Quantum-to-classical transition for ekpyrotic perturbations

Abstract

We examine the processes of quantum squeezing and decoherence of density perturbations produced during a slowly contracting ekpyrotic phase in which entropic perturbations are converted to curvature perturbations before the bounce to an expanding phase. During the generation phase, the entropic fluctuations evolve into a highly squeezed quantum state, analogous to the evolution of inflationary perturbations. Subsequently, during the conversion phase, quantum coherence is lost very efficiently due to the interactions of entropy and adiabatic modes. Moreover, while decoherence occurs, the adiabatic curvature perturbations inherit their semiclassicality from the entropic perturbations. Our results confirm that, just as for inflation, an ekpyrotic phase can generate nearly scale-invariant curvature perturbations which may be treated as a statistical ensemble of classical density perturbations, in agreement with observations of the cosmic background radiation.

On the creation of the universe via ekpyrotic instantons

“…flatness and horizon problems) can be resolved. Moreover, simple extensions can lead to the amplification of scalar quantum fluctuations in order to produce classical density perturbations in agreement with (trustworthy) cosmic microwave background measurements [8][9][10][11][12][13]. If such an ekpyrotic phase is followed by a bounce into a phase of radiation dominated expansion [14][15][16][17][18], then these models provide an interesting alternative cosmological history compared to the popular assumption of an early inflationary phase.…”

“…Thus, we can ignore these saddle points here. Second, it is important to realise that the minisuperspace approximation is justified for the contracting histories we are interested in: during a single-field ekpyrotic phase, gauge-invariant curvature perturbations are not amplified [11], and consequently do not destabilise the ekpyrotic scaling solution. This is another manifestation of the ekpyrotic attractor mechanism.…”

Ekpyrotic perturbations with small non-Gaussian corrections

“…During the ekpyrotic phase, curvature perturbations at linear order are not amplified [12,35], nor are they sourced by the entropy perturbations. However, at second order curvature perturbations can be sourced by the entropy fluctuations [13].…”

“…In the presence of a second scalar field χ with a canonical kinetic term and an unstable direction in the potential V ;χχ < 0, nearly scale-invariant (quantum) entropy fluctuations get amplified to become classical perturbations, which can then be converted into curvature perturbations before the Big Bang [9][10][11][12]. In this "entropic mechanism" the spectrum is predicted to be close to scale invariant, while the local bispectrum can take on a range of values [13][14][15], including the values favored by PLANCK [3,16].…”

No-go theorems for ekpyrosis from ten-dimensional supergravity

“…The occurrence of blueshift scalar perturbation is nonetheless a minor flaw of ekpyrotic theory which can be easily corrected. Hence, new ekpyrotic theory considers multiple scalar field and successfully generated a scalar spectrum which is scale invariant and slightly redshifted [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Producing primordial gravitational waves sourced by the gauge field in the ekpyrotic scenario was also studied in [31].…”