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.

A Cosmic Microwave Background Feature Consistent with a Cosmic Texture

Abstract: The Cosmic Microwave Background provides our most ancient image of the universe and our best tool for studying its early evolution. Theories of high-energy physics predict the formation of various types of topological defects in the very early universe, including cosmic texture, which would generate hot and cold spots in the Cosmic Microwave Background. We show through a Bayesian statistical analysis that the most prominent 5°-radius cold spot observed in all-sky images, which is otherwise hard to explain, is compatible with having being caused by a texture. From this model, we constrain the fundamental symmetry-breaking energy scale to be ϕ0 ≈ 8.7 × 1015 gigaelectron volts. If confirmed, this detection of a cosmic defect will probe physics at energies exceeding any conceivable terrestrial experiment.

Weak lensing probes of modified gravity

Abstract: We study the effect of modifications to general relativity on large-scale weak lensing observables. In particular, we consider three modified gravity scenarios: $f(R)$ gravity, the Dvali-Gabadadze-Porrati model, and tensor-vector-scalar theory. Weak lensing is sensitive to the growth of structure and the relation between matter and gravitational potentials, both of which will in general be affected by modified gravity. Restricting ourselves to linear scales, we compare the predictions for galaxy-shear and shear-shear correlations of each modified gravity cosmology to those of an effective dark energy cosmology with the same expansion history. In this way, the effects of modified gravity on the growth of perturbations are separated from the expansion history. We also propose a test which isolates the matter-potential relation from the growth factor and matter power spectrum. For all three modified gravity models, the predictions for galaxy and shear correlations will be discernible from those of dark energy with very high significance in future weak lensing surveys. Furthermore, each model predicts a measurably distinct scale dependence and redshift evolution of galaxy and shear correlations, which can be traced back to the physical foundations of each model. We show that the signal-to-noise for detecting signatures of modified gravity is much higher for weak lensing observables as compared to the integrated Sachs-Wolfe effect, measured via the galaxy-cosmic microwave background cross-correlation.

A wide-field HI mosaic of Messier 31. II.The disk warp, rotation and the dark matter halo

Abstract: We test cosmological models of structure formation using the rotation curve of the nearest spiral galaxy, M31, determined using a recent deep, full-disk 21-cm imaging survey smoothed to 466 pc resolution. We fit a tilted ring model to the HI data from 8 to 37 kpc. The disk of M31 warps from 25 kpc outwards and becomes more inclined with respect to our line of sight. Newtonian dynamics without a dark matter halo provide a very poor fit to the rotation curve derived using the warp model. In the framework of modified Newtonian dynamic however the 21-cm rotation curve is well fitted by the gravitational potential traced by the baryonic matter density alone. The inclusion of a dark matter halo with a density profile as predicted by structure formation in a hierarchical clustering LambdaCDM cosmology makes the mass model in newtonian dynamic compatible with the rotation curve data. The dark halo concentration for the best fit is C=12 and its total mass is 1.2 10^{12} Msun. If a dark halo model with a constant density core is considered, the core radius has to be larger than 20 kpc in order for the model to fit to the data. We extrapolate the best-fit LambdaCDM and constant-density core mass models to very large galactocentric radii, comparable to the size of the dark matter halo. A comparison of the predicted mass with the M31 mass determined at such large radii using other dynamical tracers, confirms the validity of our results. In particular the LambdaCDM dark halo model which best fits the 21-cm data well reproduces the M31 mass traced out to 560 kpc. Our estimated total mass of M31 is 1.3 10^{12} Msun, with 12% baryonic fraction and only 6% of the baryons in neutral gas.

Constraints from the Lyman alpha forest power spectrum

Abstract: We use published measurements of the transmission power spectrum of the Lyman alpha forest to constrain several parameters that describe cosmology and thermal properties of the intergalactic medium (IGM). A 6 parameter grid is constructed using Particle-Mesh dark matter simulations together with scaling relations to make predictions for the gas properties. We fit for all parameters simultaneously and identify several degeneracies. We find that the temperature of the IGM can be well determined from the fall-off of the power spectrum at small scales. We find a temperature around 20.000 K, dependent on the slope of the gas equation of state. We see no evidence for evolution in the IGM temperature. We place constraints on the amplitude of the dark matter fluctuations. However, contrary to previous results, the slope of the dark matter power spectrum is poorly constrained. This is due to uncertainty in the effective Jeans smoothing scale, which depends on the temperature as well as the thermal history of the gas.

Dark energy: A Quantum fossil from the inflationary Universe?

Abstract: The discovery of dark energy (DE) as the physical cause for the accelerated expansion of the Universe is the most remarkable experimental finding of modern cosmology. However, it leads to insurmountable theoretical difficulties from the point of view of fundamental physics. Inflation, on the other hand, constitutes another crucial ingredient, which seems necessary to solve other cosmological conundrums and provides the primeval quantum seeds for structure formation. One may wonder if there is any deep relationship between these two paradigms. In this work, we suggest that the existence of the DE in the present Universe could be linked to the quantum field theoretical mechanism that may have triggered primordial inflation in the early Universe. This mechanism, based on quantum conformal symmetry, induces a logarithmic, asymptotically free, running of the gravitational coupling. If this evolution persists in the present Universe, and if matter is conserved, the general covariance of Einstein's equations demands the existence of dynamical DE in the form of a running cosmological term, Λ, whose variation follows a power law of the redshift.