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.

Does the growth of structure affect our dynamical models of the universe? The averaging, backreaction and fitting problems in cosmology

Abstract: Structure occurs over a vast range of scales in the universe. Our large-scale cosmological models are coarse-grained representations of what exists, which have much less structure than there really is. An important problem for cosmology is determining the influence the small-scale structure in the universe has on its large-scale dynamics and observations. Is there a significant, general relativistic, backreaction effect from averaging over structure? One issue is whether the process of smoothing over structure can contribute to an acceleration term and so alter the apparent value of the cosmological constant. If this is not the case, are there other aspects of concordance cosmology that are affected by backreaction effects? Despite much progress, this 'averaging problem' is still unanswered, but it cannot be ignored in an era of precision cosmology.

Black hole remnants and dark matter

Abstract: We argue that, when the gravity effect is included, the generalized uncertainty principle (CUP) may prevent black holes from total evaporation in a similar way that the standard uncertainty principle prevents the hydrogen atom from total collapse. Specifically we invoke the GUP to obtain a modified Hawking temperature, which indicates that there should exist non-radiating remnants (BHR) of about Planck mass. BHRs are an attractive candidate for cold dark matter. We investigate an alternative cosmology in which primordial BHRs are the primary source of dark matter.

Circular Polarization of Primordial Gravitational Waves in String-inspired Inflationary Cosmology

Abstract: We study a mechanism to produce the circular polarization of primordial gravitational waves. The circular polarization is generated during the superinflation driven by the Gauss-Bonnet term in the string-inspired cosmology. The instability in the tensor mode caused by the Gauss-Bonnet term and the parity violation due to the gravitational Chern-Simons term are the essential ingredients of the mechanism. We also discuss detectability of the produced circular polarization of gravitational waves. It turns out that the simple model of single-field inflation contradicts cosmic microwave background (CMB) observations. To circumvent this difficulty, we propose a two-field inflation model. In this two-field model, the circular polarization of gravitational waves is created in the frequency range designed by the big-bang observer (BBO) or the deci-hertz gravitational-wave observatory (DECIGO)

Early Supersymmetric Cold Dark Matter Substructure

Abstract: Earth-mass "microhalos" may be the first objects to virialize in the early universe. Their ability to survive the hierarchical clustering process as substructure in the larger halos that form subsequently has implications for dark matter detection experiments. We present a large N-body simulation of early substructure in a supersymmetric cold dark matter (SUSY-CDM) scenario characterized by an exponential cutoff in the power spectrum at Mc = 10-6 M☉. The simulation resolves a 0.014 M☉ parent SUSY halo at z = 75 with 14 million particles. On these scales, the effective index of the power spectrum approaches -3, and a range of mass scales collapses almost simultaneously. Compared to a z = 0 galaxy cluster, substructure within our SUSY host is less evident both in phase-space and in physical space, and it is less resistant against tidal disruption. As the universe expands by a factor of 1.3, we find that between 20% and 40% of well-resolved SUSY substructure is destroyed, compared to only ~1% in the low-redshift cluster. Nevertheless, SUSY substructure is just as abundant as in z = 0 galaxy clusters; i.e., the normalized mass and circular velocity functions are very similar. The DM self-annihilation γ-ray luminosity from bound subhalos and other deviations from a smooth spherical configuration is at least comparable to the spherically averaged signal in the SUSY host, and at least 3 times larger than the spherically averaged signal in the cluster host. Such components must be taken into account when estimating the total cosmological extragalactic γ-ray annihilation background. The relative contribution of bound substructure alone to the total annihilation luminosity is about 4 times smaller in the SUSY host than in the z = 0 cluster because of the smaller density contrast of micro-subhalos.

Lensing is Low: Cosmology, Galaxy Formation, or New Physics?

Abstract: We present high signal-to-noise galaxy-galaxy lensing measurements of the Baryon Oscillation Spectroscopic Survey constant mass (CMASS) sample using 250 deg(2) of weak-lensing data from Canada-France-Hawaii Telescope Lensing Survey and Canada-France-Hawaii Telescope Stripe 82 Survey. We compare this signal with predictions from mock catalogues trained to match observables including the stellar mass function and the projected and twodimensional clustering of CMASS. We show that the clustering of CMASS, together with standard models of the galaxy-halo connection, robustly predicts a lensing signal that is 2040 per cent larger than observed. Detailed tests show that our results are robust to a variety of systematic effects. Lowering the value of S-8 = sigma(8)root Omega(m)/0.3 compared to Planck Collaboration XIII reconciles the lensing with clustering. However, given the scale of our measurement (r < 10 h(-1) Mpc), other effects may also be at play and need to be taken into consideration. We explore the impact of baryon physics, assembly bias, massive neutrinos and modifications to general relativity on Delta Sigma and show that several of these effects may be non-negligible given the precision of our measurement. Disentangling cosmological effects from the details of the galaxy-halo connection, the effect of baryons, and massive neutrinos, is the next challenge facing joint lensing and clustering analyses. This is especially true in the context of large galaxy samples from Baryon Acoustic Oscillation surveys with precise measurements but complex selection functions.