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Register a new userMessengers from the Early Universe: Cosmic Neutrinos and Other Light Relics
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The hot dense environment of the early universe is known to have produced large numbers of baryons, photons, and neutrinos. These extreme conditions may have also produced other long-lived species, including new light particles (such as axions or sterile neutrinos) or gravitational waves. The gravitational effects of any such light relics can be observed through their unique imprint in the cosmic microwave background (CMB), the large-scale structure, and the primordial light element abundances, and are important in determining the initial conditions of the universe. We argue that future cosmological observations, in particular improved maps of the CMB on small angular scales, can be orders of magnitude more sensitive for probing the thermal history of the early universe than current experiments. These observations offer a unique and broad discovery space for new physics in the dark sector and beyond, even when its effects would not be visible in terrestrial experiments or in astrophysical environments. A detection of an excess light relic abundance would be a clear indication of new physics and would provide the first direct information about the universe between the times of reheating and neutrino decoupling one second later.
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Neutrino oscillations present the only robust example of experimentally detected physics beyond the standard model. This review discusses the established and several hypothetical beyond standard models neutrino characteristics and their cosmological effects and constraints. Particularly, the contemporary cosmological constraints on the number of neutrino families, neutrino mass differences and mixing, lepton asymmetry in the neutrino sector, neutrino masses, light sterile neutrino are briefly reviewed.
We suggest the possibility of creation in the early Universe of stable domains of radius a few kilometers wide, formed by coherently excited states of $pi$-mesons. Such domains appear dark to an external observer, since the decay rate of the said coherent pionic states into photons is vanishingly small. The related thermal insulation of the domains from the outer world could have allowed them to survive till present days. The estimated maximum radius and the period of rotation of such objects turn out to be compatible with those of certain pulsars.
A number of theoretically well-motivated additions to the standard cosmological model predict weak signatures in the form of spatially localized sources embedded in the cosmic microwave background (CMB) fluctuations. We present a hierarchical Bayesian statistical formalism and a complete data analysis pipeline for testing such scenarios. We derive an accurate approximation to the full posterior probability distribution over the parameters defining any theory that predicts sources embedded in the CMB, and perform an extensive set of tests in order to establish its validity. The approximation is implemented using a modular algorithm, designed to avoid a posteriori selection effects, which combines a candidate-detection stage with a full Bayesian model-selection and parameter-estimation analysis. We apply this pipeline to theories that predict cosmic textures and bubble collisions, extending previous analyses by using: (1) adaptive-resolution techniques, allowing us to probe features of arbitrary size, and (2) optimal filters, which provide the best possible sensitivity for detecting candidate signatures. We conclude that the WMAP 7-year data do not favor the addition of either cosmic textures or bubble collisions to the standard cosmological model, and place robust constraints on the predicted number of such sources. The expected numbers of bubble collisions and cosmic textures on the CMB sky within our detection thresholds are constrained to be fewer than 4.0 and 5.2 at 95% confidence, respectively.
In light of the recent BICEP2 B-mode polarization detection, which implies a large inflationary tensor-to-scalar ratio r_{0.05}=0.2^{+0.07}_{-0.05}, we re-examine the evidence for an extra sterile massive neutrino, originally invoked to account for the tension between the cosmic microwave background (CMB) temperature power spectrum and local measurements of the expansion rate H0 and cosmological structure. With only the standard active neutrinos and power-law scalar spectra, this detection is in tension with the upper limit of r<0.11 (95% confidence) from the lack of a corresponding low multipole excess in the temperature anisotropy from gravitational waves. An extra sterile species with the same energy density as is needed to reconcile the CMB data with H0 measurements can also alleviate this new tension. By combining data from the Planck and ACT/SPT temperature spectra, WMAP9 polarization, H_0, baryon acoustic oscillation and local cluster abundance measurements with BICEP2 data, we find the joint evidence for a sterile massive neutrino increases to DeltaNeff=0.98pm 0.26 for the effective number and ms= 0.52pm 0.13 eV for the effective mass or 3.8 sigma and 4 sigma evidence respectively. We caution the reader that these results correspond to a joint statistical evidence and, in addition, astrophysical systematic errors in the clusters and H0 measurements, and small-scale CMB data could weaken our conclusions.
Alternative cosmologies, based on extensions of General Relativity, predict modified thermal histories in the Early Universe during the pre Big Bang Nucleosynthesis (BBN) era, epoch which is not directly constrained by cosmological observations. When the expansion rate is enhanced with respect to the standard case, thermal relics typically decouple with larger relic abundances. The correct value of the relic abundance is therefore obtained for larger annihilation cross--sections, as compared to standard cosmology. A direct consequence is that indirect detection rates are enhanced. Extending previous analyses of ours, we derive updated astrophysical bounds on the dark matter annihilation cross sections and use them to constrain alternative cosmologies in the pre--BBN era. We also determine the characteristics of these alternative cosmologies in order to provide the correct value of relic abundance for a thermal relic for the (large) annihilation cross--section required to explain the PAMELA results on the positron fraction, therefore providing a cosmological boost solution to the dark matter interpretation of the PAMELA data.
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