Four-dimensional Cosmology Research Articles

Shedding light on dark bubble cosmology

Dark bubble cosmology is an alternative paradigm to compactification, which can circumvent issues of moduli stabilization and scale separation. In this paper we investigate how electromagnetic fields can be incorporated in this framework. Worldvolume fields backreact on the ambient universe in which the bubble expands, which in turn affects the energy-momentum distribution and the effective gravity induced on the brane. We compute these effects, showing that the resulting four-dimensional cosmology consistently includes electromagnetic waves.

De Sitter braneworld and gravitational waves

We study the braneworld theory constructed by multi-scalar fields. The model contains a smooth and infinitely large extra dimension, allowing background fields to propagate in it. We give a de Sitter solution for the four-dimensional cosmology as a good approximation to the early universe inflation. We show that the graviton has a localizable massless mode, and a series of continuous massive modes, separated by a mass gap. There could be a normalizable massive mode, depending on the background solution. The gravitational waves of the massless mode evolve as in the four-dimensional theory, while those of the massive modes evolve very differently from the massless mode.

De Sitter cosmology on an expanding bubble

Constructing an explicit compactification yielding a metastable de Sitter (dS) vacuum in a UV consistent string theory is an incredibly difficult open problem. Motivated by this issue, as well as the conjecture that all non-supersymmetric AdS vacua must decay, we discuss the alternative possibility of realizing an effective four-dimensional dS cosmology on a codimension-one bubble wall separating two AdS5 vacua. The construction further elaborates on the scenario of arXiv:1807.01570, where the aforementioned cosmology arises due to a non-perturbative decay and is embedded in a five-dimensional bulk in a time­ dependent way. In this paper we discuss the relation between this scenario and the weak gravity conjecture and further develop the details of the four-dimensional cosmology. We provide a bulk interpretation for the dS temperature as the Unruh temperature experienced by an accelerated observer riding the bubble. A source of four-dimensional matter arises from a string cloud in the bulk, and we examine the consequences for the particle mass spectrum. Furthermore, we show how effective four-dimensional Einstein gravity on the bubble is obtained from the five-dimensional Gauss equation. We conclude by outlining some implications that this paradigm will have for holography, inflation, the standard model, and black holes.

Scalar Tensor Cosmology With Kinetic, Gauss-Bonnet and Nonminimal Derivative Couplings and Supersymmetric Loop Corrected Potential

We discuss a particular four-dimensional cosmology based on non-minimal scalar tensor theories characterized by a supersymmetric loop corrected potential and a Hubble parameter defined as a function of the scalar field. Power-law solutions are obtained in the FRW background giving rise to acceleratedly expanding universe characterized by a scale factor and a scalar field depending both on the non-minimal coupling parameter ξ. Based on SNeIa data and on Hubble data X-ray gas mass fraction measurements, we find 0.116 < ξ < 0.225 which results on a universe dominated by vacuum energy.

Study of Branes with Variable Tension**This work was partially founded by FONDECYT Regular under Grant Nos. 1151107, 1131075, 1140296, UNAB DI-735-15/R and DPI20140115

In this work we study a brane world model with variable tension, which gives rise to four-dimensional cosmologies. The brane worlds obtained correspond to Eötvös branes whose (internal) geometry can be casted as either a four-dimensional (A)dS4 or a standard radiation period cosmology. The matter dominated period is discussed as well.

Higher-order gravity in higher dimensions: geometrical origins of four-dimensional cosmology?

