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.     

New bounds on millicharged particles from cosmology

Particles with millicharge q and sub-eV mass can be produced in photon–photon collisions, distorting the energy spectrum of the Cosmic Microwave Background. We derive the conservative bound q10⁻⁷e (as well as model-dependent bounds two orders of magnitude stronger), incompatible with proposed interpretations of the PVLAS anomaly based on millicharged production or on millicharged-mediated axion-like couplings.     

New approaches for inflationary cosmology

Extended and hyperextended inflationary models of the universe have been developed which avoid the extreme fine-tuning required by all previous approaches. The models also generate a new source of inhomogeneities that affect large-scale structure. The most surprising feature is the role that inflation can play in altering the nature of the gravitational force.This essay received the second award from the Gravity Research Foundation, 1990 —Ed.     

New gravitational tests of early universe cosmology

Gravitational waves and lenses were among the earliest predictions of general relativity. I demonstrate here how both these phenomena can, in conjunction with newly discovered astrophysical objects, be used to test fundamental aspects of early universe cosmology, including (a) scenarios for galaxy formation, and (b) nonadiabatic expansion before and after nucleosynthesis.Research supported in part by the N.S.F. under Grant No. PHY-82-15249.     

New classes of exact solutions in inflationary cosmology

The problem of determining a representation of the self-interaction potential in the form of a time dependence of the field potential energy which admits the existence of an inflationary regime and the transition of evolution to a Friedmann regime of asymptotic expansion is investigated within a cosmological model with a self-interacting scalar field. A variational formulation of the slow-roll concept is introduced, and, on the basis thereof, an exact solution is constructed for the evolution of the scale factor and the form of the self-interaction potential. A method based on representing the Einstein equations in the form of a linear second-order equation is developed for constructing and analyzing exact cosmological solutions of these equations. Selected types of potentials and the corresponding evolutions of the universe are investigated. Zh. éksp. Teor. Fiz. 114, 406–417 (August 1998)     

A new slant on hadron structure

Rather than regarding the restriction of current lattice QCD simulations to quark masses that are 5–10 times larger than those observed as a problem, we note that this presents a wonderful opportunity to deepen our understanding of QCD. Just as it has been possible to learn a great deal about QCD by treating N _c as a variable, so the study of hadron properties as a function of quark mass is leading us to a deeper appreciation of hadron structure. As examples we cite progress in using the chiral properties of QCD to connect hadron masses, magnetic moments, charge radii and structure functions calculated at large quark masses within lattice QCD with the values observed physically.     

Laguerre-Gaussian beam scintillation on slant paths

Scintillation evaluations for Laguerre-Gaussian (LG) beams for slant paths are made using Rytov approximation. On- and off-axis scintillation is formulated and calculated up to several tens of kilometers of slant distances for different zenith angles. Scintillation index variations against radial receiver point and different source sizes are also investigated. In all cases evaluated, it is found that LG beams with higher radial mode numbers result in less scintillation than Gaussian beam. Kolmogorov spectrum function is utilized in the scintillation calculations.     

A slant on warped extra dimensions

We propose an orbifolded, warped, extra dimension scenario in which the visible brane is not parallel to the hidden brane. This leads automatically to Lorentz violation in the visible, four-dimensional world. The background solution to the Einstein equations is a function of a parameter that can be identified with the amount of ‘tilting’ of the brane. The cosmological constant is found to coincide with the classic Randall–Sundrum value to the first order in this tilt. Lorentz violating effects induced in the Standard Model are considered. We find that the strongest constraint on the tilt comes from determinations of the electron–proton mass ratio in six quasar spectra (four optical and two radio). Measurements of a third radio source could improve this by an order of magnitude.     

Comments on gauge-invariance in cosmology

We revisit the gauge issue in cosmological perturbation theory, and highlight its relation to the notion of covariance in general relativity. We also discuss the similarities and differences of the covariant approach in perturbation theory to the Bardeen or metric approach in a non-technical fashion.     

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.     

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.     

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

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.     

Standpoint cosmology

An unorthodox cosmology is based on a notion of standpoint, distinguishing past from future, realized through Hilbert-space representation of the complex conformai group for 3+1spacetime and associated coherent states. Physical (approximate) symmetry attaches to eight-parameter complex Poincaré displacements, interpretable as growth of standpoint age (one parameter), boost of matter energy-momentum in standpoint rest frame (three parameters) and displacement of matter location in a compact U(1)O(4)/O(3) spacetime attached to standpoint (four parameters). An initial condition (at big bang) is characterized by a huge dimensionless parameter that breaks dilation invariance. Four major length scales are recognized, called Planck, particle, lab, and Hubble, with separations controlled by ; all physical concepts, including spacetime, depend on wideness of scale separation.     

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.     

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.