arXiv subjects

R. C. Santos

Publications and source records attributed to R. C. Santos.

At least 19 recordsLinked to original sources

arXiv2026

Cosmic adiabatic photon creation: Temperature law and blackbody spectrum

The temperature-redshift relation (cosmic thermometer) is rediscussed in the framework of gravitationally induced photon production in the expanding universe. It is found that the same extended temperature law can be derived based on three independent methods, namely: (i) through a simple heuristic approach, (ii) based on macroscopic irreversible thermodynamics with photon creation, and (iii) using a kinetic relativistic Boltzmann equation incorporating gravitational photon creation. All these methods yield the same answer. The modified kinetic treatment reveals not only that the Planckian blackbody spectrum endowed with the adiabatic creation is preserved in the course of the expansion but also that CMB anisotropies and distortions can be treated properly. The new cosmic thermometer suggests a crucial test to the standard cosmic concordance model in the thermal sector. It is also argued that the present results may open a new route for scrutinising the H0 Supernovae-CMB tension challenging the LambdaCDM model.

physics.gen-ph↗

arXiv2026

YNiSn$_2$: A candidate Dirac semimetal

We report the synthesis and physical properties of the new compound YNiSn$_2$, which crystallizes in the orthorhombic \textit{Cmcm} structure. The material exhibits semimetallic behavior and develops a giant positive magnetoresistance approaching 1200\% at $B = 16$ T. Pronounced de Haas-van Alphen and Shubnikov-de Haas oscillations reveal a dominant quasi-two-dimensional Fermi surface with an exceptionally small cyclotron effective mass of $m^{*} = 0.08 m{0}$, indicating light carriers and a tiny Fermi surface pocket. The strong anisotropy revealed by Shubnikov-de Haas quantum oscillation measurements highlights the low-dimensional electronic character of YNiSn$_2$, positioning it as a promising Dirac semimetal candidate.

cond-mat.str-el↗

arXiv2025

From Bulk to Nanowire: A Dimensional Journey with CeIn$_3$

In this work, we have explored the Metallic-Flux Nanonucleation method to synthesize single crystals and nanowires (diameter $\approx$ 170 nm) of CeIn$_{3}$ and compare their properties. The effects of reduced dimensionality were systematically investigated using Energy Dispersive Spectroscopy (EDS), Selected area electron diffraction (SAED), magnetic susceptibility, heat capacity, and Nuclear Magnetic Resonance (NMR). Semi-quantitative EDS analysis revealed a Ce:In ratio of 1:3.1(1), and the SAED results confirmed that the nanowires are polycrystalline with a cubic unit cell. Magnetic susceptibility, specific heat, and NMR data indicated a suppression of the antiferromagnetic transition to $T_N$ $\approx$ 2.4 K compared to the bulk value ($\approx$ 10 K). Furthermore, NMR analysis at temperatures below 2.8 K showed a reduced quadrupole frequency, $ν_Q$ $\approx$ 1.77(2) MHz, and provided evidence of polycrystalline nanowires formed within the nanoporous alumina template, in agreement with SAED results. We attribute these findings to an increasing magnetic order frustration induced by dimensionality in CeIn$_{3}$ nanowires.

cond-mat.str-el↗

arXiv2024

From de Sitter to de Sitter: A Thermal Approach to Running Vacuum Cosmology and the Non-Canonical Scalar Field Description

The entire classical cosmological history between two extreme de Sitter vacuum solutions is discussed based on Einstein's equations and non-equilibrium thermodynamics. The initial non-singular de Sitter state is characterised by a very high energy scale which is equal or smaller than the reduced Planck mass. It is structurally unstable and all the continuous created matter, energy and entropy of the material component comes from the irreversible flow powered by the primeval vacuum energy density. The analytical expression describing the running vacuum is obtained from the thermal approach. It opens a new perspective to solve the old puzzles and current observational challenges plaguing the cosmic concordance model driven by a rigid vacuum. Such a scenario is also modelled through a non-canonical scalar field. It is demonstrated that the resulting scalar field model is shown to be step by step a faithful analytical representation of the thermal running vacuum cosmology.

astro-ph.CO↗

arXiv2019

From Solar Eclipse of 1919 to the Spectacle of Gravitational Lensing

A century after observing the deflection of light emitted by distant stars during the solar eclipse of 1919, it is interesting to know the concepts emerged from the experiment and the theoretical and observational consequences for modern cosmology and astrophysics. In addition to confirming Einstein's gravitational theory, its greatest legacy was the construction of a new research area to cosmos science dubbed gravitational lensing. The formation and magnification of multiple images (mirages) by the gravitational field of a compact or extended lens are among the most striking phenomena of nature. This article presents a pedagogical view of the first genuine gravitational lens effect, the double quasar QSO 0957 + 561. We also describe the formation of rings, giant arcs, arclets and multiple Supernova images. It is also surprising that the Hubble constant and the amount of dark matter in the Universe can be measured by the same technique. Finally, the lensing of gravitational waves, a possible but still not yet detected effect, is also briefly discussed.

