arXiv subjects

Reginald Christian Bernardo

Publications and source records attributed to Reginald Christian Bernardo.

44 records · Page 3Linked to original sources

arXiv2021

Towards a model-independent reconstruction approach for late-time Hubble data

Gaussian processes offers a convenient way to perform nonparametric reconstructions of observational data assuming only a kernel which describes the covariance between neighbouring points in a data set. We approach the ambiguity in the choice of kernel in Gaussian processes with two methods -- (a) approximate Bayesian computation with sequential Monte Carlo sampling and (b) genetic algorithm -- and use the overall resulting method to reconstruct the cosmic chronometers and supernovae type Ia data sets. The results have shown that the Matérn$\left( ν= 5/2 \right)$ kernel emerges on top of the two-hyperparameter family of kernels for both cosmological data sets. On the other hand, we use the genetic algorithm in order to select a most naturally-fit kernel among a competitive pool made up of a ten-hyperparameters class of kernels. Imposing a Bayesian information criterion-inspired measure of the fitness, the results have shown that a hybrid of the Radial Basis Function and the Matérn$\left( ν= 5/2 \right)$ kernel best represented both data sets. The kernel selection problem is not totally closed and may benefit from further analysis using other strategies to resolve an optimal kernel for a particular data set.

astro-ph.CO↗

arXiv2021

Stealth black hole perturbations in kinetic gravity braiding

We study stealth black hole perturbations in shift symmetric kinetic gravity braiding and obtain its analogous Regge-Wheeler and Zerilli master equations for the odd and even parity sectors. We show that the nontrivial hair of static and spherically symmetric stealth black holes contributes only an additional source term to the even parity master equation. Furthermore, we derive exact solutions to the monopolar and dipolar perturbations and show that they are generally pathological non-gauge modes, or else reduce to the pure-gauge low-order multipoles of general relativity.

gr-qc↗

arXiv2021

Gravitational wave signatures from dark sector interactions

We show that the gravitational waves generated by the perturbations of general relativistic black holes can be considered as a direct probe of the existence of dark sector interactions. Working within the framework of Horndeski theory and linear perturbations, we show that dark sector interactions effectively reduce to an interaction charge that influences both scalar and tensor waveforms. Furthermore, we show that the total dark matter field, including the effects of dark sector interactions, satisfies a conservation equation embodying the equivalence principle. We exploit this realization to setup the Regge-Wheeler equation and the coupled Zerilli and scalar wave equations for a Schwarzschild-(anti) de Sitter black hole. We then present numerical integration of the coupled even-parity wave equations for the case of a dark matter particle falling straight down into a Schwarzschild black hole.

gr-qc↗

arXiv2021

Self-tuning kinetic gravity braiding: Cosmological dynamics, shift symmetry, and the tadpole

We study the self-tuning subclass of kinetic gravity braiding and obtain robust predictions on self-tuning and dynamics in the tadpole-free shift symmetric sector of the theory. In particular, we show inevitability of cosmic acceleration, prove the dynamical stability of this late-time asymptotic state, and derive ghost and gradient stability constraints on the self-tuning vacuum. We discuss the results concretely in the context of generalized cubic covariant Galileon theory and an exponential kinetic gravity braiding.

gr-qc↗

arXiv2019

Stealth black holes in shift symmetric kinetic gravity braiding

We derive all hairy stealth black holes in the most general second-order, shift symmetric, scalar-tensor theory with luminally propagating gravitational waves, often called kinetic gravity braiding. Our approach exploits an overlooked loophole in a recently obtained no-go statement which claims shift symmetry breaking to be necessary for stealth solutions to exist in kinetic gravity braiding. We highlight the essential role played by a covariantly constant kinetic density in obtaining these solutions. Lastly, we propose a parametrization of the theories based on the asymptotics of its stealth solutions and comment on the intriguing singular effective metric for scalar perturbations in stealth black holes.

gr-qc↗

arXiv2019

Tailoring cosmologies in cubic shift-symmetric Horndeski gravity

We present a method for furnishing flat Friedman-Robertson-Walker spacetimes with nearly arbitrary dynamics in an important subclass of cubic Horndeski theory -- specifically shift-symmetric, cubic Horndeski theory with a vanishing conserved current. This builds on insight from previous work on the construction of static and spherically-symmetric hairy spacetimes in the same sector. The method is explicitly demonstrated by deriving exact analytical solutions describing an inflating universe and several power-law expansion scenarios, and by showing how the predicted evolution of the Hubble parameter in $Λ$CDM can be fit to a particular choice of Horndeski model function. We fully characterize the classes of cosmological models that cannot be generated purely by selecting a Horndeski model function.

gr-qc↗

arXiv2019

Hair-dressing Horndeski: an approach to hairy solutions in cubic Horndeski gravity

In obtaining exact solutions in gravitational theories containing arbitrary model functions, such as Horndeski gravity, one usually starts by prescribing the model functions of the theory and then goes on to solving their corresponding field equations. In this paper, we explore the extent to which the reverse procedure can be useful, whereby one starts with desired solutions and then determines the models that support them. Working within the phenomenologically interesting cubic and shift-symmetric sector of Horndeski gravity, we develop a method for obtaining exact static and spherically-symmetric solutions, one of which happens to be a new hairy black hole. We study this black hole and its properties. We also discuss the limitations of the method and its possible extension to other Horndeski sectors.

gr-qc↗

arXiv2016

Exactly solvable dynamical models with a minimal length uncertainty

We present exact analytical solutions to the classical equations of motion and analyze the dynamical consequences of the existence of a minimal length for the free particle, particle in a linear potential, anti-symmetric constant force oscillator, harmonic oscillator, vertical harmonic oscillator, linear diatomic chain, and linear triatomic chain. It turns out that a minimal length increases the speed of a free particle and the rate of fall of a particle that is subject to the influence of a linear potential. Our results suggest that the characteristic frequency of systems tend to increase when there is a minimal length. This is a common feature that we observed for the oscillator systems that we have considered.

hep-th↗