Neutrino cosmology

We present a class of non-stationary cosmological solutions of coupled Einstein-Dirac equations which correspond to an Universe filled solely with neutrinos of right and/or left helicity.     

String cosmology

In this review, we discuss various cosmological issues related to our Universe from a string theoretic perspective. We analyse the pre-big bang cosmological scenario which appears naturally in this context due to the existence of scale factor duality symmetry in string theory. We then discuss some of the attractive and problematic features of this scenario. Finally, we introduce a method which is powerful enough to search for cosmological solutions in various low energy limits of string theories.     

Laser cosmology

Recent years have witnessed tremendous progress in our understanding of the cosmos, which in turn points to even deeper questions to be further addressed. Concurrently the laser technology has undergone dramatic revolutions, providing exciting opportunity for science applications. History has shown that the symbiosis between direct observations and laboratory investigation is instrumental in the progress of astrophysics. We believe that this remains true in cosmology. Current frontier phenomena related to particle astrophysics and cosmology typically involve one or more of the following conditions: (1) extremely high energy events;(2) very high density, high temperature processes; (3) super strong field environments. Laboratory experiments using high intensity lasers can calibrate astrophysical observations, investigate underlying dynamics of astrophysical phenomena, and probe fundamental physics in extreme limits. In this article we give an overview of the exciting prospect of laser cosmology. In particular, we showcase its unique capability of investigating frontier cosmology issues such as cosmic accelerator and quantum gravity.     

Real-time cosmology

In recent years, improved astrometric and spectroscopic techniques have opened the possibility of measuring the temporal change of radial and transverse position of sources in the sky over relatively short time intervals. This has made at least conceivable to establish a novel research domain, which we dub “real-time cosmology”. We review for the first time most of the work already done in this field, analysing the theoretical framework as well as some foreseeable observational strategies and their capability to constrain models. We first focus on real-time measurements of the overall redshift drift and angular separation shift in distant sources, which allows the observer to trace the background cosmic expansion and large scale anisotropy, respectively. We then examine the possibility of employing the same kind of observations to probe peculiar and proper accelerations in clustered systems, and therefore their gravitational potential. The last two sections are devoted to the future change of the cosmic microwave background on “short” time scales, as well as to the temporal shift of the temperature anisotropy power spectrum and maps. We conclude revisiting in this context the usefulness of upcoming experiments (like CODEX and Gaia) for real-time observations.     

Brane cosmology

We summarize the main ideas underlying brane cosmology, or the cosmology of a Universe considered as a submanifold of a higher-dimensional spacetime and where ordinary matter is supposed to be confined. This new scenario, motivated by recent developments in string theory, leads to several specific features that could allow, via forthcoming high precision cosmological observations, to distinguish it from the traditional cosmological scenario. To cite this article: P. Binétruy et al., C. R. Physique 4 (2003).     

Neutrino cosmology

Cosmological data are reviewed questioning whether the universe may be open and dominated by neutrinos and gravitons rather than by baryons. The thermal history of the Lepton Era is investigated incorporating the effects of neutral currents, additional neutrinos, and a small neutrino mass. In the canonical version of Big Bang cosmology (equal numbers of neutrinos and antineutrinos), the neutrino number and energy density is, like that of photons, gravitationally insignificant unless the neutrino has a small mass (10 eV). The neutrino sea can be cosmologically significant if it is degenerate (so that the net leptonic or muonic charge is nonzero) with7×10 ⁵ neutrinos (or antineutrinos) per cm.³ This density homogeneously spread out is still so low that even the most energetic cosmic ray protons will not be stopped, even if neutral currents exist with the usual weak strength. If these degenerate neutrinos have a small mass (0.5 eV), they will condense into degenerate neutrino superstars of the size and mass of galactic clusters. If neutral currents make the (ev) (ev) coupling five times greater than what it is in V — A theory, nucleosynthesis commences a little earlier than conventionally assumed. This increases the cosmological He⁴ abundance predicted only slightly from Y= 0.27 to Y= 0.29. An appendix reviews the effect of neutral currents on neutrino processes in stars.Supported in part by the U.S.A.E.C.     

