Observable universe

About: Observable universe is a research topic. Over the lifetime, 690 publications have been published within this topic receiving 16231 citations. The topic is also known as: metagalaxy & our observable universe.

A General Analytic Formula for the Spectral Index of the Density Perturbations Produced during Inflation.

Abstract: During inflation'> vacuum fluctuations on scales less than the Hubble radius in scalar fields with effective masses much less than the Hubble parameter**> are magnified into classical perturbations in the scalar fields on scales larger than the Hubble radius. These classical perturbations in the scalar fields can then change the number of e-folds of expansion and so lead to classical curvature/density perturba­ tions after inflation. These density perturbations are thought to be responsible for the formation of galaxies and the large scale structure of the observable Universe as well as, in combination with the gravitational waves produced during inflation, for the anisotropies in the cosmic microwave background. The standard calculation 7 H 1 > of the spectrum of density perturbations produced during inflation assumes that there is only one real dynamical degree of freedom during inflation. Although this is the case in most of the models of inflation con­ structed up to now, it is by assumption rather than prediction. When one tries to construct models of inflation 3 >,s>. 12 > that might arise naturally in realistic models of particle physics, such as the low energy effective supergravity theories derived from superstrings, one often gets more than one dynamical degree of freedom during inflation. The standard calculation is then generally not applicable. In this paper we derive general analytic formulae for the spectrum and spectral index of the density perturbations produced during inflation. This work is based on earlier work by Starobinsky. 13 > See also Ref. 14). While this work was in slow preparation three other related papers

The trouble with $H_0$

Abstract: We perform a comprehensive cosmological study of the H0 tension between the direct local measurement and the model-dependent value inferred from the Cosmic Microwave Background. With the recent measurement of H0 this tension has raised to more than 3 σ. We consider changes in the early time physics without modifying the late time cosmology. We also reconstruct the late time expansion history in a model independent way with minimal assumptions using distance measurements from Baryon Acoustic Oscillations and Type Ia Supernovae, finding that at z < 0.6 the recovered shape of the expansion history is less than 5% different than that of a standard ΛCDM model. These probes also provide a model insensitive constraint on the low-redshift standard ruler, measuring directly the combination rsh where H0 = h × 100 Mpc−1km/s and rs is the sound horizon at radiation drag (the standard ruler), traditionally constrained by CMB observations. Thus rs and H0 provide absolute scales for distance measurements (anchors) at opposite ends of the observable Universe. We calibrate the cosmic distance ladder and obtain a model-independent determination of the standard ruler for acoustic scale, rs. The tension in H0 reflects a mismatch between our determination of rs and its standard, CMB-inferred value. Without including high-l Planck CMB polarization data (i.e., only considering the ``recommended baseline" low-l polarisation and temperature and the high l temperature data), a modification of the early-time physics to include a component of dark radiation with an effective number of species around 0.4 would reconcile the CMB-inferred constraints, and the local H0 and standard ruler determinations. The inclusion of the ``preliminary" high-l Planck CMB polarisation data disfavours this solution.

Hilltop Inflation

Abstract: We study `hilltop' inflation, in which inflation takes place near a maximum of the potential Viewed as a model of inflation after the observable Universe leaves the horizon (observable inflation) hilltop inflation is rather generic If the potential steepens monotonically, observable hilltop inflation gives a tiny tensor fraction (r< 0002) The usual F- and D-term models may easily be transmuted to hilltop models by Planck-suppressed terms, making them more natural The only commonly-considered model of observable inflation which is definitely not hilltop is tree-level hybrid inflation Viewed instead as an initial condition, we explain that hilltop inflation is more generic than seems to have been previously recognized, adding thereby to the credibility of the idea that eternal inflation provides the pre-inflationary initial condition