Determining the cosmological field equations is still very much debated and led to a wide discussion around different theoretical proposals. A suitable conceptual scheme could be represented by gravity models that naturally generalize Einstein theory like higher-order gravity theories and higher-dimensional ones. Both of these two different approaches allow one to define, at the effective level, Einstein field equations equipped with source-like energy-momentum tensors of geometrical origin. In this paper, the possibility is discussed to develop a five-dimensional fourth-order gravity model whose lower-dimensional reduction could provide an interpretation of cosmological four-dimensional matter–energy components. We describe the basic concepts of the model, the complete field equations formalism and the 5-D to 4-D reduction procedure. Five-dimensional f(R) field equations turn out to be equivalent, on the four-dimensional hypersurfaces orthogonal to the extra coordinate, to an Einstein-like cosmological model with three matter–energy tensors related with higher derivative and higher-dimensional counter-terms. By considering the gravity model with f(R)=f_0R^n the possibility is investigated to obtain five-dimensional power law solutions. The effective four-dimensional picture and the behaviour of the geometrically induced sources are finally outlined in correspondence to simple cases of such higher-dimensional solutions.

Cosmic Initial Singularities in a Single Repeating Universe as Opposed to Their Behavior in a Multiverse

When the initial radius of the universe is set in four dimensions and if there is only ONE repeating universe, then the initial radii of the universe is R → 0 or gets very close to zero if we use the Einstein Equations modified by Stoica. The Einstein Equations are reset by Stoical in a formalism which removes in four dimensions, the big bang singularity pathology. So then the reason for Planck length no longer holds. This manuscript assumes a repeating single universe. We present entanglement entropy in the early universe with a shrinking scale factor, due to Muller and Lousto, and show that there are consequences due to initial entangled for a time dependent horizon radius in cosmology, with (flat space conditions) for conformal time. Even if the 3-dimensional spatial length goes to zero. Our new manuscript presentation sets as a starting point a cosmology with a non-zero Λ vacuum energy. The non-zero Λ vacuum energy, initial configuration of the universe permits us to keep in an information theory stand point (information theory), computational bits for our configuration of cosmological expansion. This assemblage of computational bits occurs in cosmological evolution even if in an initial four-dimensional cosmology, we have the initial radii of the universe R → 0. We also find that in the case of a multiverse, such considerations will not hold and that cosmic singularities have a more different characteristic in the multiverse setting than in the single universe repeated over and over again, i.e. using an argument borrowed and modified from Kauffman, the multiverse will not mandate “perfect” singularities. The existence of a multiverse may allow for non zero singularities in lieu with the Kauffman argument cited at the end of the document, plus the lower pre big bang temperatures which may allow for the survival of gravitons just before the onset of the cosmological expansion phase, if a multiverse exists embedding our present universe.

Matrix model approach to cosmology

We perform a systematic search for rotationally invariant cosmological solutions to matrix models, or more specifically the bosonic sector of Lorentzian IKKT-type matrix models, in dimensions $d$ less than ten, specifically $d=3$ and $d=5$. After taking a continuum (or commutative) limit they yield $d-1$ dimensional space-time surfaces, with an attached Poisson structure, which can be associated with closed, open or static cosmologies. For $d=3$, we obtain recursion relations from which it is possible to generate rotationally invariant matrix solutions which yield open universes in the continuum limit. Specific examples of matrix solutions have also been found which are associated with closed and static two-dimensional space-times in the continuum limit. The solutions provide for a matrix resolution of cosmological singularities. The commutative limit reveals other desirable features, such as a solution describing a smooth transition from an initial inflation to a noninflationary era. Many of the $d=3$ solutions have analogues in higher dimensions. The case of $d=5$, in particular, has the potential for yielding realistic four-dimensional cosmologies in the continuum limit. We find four-dimensional de Sitter $dS^4$ or anti-de Sitter $AdS^4$ solutions when a totally antisymmetric term is included in the matrix action. A nontrivial Poisson structure is attached to these manifolds which represents the lowest order effect of noncommutativity. For the case of $AdS^4$, we find one particlular limit where the lowest order noncommutativity vanishes at the boundary, but not in the interior.

A FIVE-DIMENSIONAL MODEL WITH A FIFTH DIMENSION AS FUNDAMENTAL AS TIME IN TERMS OF A COSMOLOGICAL APPROACH

In five-dimensional cosmological models, the convention is to include the fifth dimension in a way similar to the other space dimensions. In this paper, we attempt to introduce the fifth dimension in a way that a time dimension would be introduced. With such an internal space, we are able to obtain accelerated expansion without introducing dark energy. We obtain a five-dimensional flat, meaning both Ricci flat and conformally flat, spacetime into which all relevant four-dimensional cosmologies can be locally embedded. We also argue on the choice of the cosmological frame. The choice that is simplest and most convenient in terms of dimensional analysis, amounts to a linearly expanding universe.

