There are important avenues where string theory can actually help us in our understand- ing the inflation dynamics and various cosmological consequences. • UV completion: • Initial conditions: The UV completion of an inflationary potential is related to the fact that whether the potential can be kept flat enough during the interval of slow-roll inflation or not at VEVs close to the cut-off of the theory. For instance, the slow-roll parameters, i.e. ϵ ≤ 1 , η ≤ 1, ought to be maintained during sufficiently large e-foldings of slow-roll inflation. One particular example is the realization of chaotic inflation [4] in string theory, but below the Planckian VEV, with the help of assisted inflation [157]. In string theory it is possible to realize N number of string axions arising from part- ners of K¨ ahler moduli, which can collectively drive inflation below the 4 d Planck scale [285]. These axions have shift symmetry from 10 d gauge invariance, which is broken non-perturbatively, and avoids the SUGRA η problem, thus keeping the po- tential sufficiently flat enough for the success of inflation [287, 557]. String theory provides multiple vacua with all possible values of the cosmological constant [56, 58, 875, 876], for reviews, see [45, 57]. There has been a new revelation in string theory after the advent of a KKLT scenario [55], where the moduli are stabilized in an anti de Sitter (AdS) space with a negative cosmological constant. The initial configuration is stabilized with the help of various flux vacua [43]. Various non- perturbative effects such as gaugino condensation and/or warped brane instantons lift the vacuum from AdS to de Sitter (dS) with a meta stable minimum with a life time greater than the present universe. These numerous vacua eliminate the problem of initial conditions for inflation [58, 877, 878]. Our patch of the universe could emerge from one such initial vacuum with a large cosmological constant. It is also quite plausible that string theory would provide some hints on the nature of trans-Planckian time dimensions. For instance, gravitons are allowed to propagate in the entire space-time dimensions, while the gauge fields including photons are localized on the brane position where the open strings must end [866, 867]. This has lead to a number of paradigms such as; a low string scale : the string scale could be as low as TeV, the large extra dimensions could be as large as the micron scale [535, 868–870], or at intermediate scales [871], and cosmological constant problem : a possible solution emerges where it is possible to decouple 4 d vacuum energy from the 4 d curvature [872–874]. 222