physics.pop-ph↗

arXiv2018

From de Sitter to de Sitter: decaying vacuum models as a possible solution to the main cosmological problems

Decaying vacuum cosmological models evolving smoothly between two extreme (very early and late time) de Sitter phases are capable to solve or at least to alleviate some cosmological puzzles, among them: (i) the singularity, (ii) horizon, (iii) graceful-exit from inflation, and (iv) the baryogenesis problem. Our basic aim here is to discuss how the coincidence problem based on a large class of running vacuum cosmologies evolving from de Sitter to de Sitter can also be mollified. It is also argued that even the cosmological constant problem become less severe provided that the characteristic scales of the two limiting de Sitter manifolds are predicted from first principles.

gr-qc↗

arXiv2017

100 Years of Relativistic Cosmology (1917-2017). Part I: From Origins to the Discovery of Universal Expansion (1929)

We are experiencing a period of extreme intellectual effervescence in the area of cosmology. A huge volume of observational data in unprecedented quantity and quality and a more consistent theoretical framework propelled cosmology to an era of precision, turning the discipline into a cutting-edge area of contemporary science. Observations with type Ia Supernovae (SNe Ia), showed that the expanding Universe is accelerating, an unexplained fact in the traditional decelerated model. Identifying the cause of this acceleration is the most fundamental problem in the area. As in the scientific renaissance, the solution will guide the course of the discipline in the near future and the possible answers (whether dark energy, some extension of general relativity or a still unknown mechanism) should also leverage the development of physics. In this context, without giving up a pedagogical approach, we present an overview of both the main theoretical results and the most significant observational discoveries of cosmology in the last 100 years. The saga of cosmology will be presented in a trilogy. In this article (Part I), based on the articles by Einstein, de Sitter, Friedmann, Lemaître and Hubble, we will describe the period between the origins of cosmology and the discovery of Universal expansion (1929). In Part II, we will see the period from 1930 to 1997, closing with the old standard decelerated model. The Part III will be entirely devoted to the accelerated model of the universe, the cosmic paradigm of the XXI century.

physics.hist-ph↗

arXiv2016

Is $Λ$CDM an effective CCDM cosmology?

We show that a cosmology driven by gravitationally induced particle production of all non-relativistic species existing in the present Universe mimics exactly the observed flat accelerating $Λ$CDM cosmology with just one dynamical free parameter. This kind of scenario includes the creation cold dark matter (CCDM) model [Lima, Jesus & Oliveira, JCAP 011(2010)027] as a particular case and also provides a natural reduction of the dark sector since the vacuum component is not needed to accelerate the Universe. The new cosmic scenario is equivalent to $Λ$CDM both at the background and perturbative levels and the associated creation process is also in agreement with the universality of the gravitational interaction and equivalence principle. Implicitly, it also suggests that the present day astronomical observations cannot be considered the ultimate proof of cosmic vacuum effects in the evolved Universe because $Λ$CDM may be only an effective cosmology.

gr-qc↗

arXiv2014

Is the transition redshift a new cosmological number?

Observations from Supernovae Type Ia (SNe Ia) provided strong evidence for an expanding accelerating Universe at intermediate redshifts. This means that the Universe underwent a transition from deceleration to acceleration phases at a transition redshift $z_t$ of the order unity whose value in principle depends on the cosmology as well as on the assumed gravitational theory. Since cosmological accelerating models endowed with a transition redshift are extremely degenerated, in principle, it is interesting to know whether the value of $z_t$ itself can be observationally used as a new cosmic discriminator. After a brief discussion of the potential dynamic role played by the transition redshift, it is argued that future observations combining SNe Ia, the line-of-sight (or "radial") baryon acoustic oscillations, the differential age of galaxies, as well as the redshift drift of the spectral lines may tightly constrain $z_t$, thereby helping to narrow the parameter space for the most realistic models describing the accelerating Universe.

astro-ph.CO↗

arXiv2014

Studying light propagation in a locally homogeneous universe through an extended Dyer-Roeder approach