The Quest for a Realistic Four-dimensional Cosmology

The existence of a small and positive cosmological constant attributed to gravitational vacuum energy (or dark energy) in the present-day universe appears to be the most pressing obstacle as well as opportunity to significantly improving the models of four-dimensional cosmology from fundamental theories of gravity, including string theory and modern Kaluza-Klein theories. In seeking to resolve this problem, one naturally wonders if the real world can somehow be interpreted as an inflating de Sitter "brane" embedded in a five or even higher-dimensional space-time described by warped or non-factorizable geometry. In this scenario, the four-dimensional cosmological constant may well be determined in terms of two length scales: one is a scale associated with the size of extra dimensions and the other is a scale associated with the expansion rate of the universe, which is also related to the warping of extra spaces.Key words: CosmologyThe Himalayan Physics Vol.1, No.1, May, 2010Page: 18-24Uploaded Date: 28 July, 2011

Quintessence cosmology with a traversable wormhole, massive decaying gravitons and with varying G and Λ

In this work, we explore many cosmological implications of a four-dimensional cosmology dominated by quintessence with a static traversable wormhole, spatio-temporal varying G and by taking into account a decaying cosmological constant and a decaying graviton mass by means of an additional bimetric tensor in Einstein’s field equations proposed by Visser in 1998.

Accelerating universe from warped extra dimensions

Accelerating universe or the existence of a small and positive cosmological constant is probably the most pressing obstacle as well as opportunity to significantly improving the models of four-dimensional cosmology from fundamental theories of gravity, including string theory. In seeking to resolve this problem, one naturally wonders if the real world can somehow be interpreted as an inflating de Sitter brane embedded in a higher dimensional spacetime described by warped geometry. In this scenario, the four-dimensional cosmological constant may be uniquely determined in terms of two length scales: one is a scale associated with the size of extra dimensions and the other is a scale associated with the expansion rate of our universe.

Microscopic primordial black holes and extra dimensions

We examine the production and evolution of microscopic black holes in the early universe in the large extra dimensions scenario. We demonstrate that, unlike in the standard four-dimensional cosmology, in large extra dimensions absorption of matter from the primordial plasma by the black holes is significant and can lead to rapid growth of the black hole mass density. This effect can be used to constrain the conditions present in the very early universe. We demonstrate that this constraint is applicable in regions of parameter space not excluded by existing bounds.

Slow-roll corrections to inflaton fluctuations on a brane

Quantum fluctuations of an inflaton field, slow rolling during inflation are coupled to metricfluctuations. In conventional four-dimensional cosmology one can calculate the effect ofscalar metric perturbations as slow-roll corrections to the evolution of a massless free fieldin de Sitter spacetime. This gives the well-known first-order corrections to the fieldperturbations after horizon exit. If inflaton fluctuations on a four-dimensional braneembedded in a five-dimensional bulk spacetime are studied to first order in slow roll, thenwe recover the usual conserved curvature perturbation on superhorizon scales. But on smallscales, at high energies, we find that the coupling to the bulk metric perturbationscannot be neglected, leading to a modified amplitude of vacuum oscillations onsmall scales. This is a large effect which casts doubt on the reliability of the usualcalculation of inflaton fluctuations on the brane neglecting their gravitational coupling.

Axino dark matter in brane world cosmology

We discuss dark matter in the brane world scenario. We work in the Randall–Sundrumtype II brane world and assume that the lightest supersymmetric particle is the axino.We find that the axinos can play the role of cold dark matter in the universe,provided that the five-dimensional Planck mass is bounded both from below andfrom above. This is possible for higher reheating temperatures compared to theconventional four-dimensional cosmology due to a novel expansion law for the universe.

