arXiv subjects

S. H. Pereira

Publications and source records attributed to S. H. Pereira.

63 records · Page 4Linked to original sources

arXiv2009

Can Dark Matter Decay in Dark Energy?

We analyze the interaction between Dark Energy and Dark Matter from a thermodynamical perspective. By assuming they have different temperatures, we study the possibility of occurring a decay from Dark Matter into Dark Energy, characterized by a negative parameter $Q$. We find that, if at least one of the fluids has non vanishing chemical potential, for instance $μ_x<0$ and $μ_{dm}=0$ or $μ_x=0$ and $μ_{dm}>0$, the decay is possible, where $μ_x$ and $μ_{dm}$ are the chemical potentials of Dark Energy and Dark Matter, respectively. Using recent cosmological data, we find that, for a fairly simple interaction, the Dark Matter decay is favored with a probability of $\sim 93%$ over the Dark Energy decay. This result comes from a likelihood analysis where only background evolution has been considered.

astro-ph↗

arXiv2009

Hard thermal loops in static external fields

We study, in the imaginary-time formalism, the high temperature behavior of n-point thermal loops in static Yang-Mills and gravitational fields. We show that in this regime, any hard thermal loop gives the same leading contribution as the one obtained by evaluating the loop at zero external energies and momenta.

hep-th↗

arXiv2008

On Phantom Thermodynamics

The thermodynamic properties of dark energy fluids described by an equation of state parameter $ω=p/ρ$ are rediscussed in the context of FRW type geometries. Contrarily to previous claims, it is argued here that the phantom regime $ω<-1$ is not physically possible since that both the temperature and the entropy of every physical fluids must be always positive definite. This means that one cannot appeal to negative temperature in order to save the phantom dark energy hypothesis as has been recently done in the literature. Such a result remains true as long as the chemical potential is zero. However, if the phantom fluid is endowed with a non-null chemical potential, the phantom field hypothesis becomes thermodynamically consistent, that is, there are macroscopic equilibrium states with $T>0$ and $S>0$ in the course of the Universe expansion. Further, the negative value of the chemical potential resulting from the entropy constraint ($S>0$) suggests a bosonic massless nature to the phantom particles.

gr-qc↗

arXiv2008

Phantom Accretion by Black Holes and the Generalized Second Law of Thermodynamics

The accretion of a phantom fluid with non-zero chemical potential by black holes is discussed with basis on the Generalized Second Law of thermodynamics. For phantom fluids with positive temperature and negative chemical potential we demonstrate that the accretion process is possible, and that the condition guaranteeing the positiveness of the phantom fluid entropy coincides with the one required by Generalized Second Law. In particular, this result provides a complementary confirmation that cosmological phantom fluids do not need to have negative temperatures.

gr-qc↗

arXiv2008

On the Chemical Potential of Dark Energy

It is widely assumed that the observed universe is accelerating due to the existence of a new fluid component called dark energy. In this article, the thermodynamics consequences of a nonzero chemical potential on the dark energy component is discussed with special emphasis to the phantom fluid case. It is found that if the dark energy fluid is endowed with a negative chemical potential, the phantom field hypothesis becomes thermodynamically consistent with no need of negative temperatures as recently assumed in the literature.

astro-ph↗

arXiv2008

Chemical Potential and the Nature of the Dark Energy: The case of phantom

The influence of a possible non zero chemical potential $μ$ on the nature of dark energy is investigated by assuming that the dark energy is a relativistic perfect simple fluid obeying the equation of state (EoS), $p=ωρ$ ($ω<0, constant$). The entropy condition, $S \geq 0$, implies that the possible values of $ω$ are heavily dependent on the magnitude, as well as on the sign of the chemical potential. For $μ>0$, the $ω$-parameter must be greater than -1 (vacuum is forbidden) while for $μ< 0$ not only the vacuum but even a phantomlike behavior ($ω<-1$) is allowed. In any case, the ratio between the chemical potential and temperature remains constant, that is, $μ/T=μ_0/T_0$. Assuming that the dark energy constituents have either a bosonic or fermionic nature, the general form of the spectrum is also proposed. For bosons $μ$ is always negative and the extended Wien's law allows only a dark component with $ω< -1/2$ which includes vacuum and the phantomlike cases. The same happens in the fermionic branch for $μ<0$. However, fermionic particles with $μ>0$ are permmited only if $-1 < ω< -1/2$. The thermodynamics and statistical arguments constrain the EoS parameter to be $ω< -1/2$, a result surprisingly close to the maximal value required to accelerate a FRW type universe dominated by matter and dark energy ($ω\lesssim -10/21$).

astro-ph↗

arXiv2005

Quantum behavior of a charged particle in an axial magnetic field

We study some aspects of the quantum theory of a charged particle moving in a time-independent, uni-directional magnetic field. When the field is uniform, we make a few clarifying remarks on the use of angular momentum eigenstates and momentum eigenstates with the diamagnetism of a free electron gas as an example. When the field is non-uniform but weakly varying, we discuss both perturbative and non-perturbative methods for studying a quantum mechanical system. As an application, we derive the quantized energy levels of a charged particle in a Helmholtz coil, which go over to the usual Landau levels in the limit of a uniform field.

hep-th↗

arXiv2004

Coordinate noncommutativity in strong non-uniform magnetic fields

Noncommuting spatial coordinates are studied in the context of a charged particle moving in a strong non-uniform magnetic field. We derive a relation involving the commutators of the coordinates, which generalizes the one realized in a strong constant magnetic field. As an application, we discuss the noncommutativity in the magnetic field present in a magnetic mirror.

hep-th↗

arXiv2003

Background field quantization and non-commutative Maxwell theory

We quantize non-commutative Maxwell theory canonically in the background field gauge for weak and slowly varying background fields. We determine the complete basis for expansion under such an approximation. As an application, we derive the Wigner function which determines the leading order high temperature behavior of the perturbative amplitudes of non-commutative Maxwell theory. To leading order, we also give a closed form expression for the distribution function for the non-commutative $U (1)$ gauge theory at high temperature.

hep-th↗