inflationary universes.

astro-ph↗

arXiv1996

Density Perturbations from Two-field Inflation

We discuss metric perturbations produced during a period of inflation in the early universe where two scalar fields evolve. The final scalar perturbation spectrum can be calculated in terms of the perturbed expansion along neighbouring trajectories in field-space. In the usual single field case this is fixed by the values of the fields at horizon-crossing, but in the presence of more than one field there is no longer a unique slow-roll trajectory. The presence of entropy as well as adiabatic fluctuations means that the super-horizon-sized metric perturbation $ζ$ may no longer be conserved and the evolution must be integrated along the whole of the subsequent trajectory. In general there is an inequality between the ratio of tensor to scalar perturbations and the tilt of the gravitational wave spectrum, which becomes an equality when only adiabatic perturbations are possible and $ζ$ is conserved.

astro-ph↗

arXiv1996

The spectrum of curvature perturbations from hybrid inflation

We study the amplitude and spectral tilt of density perturbations in the simplest hybrid inflation models. We give an exact expression for the amplitude of quantum fluctuations on all scales in the limit where we can neglect the backreaction on the metric. This is a very good approximation for values of the inflaton field well below the Planck scale and our results remain valid far from the usual massless limit. We confirm that the primordial density spectrum in this model has a constant spectral index n>1 over all observable scales. For the small values of the tilt (n<1.4) required by observations, the results remain close to those obtained using the quasi-massless approximation.

astro-ph↗

arXiv1996

Density Perturbations and Black Hole Formation in Hybrid Inflation

We investigate the recently proposed hybrid inflation models with two stages of inflation. We show that quantum fluctuations at the time corresponding to the phase transition between the two inflationary stages can trigger the formation of a large number of inflating topological defects. In order to study density perturbations in these models we develop a new method to calculate density perturbations in a system of two scalar fields. We show that density perturbations in hybrid inflation models of the new type can be very large on the scale corresponding to the phase transition. The resulting density inhomogeneities lead to a copious production of black holes. This could be an argument against hybrid inflation models with two stages of inflation. However, we find a class of models where this problem can be easily avoided. The number of black holes produced in these models can be made extremely small, but in general it could be sufficiently large to have important cosmological and astrophysical implications. In particular, for certain values of parameters these black holes may constitute the dark matter in the universe. It is also possible to have hybrid models with two stages of inflation where the black hole production is not suppressed, but where the typical masses of the black holes are very small. Such models lead to a completely different thermal history of the universe, where post-inflationary reheating occurs via black hole evaporation.

astro-ph↗

arXiv1996

Metric Perturbations from Quantum Tunneling in Open Inflation

We study the effect that quantum fluctuations produced during the nucleation of a single-bubble open inflationary universe have on the amplitude of temperature anisotropies in the microwave background. We compute the instanton action for the quantum tunneling between the false and true vacua in open inflation models and show that the amplitude of quantum fluctuations of the bubble wall is very sensitive to the gravitational effects of the true vacuum. We study the spectrum of quantum fluctuations of the bubble wall and confirm that there is only an inhomogeneous ($k^2 = -3$) discrete mode associated with transverse traceless fluctuations of the bubble wall. This super-curvature mode could in principle distort the anisotropy of the microwave background. We compute the amplitude of the gauge invariant metric perturbations induced by the bubble wall fluctuations on a comoving hypersurface, and calculate the induced amplitude of temperature fluctuations in the microwave background, for arbitrary values of $Ω_0$. We find that in the limit $Ω_0\simeq 1$, the quadrupole dominates the angular power spectrum, like in the usual Grishchuk-Zel'dovich effect. The resulting bounds on the amplitude of quantum fluctuations of the bubble wall from the absence of such an effect in the observed microwave background anisotropies are quite strong. We also study the contribution from a discrete long wavelength super-curvature mode ($k^2 \simeq 2m^2/3H^2$) that appears in the spectrum of open de Sitter vacuum fluctuations. We constrain the parameters of the models of open inflation so that these modes do not distort the observed temperature anisotropy.

