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Juan Garcia-Bellido

Publications and source records attributed to Juan Garcia-Bellido.

186 records · Page 11Linked to original sources
arXiv1994

Jordan-Brans-Dicke Stochastic Inflation

We study stochastic inflation in the presence of a dynamical gravitational constant. We describe the Arnowitt--Deser--Misner formalism for Jordan--Brans--Dicke theory of gravity with an inflaton field. The inflaton and dilaton scalar fields can be separated into coarse-grained background fields and quantum fluctuations. We compute the amplitude of the perturbations generated by those quantum fluctuations in JBD theory with an arbitrary potential for the inflaton field. The effect of the quantum fluctuations on the background fields is equivalent to a Brownian motion of the scalar fields, which can be described with the use of a Fokker--Planck diffusion equation. The probability to find a given value of the fields in the comoving frame can be written as a Gaussian distribution centered on their classical trajectory, with decreasing dispersion along both field directions. We also calculate the condition for the Universe to enter a self-regenerating inflationary phase. The probability distribution in the physical frame, which takes into account the expansion of the proper volume of the inflationary domains, will be concentrated at the Planck boundary and will move along it towards large values of the fields.

astro-ph↗
arXiv1992

Jordan-Brans-Dicke Quantum Wormholes and Coleman's Mechanism

We consider the quantum gravity and cosmology of a Jordan-Brans-Dicke theory, predicted by string effective actions. We study its canonical formalism and find that the constraint algebra is that of general relativity, as a consequence of the general covariance of scalar-tensor theories. We also analyze the problem of boundary conditions and propose that they must be imposed in the Jordan frame, in which particles satisfy the strong equivalence principle. Specifically, we discuss both Hartle-Hawking and wormhole boundary conditions in the context of quantum cosmology. We find quantum wormhole solutions for Jordan-Brans-Dicke gravity even in the absence of matter. Wormholes may affect the constants of nature and, in particular, the Brans-Dicke parameter. Following Coleman's mechanism, we find a probability distribution which is strongly peaked at zero cosmological constant and infinite Brans-Dicke parameter. That is, we recover general relativity as the effective low energy theory of gravity.

gr-qc↗
arXiv1992

Axions with Variable Masses

Axions with variable masses, in the context of a scalar--tensor gravity theory, give a large entropy production during the matter era. The subsequent axion dilution is proportional to their present energy density. Depending on the parameters ($β_I,β_V$) of the model, this dilution relaxes or even eludes the cosmological bound on the axion mass, therefore opening the so--called ``axion window".

hep-ph↗
arXiv1992

Dark Matter with Variable Masses

String effective theories contain a dilaton scalar field which couples to gravity, matter and radiation. In general, particle masses will have different dilaton couplings. We can always choose a conformal frame in which baryons have constant masses while (non--baryonic) dark matter have variable masses, in the context of a scalar--tensor gravity theory. We are interested in the phenomenology of this scenario. Dark matter with variable masses could have a measurable effect on the dynamical motion of the halo of spiral galaxies, which may affect cold dark matter models of galaxy formation. As a consequence of variable masses, the energy--momentum tensor is not conserved; there is a dissipative effect, due to the dilaton coupling, associated with a ``dark entropy" production. In particular, if axions had variable masses they could be diluted away, thus opening the ``axion window". Assuming that dark matter with variable masses dominates the cosmological evolution during the matter era, it will affect the primordial nucleosynthesis predictions on the abundances of light elements. Furthermore, the dilaton also couples to radiation in the form of a variable gauge coupling. Experimental bounds will constrain the parameters of this model.

hep-ph↗
arXiv1992

String Effective Actions and Cosmological Stability of Scalar Potentials

The cosmology of the string effective action, including one loop string threshold corrections, is analyzed for static compactifications. The stability of the minima of a general supersymmetry breaking potential is studied in the presence of radiation. In particular, it is shown that the radiation bath makes the minima with negative cosmological constant unstable.

hep-th↗