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.

Reionization and Cosmology with 21-cm Fluctuations

Abstract: Measurement of the spatial distribution of neutral hydrogen via the redshifted 21-cm line promises to revolutionize our knowledge of the epoch of reionization and the first galaxies, and may provide a powerful new tool for observational cosmology from redshifts 1

The Hubble Diagram to Redshift >6 from 69 Gamma-Ray Bursts

Abstract: One of the few ways to measure the properties of dark energy is to extend the Hubble diagram (HD) to higher redshifts with gamma-ray bursts (GRBs). GRBs have at least five properties (their spectral lag, variability, spectral peak photon energy, time of the jet break, and the minimum rise time) that have correlations to the luminosity of varying quality. In this paper I construct a GRB HD with 69 GRBs over a redshift range from 0.17 to >6, with half the bursts having a redshift larger than 1.7. This paper uses over 3.6 times as many GRBs and 12.7 times as many luminosity indicators as any previous GRB HD work. For the gravitational lensing and Malmquist biases, I find that the biases are small, with an average of 0.03 mag and an rms scatter of 0.14 mag in the distance modulus. The GRB HD is well behaved and nicely delineates the shape of the HD. The reduced χ2 for the fit to the concordance model is 1.05, and the rms scatter about the concordance model is 0.65 mag. This accuracy is just a factor of 2.0 times that gotten for the same measure from all the big supernova surveys. I fit the GRB HD to a variety of models, including where the dark energy has its equation of state parameter varying as w(z) = w0 + waz/(1 + z). I find that the concordance model is consistent with the data, that is, the dark energy can be described well as a cosmological constant that does not change with time.

Dark Energy from structure: a status report

Abstract: The effective evolution of an inhomogeneous universe model in any theory of gravitation may be described in terms of spatially averaged variables. In Einstein’s theory, restricting attention to scalar variables, this evolution can be modeled by solutions of a set of Friedmann equations for an effective volume scale factor, with matter and backreaction source terms. The latter can be represented by an effective scalar field (“morphon field”) modeling Dark Energy. The present work provides an overview over the Dark Energy debate in connection with the impact of inhomogeneities, and formulates strategies for a comprehensive quantitative evaluation of backreaction effects both in theoretical and observational cosmology. We recall the basic steps of a description of backreaction effects in relativistic cosmology that lead to refurnishing the standard cosmological equations, but also lay down a number of challenges and unresolved issues in connection with their observational interpretation. The present status of this subject is intermediate: we have a good qualitative understanding of backreaction effects pointing to a global instability of the standard model of cosmology; exact solutions and perturbative results modeling this instability lie in the right sector to explain Dark Energy from inhomogeneities. It is fair to say that, even if backreaction effects turn out to be less important than anticipated by some researchers, the concordance high-precision cosmology, the architecture of current N-body simulations, as well as standard perturbative approaches may all fall short in correctly describing the Late Universe.

Dark energy: Vacuum fluctuations, the effective phantom phase, and holography

Abstract: We aim at the construction of dark energy models without exotic matter but with a phantomlike equation of state (an effective phantom phase) The first model we consider is decaying vacuum cosmology where the fluctuations of the vacuum are taken into account In this case, the phantom cosmology (with an effective, observational $\ensuremath{\omega}$ being less than $\ensuremath{-}1$ ) emerges even for the case of a real dark energy with a physical equation of state parameter $\ensuremath{\omega}$ larger than $\ensuremath{-}1$ The second proposal is a generalized holographic model, which is produced by the presence of an infrared cutoff It also leads to an effective phantom phase, which is not a transient one as in the first model However, we show that quantum effects are able to prevent its evolution towards a big rip singularity

Accurate universal models for the mass accretion histories and concentrations of dark matter halos

Abstract: A large amount of observations have constrained cosmological parameters and the initial density fluctuation spectrum to a very high accuracy. However, cosmological parameters change with time and the power index of the power spectrum varies with mass scale dramatically in the so-called concordance Lambda CDM cosmology. Thus, any successful model for its structural evolution should work well simultaneously for various cosmological models and different power spectra. We use a large set of high-resolution N-body simulations of a variety of structure formation models (scale-free, standard CDM, open CDM, and Lambda CDM) to study the mass accretion histories (MAHs), the mass and redshift dependence of concentrations and the concentration evolution histories of dark matter halos. We find that there is significant disagreement between the much-used empirical models in the literature and our simulations. According to two simple but tight correlations we find from the simulation results, we develop new empirical models for both the MAHs and the concentration evolution histories of dark matter halos, and the latter can also be used to predict the mass and redshift dependence of halo concentrations. These models are accurate and universal: the same set of model parameters works well for different cosmological models and for halos of different masses at different redshifts and the model predictions are highly accurate even when the histories are traced to very high redshift. These models are also simple and easy to implement. A web calculator and a user-friendly code to make the relevant calculations are available from this http URL . We explain why Lambda CDM halos on nearly all mass scales show two distinct phases in their evolution histories.