astro-ph↗

arXiv1995

Open Inflation

Open Inflation has recently been suggested as a possible way out of the age crisis caused by observations of a large rate of expansion of the universe, in conflict with the existence of very old globular clusters. It proposes that our local patch of the universe originated in a quantum tunneling event, with the formation of a single bubble within which our universe inflated to almost flatness. I review the different models proposed together with their predictions for the amplitude of temperature anisotropies in the cosmic microwave background.

astro-ph↗

arXiv1995

Metric perturbations in two-field inflation

We study the metric perturbations produced during inflation in models with two scalar fields evolving simultaneously. In particular, we emphasize how the large-scale curvature perturbation $ζ$ on fixed energy density hypersurfaces may not be conserved in general for multiple field inflation due to the presence of entropy as well as adiabatic fluctuations. We show that the usual method of solving the linearized perturbation equations is equivalent to the recently proposed analysis of Sasaki and Stewart in terms of the perturbed expansion along neighboring trajectories in field-space. In the case of a separable potential it is possible to compute in the slow-roll approximation the spectrum of density perturbations and gravitational waves at the end of inflation. In general there is an inequality between the ratio of tensor to scalar perturbations and the tilt of the gravitational wave spectrum, which becomes an equality when only adiabatic perturbations are possible and $ζ$ is conserved.

astro-ph↗

arXiv1995

The Open Universe Grishchuk-Zel'dovich Effect

The Grishchuk--Zel'dovich effect is the contribution to the microwave background anisotropy from an extremely large scale adiabatic density perturbation, on the standard hypothesis that this perturbation is a typical realization of a homogeneous Gaussian random field. We analyze this effect in open universes, corresponding to density parameter $Ω_0<1$ with no cosmological constant, and concentrate on the recently discussed super-curvature modes. The effect is present in all of the low multipoles of the anisotropy, in contrast with the $Ω_0=1$ case where only the quadrupole receives a contribution. However, for no value of $Ω_0$ can a very large scale perturbation generate a spectrum capable of matching observations across a wide range of multipoles. We evaluate the magnitude of the effect coming from a given wavenumber as a function of the magnitude of the density perturbation, conveniently specified by the mean-square curvature perturbation. From the absence of the effect at the observed level, we find that for $0.25\leqΩ_0\leq 0.8$, a curvature perturbation of order unity is permitted only for inverse wavenumbers more than one thousand times the size of the observable universe. As $Ω_0$ tends to one, the constraint weakens to the flat space result that the inverse wavenumber be more than a hundred times the size of the observable universe, whereas for $Ω_0 < 0.25$ it becomes stronger. We explain the physical meaning of these results, by relating them to the correlation length of the perturbation. Finally, in an Appendix we consider the dipole anisotropy and show that it always leads to weaker constraints.

astro-ph↗

arXiv1995

Constraints from Inflation on Scalar-Tensor Gravity Theories

We show how observations of the perturbation spectra produced during inflation may be used to constrain the parameters of general scalar-tensor theories of gravity, which include both an inflaton and dilaton field. An interesting feature of these models is the possibility that the curvature perturbations on super-horizon scales may not be constant due to non-adiabatic perturbations of the two fields. Within a given model, the tilt and relative amplitude of the scalar and tensor perturbation spectra gives constraints on the parameters of the gravity theory, which may be comparable with those from primordial nucleosynthesis and post-Newtonian experiments.

gr-qc↗

arXiv1995

STATIONARY SOLUTIONS IN BRANS-DICKE STOCHASTIC INFLATIONARY COSMOLOGY

In Brans-Dicke theory the Universe becomes divided after inflation into many exponentially large domains with different values of the effective gravitational constant. Such a process can be described by diffusion equations for the probability of finding a certain value of the inflaton and dilaton fields in a physical volume of the Universe. For a typical chaotic inflation potential, the solutions for the probability distribution never become stationary but grow forever towards larger values of the fields. We show here that a non-minimal conformal coupling of the inflaton to the curvature scalar, as well as radiative corrections to the effective potential, may provide a dynamical cutoff and generate stationary solutions. We also analyze the possibility of large nonperturbative jumps of the fluctuating inflaton scalar field, which was recently revealed in the context of the Einstein theory. We find that in the Brans--Dicke theory the amplitude of such jumps is strongly suppressed.

