We examine the possibility that a significant component of the energy density of the universe has an equation-of-state different from that of matter, radiation or cosmological constant (Λ). An example is a cosmic scalar field evolving in a potential, but our treatment is more general. Including this component alters cosmic evolution in a way that fits current observations well. Unlike Λ, it evolves dynamically and develops fluctuations, leaving a distinctive imprint on the microwave background anisotropy and mass power spectrum. PACS number(s): 95.35.+d,98.70.Vc,98.65.Dx,98.80.Cq Inflationary cosmology predicts that the universe is spatially flat and that the total energy density of the universe is equal to the critical density. This prediction is consistent with current measurements of the cosmic microwave background (CMB) anisotropy and may be verified with high precision in the next generation of CMB satellite experiments. At the same time, there is growing observational evidence that the total matter density of the universe is significantly less than the critical density. 1 If this latter result holds and the CMB anisotropy establishes that the universe is flat, then there must be another contribution to the energy density of the universe. One candidate that is often considered is a cosmological constant, Λ, or vacuum energy density. The vacuum density is a spatially uniform, time-independent component. Cold dark matter models with a substantial cosmological constant (ΛCDM) are among the models which best fit existing observational data. 1 However, it should be emphasized that the fit depends primarily on the fact that the models have low matter density and are spatially flat; the fit is not a sensitive test of whether the additional energy contribution is vacuum energy.In this paper, we consider replacing Λ with a dynamical, time-dependent and spatially inhomogeneous component whose equation-of-state is different from baryons, neutrinos, dark matter, or radiation. The equation-ofstate of the new component, denoted as w, is the ratio of its pressure to its energy density. This fifth contribution to the cosmic energy density, referred to here as "quintessence" or Q-component, is broadly defined, allowing a spectrum of possibilities including an equationof-state which is constant, uniformly evolving or oscillatory. Examples of a Q-component are fundamental fields (scalar, vector, or tensor) or macroscopic objects, such as a network of light, tangled cosmic strings. 2 The analysis in the present paper applies to any component whose hydrodynamic properties can be mimicked by a scalar field evolving in a potential which couples to matter only through gravitation. In particular, we focus on equations-of-state with −1 < w < 0 because this range fits current cosmological observations best. [3][4][5][6][7] This has motivated several investigations 3,4,8,9 of components with w < 0 in which a spatially uniform distribution has been assumed, e.g., a decaying Λ or smooth component.In this Letter, we begin by arguing t...
Some form of missing energy may account for the difference between the observed cosmic matter density and the critical density. Two leading candidates are a cosmological constant and quintessence (a time-varying, inhomogenous component with negative pressure). We show that an ideal, full-sky cosmic background anisotropy experiment may not be able to distinguish the two and, due to this ambiguity, may not determine the matter density or Hubble constant. We further show that degeneracy may remain even after considering classical cosmological tests and measurements of large scale structure.This paper looks ahead a few years to a time when highly precise, full-sky maps of the cosmic microwave background (CMB) anisotropy become available from satellite experiments such as the NASA Microwave Anisotropy Probe (MAP) and the ESA Planck mission. The goal is to determine if measurements of the anisotropy by itself or combined with other cosmological constraints can resolve between competing models for the "missing energy" of the universe. The missing energy problem arises because inflationary cosmology and some current microwave anisotropy measurements suggest that the universe is flat at the same that a growing number of observations indicate that the matter density (baryonic and nonbaryonic) is below the critical density (Ω m < 1). These two trends can be reconciled if there is another contribution to the energy density of the universe besides matter. One candidate for the missing energy is a vacuum density or cosmological constant (Λ).1 A second candidate is quintessence, a time-varying, spatially inhomogeneous component with negative pressure.2 Both models fit all current observations well. 1, 3If current observational trends continue, determining the nature of the missing energy will emerge as one of cosmology's most important challenges. The issue must be decided in order to understand the energy composition of the universe. Also, as shown below, ambiguity concerning the missing energy leads to large uncertainties in two key parameters: Ω m and h (the Hubble constant in units of 100 km sec −1 Mpc −1 ). In this Letter, we show that, despite extraordinary advances in measurements of the CMB anisotropy and large-scale structure anticipated in the near future, the missing energy problem and, consequently Ω m and h, may remain unresolved in some circumstances.The key differences between quintessence and vacuum density are: (1) quintessence has an equation-of-state w (equal to the ratio of pressure to energy density) greater than −1, whereas vacuum density has w precisely equal to −1; (2) the energy density for quintessence varies with time whereas the vacuum density is constant; and (3), quintessence is spatially inhomogeneous and can cluster gravitationally, whereas vacuum density remains spatially uniform. The first two properties result in different predictions for the expansion rate. The third property results in a direct imprint of quintessence fluctuations on the CMB and large scale structure.For the purposes of thi...
We present a methodology to discover outliers in catalogues of periodic light curves. We use a cross‐correlation as the measure of ‘similarity’ between two individual light curves, and then classify light curves with lowest average ‘similarity’ as outliers. We performed the analysis on catalogues of periodic variable stars of known type from the MACHO and OGLE projects. This analysis was carried out in Fourier space and we established that our method correctly identifies light curves that do not belong to those catalogues as outliers. We show how an approximation to this method, carried out in real space, can scale to large data sets that will be available in the near future such as those anticipated from the Panoramic Survey Telescope & Rapid Response System (Pan‐STARRS) and Large Synoptic Survey Telescope (LSST).
No abstract
We analyze the evolution of energy density fluctuations in cosmological scenarios with a mixture of cold dark matter and quintessence, in which the quintessence field is modeled by a constant equation of state. We obtain analytic expressions for the time evolution of the quintessence perturbations in models with light fields. The fluctuations behave analogously to a driven harmonic oscillator, where the driving term arises from the inhomogeneities in the surrounding cosmological fluid. We demonstrate that the homogeneous solution, determined by the initial conditions, is completely sub-dominant to the inhomogeneous solution for physically realistic scenarios. Thus we show that the cosmic microwave background anisotropy predicted for such models is highly insensitive to the initial conditions in the quintessence field.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.