1,721,000 research outputs found
Inhomogeneous post-inflationary Lambda-CDM cosmology as a moduli space expansion
No full textWe model the large scale late time universe as a Lambda-CDM cosmology driven by cosmological constant and perfect dust fluid. Our aim is to find new solutions in the matter and Lambda epoch consistent with inflationary initial conditions, namely that to the far past in the matter era the cosmology tends to a flat FLRW solution. We identify the moduli degrees of freedom that parametrize the flat Lambda-dust FLRW solution and then promote these moduli to slowly varying functions of the spatial coordinates and show how to solve the Einstein equations in a comoving gradient expansion, controlled by the cosmological constant length scale. Our initial conditions ensure that the approximation remains under control to the far past of the matter era, and to the far future of Lambda domination. The solution is fully non-perturbative in the amplitude of the metric deformation, and we explicitly construct it to fourth order in derivatives. A general Lambda-dust universe dominated by Lambda in the future is characterized by a 3-metric and a stress tensor (with positive trace) defined on the future conformal boundary. The new cosmologies with inflationary initial conditions are characterized only by the boundary 3-metric, the stress tensor being locally determined entirely in terms of that metric
Holographic renormalization for coincident Dp-branes
No full textWe consider holographic renormalization for the decoupling limit of coincident Dp-branes. We truncate the theory to the supergravity sector which is homogeneous on the (8-p)-sphere and carries only RR electric (p+2)-flux, leaving a graviton and two scalar degrees of freedom associated to the dilaton and the sphere radius. We non-linearly construct the asymptotic graviton and dilaton deformations - the analog of the Graham-Fefferman expansion for AdS/CFT - and compute counterterms to give a finite renormalized bulk action and dual one point functions. Restricting to linear deformations we find additional counterterms to include the remaining sphere deformations which strongly deform the asymptotic behaviour
Late time solutions for inhomogeneous<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Λ</mml:mi><mml:mi>CDM</mml:mi></mml:math>cosmology, their characterization, and observation
No full textAssuming homogeneous isotropic Lambda-CDM cosmology allows Lambda, spatial curvature and dark matter density to be inferred from large scale structure observations such as supernovae. The purpose of this paper is to extend this to allow observations to measure or constrain inhomogeneity and anisotropy. We obtain the general inhomogeneous anisotropic Lambda-CDM solution which is locally asymptotic to an expanding de Sitter solution as a late time expansion using Starobinsky's method (analogous to the `holographic renormalization' technique in AdS/CFT) together with a resummation of the series. The dark matter is modeled as perfect dust fluid. The terms in the expansion systematically describe inhomogeneous and anisotropic deformations of an expanding FLRW solution, and are given as a spatial derivative expansion in terms of data characterizing the solution - a 3-metric and a perturbation of that 3-metric. Leading terms describe inhomogeneity and anisotropy on the scale set by the cosmological constant, approximately the horizon scale today. Higher terms in the expansion describe shorter scale variations. We compute the luminosity distance-redshift relation and argue that comparison with current and future observation would allow a partial reconstruction of the characterizing data. We also comment on smoothing these solutions noting that geometric flows (such as Ricci flow) applied to the characterizing data provide a canonical averaging method
Warm p-soup and near extremal black holes
No full textWe consider a model of D-dimensional supergravity coupled to elementary p-branes. We use gravitational arguments to deduce the low energy effective theory of N nearly parallel branes. This is a (p+1)-dimensional scalar field theory, where the scalars represent the positions of the branes in their transverse space. We propose that the same theory in a certain temperature regime describes a `soup' of strongly interacting branes, giving a microscopic description of near extremal black p-branes. We use natural approximations to estimate the energy density of this soup as a function of the physical parameters; N, temperature, brane tension and gravitational coupling. We also characterise the horizon radius, measured in the metric natural to the branes, with the thermal vev of the scalars. For both quantities we find agreement with the corresponding supergravity black brane results. Surprisingly, beyond the physical parameters, we are naturally able to reproduce certain irrational factors such as pi's. We comment on how these ideas may explain why black hole thermodynamics arises in gauge theories with holographic duals at finite temperature
TeVeS gets caught on caustics
