Dark Energy From Transplanckian Physics

Bastero-Gil, Mar; Mersini, Laura

doi:10.1142/9789812791313_0002

Cited by 11 publications

(34 citation statements)

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“…We revisit this issue further in Section IV, where we prove that the effective energy-momentum tensor we derived earlier is well-behaved, thus disproving an improper claim of Ref. [3] and confirming our earlier criticisms. We provide a summary of our conclusions in Section V.…”

Section: Introductionsupporting

confidence: 75%

“…Therefore, the scenario of Refs. [1,2,3] does not improve the situation. At this point, it should be emphasized that the above problem is a generic feature of the dispersion relation used in Refs.…”

Section: Fine-tuning Of the Initial Conditionsmentioning

confidence: 90%

“…At this point, it should be emphasized that the above problem is a generic feature of the dispersion relation used in Refs. [1,2,3]. In order for the trans-Planckian effects to modify the power spectrum of the fluctuations, the physical modes of interest must spend some time in a region where the WKB approximation is violated.…”

Section: Fine-tuning Of the Initial Conditionsmentioning

confidence: 99%

“…Therefore the class of dispersion relations used in Refs. [1,2,3] suffer in a generic way from the problem discussed in Ref. [8].…”

Section: Fine-tuning Of the Initial Conditionsmentioning

confidence: 99%

“…[1,2,3] rests breaks down. In an earlier study, devoted to constructing an effective stress-energy tensor for theories with non-linear dispersion relations [6], we already criticized this model by arguing that it led generically to the wrong equation of state.…”

Section: Introductionmentioning

confidence: 99%

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Trans-Planckian dark energy?

2003

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It has recently been proposed in Refs. [1,2,3] that the dark energy could be attributed to the cosmological properties of a scalar field with a non-standard dispersion relation that decreases exponentially at wave-numbers larger than Planck scale (k phys > M Pl ). In this scenario, the energy density stored in the modes of trans-Planckian wave-numbers but sub-Hubble frequencies produced by amplification of the vacuum quantum fluctuations would account naturally for the dark energy. The present article examines this model in detail and shows step by step that it does not work. In particular, we show that this model cannot make definite predictions since there is no welldefined vacuum state in the region of wave-numbers considered, hence the initial data cannot be specified unambiguously. We also show that for most choices of initial data this scenario implies the production of a large amount of energy density (of order ∼ M 4 Pl ) for modes with momenta ∼ M Pl , far in excess of the background energy density. We evaluate the amount of fine-tuning in the initial data necessary to avoid this back-reaction problem and find it is of order H/M Pl . We also argue that the equation of state of the trans-Planckian modes is not vacuum-like. Therefore this model does not provide a suitable explanation for the dark energy.

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Section: Introductionsupporting

confidence: 75%

Section: Fine-tuning Of the Initial Conditionsmentioning

confidence: 90%

Section: Fine-tuning Of the Initial Conditionsmentioning

confidence: 99%

“…Therefore the class of dispersion relations used in Refs. [1,2,3] suffer in a generic way from the problem discussed in Ref. [8].…”

Section: Fine-tuning Of the Initial Conditionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Trans-Planckian dark energy?

2003

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The Arrow of Time In a Universe with a Positive Cosmological Constant Λ

Mersini-Houghton

2011

Cosmic Update

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Motivated by the mounting evidence for dark energy, here we explore the consequences of a fundamental cosmological constant Λ for our universe. We show that when the gravitational entropy of a pure DeSitter state ultimately wins over matter, then the thermodynamic arrow of time in our universe must reverse in scales of order a Hubble time. We find that due to the dynamics of gravity and nonlocal entanglement, a finite size system such as a DeSitter patch with horizon size H −1 0 has a finite lifetime ∆t. This phenomenon arises from the dynamic gravitational instabilities that develop during a DeSitter epoch and turn catastrophic. A reversed arrow of time is clearly in disagreement with observations. Thus we are led to conclude: Nature forbids a fundamental Λ. Or else general relativity must be modified in the IR regime when Λ dominates the expansion of the Universe.

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How to test stringy dark energy?

Frampton

2003

Physics Letters B

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It is suggested, by using a covariant lagrangian formalism to estimate the equation of state w = p/ρ, that stringy dark energy predicts w < −1, a negative pressure larger in magnitude than that for a cosmological constant or quintessence. This would lead to a later transition from decelerating to accelerating cosmological expansion; w = −4/3 is briefly considered as one illustrative example.The last few years have witnessed a revolution in our knowledge of the universe. Paramount among recent observational discoveries the phenomenon of dark energy, probably the most surprising discovery in physics or astronomy since parity violation almost a half a century ago. As in that case, the explanation of dark energy will surely impact a wide range of disciplines.It was recently proposed that the cosmological dark energy which causes the present accelerated expansion of the universe may arise from a stringy origin [1]. In particular, in a toroidal closed string universe the correlation between winding and momentum modes of closed strings leads in the phase transition at the Hagedorn temperature to a condensate phenomenon.For stringy dark energy, the pressure can certainly be negative because for exponentially falling ω(κ) the group velocity v g ∝ dω(κ)/dκ is negative and can dominate the pressure expression. The detailed calculation of the value of w(Z) depends on the Hagedorn phase transition where the closed strings are strongly interacting and hence impracticable to compute. Still we can hope to test the approach already by some estimates. The idea of an exponentially decrease of ω(κ) at large transplanckian κ providing a candidate for dark energy was proposed in [2].We discuss the pressure estimate first in general terms followed by a more specific calculation. Finally, the resultant red-shift for the transition from decelerated to accelerated cosmological expansion is briefly dicussed.General dispersion relation. Consider a spatially flat FRW universe with the spacetime line element,Let φ be a field satisfying a wave equation that, either exactly or in the adiabatic approximation, has plane wave solutions,where k ≡ | k|. If the wave equation were Lorentz covariant in the flat spacetime limit, then one would have ω(k/a(t )) = (k 2 /a 2 +m 2 ) 1/2 . We will suppose that ω has this form when the momentum k/a(t) is small with respect to the Planck mass, m * . For larger momenta, we will suppose that ω(c) is a decreasing function of c ≡ k/a(t).The group velocity, v g , of a wave packet,formed from these plane waves over a narrow range of k's near the value k, is. (4) It follows that the group velocity is negative if k is larger than m * . This means that the wave packet moves in the direction opposite to the momentum, k/a(t). Consider a set of such wave packets contained at time t within a given comoving volume. When one of the wave packets passes out of the comoving volume it transfers momentum into the volume. This means that the pressure exerted by the energy density inside a comoving volume on the bounding surface of...

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Dark Energy From Transplanckian Physics

Cited by 11 publications

References 0 publications

Trans-Planckian dark energy?

Trans-Planckian dark energy?

The Arrow of Time In a Universe with a Positive Cosmological Constant Λ

How to test stringy dark energy?

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