Light is affected by local inhomogeneities in its propagation, which may alter distances and so cosmological parameter estimation. In the era of precision cosmology, the presence of inhomogeneities may induce systematic errors if not properly accounted. In this vein, a new interpretation of the conventional Dyer-Roeder (DR) approach by allowing light received from distant sources to travel in regions denser than average is proposed. It is argued that the existence of a distribution of small and moderate cosmic voids (or "black regions") implies that its matter content was redistributed to the homogeneous and clustered matter components with the former becoming denser than the cosmic average in the absence of voids. Phenomenologically, this means that the DR smoothness parameter (denoted here by $α_E$) can be greater than unity, and, therefore, all previous analyses constraining it should be rediscussed with a free upper limit. Accordingly, by performing a statistical analysis involving 557 type Ia supernovae (SNe Ia) from Union2 compilation data in a flat $Λ$CDM model we obtain for the extended parameter, $α_E=1.26^{+0.68}_{-0.54}$ ($1σ$). The effects of $α_E$ are also analyzed for generic $Λ$CDM models and flat XCDM cosmologies. For both models, we find that a value of $α_E$ greater than unity is able to harmonize SNe Ia and cosmic microwave background observations thereby alleviating the well-known tension between low and high redshift data. Finally, a simple toy model based on the existence of cosmic voids is proposed in order to justify why $α_E$ can be greater than unity as required by supernovae data.

astro-ph.CO↗

arXiv2013

A note on linearized "New Massive Gravity" in arbitrary dimensions

By means of a triple master action we deduce here a linearized version of the "New Massive Gravity" (NMG) in arbitrary dimensions. The theory contains a 4th-order and a 2nd-order term in derivatives. The 4th-order term is invariant under a generalized Weyl symmetry. The action is formulated in terms of a traceless $η^{μν}Ω_{μνρ}=0$ mixed symmetry tensor $Ω_{μνρ}=-Ω_{μρν}$ and corresponds to the massive Fierz-Pauli action with the replacement $e_{μν}=\p^ρΩ_{μνρ}$. The linearized 3D and 4D NMG theories are recovered via the invertible maps $Ω_{μνρ} = ε_{νρ}^{\quadβ}h_{βμ} $ and $Ω_{μνρ} = ε_{νρ}^{\quad γδ}T_{[γδ]μ} $ respectively. The properties $h_{μν}=h_{νμ}$ and $T_{[[γδ]μ]}=0$ follow from the traceless restriction. The equations of motion of the linearized NMG theory can be written as zero "curvature" conditions $\p_νT_{ρμ} - \p_ρT_{νμ}=0$ in arbitrary dimensions.

hep-th↗

arXiv2012

Constraining the dark energy and smoothness parameter with type Ia Supernovae and Gamma-Ray Bursts

The existence of inhomogeneities in the observed Universe modifies the distance-redshift relations thereby affecting the results of cosmological tests in comparison to the ones derived assuming spatially uniform models. By modeling the inhomogeneities through a Zeldovich-Kantowski-Dyer-Roeder (ZKDR) approach which is phenomenologically characterized by a smoothness parameter $α$, we rediscuss the constraints on the cosmic parameters based on Supernovae type Ia and Gamma-Ray Bursts (GRBs) data. The present analysis is restricted to a flat $Λ$CDM model with the reasonable assumption that $Λ$ does not clump. A $χ^{2}$-analysis using 557 SNe Ia data from the Union2 Compilation Data (Amanullah {\it et al.} 2010) constrains the pair of parameters ($Ω_m, α$) to $Ω_m=0.27_{-0.03}^{+0.08}$($2σ$) and $α\geq 0.25$. A similar analysis based only on 59 Hymnium GRBs (Wei 2010) constrains the matter density parameter to be $Ω_m= 0.35^{+0.62}_{-0.24}$ ($2σ$) while all values for the smoothness parameter are allowed. By performing a joint analysis, it is found that $Ω_m = 0.27^{+0.06}_{-0.03}$ and $α\geq 0.52$. As a general result, although considering that current GRB data alone cannot constrain the smoothness $α$ parameter our analysis provides an interesting cosmological probe for dark energy even in the presence of inhomogeneities.

astro-ph.CO↗

arXiv2011

Spin-1 duality in $D$-dimensions

It is known that the Maxwell theory in $D$ dimensions can be written in a first order form (in derivatives) by introducing a totally antisymmetric field which leads to a $(D-3)$-form dual theory. Remarkably, one can replace the antisymmetric field by a symmetric rank two tensor ($W_{μν}=W_{νμ}$). Such master action establishes the duality between the Maxwell-theory and a fourth order higher rank model in a $D$-dimensional flat space time. A naive generalization to the curved space shows a connection between the recently found D=4 critical gravity and the Maxwell theory plus a coupling term to the Ricci tensor ($R_{μν}A^μA^ν$). The mass of the spin-1 particle which appears in the D=4 critical gravity linearized around anti-de Sitter space is the same one obtained from the Ricci coupling term. We also work out, in flat space time, the explicitly massive case (Maxwell-Proca) which is dual to a second order theory for $W_{μν}$.

hep-th↗

arXiv2011

Accelerating Cosmology Driven by Gravitational Production of Dark Matter Particles: Constraints from SNe Ia and H(z) data