gr-qc↗

arXiv1995

Astrophysical and cosmological constraints on a scale-dependent gravitational coupling

We study the phenomenological consequences of the recently proposed idea of a running gravitational coupling on macroscopic scales. When applied to the rotation curves of galaxies, we find that their flatness requires the presence of baryonic dark matter. Bounds on the variation of the gravitational coupling from primordial nucleosynthesis and the change of the period of binary pulsars are analysed. We also study constraints on the variations of $G$ with scale from gravitational lensing and the cosmic virial theorem, as well as briefly discuss the implications of such a scenario for structure formation.

astro-ph↗

arXiv1995

General Relativity as an Attractor in Scalar-Tensor Stochastic Inflation

Quantum fluctuations of scalar fields during inflation could determine the very large-scale structure of the universe. In the case of general scalar-tensor gravity theories these fluctuations lead to the diffusion of fundamental constants like the Planck mass and the effective Brans--Dicke parameter, $ω$. In the particular case of Brans--Dicke gravity, where $ω$ is constant, this leads to runaway solutions with infinitely large values of the Planck mass. However, in a theory with variable $ω$ we find stationary probability distributions with a finite value of the Planck mass peaked at exponentially large values of $ω$ after inflation. We conclude that general relativity is an attractor during the quantum diffusion of the fields.

gr-qc↗

arXiv1994

Quantum Fluctuations of Planck Mass as Mutation Mechanism in a Theory of Evolution of the Universe

Contributed talk at the Seventh Marcel Grossman Meeting on Gravity, June 24-30. A theory of evolution of the universe requires both a mutation mechanism and a selection mechanism. We believe that both can be encountered in the stochastic approach to quantum cosmology. In Brans-Dicke chaotic inflation, the quantum fluctuations of Planck mass behave as mutations, such that new inflationary domains may contain values of Planck mass that differ slightly from their parent's. The selection mechanism establishes that the value of Planck mass should be such as to increase the proper volume of the inflationary domain, which will then generate more offsprings. This mechanism predicts that the effective Planck scale at the end of inflation should be much larger than any given scale in the model.

gr-qc↗

arXiv1994

Stationarity of Inflation and Predictions of Quantum Cosmology

We describe several different regimes which are possible in inflationary cosmology. The simplest one is inflation without self-reproduction of the universe. In this scenario the universe is not stationary. The second regime, which exists in a broad class of inflationary models, is eternal inflation with the self-reproduction of inflationary domains. In this regime local properties of domains with a given density and given values of fields do not depend on the time when these domains were produced. The probability distribution to find a domain with given properties in a self-reproducing universe may or may not be stationary, depending on the choice of an inflationary model. We give examples of models where each of these possibilities can be realized, and discuss some implications of our results for quantum cosmology. In particular, we propose a new mechanism which may help solving the cosmological constant problem.

hep-th↗

arXiv1994

Quantum Diffusion of Planck Mass and the Evolution of the Universe

A theory of evolution for the Universe requires both a mutation mechanism and a selection mechanism. We propose that both can be encountered in the stochastic approach to quantum cosmology. In Brans--Dicke chaotic inflation, the quantum fluctuations of Planck mass behave as mutations, such that new inflationary domains may contain values of Planck mass that differ slightly from their parent's. The selection mechanism establishes that the value of Planck mass should be such as to increase the proper volume of the inflationary domain, which will then generate more offsprings. For generic chaotic potentials we find runaway probability distributions that move towards large values of both inflaton and dilaton fields. Therefore, this mechanism predicts that the effective Planck scale should be much larger than any given scale in the problem.

astro-ph↗