arXiv subjects

Jordi Miralda-Escudé

Publications and source records attributed to Jordi Miralda-Escudé.

61 records · Page 4Linked to original sources

arXiv2010

The Invisible Tension of the Universe from Astrophysical Black Holes: a Solution to the Coincidence Problem of the Accelerated Expansion

Astronomical observations have shown that the expansion of the universe is at present accelerating, consistently with a constant negative pressure or tension. This is a major puzzle because we do not understand why this tension is so small compared to the Planck density; why, being so small, it is not exactly zero; and why it has precisely the required value to make the expansion start accelerating just at the epoch when we are observing the universe. The recently proposed conjecture by Afshordi that black holes create a gravitational aether owing to quantum gravity effects, which may be identified with this invisible tension, can solve this coincidence problem. The fact that the expansion of the universe is starting to accelerate at the epoch when we observe it is a necessity that is implied by our origin in a planet orbiting a star that formed when the age of the universe was of the same order as the lifetime of the star. This argument is unrelated to any anthropic reasoning.

astro-ph.CO↗

arXiv2009

Lyman-alpha Emission From Cosmic Structure I: Fluorescence

We present predictions for the fluorescent Lyman-alpha emission signature arising from photoionized, optically thick structures in Smoothed Particle Hydrodynamic (SPH) cosmological simulations of a Lambda-CDM universe using a Monte Carlo Lyman-alpha radiative transfer code. We calculate the expected Lyman-alpha image and 2-dimensional spectra for gas exposed to a uniform ultraviolet ionizing background as well as gas exposed additionally to the photoionizing radiation from a local quasar, after correcting for the self-shielding of hydrogen. As a test of our numerical methods and for application to current observations, we examine simplified analytic structures that are uniformly or anisotropically illuminated. We compare these results with recent observations. We discuss future observing campaigns on large telescopes and realistic strategies for detecting fluorescence owing to the ambient metagalactic ionization and in regions close to bright quasars. While it will take hundreds of hours on the current generation of telescopes to detect fluorescence caused by the ultraviolet background (UVB) alone, our calculations suggest that of order ten sources of quasar-induced fluorescent Lyman-alpha emission should be detectable after a 10 hour exposure in a 10 arcmin^2 field around a bright quasar. These observations will help probe the physical conditions in the densest regions of the intergalactic medium as well as the temporal light curves and isotropy of quasar radiation.

astro-ph.CO↗

arXiv2009

A quantitative explanation of the observed population of Milky Way satellite galaxies

We revisit the well known discrepancy between the observed number of Milky Way (MW) dwarf satellite companions and the predicted population of cold dark matter (CDM) sub-halos, in light of the dozen new low luminosity satellites found in SDSS imaging data and our recent calibration of the SDSS satellite detection efficiency, which implies a total population far larger than these dozen discoveries. We combine a dynamical model for the CDM sub-halo population with simple, physically motivated prescriptions for assigning stellar content to each sub-halo, then apply observational selection effects and compare to the current observational census. As expected, models in which the stellar mass is a constant fraction F(Omega_b/Omega_m) of the sub-halo mass M_sat at the time it becomes a satellite fail for any choice of F. However, previously advocated models that invoke suppression of gas accretion after reionization in halos with circular velocity v_c <~ 35 km/s can reproduce the observed satellite counts for -15 < M_V < 0, with F ~ 10^{-3}. Successful models also require strong suppression of star formation BEFORE reionization in halos with v_c <~ 10 km/s; models without pre-reionization suppression predict far too many satellites with -5 < M_V < 0. Our models also reproduce the observed stellar velocity dispersions ~ 5-10 km/s of the SDSS dwarfs given the observed sizes of their stellar distributions, and model satellites have M(<300 pc) ~ 10^7 M_sun as observed even though their present day total halo masses span more than two orders of magnitude. Our modeling shows that natural physical mechanisms acting within the CDM framework can quantitatively explain the properties of the MW satellite population as it is presently known, thus providing a convincing solution to the `missing satellite' problem.

astro-ph.GA↗

arXiv2001

Large-scale Correlation of Mass and Galaxies with the Lyman-alpha Forest Transmitted Flux

