a non-singular cosmological bounce. Such new physics could come from the matter sector (see, e.g., Refs. [@matterBounce-1; @matterBounce-2]), from modifications of the classical gravitational action (as for example in Horava-Lifshitz gravity [@HLbounce] or in the non-local gravity construction of Ref. [@Biswas]), or from quantum gravity effects. Examples of the latter are the bounce in loop quantum cosmology (see, e.g., Ref. [@LQCbounce] for reviews), in deformed AdS/CFT cosmology [@Elisa], the S-brane bounce of Ref. [@Costas] and the temperature bounce in String Gas Cosmology [@BV].

Concerning the robustness of the computations of the spectrum of cosmological fluctuations, an advantage of bouncing cosmologies (without an inflationary phase after the bounce) is that the physical length of modes which are probed in current observations remain in the far infrared throughout the cosmological evolution as long as the energy density at the bounce point is smaller than the Planck density. Hence, the computations can be done in the realm where effective field theory is well justified. This is in contrast to the situation in inflationary cosmology [@MB] where the physical wavelengths of even the largest scales which are currently observed are smaller than the Planck length at the beginning of inflation (provided that the inflationary 