Causality and the speed of sound

Ellis, George; Maartens, Roy; MacCallum, Malcolm

doi:10.1007/s10714-007-0479-2

Cited by 107 publications

(92 citation statements)

References 25 publications

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“…In such a picture, it is possible to exclude n < 0 which corresponds to singular scale factor. Finally, at n = 0 and n = 1 2 the sound speed provides a divergence [38]. 13 As cold dark matter appears to do, in agreement with observations [39].…”

Section: The Cosmographic Energy Conditionssupporting

confidence: 76%

On the theory and applications of modern cosmography

Dunsby

Luongo

2016

Int. J. Geom. Methods Mod. Phys.

158

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Cosmography represents an important branch of cosmology which aims to describe the universe without the need of postulating a priori any particular cosmological model. All quantities of interest are expanded as a Taylor series around here and now, providing in principle, a way of directly matching with cosmological data. In this way, cosmography can be regarded a model-independent technique, able to fix cosmic bounds, although several issues limit its use in various model reconstructions. The main purpose of this review is to focus on the key features of cosmography, emphasising both the strategy for obtaining the observable cosmographic series and pointing out any drawbacks which might plague the standard cosmographic treatment. In doing so, we relate cosmography to the most relevant cosmological quantities and to several dark energy models. We also investigate whether cosmography is able to provide information about the form of the cosmological expansion history, discussing how to reproduce the dark fluid from the cosmographic sound speed. Following this, we discuss limits on cosmographic priors and focus on how to experimentally treat cosmographic expansions. Finally, we present some of the latest developments of the cosmographic method, reviewing the use of rational approximations, based on cosmographic Padé polynomials. Future prospects leading to more accurate cosmographic results, able to better reproduce the expansion history of the universe are also discussed in detail.

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Section: The Cosmographic Energy Conditionssupporting

confidence: 76%

On the theory and applications of modern cosmography

Dunsby

Luongo

2016

Int. J. Geom. Methods Mod. Phys.

158

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“…However, analysis [30,33] of the supernova data seems to indicate that the generalized Chaplygin gas with α ≥ 1 is favored over the α → 0 model and similar conclusions were drawn in [20]. But one should bear in mind that the generalized Chaplygin gas with α > 1 has a superluminal sound speed that violates causality [34]. For a different view on this issue see [35,36,37] (see discussion in section 2).…”

Section: Introductionmentioning

confidence: 86%

“…For hydrodynamic stability we require c 2 s ≥ 0. In addition, it seems physically reasonable to require c 2 s ≤ 1 in order to avoid possible problems with causality [34]. Causality violation in relation to superluminal sound propagation has been the subject of a recent debate [34,35,36,37,53].…”

Section: K-essentialsmentioning

confidence: 99%

“…In addition, it seems physically reasonable to require c 2 s ≤ 1 in order to avoid possible problems with causality [34]. Causality violation in relation to superluminal sound propagation has been the subject of a recent debate [34,35,36,37,53]. It has been shown [36,53] that if k-essence is to solve the coincidence problem there must be an epoch when perturbations in the k-essence field propagate faster than light.…”

Section: K-essentialsmentioning

confidence: 99%

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Cosmological tachyon condensation

2009

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We consider the prospects for dark matter/energy unification in k-essence type cosmologies. General mappings are established between the k-essence scalar field, the hydrodynamic and braneworld descriptions. We develop an extension of the general relativistic dust model that incorporates the effects of both pressure and the associated acoustic horizon. Applying this to a tachyon model, we show that this inhomogeneous "variable Chaplygin gas" does evolve into a mixed system containing cold dark matter like gravitational condensate in significant quantities. Our methods can be applied to any dark energy model, as well as to mixtures of dark energy and traditional dark matter.

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“…Either it may concern the Universe or a dense compact star, we consider the tachyon matter as continuous medium and analyze its stability to the causality condition [12,13]:…”

Section: Introductionmentioning

confidence: 99%

Acoustics of tachyon Fermi gas

Trojan

Vlasov

2011

Phys. Rev. D

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We consider a Fermi gas of free tachyons as a continuous medium and find whether it satisfies the causality condition. There is no stable tachyon matter with the particle density below critical value n T and the Fermi momentum k F < 3 2 m that depends on the tachyon mass m. The pressure P and energy density E cannot be arbitrary small, but the situation P > E is not forbidden. Existence of shock waves in tachyon gas is also discussed. At low density n T < n < 3.45n T the tachyon matter remains stable but no shock wave survives.

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Causality and the speed of sound

Abstract: A usual causal requirement on a viable theory of matter is that the speed of sound be at most the speed of light. In view of various recent papers querying this limit, the question is revisited here. We point to various issues confronting theories that violate the usual constraint.

Cited by 107 publications

References 25 publications

On the theory and applications of modern cosmography

On the theory and applications of modern cosmography

Cosmological tachyon condensation

Acoustics of tachyon Fermi gas

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