arXiv subjects

M. J. Reboucas

Publications and source records attributed to M. J. Reboucas.

At least 55 records · Page 3Linked to original sources

arXiv2007

Energy conditions in f(R)-gravity

In order to shed some light on the current discussion about f(R)-gravity theories we derive and discuss the bounds imposed by the energy conditions on a general f(R) functional form. The null and strong energy conditions in this framework are derived from the Raychaudhuri's equation along with the requirement that gravity is attractive, whereas the weak and dominant energy conditions are stated from a comparison with the energy conditions that can be obtained in a direct approach via an effective energy-momentum tensor for f(R)-gravity. As a concrete application of the energy conditions to locally homogeneous and isotropic f(R)-cosmology, the recent estimated values of the deceleration and jerk parameters are used to examine the bounds from the weak energy condition on the parameters of two families of f(R)-gravity theories.

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arXiv2007

Lookback time bounds from energy conditions

In general relativity, the energy conditions are invoked to restrict general energy-momentum tensors on physical grounds. We show that in the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) approach to cosmological modeling, where the energy and matter components of the cosmic fluid are unknown, the energy conditions provide model-independent bounds on the behavior of the lookback time of cosmic sources as a function of the redshift for any value of the spatial curvature. We also confront such bounds with a lookback time sample which is built from the age estimates of 32 galaxies lying in the interval $0.11 \lesssim z \lesssim 1.84$ and by assuming the total expanding age of the Universe to be $13.7 \pm 0.2$ Gyr, as obtained from current cosmic microwave background experiments. In agreement with previous results, we show that all energy conditions seem to have been violated at some point of the recent past of cosmic evolution.

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arXiv2007

A note on the large-angle anisotropies in the WMAP cut-sky maps

Recent analyses of the WMAP data seem to indicate the possible presence of large-angle anisotropy in the Universe. If confirmed, these can have important consequences for our understanding of the Universe. A number of attempts have recently been made to establish the reality and nature of such anisotropies in the CMB data. Among these is a directional indicator recently proposed by the authors. A distinctive feature of this indicator is that it can be used to generate a sky map of the large-scale anisotropies of the CMB maps. Applying this indicator to full-sky temperature maps we found a statistically significant preferred direction. The full-sky maps used in these analyses are known to have residual foreground contamination as well as complicated noise properties. Thus, here we performed the same analysis for a map where regions with high foreground contamination were removed. We find that the main feature of the full-sky analysis, namely the presence of a significant axis of asymmetry, is robust with respect to this masking procedure. Other subtler anomalies of the full-sky are on the other hand no longer present.

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arXiv2007

Energy Conditions and Cosmic Acceleration

In general relativity, the energy conditions are invoked to restrict general energy-momentum tensors $T_{μν}$ in different frameworks, and to derive general results that hold in a variety of general contexts on physical grounds. We show that in the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) approach, where the equation of state of the cosmological fluid is unknown, the energy conditions provide model-independent bounds on the behavior of the distance modulus of cosmic sources as a function of the redshift for any spatial curvature. We use the most recent type Ia supernovae (SNe Ia) observations, which include the new Hubble Space Telescope SNe Ia events, to carry out a model-independent analysis of the energy conditions violation in the context of the standard cosmology. We show that both the null (NEC), weak (WEC) and dominant (DEC) conditions, which are associated with the existence of the so-called phantom fields, seem to have been violated only recently ($z \lesssim 0.2$), whereas the condition for attractive gravity, i.e., the strong energy condition (SEC) was firstly violated billions of years ago, at $z \gtrsim 1$.

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arXiv2007

Supernovae Constraints on DGP Model and Cosmic Topology

We study the constraints that the detection of a non-trivial spatial topology may place on the parameters of braneworld models by considering the Dvali-Gabadadze-Porrati (DGP) and the globally homogeneous Poincaré dodecahedral spatial (PDS) topology as a circles-in-the-sky observable topology. To this end we reanalyze the type Ia supernovae constraints on the parameters of the DGP model and show that PDS topology gives rise to strong and complementary constraints on the parameters of the DGP model.