We present predictions of the correlation between the Lyman-alpha forest absorption in quasar spectra and the mass within \sim 5 Mpc/h (comoving) of the line of sight, using fully hydrodynamic and hydro-PM numerical simulations of the cold dark matter model supported by present observations. The observed correlation based on galaxies and the Lya forest can be directly compared to our theoretical results, assuming that galaxies are linearly biased on large scales. Specifically, we predict the average value of the mass fluctuation, , conditioned to a fixed value of the Lya forest transmitted flux delta_F, after they have been smoothed over a 10 Mpc/h cube and line of sight interval, respectively. We find that /sigma_m as a function of delta_F/sigma_F has a slope of 0.6 at this smoothing scale, where sigma_m and sigma_F are the rms dispersions (this slope should decrease with the smoothing scale). We show that this value is largely insensitive to the cosmological model and other Lya forest parameters. Comparison of our predictions to observations should provide a fundamental test of our ideas on the nature of the Lya forest and the distribution of galaxies, and can yield a measurement of the bias factor of any type of galaxies that are observed in the vicinity of Lya forest lines of sight.

astro-ph↗

arXiv1997

Hubble Flow Broadening of the Lyman-alpha Forest and its Implications

Lyman-alpha forest lines in QSO spectra have typical widths of 20-50 km/s. Low column density absorbers in cosmological simulations are large, diffuse structures, and the Hubble flow across the spatially extended absorber is usually the dominant contribution to the width of its associated absorption line. Thermal broadening is unimportant over most of the spectrum, and peculiar velocities tend to make absorption features narrower rather than broader. As a consequence of Hubble flow broadening, there is a close relation between local Lyman-alpha optical depth and local neutral hydrogen density, which is well approximated by the Gunn-Peterson formula. The physics that governs the unshocked intergalactic medium leads to a tight correlation between the neutral hydrogen density and the underlying gas and dark matter overdensity. For many purposes, it is simpler to regard a Lyman-alpha forest spectrum as a continuous, non-linear map of the density field rather than a collection of discrete lines. This continuous field view of the Lyman-alpha forest can be applied to measurement of the baryon density parameter, testing of cosmological models, and robust determination of the shape and amplitude of the primordial mass power spectrum.

astro-ph↗

arXiv1997

On the Clustering of Lyman Alpha Clouds

We examine the correlational property of $\lya$ clouds in detail and compare it to that of mass and galaxies. We show that the correlation strength of $\lya$ clouds is somewhat weaker than that of the underlying matter, which in turn is weaker than that of galaxies (biased galaxy formation). On the scales probed, $10-300$km/s, for $\lya$ clouds, we find that higher density, higher optical depth, higher column density regions are more strongly clustered than lower density, lower optical depth, lower column density regions. Thus, a consistent picture seems to emerge: the correlation strength for a given set of objects is positively correlated with their characteristic global density and the differences among the correlations of galaxies, $\lya$ clouds and mass reflect the differences in density that each trace. Significant positive correlations with a strength of $0.1-1.0$ are found for $\lya$ clouds in the velocity range $50-300$km/s. Among the correlational measures examined, an optical depth correlation function (Equation 5) proposed here may serve as the best correlational measure. It reasonably faithfully represents the true correlation of the underlying matter, enabling a better indication of both matter correlation and the relationship between galaxies and $\lya$ clouds. Furthermore, it appears to be an alternative to the conventional line-line correlation function with the virtue that it does not require ambiguous post-observation fitting procedures such as those commonly employed in the conventional line-finding methods. Neither does it depend sensitively on the observational resolution (e.g., FWHM), insofaras the clouds are resolved Conveniently, it can be easily measured with the current observational sensitivity without being contaminated significantly by the presence of noise.

astro-ph↗

arXiv1995

The Distribution of Mass and Gas in the Center of Clusters of Galaxies Implied by X-Ray and Lensing Observations

Observations of gravitational lensing indicate that the mass distribution in clusters of galaxies (where most of the mass is dark matter) is highly peaked towards the center, while X-ray observations imply that the gas is more extended. The additional fact that the gas is cooling in the center has often led one to expect that the gas temperature should be lower near the center, and therefore the gas should be more concentrated than the dark matter. We show that such expectation is not correct, and that the gas temperature must remain approximately constant within the cooling region in order to have consistency with the observed X-ray profiles and lenses. A multiphase cooling flow naturally produces an approximately constant temperature profile, and a more extended distribution for the gas compared to the mass. Cool phases are deposited at relatively large radius, while hot phases are adiabatically heated as they flow inwards and can keep the average temperature constant. Thus, cooling flows result in an {\it increase} of the central temperature, relative to a case where there is no cooling and the gas follows the mass distribution. The increased central temperatures caused by cooling flows give a characteristic core radius to the gas profiles, which is of order the cooling radius. This provides a natural explanation for the typical cores observed in X-ray clusters. It also brings into better agreement with observations the rate of cluster evolution expected in self-similar hierarchical models. We propose that clusters having core radii much larger than their cooling radii are in the process of merging and are not in dynamical equilibrium.

astro-ph↗