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Lorenzo Sorbo

Publications and source records attributed to Lorenzo Sorbo.

71 records · Page 4Linked to original sources
arXiv2005

Of pNGB QuiNtessence

We review the pNGB quintessence models, and point out that the reason why the large decay constants $f_a \ga {\cal O}(1)M_{Pl}$ are really needed is to tame a tachyonic instability present for a wide range of initial $vev$s. Starting very close to potential maxima does not help because quantum fluctuations during early inflation at a scale $H_I$ perturb the quintessence $vev$, displacing it from the maxima. This issue is quite interesting for pNGB dark energy in light of the recently discussed difficulties with embedding models with $f_a > M_{Pl}$ in fundamental theory. A possible way around is provided by models with several ultralight pNGBs, which can drive a short burst of very late inflation {\it together} even if all of their decay constants obey $f_a < M_{Pl}$. Starting with their $vev \sim f_a$, the pNGBs will hold each other up on the potential for a longer time period. Their effective dynamics is captured by a collective mode, containing admixtures of all of the rolling pNGBs, which behaves as an ultralight field with $f_{eff} > M_{Pl}$. We point out that there may be potentially observable large scale disturbances in the sea of dark energy in such models.

astro-ph↗
arXiv2005

Locally Localized Gravity: The Inside Story

We derive the exact gravitational field of a relativistic particle localized on an $AdS$ 3-brane, with curvature radius $\ell$, in $AdS_5$ bulk with radius $L$. The solution is a gravitational shock wave. We use it to explore the dynamics of locally localized tensor gravitons over a wide range of scales. At distances below $L$ the shock wave looks exactly like the $5D$ $GR$ solution. Beyond $L$ the solution approximates very closely the shock wave in 4D $AdS$ space all the way out to distances $\ell^3/L^2$ along the brane. At distances between $L$ and $\ell$, the effective 4D graviton is a composite built of the ultralight mode and heavier gravitons, whereas between $\ell$ and $\ell^3/L^2$ it is just the ultralight mode. Finally beyond $\ell^3/L^2$ the shock reveals a glimpse of the fifth dimension, since the ultralight mode wave function decays to zero at the rate inherited from the full $5D$ geometry. We obtain the precise bulk-side formula for the 4D Planck mass, defined as the coupling of the ultralight mode, in terms of the $5D$ Planck mass and the curvature radii. It includes higher-order corrections in $L/\ell$, and reduces to the RS2 formula in the limit $\ell \to \infty$. We discuss $AdS/CFT$ interpretation of these results, and argue that the spatial variation of the effective gravitational coupling read from the shock wave amplitude corresponds to RG running driven by quantum effects in the dual $CFT$.

hep-th↗
arXiv2004

Moduli Entrapment with Primordial Black Holes

We argue that primordial black holes in the early universe can provide an efficient resolution of the Brustein-Steinhardt moduli overshoot problem in string cosmology. When the universe is created near the Planck scale, all the available states in the theory are excited by strong interactions and cosmological particle production. The heavy states are described in the low energy theory as a gas of electrically and magnetically charged black holes. This gas of black holes quickly captures the moduli which appear in the relation between black hole masses and charges, and slows them down with their vevs typically close to the Planck scale. From there, the modulus may slowly roll into a valley with a positive vacuum energy, where inflation may begin. The black hole gas will redshift away in the course of cosmic expansion, as inflation evicts black holes out of the horizon.

hep-th↗
arXiv2004

Observational Implications of Cosmological Event Horizons

In a universe dominated by a small cosmological constant or by eternal dark energy with equation of state w < -1/3, observers are surrounded by event horizons. The horizons limit how much of the universe the observers can ever access. We argue that this implies a bound N~60 on the number of e-folds of inflation that will ever be observable in our universe if the scale of the dark energy today is ~(10^{-3} eV)^4. This bound is independent of how long inflation lasted, or for how long we continue to observe the sky. The bound arises because the imprints of the inflationary perturbations thermalize during the late acceleration of the universe. They "inflate away" just like the initial inhomogeneities during ordinary inflation. Thus the current CMB data may be looking as far back in the history of the universe as will ever be possible, making our era a most opportune time to study cosmology.

astro-ph↗
arXiv2004

Can the universe afford inflation?

Cosmic inflation is envisioned as the ``most likely'' start for the observed universe. To give substance to this claim, a framework is needed in which inflation can compete with other scenarios and the relative likelihood of all scenarios can be quantified. The most concrete scheme to date for performing such a comparison shows inflation to be strongly disfavored. We analyze the source of this failure for inflation and present an alternative calculation, based on more traditional semiclassical methods, that results in inflation being exponentially favored. We argue that reconciling the two contrasting approaches presents interesting fundamental challenges, and is likely to have a major impact on ideas about the early universe.

hep-th↗
arXiv2004

Goldberger-Wise variations: stabilizing brane models with a bulk scalar

Braneworld scenarios with compact extra-dimensions need the volume of the extra space to be stabilized. Goldberger and Wise have introduced a simple mechanism, based on the presence of a bulk scalar field, able to stabilize the radius of the Randall-Sundrum model. Here, we transpose the same mechanism to generic single-brane and two-brane models, with one extra dimension and arbitrary scalar potentials in the bulk and on the branes. The single-brane construction turns out to be always unstable, independently of the bulk and brane potentials. In the case of two branes, we derive some generic criteria ensuring the stabilization or destabilization of the system.

