Search results for " Cosmology"
showing 10 items of 1486 documents
A Precise Photometric Ratio via Laser Excitation of the Sodium Layer II: Two-photon Excitation Using Lasers Detuned from 589.16 nm and 819.71 nm Reso…
2020
This article is the second in a pair of articles on the topic of the generation of a two-color artificial star (which we term a "laser photometric ratio star," or LPRS) of de-excitation light from neutral sodium atoms in the mesosphere, for use in precision telescopic measurements in astronomy and atmospheric physics, and more specifically for the calibration of measurements of dark energy using type Ia supernovae. The two techniques respectively described in both this and the previous article would each generate an LPRS with a precisely 1:1 ratio of yellow (589/590 nm) photons to near-infrared (819/820 nm) photons produced in the mesosphere. Both techniques would provide novel mechanisms f…
Cosmic Dark Radiation and Neutrinos
2013
New measurements of the cosmic microwave background (CMB) by the Planck mission have greatly increased our knowledge about the universe. Dark radiation, a weakly interacting component of radiation, is one of the important ingredients in our cosmological model which is testable by Planck and other observational probes. At the moment, the possible existence of dark radiation is an unsolved question. For instance, the discrepancy between the value of the Hubble constant, H-0, inferred from the Planck data and local measurements of H-0 can to some extent be alleviated by enlarging the minimal ACDM model to include additional relativistic degrees of freedom. From a fundamental physics point of v…
Uncertainty on w from large-scale structure
2012
We find that if we live at the center of an inhomogeneity with total density contrast of roughly 0.1, dark energy is not a cosmological constant at 95% confidence level. Observational constraints on the equation of state of dark energy, w, depend strongly on the local matter density around the observer. We model the local inhomogeneity with an exact spherically symmetric solution which features a pressureless matter component and a dark-energy fluid with constant equation of state and negligible sound speed, that reaches a homogeneous solution at finite radius. We fit this model to observations of the local expansion rate, distant supernovae and the cosmic microwave background. We conclude …
Metaphysics of cosmological models
2023
This article aims to address the metaphysical dimension of cosmological models, be they mythological, philosophical, religious or modern scientific models, using multidisciplinary and transdisciplinary methodology. Such an approach is a novelty, both in the theological field and in the philosophical field and secular sciences, which studies the origin of humans and the universe. Contribution: The originality of this article consists in introducing the concept of transcendental cosmology, which, along with spiritual cosmology, can be a serious theological and philosophical reply to the Bing Bang theory.
Black Holes in Extended Gravity Theories in Palatini Formalism
2013
We consider several physical scenarios where black holes within classical gravity theories including R 2 and Ricci-squared corrections and formulated a la Palatini can be analytically studied.
Non-supersymmetric Extremal Black Holes: First-Order Flows and Stabilisation Equations
2013
We review the results of [1, 2] on reducing the second-order equations of motion for stationary extremal black holes in four-dimensional \({\textit{N}}\,=\,2\) supergravity to first-order flow equations and further to non-differential stabilisation equations.
Excision technique in constrained formulations of Einstein equations: collapse scenario
2015
We present a new excision technique used in constrained formulations of Einstein equations to deal with black hole in numerical simulations. We show the applicability of this scheme in several scenarios. In particular, we present the dynamical evolution of the collapse of a neutron star to a black hole, using the CoCoNuT code and this excision technique.
Numerical Simulations of Relativistic Wind Accretion onto Black Holes Using Godunov-Type Methods
2001
We have studied numerically the so-called Bondi-Hoyle (wind) accretion onto a rotating black hole in general relativity. We have used the Kerr-Schild form of the Kerr metric, free of coordinate singularities at the black hole horizon. The ‘test-fluid’ approximation has been adopted, assuming no dynamical evolution of the gravitational field. We have used a formulation of the relativistic hydrodynamic equations which casts them into a first-order hyperbolic system of conservation laws. Our studies were performed using a Godunov-type scheme based on Marquina’s flux-formula.
NONSINGULAR BLACK HOLES IN PALATINI EXTENSIONS OF GENERAL RELATIVITY
2015
An introduction to extended theories of gravity formulated in metric-affine (or Palatini) spaces is presented. Focusing on spherically symmetric configurations with electric fields, we will see that in these theories the central singularity present in General Relativity is generically replaced by a wormhole structure. The resulting space-time becomes geodesically complete and, therefore, can be regarded as non-singular. We illustrate these properties considering two different models, namely, a quadratic f(R) theory and a Born-Infeld like gravity theory.
The NHXM observatory
2011
Exploration of the X-ray sky has established X-ray astronomy as a fundamental astrophysical discipline. While our knowledge of the sky below 10 keV has increased dramatically (∼8 orders of magnitude) by use of grazing incidence optics, we still await a similar improvement above 10 keV, where to date only collimated instruments have been used. Also ripe for exploration is the field of X-ray polarimetry, an unused fundamental tool to understand the physics and morphology of X-ray sources. Here we present a novel mission, the New Hard X-ray Mission (NHXM) that brings together for the first time simultaneous high-sensitivity, hard-X-ray imaging, broadband spectroscopy and polarimetry. NHXM will…