Cosmology

About: Cosmology is a research topic. Over the lifetime, 18004 publications have been published within this topic receiving 631028 citations. The topic is also known as: physical cosmology & cosmologies.

Uncorrelated estimates of dark energy evolution

Abstract: Type Ia supernova data have recently become strong enough to enable, for the first time, constraints on the time variation of the dark energy density and its equation of state. Most analyses, however, are using simple two or three-parameter descriptions of the dark energy evolution, since it is well known that allowing more degrees of freedom introduces serious degeneracies. Here we present a method to produce uncorrelated and nearly model-independent band power estimates of the equation of state of dark energy and its density as a function of redshift. We apply the method to recently compiled supernova data. Our results are consistent with the cosmological constant scenario, in agreement with other analyses that use traditional parametrizations, though we find marginal (2-sigma) evidence for w(z)<-1 at z<0.2. In addition to easy interpretation, uncorrelated, localized band powers allow intuitive and powerful testing of the constancy of either the energy density or equation of state. While we have used relatively coarse redshift binning suitable for the current set of ~150 supernovae, this approach should reach its full potential in the future, when applied to thousands of supernovae found from ground and space, combined with complementary information from other cosmological probes.

Two accurate time-delay distances from strong lensing: implications for cosmology

Abstract: Strong gravitational lenses with measured time delays between the multiple images and models of the lens mass distribution allow a one-step determination of the time-delay distance, and thus a measure of cosmological parameters. We present a blind analysis of the gravitational lens RXJ1131-1231 incorporating (1) the newly measured time delays from COSMOGRAIL, the COSmological MOnitoring of GRAvItational Lenses, (2) archival Hubble Space Telescope imaging of the lens system, (3) a new velocity-dispersion measurement of the lens galaxy of 323 +/- 20 km s(-1) based on Keck spectroscopy, and (4) a characterization of the line-of-sight structures via observations of the lens' environment and ray tracing through the Millennium Simulation. Our blind analysis is designed to prevent experimenter bias. The joint analysis of the data sets allows a time-delay distance measurement to 6% precision that takes into account all known systematic uncertainties. In combination with the Wilkinson Microwave Anisotropy Probe seven-year (WMAP7) data set in flat wCDM cosmology, our unblinded cosmological constraints for RXJ1131-1231 are H-0 = 80.0(-5.7)(+5.8) km s(-1) Mpc(-1), Omega(de) = 0.79 +/- 0.03, and w = -1.25(-0.21)(+0.17). We find the results to be statistically consistent with those from the analysis of the gravitational lens B1608+ 656, permitting us to combine the inferences from these two lenses. The joint constraints from the two lenses and WMAP7 are H-0 = 75.2(-4.2)(+4.4) km s(-1) Mpc(-1), Omega(de) = 0.76(-0.03)(+0.02), and w = -1.14(-0.20)(+0.17) in flat wCDM, and H-0 = 73.1(-3.6)(+2.4) km s(-1) Mpc(-1), Omega(Lambda) = 0.75(-0.02)(+0.01), and Omega(k) = 0.003(-0.006)(+0.005) in open Lambda CDM. Time-delay lenses constrain especially tightly the Hubble constant H0 (5.7% and 4.0% respectively in wCDM and open Lambda CDM) and curvature of the universe. The overall information content is similar to that of Baryon Acoustic Oscillation experiments. Thus, they complement well other cosmological probes, and provide an independent check of unknown systematics. Our measurement of the Hubble constant is completely independent of those based on the local distance ladder method, providing an important consistency check of the standard cosmological model and of general relativity.

The stochastic gravitational-wave background from massive black hole binary systems: implications for observations with Pulsar Timing Arrays

Abstract: Massive black hole binary systems, with masses in the range � 10 4 10 10 M⊙, are among the primary sources of gravitational waves in the frequency window � 10 −9 Hz 0.1Hz. Pulsar Timing Arrays (PTAs) and the Laser Interferometer Space Antenna (LISA) are the observational means by which we will be able to observe gravitational radiation from these systems. We carry out a systematic study of the generation of the stochastic gravitational-wave background from the cosmic population of massive black hole binaries. We consider a wide variety of assembly scenarios and we estimate the range of signal strength in the frequency band accessible to PTAs. We show that regardless of the specific model of massive black hole binaries formation and evolution, the characteristic amplitude hc of the gravitational wave stochastic background at 10 −8 Hz varies by less than a factor of 2. However, taking into account the uncertainties surrounding the actual key model parameters, the amplitude lies in the interval hc(f = 10 −8 Hz) � 5×10 −16 8×10 −15 . The most optimistic predictions place the signal level at a factor of � 3 below the current sensitivity of Pulsar Timing Arrays, but within the detection range of the complete Parkes PTA for a wide variety of models, and of the future Square-Kilometer-Array PTA for all the models considered here. We also show that at frequencies > 10 −8 Hz the frequency dependency of the generated background follows a power-law significantly steeper than hc / f −2/3 , that has been considered so far; the value of the spectral index depends on the actual assembly scenario and provides therefore an additional opportunity to extract astrophysical information about the cosmic population of massive black holes. Finally we show that LISA observations of individual resolvable massive black hole binaries are complementary and orthogonal to PTA observations of a stochastic background from the whole population in the Universe. In fact, the detection of gravitational radiation in both frequency windows will enable us to fully characterise the cosmic history of massive black holes.

Dark Energy vs. Modified Gravity

Abstract: Understanding the reason for the observed accelerated expansion of the Universe represents one of the fundamental open questions in physics. In cosmology, a classification has emerged among physical models for the acceleration, distinguishing between Dark Energy and Modified Gravity. In this review, we give a brief overview of models in both categories as well as their phenomenology and characteristic observable signatures in cosmology. We also introduce a rigorous distinction between Dark Energy and Modified Gravity based on the strong and weak equivalence principles.