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arXiv2007

Supernovae Constraints on Cosmological Density Parameter and Cosmic Topology

We illustrate the constraints that a possible detection of a non-trivial spatial topology may place on the cosmological density parameters by considering the $Λ$CDM model Poincaré dodecahedal space (PDS) topology as a circles-in-the-sky detectable topology. To this end we reanalyze the type Ia supernovae constraints on the density parameter plane $Ω_k$--$Ω_Λ$ and show that a circles-in-the-sky detectable PDS topology gives rise to important constraints on this parameters plane.

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arXiv2006

Mapping the large-scale anisotropy in the WMAP data

Analyses of recent cosmic microwave background (CMB) observations have provided increasing indications for the existence of large scale anisotropy in the universe. Given the far reaching consequences of such an anisotropy for our understanding of the universe, it is important to employ alternative indicators in order to determine whether the reported anisotropy is cosmological in origin, and if so extract further information that may be helpful for identifying its causes. Here we propose a new directional indicator, based on angular-separation histograms of pairs of pixels with similar temperatures in the CMB map, which provides a measure of departure from statistical isotropy. The main advantage of this indicator is that it can be used to generate a sky map of large-scale anisotropies in the CMB temperature map, thus allowing a possible additional window into their causes. Using this indicator, we find a statistically significant (at 95% CL) preferred direction in the CMB data and discuss how it compares with other such axes recently reported. We also show that our findings are robust with respect to both the details of the method used, and the choice of the WMAP CMB maps employed, including the three-year CMB data released recently.

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arXiv2006

Energy Conditions and Supernovae Observations

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arXiv2006

Constraints on the cosmological density parameters and cosmic topology

A nontrivial topology of the spatial section of the universe is an observable, which can be probed for all locally homogeneous and isotropic universes, without any assumption on the cosmological density parameters. We discuss how one can use this observable to set constraints on the density parameters of the Universe by using a specific spatial topology along with type Ia supenovae and X-ray gas mass fraction data sets.

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arXiv2006

Supernovae observations and cosmic topology

Two fundamental questions regarding our description of the Universe concern the geometry and topology of its 3-dimensional space. While geometry is a local characteristic that gives the intrinsic curvature, topology is a global feature that characterizes the shape and size of the 3-space. The geometry constrains, but does not dictate the the spatial topology. We show that, besides determining the spatial geometry, the knowledge of the spatial topology allows to place tight constraints on the density parameters associated with dark matter ($Ω_m$) and dark energy ($Ω_Λ$). By using the Poincaré dodecahedral space as the observable spatial topology, we reanalyze the current type Ia supenovae (SNe Ia) constraints on the density parametric space $Ω_{m} - Ω_Λ$. From this SNe Ia plus cosmic topology analysis, we found best fit values for the density parameters, which are in agreement with a number of independent cosmological observations.

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arXiv2006

A note on cosmological parameters and the topology of the universe

Given the wealth of increasingly accurate cosmological observations, especially the recent results from the WMAP, and the development of methods and strategies in the search for cosmic topology, it is reasonable to expect that we should be able to detect the spatial topology of the Universe in the near future. Motivated by this, we examine to what extent a possible detection of a nontrivial topology of positively curved universe may be used to place constraints on the matter content of the Universe. We show through concrete examples that the knowledge of the spatial topology allows to place constraints on the density parameters associated to dark matter ($Ω_m$) and dark energy ($Ω_Λ$)

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arXiv2006

Cosmological Parameters and Cosmic Topology

Geometry constrains but does not dictate the topology of the 3--dimensional space. In a locally spatially homogeneous and isotropic universe, however, the topology of its spatial section dictates its geometry. We show that, besides determining the geometry, the knowledge of the spatial topology through the circles--in--the--sky offers an effective way of setting constraints on the density parameters associated with dark matter ($Ω_m$) and dark energy ($Ω_Λ$). By assuming the Poincaré dodecahedral space as the circles--in--the--sky detectable topology of the spatial sections of the Universe, we re-analyze the constraints on the density parametric plane $Ω_{m}$--$ Ω_Λ $ from the current type Ia supenovae (SNe Ia) plus X-ray gas mass fraction data, and show that a circles--in--the sky detection of the dodecahedral space topology give rise to strong and complementary constraints on the region of the density parameter plane currently allowed by these observational data sets.

