Bimetric structure formation: Non-Gaussian predictions

Magueijo, João; Noller, Johannes; Piazza, Federico

doi:10.1103/physrevd.82.043521

Cited by 30 publications

(66 citation statements)

References 65 publications

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“…In a recent paper [15] it was shown that the cuscuton-like model also leaves a potentially observable non-Gaussian signature in the CMB anisotropy field, f NL ∼ O(1), which is slightly different from the signatures of other superluminal models discussed in the literature [16,17]. This difference is due to the contribution of the termζ 3 in the cubic action [18,19] to the full non-Gaussian amplitude, giving rise to an extra term with a linear dependence on the flow parameters and s. This dependence is not present either in DBI models (since the coefficient ofζ 3 in the third-order action is identically null in this case), or in disformal bimetric model [16], since by projecting its scalar-field action in the Einstein frame one obtains a DBI-like action. Therefore, the cuscuton tachyacoustic model "inherits" an extra contribution from theζ 3 term, providing a different value for f NL compared to the other superluminal models.…”

Section: Introductionmentioning

confidence: 90%

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Attractor solutions in tachyacoustic cosmology

Bessada

Kinney

2012

Phys. Rev. D

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We study the dynamical stability of "tachyacoustic" cosmological models, in which primordial perturbations are generated by a shrinking sound horizon during a period of decelerating expansion. Such models represent a potential alternative to inflationary cosmology, but the phase-space behavior of tachyacoustic solutions has not previously been investigated. We numerically evaluate the dynamics of two non-canonical Lagrangians, a cuscuton-like Lagrangian and a Dirac-Born-Infeld Lagrangian, which generate a scale-invariant spectrum of perturbations. We show that the power-law background solutions in both cases are dynamical attractors.

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Section: Introductionmentioning

confidence: 90%

“…(16). The resulting model is tachyacoustic, that is, has a decaying, superluminal speed of sound and shrinking acoustic horizon in comoving units.…”

Section: Attractor Behavior a The Cuscuton Casementioning

confidence: 99%

Attractor solutions in tachyacoustic cosmology

Bessada

Kinney

2012

Phys. Rev. D

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“…The trick was used in [21,22] to derive the cubic action for theories with a varying speed of sound [2,3] and their non-Gaussianities.…”

Section: Rainbow Gravity and Linearizing Variablesmentioning

confidence: 99%

Rainbow gravity and scale-invariant fluctuations

et al. 2013

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We re-examine a recently proposed scenario where the deformed dispersion relations associated with a flow of the spectral dimension to a UV value of 2 leads to a scale-invariant spectrum of cosmological fluctuations, without the need for inflation. In that scenario Einstein gravity was assumed. The theory displays a wavelength-dependent speed of light but by transforming to a suitable "rainbow frame" this feature can be removed, at the expense of modifying gravity. We find that the ensuing rainbow gravity theory is such that gravity switches off at high energy (or at least leads to a universal conformal coupling). This explains why the fluctuations are scale-invariant on all scales: there is no horizon scale as such. For dispersion relations that do not lead to exact scale invariance we find instead esoteric inflation in the rainbow frame. We argue that these results shed light on the behaviour of gravity under the phenomenon of dimensional reduction.

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“…Bimetric varying speed of light theories [27] and tachyacoustic cosmology [28] are two possible ways of putting this mechanism to work. The minimal bimetric theory leads to scale-invariant perturbations but it is also possible to obtain deviations from strict scale-invariance, subject to consistency relations involving the 3-point function [29,30]. It is also possible to implement this mechanism via deformed dispersion relations [31], as we now review.…”

Section: Cosmological Perturbationsmentioning

confidence: 99%

Dimensional reduction in the sky

et al. 2013

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We explore the cosmological implications of a mechanism found in several approaches to quantum-gravity, whereby the spectral dimension of spacetime runs from the standard value of 4 in the infrared (IR) to a smaller value in the ultraviolet (UV). Specifically, we invoke the picture where the phenomenon is associated with modified dispersion relations. With minimal assumptions, we find that UV behaviour leading to 2 spectral dimensions results in an exactly scale-invariant spectrum of vacuum scalar and tensor fluctuations, regardless of the equation of state. The fluctuation production mechanism is analogous to the one known for varying speed of sound/light models and, unlike in inflation, the spectrum is already scale-invariant before leaving the horizon, remaining so after freeze-in. In the light of Planck's recent results we also discuss scenarios that break exact scale-invariance, such as the possibility that the spectral dimension runs down to a value slightly higher than 2, or runs down to 2 but with an extremely slow transient. We further show that the tensor to scalar ratio is fixed by the UV ratio between the speed of gravity and the speed of light. Not only does our model not require inflation, but at its most minimal it seems incompatible with it. In contrast, we find that running spectral dimensions can improve the outlook of the cyclic/ekpyrotic scenario, solving the main problems present in its simplest and most appealing realisations.

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Bimetric structure formation: Non-Gaussian predictions

Cited by 30 publications

References 65 publications

Attractor solutions in tachyacoustic cosmology

Attractor solutions in tachyacoustic cosmology

Rainbow gravity and scale-invariant fluctuations

Dimensional reduction in the sky

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