1,721,029 research outputs found
The extreme limit of the generalised Chaplygin gas
No full textUnified Dark Matter models describe Dark Matter and Dark Energy as a single entity which is, in the simplest case, embodied in a perfect barotropic fluid. It is a well-established fact that small adiabatic perturbations of Unified Dark Matter have an evolution characterised by oscillations and decay which provide predictions on the Cosmic Background Radiation anisotropies which are in poor agreement with observation. In this paper we investigate the generalised Chaplygin gas and we find that the Integrated Sachs-Wolfe effect excludes the model for 10^{-3} < alpha < 350. We discuss the implications on the background evolution of the Universe if large values of are considered. In this case, the Universe expansion mimics a matter-dominated phase abruptly followed by a de Sitter one at the transition redshift . Thanks to an analysis of the type Ia supernovae Constitution set we are able to place
COSMOLOGY AND UNIFIED DARK MATTER: The Chaplygin gas and beyond
No full textUnified Dark Matter models aim to describe thedynamical properties of Dark Matter and Dark Energyas different aspects of the same entity which is, inthe simplest case, embodied in a perfect barotropicfluid.Such models, though successful in passingobservational tests on the expansion rate of theUniverse, may present drawbacks in their perturbativestability. Indeed, to be in agreement withobservation, they must possess a sufficiently smallspeed of sound. In this thesis, two counterexamples to this generalconclusion are put forward. First, addressing the generalised Chaplygin gas, onefinds strong hints on the viability of the model alsowhen its asymptotic speed of sound is sufficiently large.Second, a new class of Unified Dark Matter models isintroduced. They are characterised by a fasttransition from a matter-dominated era to a phasewith constant negative pressure. It turns out that, if the transition is fast enough,these models may predict a satisfactory structureformation process and cosmic background radiation nisotropies pattern
Does the cosmological constant stay hidden?
No full textWe elaborate on the proposal of [Phys. Rev. Lett. 123, 131302 (2019). PRLTAO0031-900710.1103/PhysRevLett.123.131302], about the possibility of hiding the cosmological constant in the complicated topology that one expects to exist at the Planck scale. We build a differential equation ruling the time evolution of ⟨K⟩, the spatial average of the expansion scalar. Supposing that the solution ⟨K⟩=0 exists despite the presence of a large cosmological constant Λ, we show that such a solution seems to be unstable
Lecture notes in cosmology
No full textCosmology has become a very active research field in the last decades thanks to the impressing improvement of our observational techniques which have led to landmark discoveries such as the accelerated expansion of the universe, and have put physicists in front of new mysteries to unveil, such as the quest after the nature of dark matter and dark energy. These notes offer an approach to cosmology, covering fundamental topics in the field: the expansion of the universe, the thermal history, the evolution of small cosmological perturbations and the anisotropies in the cosmic microwave background radiation. Some extra topics are presented in the penultimate chapter and some standard results of physics and mathematics are available in the last chapter in order to provide a self-contained treatment. These notes offer an in-depth account of the above-mentioned topics and are aimed to graduate students who want to build an expertise in cosmology
Erratum: Lensing in the McVittie metric (Physical Review D - Particles, Fields, Gravitation and Cosmology (2016) 93 (024020) DOI: 10.1103/PhysRevD.93.024020)
No full textLate-times asymptotic equation of state for a class of nonlocal theories of gravity
No full textWe investigate the behavior of the asymptotic late-times effective equation of state for a class of nonlocal theories of gravity. These theories modify the Einstein-Hilbert Lagrangian introducing terms containing negative powers of the d’Alembert operator acting on the Ricci scalar. We find that imposing vanishing initial conditions for the nonlocal content during the radiation-dominated epoch implies the same asymptotic late-times behavior for most of these models. In terms of the effective equation of state of the Universe, we find that asymptotically
ωeff→−1, approaching the value given by a cosmological constant. On the other hand, unlike in the case of Λ CDM, the Hubble factor is a monotonic growing function that diverges asymptotically. We argue that this behavior is not a coincidence and discuss under which conditions this is to be expected in these nonlocal models
On the Effect of the Cosmological Expansion on the Gravitational Lensing by a Point Mass
No full textWe analyse the effect of the cosmological expansion on the deflection of light caused by a point mass, adopting the McVittie metric as the geometrical description of a point-like lens embedded in an expanding universe. In the case of a generic, non-constant Hubble parameter, H, we derive and approximately solve the null geodesic equations, finding an expression for the bending angle δ, which we expand in powers of the mass-to-closest approach distance ratio and of the impact parameter-to-lens distance ratio. It turns out that the leading order of the aforementioned expansion is the same as the one calculated for the Schwarzschild metric and that cosmological corrections contribute to δ only at sub-dominant orders. We explicitly calculate these cosmological corrections for the case of the H constant and find that they provide a correction of order 10−11 on the lens mass estimate
Asymptotically Schwarzschild solutions in f (R) extension of general relativity
No full textWe address the question of how to build a class of f(R) extensions of General Relativity which are compatible with solar system experiments, without making any preliminary assumption on the properties of f. The aim is reached by perturbatively solving the modified Einstein equations around a Schwarzschild background and retrieving a posteriori the corresponding f(R). This turns out to be non analytical in R=0 and should be intended as the leading correction to the Einstein-Hilbert action in the low curvature limit. The parameters characterizing the f(R) class are then set by constraining the corrections to four different local tests with the observations. The result is a class of f(R) theories built up from a purely bottom-up approach and compatible with the local tests. At a more general level, this result can help constraining exact f(R) models working in Cosmology, since it provides the correct local limit. Further developments and possible extensions of the approach to Cosmology are also discussed
Redshift drift of gravitational lensing
No full textWe investigate the effect of the redshift drift in strong gravitational lensing. The redshift drift produces a time variation of (i) the apparent position of a lensed source and (ii) the time delay among incoming signals from different images. We dub these effects as angular drift and time delay drift and show that they are of order of 10-10 arc seconds per year and 10-3 seconds per year, respectively. With present techniques there is apparently no hope of detecting such small changes for time intervals smaller than 109 years
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