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arXiv2005

Constraints from the Detection of Cosmic Topology on the Generalized Chaplygin Gas

Despite our present-day inability to predict the topology of the universe one may expect that we should be able to detect it in the near future, given the increasing accuracy in the astro-cosmological observations. Motivated by this, we examine to what extent a possible detection of a non-trivial topology of a low curvature (Omega_0 ~ 1) universe, suggested by a diverse set of current observations, may be used to place constraints on the matter content of the universe, focusing our attention on the generalized Chaplgygin gas (GCG) model, which unifies dark matter and dark energy in a single matter component. We show that besides constraining the GCG free parameters, the detection of a nontrivial topology also allows to set bounds on the total density parameter Omega_0. We also study the combination of the bounds from the topology detection with the limits that arise from current data on 194 SNIa, and show that the determination of a given nontrivial topology sets complementary bounds on the GCG parameters (and on Omega_0) to those obtained from the SNIa data.

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arXiv2005

Detectability of Cosmic Topology in Generalized Chaplygin Gas Models

If the spatial section of the universe is multiply connected, repeated images or patterns are expected to be detected observationally. However, due to the finite distance to the last scattering surface, such pattern repetitions could be unobservable. This raises the question of whether a given cosmic topology is detectable, depending on the values of the parameters of the cosmological model. We study how detectability is affected by the choice of the model itself for the matter-energy content of the universe, focusing our attention on the generalized Chaplygin gas (GCG) model for dark matter and dark energy unification, and investigate how the detectability of cosmic topology depends on the GCG parameters. We determine to what extent a number of topologies are detectable for the current observational bounds on these parameters. It emerges from our results that the choice of GCG as an alternative to the $Λ$CDM matter-energy content model has an impact on the detectability of cosmic topology.

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arXiv2005

A brief introduction to cosmic topology

Whether we live in a spatially finite universe, and what its shape and size may be, are among the fundamental long-standing questions in cosmology. These questions of topological nature have become particularly topical, given the wealth of increasingly accurate astro-cosmological observations, especially the recent observations of the cosmic microwave background radiation. An overview of the basic context of cosmic topology, the detectability constraints from recent observations, as well as the main methods for its detection and some recent results are briefly presented.

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arXiv2005

The Local Shape of the Universe in the Inflationary Limit

Recent high precision data by WMAP and SDSS have provided strong evidence to suggest that the universe is nearly flat. They are also making it possible to probe the topology of the universe. Motivated by these results, we have recently studied the consequences of taking the inflationary limit, i.e. $|Ω_0 - 1| << 1$. We have shown that in this limit a generic detectable spherical or hyperbolic topology is locally indistinguishable from either $R^2 x S^1$ or $R x T^2$, irrespective of its global shape. Here we briefly present these results and further discuss their observational implications.

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arXiv2004

Relativistic Effects of our Galaxy's Motion on Circles-in-the-sky

We study the geometric effects of our galaxy's peculiar motion on the circles-in-the-sky. We show that the shape of these circles-in-the-sky remains circular, as detected by a local observer with arbitrary peculiar velocity. Explicit expressions for the radius and center position of such an observed circle-in-the-sky, as well as for the angular displacement of points on the circle, are derived. In general, a circle is detected as a circle of different radius, displaced relative to its original position, and centered at a point which does not correspond to its detected center in the comoving frame. Further, there is an angular displacement of points on the circles. These effects all arise from aberration of cosmic microwave background radiation, exhausting the purely geometric effects due to the peculiar motion of our galaxy, and are independent of both the large scale curvature of space and the expansion of the universe, since aberration is a purely local phenomenon. For a Lorentz-boosted observer with the speed of our entire galaxy, the maximum (detectable) changes in the angular radius of a circle, its maximum center displacement, as well as the maximum angular distortion are shown all to be of order $β=(v/c)$ radians. In particular, two back-to-back matching circles in a finite universe will have an upper bound of $2|β|$ in the variation of either their radii, the angular position of their centers, or the angular distribution of points.

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arXiv2004

A Note on the Robustness of Pair Separations Methods in Cosmic Topology

The pair separations statistical methods devised to detect the topology of the universe rely on the accurate knowledge of the three-dimensional positions of the cosmic sources. The determination of these positions, however, involves inevitable observational uncertainties. The only significant (measurable) sign of a nontrivial topology in PSH's was shown to be spikes. We briefly report our results concerning the sensitivity of the topological spikes in the presence of the uncertainties in the positions of the cosmic sources, which arise from uncertainties in the values of the density parameters.

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