arXiv subjects

Roberto Casadio

Publications and source records attributed to Roberto Casadio.

At least 109 records · Page 6Linked to original sources

arXiv2014

Self-healing of unitarity in Higgs inflation

We reconsider perturbative unitarity violation in the standard model Higgs inflation model. We show that the Cutkosky cutting rule implied by perturbative unitarity is fulfilled at one-loop. This is a strong indication that unitarity is restored order by order in perturbation theory. We then resum certain one-loop diagrams and show that the relevant dressed amplitude fulfills the Cutkosky rule exactly. This is an example of the self-healing mechanism. The original Higgs inflation model is thus consistent and does not require any new physics beyond the standard model at least up to the Planck scale.

hep-ph↗

arXiv2014

Quantum hoop conjecture: Black hole formation by particle collisions

We address the issue of (quantum) black hole formation by particle collision in quantum physics. We start by constructing the horizon wave-function for quantum mechanical states representing two highly boosted non-interacting particles that collide in flat one-dimensional space. From this wave-function, we then derive a probability that the system becomes a black hole as a function of the initial momenta and spatial separation between the particles. This probability allows us to extend the hoop conjecture to quantum mechanics and estimate corrections to its classical counterpart.

hep-th↗

arXiv2013

Quantum Harmonic Black Holes (Proceeding of the Karl Schwarzschild Meeting 2013)

Inspired by the recent conjecture that black holes are condensates of gravitons, we investigate a simple model for the black hole degrees of freedom that is consistent both from the point of view of Quantum mechanics and of General Relativity. Since the two perspectives should "converge" into a unified picture for small, Planck size, objects, we expect our construction is a useful step for understanding the physics of microscopic, quantum black holes. In particular, we show that a harmonically trapped condensate gives rise to two horizons, whereas the extremal case (corresponding to a remnant with vanishing Hawking temperature) is not contained in the spectrum.

gr-qc↗

arXiv2013

Black Strings from Minimal Geometric Deformation in a Variable Tension Brane-World

We study brane-world models with variable brane tension and compute corrections to the horizon of a black string along the extra dimension. The four-dimensional geometry of the black string on the brane is obtained by means of the minimal geometric deformation approach, and the bulk corrections are then encoded in additional terms involving the covariant derivatives of the variable brane tension. Our investigation shows that the variable brane tension strongly affects the shape and evolution of the black string horizon along the extra dimension, at least in a near-brane expansion. In particular, we apply our general analysis to a model motivated by the Eötvös branes, where the variable brane tension is related to the Friedmann-Robertson-Walker brane-world cosmology. We show that for some stages in the evolution of the universe, the black string warped horizon collapses to a point and the black string has correspondingly finite extent along the extra dimension. Furthermore, we show that in the minimal geometric deformation of a black hole on the variable tension brane, the black string has a throat along the extra dimension, whose area tends to zero as time goes to infinity.

gr-qc↗

arXiv2013

Lorentz invariance without trans-Planckian physics?

We explore the possibility that, in a quantum field theory with Planck scale cutoff Lambda=Mp, observable quantities for low-energy processes respect the Lorentz symmetry. In particular, we compute the one-loop radiative correction Pi to the self-energy of a scalar field with lambda phi^4 interaction, using a modified (non-invariant) propagator which vanishes in the trans-Planckian regime, as expected in the "classicalization" scenario. We then show that, by imposing the result does not depend on Lambda (in the limit Lambda to Mp), an explicit (albeit not unique) expression for Pi can be derived, which is similar to the one simply obtained with the standard Feynman propagator and a cutoff Lambda=Mp.

hep-th↗

arXiv2013

Brane-world stars from minimal geometric deformation, and black holes

We build analytical models of spherically symmetric stars in the brane-world, in which the external space-time contains both an ADM mass and a tidal charge. In order to determine the interior geometry, we apply the principle of minimal geometric deformation, which allows one to map General Relativistic solutions to solutions of the effective four-dimensional brane-world equations. We further restrict our analysis to stars with a radius linearly related to the total General Relativistic mass, and obtain a general relation between the latter, the brane-world ADM mass and the tidal charge. In these models, the value of the star's radius can then be taken to zero smoothly, thus obtaining brane-world black hole metrics with a tidal charge solely determined by the mass of the source and the brane tension. General conclusions regarding the minimum mass for semiclassical black holes will also be drawn.

