Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Matter Bounce in Horava-Lifshitz Cosmology

Robert Brandenberger

TL;DR

The paper proposes that Horava-Lifshitz gravity can realize a nonsingular matter-dominated bounce, providing an inflation-free mechanism to generate a scale-invariant spectrum of curvature perturbations. It shows that a negative dark-radiation term (requiring nonzero spatial curvature) enables the bounce in a contracting phase with a massive scalar field, and that the resulting fluctuations remain scale-invariant through the bounce. The analysis predicts enhanced non-Gaussianities with a characteristic bispectrum distinguishable from inflationary predictions, and discusses observational prospects and theoretical caveats, including energy transfer to standard matter and initial conditions. Overall, the work offers a UV-complete alternative to inflation with testable signatures in the CMB bispectrum.

Abstract

Horava-Lifshitz gravity, a recent proposal for a UV-complete renormalizable gravity theory, may lead to a bouncing cosmology. In this note we argue that Horava-Lifshitz cosmology may yield a concrete realization of the matter bounce scenario, and thus give rise to an alternative to inflation for producing a scale-invariant spectrum of cosmological perturbations. In this scenario, quantum vacuum fluctuations exit the Hubble radius in the pre-bounce phase and the spectrum is transformed into a scale-invariant one on super-Hubble scales before the bounce because the long wavelength modes undergo squeezing of their wave-functions for a longer period of time than shorter wavelength modes. The scale-invariance of the spectrum of curvature fluctuations is preserved during and after the bounce. A distinctive prediction of this scenario is the amplitude and shape of the bispectrum.

Matter Bounce in Horava-Lifshitz Cosmology

TL;DR

The paper proposes that Horava-Lifshitz gravity can realize a nonsingular matter-dominated bounce, providing an inflation-free mechanism to generate a scale-invariant spectrum of curvature perturbations. It shows that a negative dark-radiation term (requiring nonzero spatial curvature) enables the bounce in a contracting phase with a massive scalar field, and that the resulting fluctuations remain scale-invariant through the bounce. The analysis predicts enhanced non-Gaussianities with a characteristic bispectrum distinguishable from inflationary predictions, and discusses observational prospects and theoretical caveats, including energy transfer to standard matter and initial conditions. Overall, the work offers a UV-complete alternative to inflation with testable signatures in the CMB bispectrum.

Abstract

Horava-Lifshitz gravity, a recent proposal for a UV-complete renormalizable gravity theory, may lead to a bouncing cosmology. In this note we argue that Horava-Lifshitz cosmology may yield a concrete realization of the matter bounce scenario, and thus give rise to an alternative to inflation for producing a scale-invariant spectrum of cosmological perturbations. In this scenario, quantum vacuum fluctuations exit the Hubble radius in the pre-bounce phase and the spectrum is transformed into a scale-invariant one on super-Hubble scales before the bounce because the long wavelength modes undergo squeezing of their wave-functions for a longer period of time than shorter wavelength modes. The scale-invariance of the spectrum of curvature fluctuations is preserved during and after the bounce. A distinctive prediction of this scenario is the amplitude and shape of the bispectrum.

Paper Structure

This paper contains 6 sections, 25 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Review of Hořava-Lifshitz Gravity
  3. Matter Bounce in Hořava-Lifshitz Cosmology
  4. Fluctuations in the Matter Bounce Scenario
  5. Evolving Fluctuations through the Bounce
  6. Conclusions and Discussion

Figures (1)

  • Figure 1: A space-time sketch of the matter bounce scenario. The vertical axis is time, with $t = 0$ being the bounce time. The horizontal axis denotes co-moving distance. The curve with the label $H$ is the Hubble radius $H^{-1}$ - in co-moving coordinates, the vertical line labeled by $k$ denotes the co-moving wavelength of a fluctuation mode. This mode crosses the Hubble radius in the contracting phase before the time $-t_c$, when the period of matter-domination ends.