No full textTeVeS uses a dynamical vector field with timelike unit norm constraint to specify a preferred local frame. When matter moves slowly in this frame - the so-called quasi-static regime - Modified Newtonian Dynamics (MoND) results. Theories with such vectors (such as Einstein-aether) are prone to the vector dynamics forming singularities which render their classical evolution problematic. Here we analyse the dynamics of the vector in TeVeS in various situations. We find that, quite generically, the vector field develops caustic singularities on time scales of order the gravitational in-fall time. Having shown singularity formation is generic with or without matter, Bekenstein's original formulation of TeVeS appears dynamically problematic. We argue that by modifying the vector field kinetic terms to the more general form used by Einstein-Aether this problem may be avoided
Moduli dynamics as a predictive tool for thermal maximally supersymmetric Yang-Mills at large N
Full text linkMaximally supersymmetric (p+1)-dimensional Yang-Mills theory at large N and finite temperature, with possibly compact spatial directions, has a rich phase structure. Strongly coupled phases may have holographic descriptions as black branes in various string duality frames, or there may be no gravity dual. In this paper we provide tools in the gauge theory which give a simple and unified picture of the various strongly coupled phases, and transitions between them. Building on our previous work we consider the effective theory describing the moduli of the gauge theory, which can be computed precisely when it is weakly coupled far out on the Coulomb branch. Whilst for perturbation theory naive extrapolation from weak coupling to strong gives little information, for this moduli theory naive extrapolation from its weakly to its strongly coupled regime appears to encode a surprising amount of information about the various strongly coupled phases. We argue it encodes not only the parametric form of thermodynamic quantities for these strongly coupled phases, but also certain transcendental factors with a geometric origin, and allows one to deduce transitions between the phases. We emphasise it also gives predictions for the behaviour of a large class of local operators in these phases
Physical constraints from holographic duality
Full text linkThe material presented in this thesis mainly consists of two parts under the umbrella topic of Holographic Duality or the AdS/CFT correspondence.
In Part II we are mainly interested in the properties of energetic quantities, namely, the energy and free energy of Conformal Field Theory (CFT) under de- formations of spacetime geometry in (2+1) dimensions. By using holography together with analytic and numerical techniques, we find many results on monotonic decreasing of the energy/free energy for static solutions of gravitational theory in asymptotically locally Anti-de Sitter (AlAdS) spacetime and correspondingly for a CFT dual in many different scenarios. These results put nontrivial physical bounds on the energy/free energy of holographic CFT in curved spaces.
In Part III, in chapter 6 we study the Gregory-Laflamme (GL) instability of charged black strings in five dimensions. The physical properties of GL unstable mode perturbations and the consequences of the instability are studied in detail. We also solve for numerical solutions of nonuniform phase of charged black strings and obtain a thermodynamic phase structure of charged black strings. In chapter 7, we apply the holographic duality to a black string solution with an extra string charge in the type-IIA supergravity. We utilize the similarity between the properties of charged black strings in five dimensions and our system in type-IIA supergravity to obtain the result on thermodynamics of its holographic dual which is a two- dimensional supersymmetric Yang-Mills theory on a circle in a boosted frame.Open Acces
Gravitation and the fundamental nature of spacetime
No full textThis Thesis investigates three gravitational theories that propose alternatives to the notion of spacetime as four-dimensional, diffeomorphism-invariant manifold.
Firstly, we study spherically symmetric configurations in ghost‐free de Rham–Gabadadze–Tolley massive gravity—a bimetric theory that breaks General Relativity’s diffeomorphism invariance by giving the graviton a mass. In the limit of small graviton mass, our result restricts non-relativistic matter so that the pressure is bounded from below in terms of the density and graviton mass in a manner that is at odds with a reasonable phenomenology.
Then, we examine higher-dimensional gravity, with extra compact dimensions, in the context of Kaluza-Klein theory. Focusing on vacuum solutions in five dimensions, we consider static black hole configurations and conjecture that, for fixed mass and sufficiently small compactification radius, the only solutions are homogeneous black strings. Supporting this conjecture, we derive bounds on inhomogeneities and develop a local rigidity theorem through elliptic analysis.
Finally, we turn to Causal Set Theory, a quantum gravity framework in which spacetime is fundamentally discrete at the Planck scale and is modelled as an irregular Lorentzian lattice. Within this setting, we develop a quantum field theory formalism, deriving a diagrammatic expansion for in-in correlators in local scalar field theories with finite polynomial interactions. This result exhibits manifest causality and terminates at finite order in the interaction coupling, highlighting how the fundamental discreteness acts as a natural cut-off, eliminating the UV divergences.Open Acces
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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