Nonlocal cosmology

We explore nonlocally modified models of gravity, inspired by quantum loop corrections, as a mechanism for explaining current cosmic acceleration. These theories enjoy two major advantages: they allow a delayed response to cosmic events, here the transition from radiation to matter dominance, and they avoid the usual level of fine-tuning; instead, emulating Dirac's dictum, the required large numbers come from the large time scales involved. Their solar system effects are safely negligible, and they may even prove useful to the black hole information problem.     

New cosmology

We propose a model of our universe as a 3-sphere resting on the surface of a black hole which exists in a spacetime consisting of four space dimensions and one time dimension. The matter and energy within our universe exist as stationary solutions to the field equations in the Rindler coordinates just above the horizon of the black hole. Each solution may be though of as a standing wave consisting of a wave propagating toward the horizon superposed with its time-reversed twin propagating away from the horizon. As matter and energy from the greater five-dimensional spacetime fall into the black hole, its radius increases and our universe expands. This mechanism of expansion allows the model to describe a universe which is older than its oldest stars and homogeneous without inflation. It also predicts galaxy counts at high redshift which agree with observation.     

Towards conformal cosmology

Approximate de Sitter symmetry of inflating Universe is responsible for the approximate flatness of the power spectrum of scalar perturbations. However, this is not the only option. Another symmetry that can explain nearly scale-invariant power spectrum is conformal invariance. We give a short review of models based on conformal symmetry that lead to the scale-invariant spectrum of the scalar perturbations. We discuss also potentially observable features of these models.     

Magnetohydrodynamics and cosmology

It is well known that the Maxwell equations are connected to Minkowski space-time and to the Poincaré group. If we pass to the De Sitter universe with constant curvature, i.e., to the projective relativity, we must generalize the Maxwell equations in such a way to make them invariants for the Fantappié group. We thus obtain more general equations which can be interpreted as equations of magnetohydrodynamics and which reunite in a single theory electromagnetism and relativistic hydrodynamics.     

Causal dissipative cosmology

The full version of the causal thermodynamics of non-equilibrium phenomena is discussed in the context of the flat Friedmann-Robertson-Walker cosmological model. Power law solutions for the scale factor are shown to exist. It is also shown that the temporal behaviour of the temperature depends on the functional dependence of the coefficient of bulk viscosity on density.     

More qualitative cosmology

Standard geometric techniques of differential equation theory are employed to determine the qualitative behaviour of a set of non-rotating perfect-fluid cosmologies, whose spatially homogeneous hypersurfaces admit a 3-parameter group of isometries of Bianchi types I, II, III, V, or VI. In this way we are led to some new exact solutions of the field equations.The field equations for a broad class of cosmological models are presented in a regularised form, limitations on the use of this procedure are examined, and some suggestions are made of ways of avoiding the difficulties that arise.     

Supernovae and cosmology

The extreme luminosity and their fairly unique temporal behaviour have made supernovae a superb tool to measure distances in the universe. As complex astrophysical events they provide interesting insights into explosion physics, explosive nucleosynthesis, hydrodynamics of the explosion and radiation transport. They are an end product of stellar evolution and provide clues to the stellar composition. Since they can be observed at large distances they have become critical probes to further explore astrophysical effects, like dust properties in external galaxies and the star formation history of galaxies. Some of the astrophysics interferes with the cosmological applications of supernovae. The local velocity field, distorted by the gravitational attraction of the local large scale structure, and the reddening law appear at the moment the major limitations in the accuracy with which cosmological parameters can be determined. These absorption effects can introduce a secondary bias into the observations of the distant supernovae, which needs to be carefully evaluated. Supernovae have been used for the measurement of the Hubble constant, i.e. the current expansion rate of the universe, and the accelerated cosmic expansion directly inferred from the apparent faintness of the distant supernovae.     

Quantum codimension-two cosmology

We generalize the 2-codimensional quantum cosmological model with the vacuum seed instanton to that with a non-vacuum seed instanton associated with the Gibbons–Townsend monopole. The confusion in the literature that substituting a field configuration into an action and varying that action is not equivalent to substituting it into the field equations is clarified.     

Anisotropic Lyra cosmology

Anisotropic Bianchi Type-I cosmological models have been studied on the basis of Lyra’s geometry. Two types of models, one with constant deceleration parameter and the other with variable deceleration parameter have been derived by considering a time-dependent displacement field.