hep-th↗
arXiv2004

Moving sources in a ghost condensate

Ghost condensation has been recently proposed as a mechanism inducing the spontaneous breaking of Lorentz symmetry. Corrections to the Newton potential generated by a static source have been computed: they yield a limit M < 10 MeV on the symmetry breaking scale, and - if the limit is saturated - they are maximal at a distance L ~ 1000 km from the source. However, these corrections propagate at a tiny velocity, v_s ~ 10^{-12} m/s, many orders of magnitude smaller than the velocity of any plausible source. We compute the gravitational potential taking the motion of the source into account: the standard Newton law is recovered in this case, with negligible corrections for any distance from the source up to astrophysical scales. Still, the vacuum of the theory is unstable, and requiring stability over the lifetime of the Universe gives a limit for M which is not too far from the one given above. In the absence of a direct coupling of the ghost to matter, signatures of this model will have to be searched in the form of exotic astrophysical events.

hep-th↗
arXiv2003

Bulk gravitons from a cosmological brane

We investigate the emission of gravitons by a cosmological brane into an Anti de Sitter five-dimensional bulk spacetime. We focus on the distribution of gravitons in the bulk and the associated production of `dark radiation' in this process. In order to evaluate precisely the amount of dark radiation in the late low-energy regime, corresponding to standard cosmology, we study numerically the emission, propagation and bouncing off the brane of bulk gravitons.

hep-th↗
arXiv2002

Leptogenesis and rescattering in supersymmetric models

The observed baryon asymmetry of the Universe can be due to the $B-L$ violating decay of heavy right handed (s)neutrinos. The amount of the asymmetry depends crucially on their number density. If the (s)neutrinos are generated thermally, in supersymmetric models there is limited parameter space leading to enough baryons. For this reason, several alternative mechanisms have been proposed. We discuss the nonperturbative production of sneutrino quanta by a direct coupling to the inflaton. This production dominates over the corresponding creation of neutrinos, and it can easily (i.e. even for a rather small inflaton-sneutrino coupling) lead to a sufficient baryon asymmetry. We then study the amplification of MSSM degrees of freedom, via their coupling to the sneutrinos, during the rescattering phase which follows the nonperturbative production. This process, which mainly influences the (MSSM) $D-$flat directions, is very efficient as long as the sneutrinos quanta are in the relativistic regime. The rapid amplification of the light degrees of freedom may potentially lead to a gravitino problem. We estimate the gravitino production by means of a perturbative calculation, discussing the regime in which we expect it to be reliable.

hep-ph↗
arXiv2002

Cosmology of a brane radiating gravitons into the extra dimension

We study in a self-consistent way the impact of the emission of bulk gravitons on the (homogeneous) cosmology of a three-brane embedded in a five-dimensional spacetime. In the low energy regime, we recover the well known result that the bulk affects the Friedmann equation only via a radiation-like term $\C/a^4$, called dark or Weyl radiation. By contrast, in the high energy regime, we find that the Weyl parameter $\C$ is no longer constant but instead grows very rapidly as $\C\propto a^4$. As a consequence, the value of $\C$ today is not a free parameter as usually considered but is a fixed number, which, generically, depends only on the number of relativistic degrees of freedom at the high/low energy transition. Our estimated amount of Weyl radiation satisfies the present nucleosynthesis bounds.

hep-th↗
arXiv2002

Moduli from Cosmic Strings

Moduli fields are generally present in superstring--inspired models. They are typically characterized by masses of the order of the supersymmetry breaking scale and by interactions of gravitational strength to ordinary matter. If stable, they can easily overclose the Universe. If unstable, strong limits on their abundance have to be imposed not to disrupt the successful predictions of nucleosynthesis. We discuss the production of moduli quanta from loops of cosmic strings, which may form at phase transitions in the early history of the Universe. As an application, we then focus on strings formed at the end of hybrid inflation, showing that in this case the coupling of the moduli to the field which forms the string network has to be much weaker than the typical gravitational interactions not to exceed the bounds from nucleosynthesis. Alternatively, if the relevant coupling is of gravitational order, an upper bound of about 10^13 GeV is imposed on the string energy scale.

hep-ph↗
arXiv2002

Effective action for the homogeneous radion in brane cosmology

We consider cosmological two-brane models with AdS bulk, for which the radion, i.e. the separation between the two branes, is time dependent. In the case of two de Sitter branes (including Minkowski branes as a limiting case), we compute explicitly, without any approximation, the effective four-dimensional action for the radion. With the scale factor on-shell, this provides the non-perturbative dynamics for the radion. We discuss the differences between the dynamics derived from the four-dimensional action with the scale factor off-shell and the true five-dimensional dynamics.

hep-th↗
arXiv2001

Fermion production from preheating-amplified metric perturbations

We study gravitational creation of light fermions in the presence of classical scalar metric perturbations about a flat Friedmann-Lemaitre- Robertson-Walker (FLRW) background. These perturbations can be large during preheating, breaking the conformal flatness of the background spacetime. We compute numerically the total number of particles generated by the modes of the metric perturbations which have grown sufficiently to become classical. In the absence of inhomogeneities massless fermions are not gravitationally produced, and then this effect may be relevant for abundance estimates of light gravitational relics.

hep-ph↗
arXiv2001

Coupled fields in external background with application to nonthermal production of gravitinos

We provide the formalism for the quantization of systems of coupled bosonic and fermionic fields in a time dependent classical background. The occupation numbers of the particle eigenstates can be clearly defined and computed, through a generalization of the standard procedure valid for a single field in which Bogolyubov coefficients are employed. We apply our formalism to the problem of nonthermal gravitino production in a two-fields model where supersymmetry is broken gravitationally in the vacuum. Our explicit calculations show that this production is strongly suppressed in the model considered, due to the weak coupling between the sector which drives inflation and the one responsible for supersymmetry breakdown.

hep-th↗
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↗