The free parameters of a flat accelerating model without dark energy are constrained by using Supernovae type Ia and observational H(z) data. Instead of the vacuum dominance, the present accelerating stage in this modified Einstein-de Sitter cosmology is a consequence of the gravitationally-induced particle production of cold dark matter. The model present a transition from a decelerating to an accelerating regime at low redshifts, and is also able to harmonize a cold dark matter picture with the latest measurements of the Hubble constant H_0, the Supernovae observations (Constitution sample), and the H(z) data.

astro-ph.CO↗

arXiv2011

Comment on "Constraining the smoothness parameter and dark energy using observational H(z) data"

In this Comment we discuss a recent analysis by Yu et al. [RAA 11, 125 (2011)] about constraints on the smoothness $α$ parameter and dark energy models using observational $H(z)$ data. It is argued here that their procedure is conceptually inconsistent with the basic assumptions underlying the adopted Dyer-Roeder approach. In order to properly quantify the influence of the $H(z)$ data on the smoothness $α$ parameter, a $χ^2$-test involving a sample of SNe Ia and $H(z)$ data in the context of a flat $Λ$CDM model is reanalyzed. This result is confronted with an earlier approach discussed by Santos et al. (2008) without $H(z)$ data. In the ($Ω_m, α$) plane, it is found that such parameters are now restricted on the intervals $0.66 \leq α\leq 1.0$ and $0.27 \leq Ω_m \leq 0.37$ within 95.4% confidence level (2$σ$), and, therefore, fully compatible with the homogeneous case. The basic conclusion is that a joint analysis involving $H(z)$ data can indirectly improve our knowledge about the influence of the inhomogeneities. However, this happens only because the $H(z)$ data provide tighter constraints on the matter density parameter $Ω_m$.

astro-ph.CO↗

arXiv2008

An Accelerating Cosmology Without Dark Energy

The negative pressure accompanying gravitationally-induced particle creation can lead to a cold dark matter (CDM) dominated, accelerating Universe (Lima et al. 1996) without requiring the presence of dark energy or a cosmological constant. In a recent study Lima et al. (2008, LSS) demonstrated that particle creation driven cosmological models are capable of accounting for the SNIa observations of the recent transition from a decelerating to an accelerating Universe. Here we test the evolution of such models at high redshift using the constraint on z_eq, the redshift of the epoch of matter radiation equality, provided by the WMAP constraints on the early Integrated Sachs-Wolfe effect. Since the contribution of baryons and radiation was ignored in the work of LSS, we include them in our study of this class of models. The parameters of these more realistic models with continuous creation of CDM is tested and constrained at widely-separated epochs (z = z_eq and z = 0) in the evolution of the Universe. This comparison reveals a tension between the high redshift CMB constraint on z_eq and that which follows from the low redshift SNIa data, challenging the viability of this class of models.

astro-ph↗

arXiv2008

New coupled quintessence cosmology

A component of dark energy has been recently proposed to explain the current acceleration of the Universe. Unless some unknown symmetry in Nature prevents or suppresses it, such a field may interact with the pressureless component of dark matter, giving rise to the so-called models of coupled quintessence. In this paper we propose a new cosmological scenario where radiation and baryons are conserved, while the dark energy component is decaying into cold dark matter (CDM). The dilution of CDM particles, attenuated with respect to the usual $a^{-3}$ scaling due to the interacting process, is characterized by a positive parameter $ε$, whereas the dark energy satisfies the equation of state $p_x=ωρ_x$ ($ω< 0$). We carry out a joint statistical analysis involving recent observations from type Ia supernovae, baryon acoustic oscillation peak, and Cosmic Microwave Background shift parameter to check the observational viability of the coupled quintessence scenario here proposed.

astro-ph↗

arXiv2008

Accelerating Cold Dark Matter Cosmology ($Ω_Λ\equiv 0$)

A new kind of accelerating flat model with no dark energy that is fully dominated by cold dark matter (CDM) is investigated. The number of CDM particles is not conserved and the present accelerating stage is a consequence of the negative pressure describing the irreversible process of gravitational particle creation. A related work involving accelerating CDM cosmology has been discussed before the SNe observations [Lima, Abramo & Germano, Phys. Rev. D53, 4287 (1996)]. However, in order to have a transition from a decelerating to an accelerating regime at low redshifts, the matter creation rate proposed here includes a constant term of the order of the Hubble parameter. In this case, $H_0$ does not need to be small in order to solve the age problem and the transition happens even if the matter creation is negligible during the radiation and part of the matter dominated phase. Therefore, instead of the vacuum dominance at redshifts of the order of a few, the present accelerating stage in this sort of Einstein-de Sitter CDM cosmology is a consequence of the gravitational particle creation process. As an extra bonus, in the present scenario does not exist the coincidence problem that plagues models with dominance of dark energy. The model is able to harmonize a CDM picture with the present age of the universe, the latest measurements of the Hubble parameter and the Supernovae observations.

astro-ph↗