gr-qc↗

arXiv2012

Gravitational renormalization of quantum field theory

We propose to include gravity in quantum field theory non-perturbatively, by modifying the propagators so that each virtual particle in a Feynman graph move in the space-time determined by the four-momenta of the other particles in the same graph. By making additional working assumptions, we are able to put this idea at work in a simplified context, and obtain a modified Feynman propagator for the massless neutral scalar field. Our expression shows a suppression at high momentum, strong enough to entail finite results, to all loop orders, for processes involving at least two virtual particles.

hep-th↗

arXiv2011

CMB acoustic scale in the entropic-like accelerating universe

We consider generalizations of the entropic accelerating universe recently proposed in Ref. [4,5] and show that their background equations can be made equivalent to a model with a dark energy component with constant parameter of state $w_{X} = -1 + 2\, γ/3$, where $γ$ is related to the coefficients of the new terms in the Friedman equations. After discussing all the Friedman equations for an arbitrary $γ$, we show how to recover the standard scalings for dust and radiation. The acoustic scale $\ell_A$, related to the peak positions in the pattern of the angular power spectrum of the Cosmic Microwave Background anisotropies, is also computed and yields the stringent bound $|γ|\ll 1$. We then argue that future data might be able to distinguish this model from pure $Λ$CDM (corresponding to $γ=0$).

gr-qc↗

arXiv2010

Asymptotic Safety, Singularities, and Gravitational Collapse

Asymptotic safety (an ultraviolet fixed point with finite-dimensional critical surface) offers the possibility that a predictive theory of quantum gravity can be obtained from the quantization of classical general relativity. However, it is unclear what becomes of the singularities of classical general relativity, which, it is hoped, might be resolved by quantum effects. We study dust collapse with a running gravitational coupling and find that a future singularity can be avoided if the coupling becomes exactly zero at some finite energy scale. The singularity can also be avoided (pushed off to infinite proper time) if the coupling approaches zero sufficiently rapidly at high energies. However, the evolution deduced from perturbation theory still implies a singularity at finite proper time.

gr-qc↗

arXiv2010

Affect of brane thickness on microscopic tidal-charged black holes

We study the phenomenological implications stemming from the dependence of the tidal charge on the brane thickness $L$ for the evaporation and decay of microscopic black holes. In general, the larger $L$, the longer are the black hole life-times and the greater their maximum mass for those cases in which the black hole can grow. In particular, we again find that tidal-charged black holes might live long enough to escape the detectors and even the gravitational field of the Earth, thus resulting in large amounts of missing energy. However, under no circumstances could TeV-scale black holes grow enough to enter the regime of Bondi accretion.

hep-ph↗

arXiv2010

A note on second-order perturbations of non-canonical scalar fields

We study second-order perturbations for a general non-canonical scalar field, minimally coupled to gravity, on the unperturbed FRW background, where metric fluctuations are neglected a priori. By employing different approaches to cosmological perturbation theory, we show that, even in this simplified set-up, the second-order perturbations to the stress tensor, the energy density and the pressure display potential instabilities, which are not present at linear order. The conditions on the Lagrangian under which these instabilities take place are provided. We also discuss briefly the significance of our analysis in light of the possible linearization instability of these fields about the FRW background.

astro-ph.CO↗

arXiv2010

Ambiguities in second-order cosmological perturbations for non-canonical scalar fields

Over the last few years, it was realised that non-canonical scalar fields can lead to the accelerated expansion in the early universe. The primordial spectrum in these scenarios not only shows near scale-invariance consistent with CMB observations,but also large primordial non-Gaussianity. Second-order perturbation theory is the primary theoretical tool to investigate such non-Gaussianity. However, it is still uncertain which quantities are gauge-invariant at second-order and their physical understanding therefore remains unclear. As an attempt to understand second order quantities, we consider a general non-canonical scalar field, minimally coupled to gravity, on the unperturbed FRW background where metric fluctuations are neglected a priori. In this simplified set-up, we show that there arise ambiguities in the expressions of physically relevant quantities, such as the effective speeds of the perturbations. Further, the stress tensor and energy density display a potential instability which is not present at linear order.

