Search results for "Equation of State"
showing 10 items of 160 documents
Properties of the isolated neutron star RX J185635-3754
2004
Abstract Despite improved observational capabilities, the surface properties of neutron stars remain uncertain. In principle, multiwavelength spectra can reveal their compositions and angular diameters and constrain the interior equation of state. The Chandra LETG spectrum of the brightest isolated neutron star is a featureless continuum. The X-ray blackbody fit underpredicts the observed optical fluxes, but the spectral energy distribution can be fit with a two blackbody model. No pulsations are seen. We discuss model atmosphere fits. We conclude there is no need to invoke exotic physics: RX J185635-3754 is likely a 0.5 × 10 6 years old pulsar.
Soundness of Dark Energy properties
2020
Type Ia Supernovae (SNeIa) used as standardizable candles have been instrumental in the discovery of cosmic acceleration, usually attributed to some form of dark energy (DE). Recent studies have raised the issue of whether intrinsic SNeIa luminosities might evolve with redshift. While the evidence for cosmic acceleration is robust to this possible systematic, the question remains of how much the latter can affect the inferred properties of the DE component responsible for cosmic acceleration. This is the question we address in this work. We use SNeIa distance moduli measurements from the Pantheon and JLA samples. We consider models where the DE equation of state is a free parameter, either …
Observational Constraints on Undulant Cosmologies
2005
In an undulant universe, cosmic expansion is characterized by alternating periods of acceleration and deceleration. We examine cosmologies in which the dark-energy equation of state varies periodically with the number of e-foldings of the scale factor of the universe, and use observations to constrain the frequency of oscillation. We find a tension between a forceful response to the cosmic coincidence problem and the standard treatment of structure formation.
More about a successful vector-tensor theory of gravitation
2016
The vector-tensor (VT) theory of gravitation revisited in this article was studied in previous papers, where it was proved that VT works and deserves attention. New observational data and numerical codes have motivated further development which is presented here. New research has been planed with the essential aim of proving that current cosmological observations, including Planck data, baryon acoustic oscillations (BAO), and so on, may be explained with VT, a theory which accounts for a kind of dark energy which has the same equation of state as vacuum. New versions of the codes CAMB and COSMOMC have been designed for applications to VT, and the resulting versions have been used to get the…
Neutron-skin uncertainties of Skyrme energy density functionals
2013
Background: Neutron-skin thickness is an excellent indicator of isovector properties of atomic nuclei. As such, it correlates strongly with observables in finite nuclei that depend on neutron-to-proton imbalance and the nuclear symmetry energy that characterizes the equation of state of neutron-rich matter. A rich worldwide experimental program involving studies with rare isotopes, parity violating electron scattering, and astronomical observations is devoted to pinning down the isovector sector of nuclear models. Purpose: We assess the theoretical systematic and statistical uncertainties of neutron-skin thickness and relate them to the equation of state of nuclear matter, and in particular…
Morphology and Dynamics of Relativistic Jets
1997
We present a comprehensive analysis of the morphology and dynamics of relativistic pressure-matched axisymmetric jets. The numerical simulations have been carried out with a high-resolution shock-capturing hydrocode based on an approximate relativistic Riemann solver derived from the spectral decomposition of the Jacobian matrices of relativistic hydrodynamics. We discuss the dependence of the jet morphology on several parameters, paying special attention to the relativistic effects caused by high Lorentz factors and large internal energies of the beam flow. The parameter space of our analysis is spanned by the ratio of the beam and ambient medium rest mass density (η), the beam Mach number…
BoltzmaNN: Predicting effective pair potentials and equations of state using neural networks
2019
Neural networks (NNs) are employed to predict equations of state from a given isotropic pair potential using the virial expansion of the pressure. The NNs are trained with data from molecular dynamics simulations of monoatomic gases and liquids, sampled in the NVT ensemble at various densities. We find that the NNs provide much more accurate results compared to the analytic low-density limit estimate of the second virial coefficient and the Carnahan-Starling equation of state for hard sphere liquids. Furthermore, we design and train NNs for computing (effective) pair potentials from radial pair distribution functions, g(r), a task that is often performed for inverse design and coarse-graini…
Probing the Equation of State of Ultradense Matter with a Submillisecond Pulsar Search Experiment
1997
Current ideas about the equation of state for the ultradense matter constituting neutron stars provide models with a range of neutron star radii for a given mass. This implies different estimates for the maximum angular velocity that such an object could attain. The fastest and the slowest angular velocity differ by a significant amount, depending on the equation of state adopted. In particular, the identification of a submillisecond pulsar would allow us to constrain the equation of state of dense matter. In this paper, we discuss a possible evolutionary scenario resulting in a submillisecond pulsar, taking into account current ideas about the evolution of the magnetic field of neutron sta…
A density functional for liquid3He
1993
We present a density functional for the description of liquid3He properties at zero temperature in a mean field approximation. Its basic ingredients are a zero-range, particle- and spin-density dependent effective interaction of Skyrme type, and a long-range effective interaction of Lennard-Jones type supplemented with a weighted density approximation similar to the one used in the study of classical fluids, to phenomenologically account for short range correlations. After fixing the value of its parameters, the functional yields a good description of the equation of state and Landau parameters (spin symmetric and spin antisymmetric as well) from saturation to solidification densities. The …
A generalization of the Carnahan–Starling approach with applications to four- and five-dimensional hard spheres
2018
Abstract Development of good equations of state for hard spheres is an important task in the study of real fluids. In a way consistent with other theoretical results, we generalize the famous Carnahan–Starling approach for arbitrary dimensions and apply it to four- and five-dimensional hard spheres. We obtain simple and integer representations for virial coefficients of lower orders and accurate equations of state. Since theoretically and practically validated, these results improve understanding of hard sphere fluids.