On detecting diffuse ultraviolet radiation of cosmological origin

Summary The intensity of diffuse extragalactic background radiation is determined by the integrated emission of all sources along a given line of sight through the Universe and by absorption due to intervening cool gas and dust. Due to red-shift and lookback-time effects, more distant objects contribute at a given wave-length today with light emitted at earlier epochs at shorter wave-lengths. Possible sources of ultraviolet (100÷3000) ? emission include, but may not be restricted to, galaxies and quasars, a lukewarm or photoionized dense intergalactic medium and a cosmological sea of decaying massive neutrinos and photinos formed in the hot big bang. The resultant complex spatial intensity pattern and spectral distribution will appear superimposed on an intense galactic emission source whose main characteristics have yet to be clearly and satisfactorily established. Although substantial progress has been made recently in understanding galactic emission mechanisms, the remaining uncertainties substantially limit the accuracy with which a cosmologically significant signal may be disentangled from the diffuse galactic background ?noise?. On assignment from the Astrophysics Division, Space Science Department, European Space Agency.     

High-energy gamma-ray sources of cosmological origin

The current generation of instruments in gamma-ray astrophysics launched a new era in the search for a dark matter signal in the high-energy sky. Such searches are said indirect, in the sense that the presence of a dark matter particle is inferred from the detection of products of its pair-annihilation or decay. They have recently started to probe the natural domain of existence for weakly interacting massive particles (WIMPs), the favorite dark matter candidates today. In this article, we review the basic framework for indirect searches and we present a status of current limits obtained with gamma-ray observations. We also devote a section to another possible class of cosmological gamma-ray sources, primordial black holes, also considered as a potential constituent of dark matter.     

On the vanishing of the cosmological constant

A lagrangian is proposed for a Kaluza-Klein theory of gravity which has a vanishing cosmological constant as stable fixed point.     

On some cosmological problems

A regular solution describing a periodic expansion – contraction process of the Universe is derived. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 46–51, December, 2008.     

On the dynamics of cosmological particle production

The time evolution of the particle number density and spectrum of massive scalar particles, coupled conformally to a classical Friedman-Robertson-Walker gravitational field is followed numerically. Not only for a pure radiation cosmos, but also for one with an inflationary interlude, the density of particles produced remains constant in time. This results in a constant equivalent temperature scale up to the Compton time of the massive particles, and opens the possibility that during a sufficiently long inflation the energy density of the particles produced can exceed that of the radiation background.     

On the cosmological constant problem and brane-world geometry

The cosmological constant problem is examined within the context of the covariant brane-world gravity, based on Nash’s embedding theorem for Riemannian geometries. We show that the vacuum structure of the brane-world is more complex than General Relativity’s because it involves extrinsic elements, in specific, the extrinsic curvature. In other words, the shape (or local curvature) of an object becomes a relative concept, instead of the “absolute shape” of General Relativity. We point out that the immediate consequence is that the cosmological constant and the energy density of the vacuum quantum fluctuations have different physical meanings: while the vacuum energy density remains confined to the four-dimensional brane-world, the cosmological constant is a property of the bulk’s gravitational field that leads to the conclusion that these quantities cannot be compared, as it is usually done in General Relativity. Instead, the vacuum energy density contributes to the extrinsic curvature, which in turn generates Nash’s perturbation of the gravitational field. On the other hand, the cosmological constant problem ceases to be in the brane-world geometry, reappearing only in the limit where the extrinsic curvature vanishes.     

On the concept of theJ/gy formation time in quark-gluon plasma

We analyze the rate with which theJ/ψ content of an originally narrow wave packet of charmed quark-antiquark pair is diminishing in a medium which screens colour charges. TheJ/ψ content decreases to 50% within a typical time of 0.2–0.3 fm. Taking this into account, we find thep _T -dependence ofJ/ψ suppression by quark-gluon plasma to be much flatter than in simplified calculations based on the semi-classical picture ofJ/ψ formation time.     

On the variation of the effective cosmological term

In the framework of classical field theory, we try to explain why the effective cosmological constant is so small. The basis of the attempt is a Higgs field that shall determine the global structure of the universe. Einstein's theory of gravitation does not allow one to realize the idea. But we are successful if we start from some variant of the scalar-tensor theory of gravitation, choosing for the parameters that enter the Lagrangean of the Higgs field the Compton length of a proton and Eddington's number as self-coupling constant.     

On the thermodynamic and quantum fluctuations of the cosmological constant

We discuss, from a condensed-matter point of view, the recent idea that the Poisson fluctuations of the cosmological constant about zero could be a source of the observed dark energy [1, 2]. We argue that the thermodynamic fluctuations of Λ are much bigger. Since the amplitude of the fluctuations is ∝ V^?1/2, where V is the volume of the universe, the present constraint on the cosmological constant provides a lower limit for V that is much larger than the volume within the cosmological horizon.     

On the invariant content of the concept of a reference system of world time

In this paper a reference system of world time is studied. It is shown that the Lorentz gauge conditions, generally speaking, determine its basic tetrads up to rigid spatial rotations with constant coefficients.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 49–52, February, 1978.     

On quintessential cosmological models and exponential potentials

We first study dark energy models with a minimally-coupled scalar field and generalized exponential potentials, admitting exact solutions for the cosmological equations: actually, it turns out that for this class of potentials the Einstein field equations exhibit alternative Lagrangians, and are completely integrable and separable. We analyze their analytical solutions, especially discussing when they are compatible with a late time quintessential expansion of the universe. As a further issue, we discuss how quintessential scalar fields with exponential potentials can be connected to the inflationary phase, building up a quintessential inflationary scenario: actually, it turns out that the transition from inflation toward late-time exponential quintessential tail admits a kination period, which is an indispensable ingredient of this kind of theoretical models. All such considerations have been made by including also radiation into the model.     

The osgood criterion and finite‐time cosmological singularities

In this paper, we apply Osgood's criterion from the theory of ordinary differential equations to detect finite‐time singularities in a spatially flat FLRW universe in the context of a perfect fluid, a perfect fluid with bulk viscosity, and a Chaplygin and anti‐Chaplygin gas. In particular, we applied Osgood's criterion to demonstrate singularity behaviour for Type 0/big crunch singularities as well as Type II/sudden singularities. We show that in each case the choice of initial conditions is important as a certain number of initial conditions leads to finite‐time, Type 0 singularities, while other precise choices of initial conditions which depend on the cosmological matter parameters and the cosmological constant can avoid such a finite‐time singularity. Osgood's criterion provides a powerful and yet simple way of deducing the existence of these singularities, and also interestingly enough, provides clues of how to eliminate singularities from certain cosmological models.     

On the induced cosmological term in quantum gravity

Tadpole diagrams where zero-momentum gravitons couple to massive matter loops lead to divergences which are not a consequence of infinite momentum integrals, but of the masslessness of the gravitons. It is shown that there exists no definition of these diagrams consistent with the Ward identities. They can be eliminated by an appropriate gauge choice, but then the BRS symmetry is spontaneously broken. Also in the scalar-tensor, conformally invariant formulation of quantum gravity, the tadpole problem does survive. The tadpole diagrams can, however, be cancelled by a cosmological counter-term. In that case, the Ward identities are satisfied.