astro-ph.CO↗

arXiv2010

Theoretical survey of tidal-charged black holes at the LHC

We analyse a family of brane-world black holes which solve the effective four-dimensional Einstein equations for a wide range of parameters related to the unknown bulk/brane physics. We first constrain the parameters using known experimental bounds and, for the allowed cases, perform a numerical analysis of their time evolution, which includes accretion through the Earth. The study is aimed at predicting the typical behavior one can expect if such black holes were produced at the LHC. Most notably, we find that, under no circumstances, would the black holes reach the (hazardous) regime of Bondi accretion. Nonetheless, the possibility remains that black holes live long enough to escape from the accelerator (and even from the Earth's gravitational field) and result in missing energy from the detectors.

hep-th↗

arXiv2009

Possibility of Catastrophic Black Hole Growth in the Warped Brane-World Scenario at the LHC

In this paper we present the results of our analysis of the growth and decay of black holes possibly produced at the Large Hadron Collider, based on our previous study of black holes in the context of the warped brane-world scenario. The black hole mass accretion and decay is obtained as a function of time, and the maximum black hole mass is obtained as a function of a critical mass parameter. The latter occurs in our expression for the luminosity and is related to the size of extra-dimensional corrections to Newton's law. Based on this analysis, we argue against the possibility of catastrophic black hole growth at the LHC.

hep-ph↗

arXiv2009

The Cosmological Constant and Horava-Lifshitz Gravity

Horava-Lifshitz theory of gravity with detailed balance is plagued by the presence of a negative bare (or geometrical) cosmological constant which makes its cosmology clash with observations. We argue that adding the effects of the large vacuum energy of quantum matter fields, this bare cosmological constant can be approximately compensated to account for the small observed (total) cosmological constant. Even though we cannot address the fine-tuning problem in this way, we are able to establish a relation between the smallness of observed cosmological constant and the length scale at which dimension 4 corrections to the Einstein gravity become significant for cosmology. This scale turns out to be approximately 5 times the Planck length for an (almost) vanishing observed cosmological constant and we therefore argue that its smallness guarantees that Lorentz invariance is broken only at very small scales. We are also able to provide a first rough estimation for the infrared values of the parameters of the theory $μ$ and $Lambda_w$.

hep-th↗

arXiv2009

Point-like sources and the scale of quantum gravity

We review the General Relativistic model of a (quasi) point-like particle represented by a massive shell of neutral matter which has vanishing total energy in the small-volume limit. We then show that, by assuming a Generalised Uncertainty Principle, which implies the existence of a minimum length of the order of the Planck scale, the total energy instead remains finite and equal to the shell's proper mass both for very heavy and very light particles. This suggests that the quantum structure of space-time might be related to the classical Equivalence Principle and possible implications for the late stage of evaporating black holes are briefly mentioned.

gr-qc↗

arXiv2009

Gravitational renormalization of quantum field theory: a "conservative" approach

We propose general guidelines in order to incorporate the geometrical description of gravity in quantum field theory and address the problem of UV divergences non-perturbatively. In our aproach, each virtual particle in a Feynman graph should be described by a modified propagator and move in the space-time generated by the other particles in the same graph according to Einstein's (semiclassical) equations.

gr-qc↗

arXiv2008

A radiation-like era before inflation

We show that the semiclassical approximation to the Wheeler-DeWitt equation for the minisuperspace of a minimally coupled scalar field in the spatially flat de Sitter Universe prompts the existence of an initial power-law evolution driven by non-adiabatic terms from the gravitational wavefunction which act like radiation. This simple model hence describes the onset of inflation from a previous radiation-like expansion during which the cosmological constant is already present but subleading.

gr-qc↗