Hubble law and brane matter after ekpyrosis

Enqvist, Kari; Keski-Vakkuri, Esko; Räsänen, Syksy

doi:10.1016/s0550-3213(01)00412-6

Cited by 19 publications

(35 citation statements)

References 38 publications

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“…One might hope that the singularity and thus the divergence of perturbations is absent. However, merely going to the higher dimensional setting, for example, in the ekpyrotic universe [396], and properly incorporating metric perturbations does not necessarily alleviate this problem, as shown in [397]. A similar problem is present for vector perturbations in a contracting universe [245].…”

Section: Cosmological Perturbationsmentioning

confidence: 99%

A critical review of classical bouncing cosmologies

2015

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Given the proliferation of bouncing models in recent years, we gather and critically assess these proposals in a comprehensive review. The PLANCK data shows an unmistakably red, quasi scaleinvariant, purely adiabatic primordial power spectrum and no primary non-Gaussianities. While these observations are consistent with inflationary predictions, bouncing cosmologies aspire to provide an alternative framework to explain them. Such models face many problems, both of the purely theoretical kind, such as the necessity of violating the NEC and instabilities, and at the cosmological application level, as exemplified by the possible presence of shear. We provide a pedagogical introduction to these problems and also assess the fitness of different proposals with respect to the data. For example, many models predict a slightly blue spectrum and must be fine-tuned to generate a red spectral index; as a side effect, large non-Gaussianities often result.We highlight several promising attempts to violate the NEC without introducing dangerous instabilities at the classical and/or quantum level. If primordial gravitational waves are observed, certain bouncing cosmologies, such as the cyclic scenario, are in trouble, while others remain valid. We conclude that, while most bouncing cosmologies are far from providing an alternative to the inflationary paradigm, a handful of interesting proposals have surfaced, which warrant further research. The constraints and lessons learned as laid out in this review might guide future research.

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Section: Cosmological Perturbationsmentioning

confidence: 99%

A critical review of classical bouncing cosmologies

2015

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“…Then one can derive, using homogeneity and isotropy in the three spatial dimensions, the general form of the metric near the bounce [30]. Under the same assumptions, the field content of the bulk can also be computed [8,30].…”

Section: Introductionmentioning

confidence: 99%

Nonsingular perturbations in a bouncing brane model

2004

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The question of how perturbations evolve through a bounce in the Cyclic and Ekpyrotic models of the Universe remains a topical one. Issues concerning singularities at the background level and at the level of perturbation theory continue to be a matter of debate. In this report we hope to demonstrate a non singular collision between the boundary branes at the background level, and circumstances under which all perturbation variables remain bounded through the collision.As expected, we find most collisions to be singular even in the full 5-D formalism, where first order perturbation theory breaks down for at least one perturbation variable. Only in the case that the boundary branes approach each other with constant velocity shortly before the bounce, can a consistent, non singular solution be found. It is then possible to follow the perturbations explicitly until the actual collision. In this case, we find that if a scale invariant spectrum developed on the hidden brane, it will get transferred to the visible brane during the bounce.

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“…where y = x 11 and a, n and b are in general functions of y and t. The relevant field equations are the Einstein equation arising from G tt , G kk , G yy , G ty and the equation for V, and are given in e. g. 8 . In addition there are the boundary conditions on the 3 branes at the orbifold planes y 1 = 0 and y 2 = πρ:…”

Section: Horava-witten M-theorymentioning

confidence: 99%

“…as a consequence of the Bianchi identities for the static solution, or alternately from the G ty equation 8 . If H is independent of y, one may integrate Eq.…”

Section: Solution Of the 5d Equationsmentioning

confidence: 99%

Cosmology in Horava-Witten M-Theory

Arnowitt¹,

Dent²,

Dutta³

2003

String Phenomenology

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The cosmology of the Horava-Witten M-Theory reduced to five dimensions retaining the volume modulus is considered. Brane matter is considered as a perturbation on the vacuum solution, and the question of under what circumstances does the theory give rise to the standard RWF cosmology is examined. It is found that for static solutions, one obtains a consistent solution of the bulk field equations and the brane boundary conditions only for pure radiation on the branes. (A similar result holds if additional 5-branes are added in the bulk.) If one stabilizes the fifth dimension in an ad hoc manner, a similar inconsistency still occurs (at least for a Hubble constant that has no dependence on y, the fifth dimension.) Within this framework, the possibility of recovering the RWF cosmology still remains if the volume modulus and /or the distance between branes becomes time dependent, under which circumstances the Hubble constant must then depend on y (unless the fifth dimension and volume modulus expand at precisely the same rate).

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Hubble law and brane matter after ekpyrosis

Cited by 19 publications

References 38 publications

A critical review of classical bouncing cosmologies

A critical review of classical bouncing cosmologies

Nonsingular perturbations in a bouncing brane model

Cosmology in Horava-Witten M-Theory

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