AR_final file_2018-19

context of inertial/Rindler observer correspon- dence in Minkowski spacetime. Generalized Schwinger effect and par- ticle production in an expanding uni- verse T. Padmanabhan and Karthik Rajeev discuss several aspects of two important examples of parti- cle production in external background: (a) quan- tum field theory of a complex scalar field in an external, homogeneous, electric field and (b) quan- tum field theory of a scalar field in an expanding Friedmann background. • They show that there exists a purely algebraic correspondence between the differential equa- tions governing the scalar field in the cases (a) and (b) mentioned above. For example, it turns out that the constant electric field case can be mapped to a radiation dominated uni- verse, while the dS universe maps back to a singular electric field. Further, the Milne uni- verse maps to an electric field in flat spacetime, which produces a Planckian spectrum of parti- cles, in a specific limit, thereby providing yet another ‘black hole analogue’. • The particle production in the expanding universe vanishes, either analytically or non- analytically when the parameter describing the expansion of the universe goes to zero. This is analogous to the case in an external electric field, where the particle production vanishes either analytically or non-analytically in the coupling constant, depending on the nature of time-variation of the electric field. They obtain a criterion for analytic versus non-analytic dependence of the coupling constant in the context of particle production in an expanding universe. • The conventional method for studying the par- ticle production in an expanding background is based on calculating the Bogoliubov coef- ficients between the in-modes and the out- modes. The straightforward application of this method works in a dS background only when ( M/H ) > 3 / 2 (where M is the mass of the scalar field quanta and H is the Hub- ble constant), and fails when ( M/H ) < 3 / 2. They provide a careful discussion of this fail- ure, and show how particle production can be computed for all values of ( M/H ) by using a different method based on the instantaneous diagonalization of the Hamiltonian. Cosmology and Structure For- mation Emulating Λ CDM-like expansion on the phantom brane Cosmological expansion appears to be speeding up. The source of cosmic acceleration may be a novel constituent called dark energy (DE) which violates the strong energy condition ρ + 3 p ≥ 0. An alter- native to this scenario rests on the possibility that general relativity (GR) inadequately describes late- time cosmic expansion and needs to be supplanted by a modified theory of gravity. Of the various DE models suggested in the literature the cosmological constant Λ occupies a special place since its equa- tion of state p = − ρ is manifestly Lorentz invari- ant. Λ, when taken together with cold dark matter (CDM), constitutes ΛCDM cosmology. The ΛCDM universe appears to agree remarkably well with a slew of cosmological observations. Yet some data sets also appear to support a phantom universe pos- sessing a strongly negative equation of state (EOS) of dark energy (DE), w < − 1 . While current data sets are unable to unambiguously differentiate be- tween these orthogonal models, high quality data expected from future DE experiments are likely to do so. Of all the phantom DE models the phantom brane is perhaps the most appealing since it has an effective equation of state whose value becomes phantom-like, w eff < − 1, at the present epoch. The phantom brane also does not possess any of the sin- gularities which usually afflict conventional phan- tom models. Satadru Bag , Swagat Mishra and Varun ( 56 )