[en] Nonlocal cosmological models are studied extensively in recent times because of their interesting cosmological consequences. In this paper, we have analysed the background cosmology on a class of non-local models which are motivated by the perturbative nature of gravity at the infrared scale. We show that inflationary solutions are possible in all constructed non-local models. However, exit from inflation to the RD era is not possible in most of the models. (paper)

[en] A list of 556 references, covering the subject of cosmology, listed in alphabetical order by first author (A to K only), is given. (U.K.)

[en] Recent observations suggest that a large fraction of the energy density of the Universe has negative pressure. One explanation is vacuum energy density; another is quintessence in the form of a scalar field slowly evolving down a potential. In either case, a key problem is to explain why the energy density nearly coincides with the matter density today. The densities decrease at different rates as the Universe expands, so coincidence today appears to require that their ratio be set to a specific, infinitesimal value in the early Universe. In this paper, we introduce the notion of a open-quotes tracker field,close quotes a form of quintessence, and show how it may explain the coincidence, adding new motivation for the quintessence scenario. copyright 1999 The American Physical Society

[en] Determining what causes cosmic acceleration is a big challenge in cosmology: it might be a cosmological constant (Λ-CDM) or a new fluid (dark energy, DE) or a new force that modifies gravity as described by Einstein (Modified Gravity, MG). Light coming from a faraway galaxy is deflected by the presence of dark matter along the way: this causes a distortion (shear) of the galaxy image we observe. Shear measurement can be used to reconstruct dark matter, i.e. the lens that deflects light. This effect is called gravitational lensing and is studied statistically, as the deflection is typically tiny (weak lensing, WL). DE and MG change, in turn, the lens and its lensing potential. WL is therefore also a powerful probe to test DE and MG. CEA plays a major role in galaxy surveys that measure WL, from the instrumentation development, to the image reconstruction of galaxy shapes, to the data interpretation. In particular, researchers at CEA have extracted information from WL data, hidden to traditional analyses. CEA researchers have compared different WL observables and identified the one that helps the most in distinguishing different theoretical models: peak count statistics. This result demonstrates how the choice of the right statistical observable plays a crucial role in data interpretation and can decide whether, with the same data, we make a discovery or not. (A.C.)

[en] We study the island universe model, in which initially the universe is in a cosmological constant sea, then the local quantum fluctuations violating the null energy condition create the islands of matter, some of which might correspond to our observable universe. We examine the possibility that the island universe model is regarded as an alternative scenario of the origin of observable universe

[en] We introduce the notion of the cosmic numbers of a cosmological model, and discuss how they can be used to naturally classify models according to their ability to solve some of the problems of the standard cosmological model

[en] We investigate the thermal history in the universe with a variable cosmological term. In particular, we show how the formation of important molecules such as H₂ and HD is influenced during the epoch for the redshift from 10000 to 1. These molecules should dominate the cooling process of primordial gas clouds which triggers the collapse of them. It is found that the molecular formation is quickened by Δz ∼ 1000 compared with the case in the standard model due to the variable cosmological term. The qualitative estimate of the mass of the first objects has been performed with use of the time scales related to the thermal evolution. It is suggested that the formation scenarios of the first objects studied so far could be changed drastically if the variable cosmological term is included in the thermal history in the universe

[en] Recently, the notion that the number of vacua is astronomical has received increased attentions, which may be regarded as a possible anthropical explanation to incredibly small cosmological constant. Further, a dynamical mechanism to implement this possibility is required. We show in an operable model of cyclic universe that the universe can experience many cycles with different vacua, which is a generic behavior independent of the details of the model. This might provide a distinct dynamical approach to an anthropically favorable vacuum

[en] In this paper we discuss a hierarchical model of Universe which was proposed by Wesson (1975). We assert that the mathematical formalism used by Wesson leads to contradiction with Einstein's equations. (orig.)

[en] We show that when the antisymmetric Kalb-Ramond field is included in the Randall-Sundrum scenario, although the hierarchy problem can be solved, it requires an extreme fine tuning of the Kalb-Ramond field (about 1 part in 1062). We interpret this as the return of the problem in disguise. Further, we show that the Kalb-Ramond field induces a small negative cosmological constant on the visible brane