A no-go theorem for accelerating cosmologies from M-theory compactifications

It is known that four-dimensional cosmologies exhibiting transient phases of acceleration can be obtained by compactifications of low-energy effective string or M-theory on time-varying manifolds. In the four-dimensional theory, the acceleration can be attributed to a quintessential scalar field with a positive effective potential. Recently, Townsend has conjectured that the potentials obtained by such compactifications cannot give rise to late-time accelerating universes which possess future event horizons. Such a ‘no-go’ result would be desirable, since current string or M-theory seems unable to provide an adequate description of space–times with future event horizons. In this Letter, we provide a proof of this conjecture for a class of warped compactifications with a single scalar modulus parametrising the volume of the compactification manifold.

Einstein spaces in warped geometries in five dimensions

We investigate five dimensional Einstein spaces in warped geometries from the point of view of the four dimensional physically relevant Robertson-Walker-Friedman cosmological metric and the Schwarzschild metric. We show that a four-dimensional cosmology with a closed spacelike section and a cosmological constant can be imbedded into five-dimensional flat space-time.

Global structure of Deffayet (Dvali-Gabadadze-Porrati) cosmologies

We detail the global structure of the five-dimensional bulk for the cosmological evolution of Dvali-Gabadadze-Porrati braneworlds. The picture articulated here provides a framework and intuition for understanding how metric perturbations leave (and possibly reenter) the brane universe. A bulk observer sees the braneworld as a relativistically expanding bubble, viewed either from the interior (in the case of the Friedmann-Lemaitre-Robertson-Walker phase) or the exterior (the self-accelerating phase). Shortcuts through the bulk in the first phase can lead to an apparent brane causality violation and provide an opportunity for the evasion of the horizon problem found in conventional four-dimensional cosmologies. Features of the global geometry in the latter phase anticipate a depletion of power for linear metric perturbations on large scales.

Supersymmetry and Brane Cosmology

We consider a five-dimensional brane world scenario where the fifth dimension is compactified on $S^1/Z_2$. We show that the familiar four-dimensional cosmology on our brane is easily recovered during a primordial stage of inflation if supersymmetry is exploited. Even if some vacuum energy density appears localized on our three brane, heavy supersymmetric bulk fields adjust themselves and acquire a nontrivial configuration along the extra-dimension. This phenomenon redistributes uniformly the energy density across the bulk and the resulting energy-momentum tensor does not display any singularity associated to the initial localized energy density on our three-brane. No jumps across the brane are present for the derivatives of the metric and Einstein's equations are solved by constant solutions along the fifth dimension. Our findings make it clear that cosmological phenomena in the supersymmetric brane world scenario must be studied taking properly into account bulk supersymmetric states. This comment is particularly relevant when applied to (super)gravity since in supersymmetric brane world scenarios, even though chiral matter and gauge fields may be restricted to live on boundaries, gravity multiplets always propagate in the bulk.

Bulk scalar field in the braneworld can mimic the 4D inflaton dynamics

Based on the recently proposed scenario of inflation driven by a bulk scalar field in the braneworld of the Randall-Sundrum (RS) type, we investigate the dynamics of a bulk scalar field on the inflating braneworld. We derive the late-time behavior of the bulk scalar field by analyzing the property of the retarded Green function. We find that the late-time behavior is basically dominated by a single (or a pair of) pole(s) in the Green function irrespective of the initial condition and of the signature of ${m}^{2}{=V}^{\ensuremath{''}}(\ensuremath{\varphi}),$ where $V(\ensuremath{\varphi})$ is the potential of the bulk scalar field. Including the lowest-order backreaction to the geometry, this late-time behavior can be well approximated by an effective four-dimensional scalar field with ${m}_{\mathrm{eff}}^{2}{=m}^{2}/2.$ The mapping to the four-dimensional effective theory is given by a simple scaling of the potential with a redefinition of the field. Our result supports the picture that the scenario of inflation driven by a bulk scalar field works in quite a similar way to that in the standard four-dimensional cosmology.