AR-2019-2020

(2.5 times) and mass (5 times) of pure discs, from z > 2 to z < 2, pure spheroids maintain roughly the same values. Additionally, while bulge host- ing discs witness an expansion in scale length (1.3 times), their bulge sizes as well as bulge to total light ratio see no evolution, suggesting that z ∼ 2 is pre-dominantly a disc formation period. This work has been done in collaboration with Sonali Sachdeva,Kanak Saha, Ajit Kembhavi,and Somak Raychaudhury. Unravelling the unusually curved X-ray spectrum of RGB J0710+591 using AstroSat observations We report the analysis of simultaneous multi- wavelength data of the high energy peaked blazar RGB J0710+591 from the LAXPC, SXT and UVIT instruments on-board AstroSat . The wide band X- ray spectrum (0.35 - 30 keV) is modelled as syn- chrotron emission from a non-thermal distribution of high energy electrons. The spectrum is unusu- ally curved, with a curvature parameter β p ∼ 6 . 4 for a log parabola particle distribution, or a high energy spectral index p 2 > 4 . 5 for a broken power- law distribution. The spectrum shows more cur- vature than an earlier quasi-simultaneous analysis of Swift -XRT/ Nu STAR data where the parameters were β p ∼ 2 . 2 or p 2 ∼ 4. It has long been known that a power-law electron distribution can be pro- duced from a region where particles are acceler- ated under Fermi process and the radiative losses in acceleration site decide the maximum attainable Lorentz factor, γ max . Consequently, this quantity decides the energy at which the spectrum curves steeply. We show that such a distribution provides a more natural explanation for the AstroSat data as well as the earlier XRT/ Nu STAR observation, making this as the first well constrained determi- nation of the photon energy corresponding to γ max . This in turn provides an estimate of the accelera- tion time-scale as a function of magnetic field and Doppler factor. The UVIT observations are consis- tent with earlier optical/UV measurements and re- confirm that they plausibly correspond to a differ- ent radiative component than the one responsible for the X-ray emission. This analysis has been done in collaboratio with Pranjupriya Goswami, Atreyee Sinha, Sunil Chandra, Ranjeev Misra, Varsha Chit- nis, et al. Gaurav Goswami Enhancement of axion decay constants in type IIA theory? We investigate the possibility of enhancement of effective axion decay constant in well controlled constructions in string theory. To this end, we study the dynamics of axions arising in the com- pactifications of type IIA string theory on Calabi- Yau orientifolds with background fluxes (with non- perturbative effects included to ensure stabilization of all moduli). In this setup, we attempt to ob- tain large effective axion decay constant in two dif- ferent ways: by searching for a direction in field space, in which the potential is sufficiently flat and by arriving at a very explicit stringy embedding of the Kim-Nilles-Peloso (KNP) alignment mech- anism. We do not find super-Planckian effective decay constants by either of the approaches. Fur- thermore, we find that the alignment angle of KNP mechanism can not be made arbitrarily small by adjusting the fluxes. Trans-Planckian censorship conjecture and non- thermal post-inflationary history The recently proposed Trans-Planckian Censorship conjecture (TCC) can be used to constrain the en- ergy scale of inflation. The conclusions, however, depend on the assumptions about post-inflationary history of the Universe. For example, in the stan- dard case of a thermal post-inflationary history in which the Universe stays radiation dominated at all times from the end of inflation to the epoch of radiation matter equality, TCC has been used to argue that the Hubble parameter during infla- tion, H inf , is below O (0 . 1) GeV. Cosmological sce- narios with a non-thermal post-inflationary history are well-motivated alternatives to the standard pic- ture, and it is interesting to find out the possible constraints which TCC imposes on such scenarios. In this work, we find out the amount of enhance- ment of the TCC compatible bound on H inf if post- inflationary history before nucleosynthesis is non- thermal. We then argue that if TCC is correct, for a large class of scenarios, it is not possible for the universe to have undergone a phase of moduli dom- ination. This work has been done in collaboration with Mansi Dhuria.