Interestingly, although the inflationary phase is responsible for the primordial perturbations, certain features from the pre-inflationary bounce may survive if the inflationary era is not too long, and be imprinted in the CMB. In such a scenario, the goal of the bounce is, in addition to overcoming the difficulties arising from classical general relativity, to bring the Universe to an inflationary phase. A natural strategy is to maintain the inflationary phase in the post-bounce era. This bounce by itself is not able to generate the primordial perturbations though, and it must be complemented with another mechanism. In LQC, the big bang singularity is replaced by a bounce ( Ashtekar et al., 2006a Ashtekar et al., 2006b Ashtekar et al., 2007 Szulc, 2007 Szulc et al., 2007 Bentivegna and Pawlowski, 2008 Martin-Benito et al., 2008 Ashtekar and Wilson-Ewing, 2009a Ashtekar and Wilson-Ewing, 2009b Garay et al., 2010 Wilson-Ewing, 2010 Pawlowski and Ashtekar, 2012) which is triggered by quantum gravitational effects. Loop quantum cosmology (LQC) uses the principles of loop quantum gravity to address this issue ( Bojowald, 2001 Ashtekar et al., 2003 Mena Marugan, 2010 Ashtekar and Singh, 2011 Banerjee et al., 2012 Agullo et al., 2014 Agullo et al., 2017a). As it is well known, general relativity, on which the inflationary scenario rests, breaks down as we approach the Planck regime. A prominent one concerns the past incompleteness of the inflationary scenario. Further, observations are consistent with the near Gaussian nature of the primordial perturbations predicted by slow-roll inflation ( Akrami, 2020).īut in spite of this success, several open questions remain. The nearly scale-invariant power spectrum predicted by slow-roll inflation has been confirmed with a significance of more than 7 σ ( Aghanim, 2018b Akrami, 2018). Observations of the cosmic microwave background (CMB) by the Planck satellite have revealed that the ΛCDM model together with the inflationary scenario checks nearly all the right boxes ( Aghanim, 2018a Akrami, 2018 Aghanim, 2020)-in the sense that it provides a detailed fit to the CMB spectrum based on a few free parameters ( Aghanim, 2018a Aghanim, 2019).
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