Towards a nonsingular bouncing cosmology

By Yi-Fu Cai, D. Easson, Robert Brandenberger. 2012

408 citations recorded by the source corpus.

We present a nonsingular bouncing cosmology using single scalar field matter with non-trivial potential and non-standard kinetic term. The potential sources a dynamical attractor solution with Ekpyrotic contraction which washes out small amplitude anisotropies. At high energy densities the field evolves into a ghost condensate, leading to a nonsingular bounce. Following the bounce there is a smooth transition to standard expanding radiation and matter dominated phases. Using linear cosmological perturbation theory we track each Fourier mode of the curvature fluctuation throughout the entire cosmic evolution. Using standard matching conditions for nonsingular bouncing cosmologies we verify that the spectral index does not change during the bounce. We show there is a controlled period of exponential growth of the fluctuation amplitude for the perturbations (but not for gravitational waves) around the bounce point which does not invalidate the perturbative treatment. This growth induces a natural suppression mechanism for the tensor to scalar ratio of fluctuations. Moreover, we study the generation of the primordial power spectrum of curvature fluctuations for various types of initial conditions. For the pure vacuum initial condition, on scales which exit the Hubble radius in the phase of Ekpyrotic contraction, the spectrum is deeply blue. For thermal particle initial condition, one possibility for generating a scale-invariant spectrum makes use of a special value of the background equation of state during the contracting Ekpyrotic phase. If the Ekpyrotic phase is preceded by a period of matter-dominated contraction, the primordial power spectrum is nearly scale-invariant on large scales (scales which exit the Hubble radius in the matter-dominated phase) but acquires a large blue tilt on small scales. Thus, our model provides a realization of the "matter bounce" scenario which is free of the anisotropy problem.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/0ebd6e69bc7d334cddd9ebb2bb7825d87cd22c96

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Citations in the corpus

The matter bounce scenario in loop quantum cosmology (2012, result): “An important point is that this cosmology can be differentiated from the one studied in this paper by observations as the predicted scalar to tensor ratio in [23] is 10 (although the predicted value of r in this setting is model dependent to some extent), almost two orders of magnitude smaller than ”
Generalized Galileons: instabilities of bouncing and Genesis cosmologies and modified Genesis (2016, background): “Known models of the bouncing Universe, employing generalized Galileons, are in fact plagued by the gradient instabilities, provided one follows the evolution for long enough time [24–28].”
Inflationary Cosmology in Modified Gravity Theories (2015, background): “In Reference [42], the reconstruction of various modified gravity theories including F (R), F (G), and F (T ) gravity theories has been performed, where F (G) is an arbitrary function of the Gauss-Bonnet invariant G, to describe the two-times bouncing phenomena, called super-bounce [103,104], and th”
Exploring bouncing cosmologies with cosmological surveys (2014, methodology): “[20], during this phase, the Hubble parameter evolves approximately as”
Horndeski theory and beyond: a review (2019, background): “g models have the same problem. Several examples show that instabilities may occur at the transition from the NEC-violating phase to some subsequent phase or even in a far future after the transition [100, 119, 120, 121, 122, 123, 124, 125]. This implies that the instabilities may not be related dir”
The Effective Field Theory of nonsingular cosmology (2016, background): “However, it seems quite difficult to avoid the gradient instability problem, which indicates a negative sound speed squared [7, 9, 29, 30] and thus leads to an exponential growth of the perturbation [31, 32].”