Astrophysics

Cosmic Microwave Background (CMB) is a powerful probe to the Universe which carries signatures of cosmic secrets over a vast range of redshifts. Along with spatial fluctuations, spectral distortions of CMB blackbody are also a rich source of cosmological information. In my talk, I will introduce a new kind of spectral distortion of CMB which can arise due to the conversion of CMB photons into Axion-Like Particles (ALPs) in the presence of an external magnetic field. This effect leads to both polarized and unpolarized spatially varying spectral distortion signal with a unique spectral shape when CMB photons undergo resonant and non-resonant conversion into ALPs in the presence of the magnetic field of Milky Way, galaxy clusters and voids. I will discuss the spatial structure of this distortion which can arise from Milky Way and galaxy clusters and will show its uniqueness from other known cosmological and astrophysical signals. I will present the first all-sky map of this distortion which is obtained using the data of Planck satellite and will discuss the capability of this new cosmological window to probe ALPs from the upcoming ground-based and space-based CMB experiments such as CMB-S4, LiteBIRD, Simons Observatory, etc.

Infrared sensitivity of the de Sitter decay rate due to particle creation requires that gravitational backreaction be taken into account on the horizon scale. At lowest order, backreaction can be studied by Linear Response of the geometry to quantum matter perturbations around the Bunch-Davies state. In Linear Response the scalar degree of freedom derived from the conformal anomaly gives rise to scalar gravitational waves that grow without bound on the de Sitter horizon scale,  which implies substantial non-linear quantum backreaction effects in cosmology. Fluctuations in the conformalon scalar are potentially responsible for the primordial density fluctuations observed in the CMB anisotropy without an inflaton, and can also lead to dynamical, spacetime dependent dark energy. Possible observational tests of the conformal origin of primordial density fluctuations is suggested by the prediction of equality of scalar and tensor weight spectral indices, and the bispectral shape function of non-Gaussian correlations in the CMB predicted by conformal invariance.

Cosmological vacuum energy does not remain constant for the same reason that a uniform electric field cannot persist indefinitely in the presence of quantum fluctuations. The decay rate of the Bunch-Davies state of QFT in de Sitter space due to particle creation is calculated in real time by the same method as that for an electric field, giving Schwinger’s result.  In both the electric field and de Sitter cases the particles created are verified to be real, in that they persist in the final asymptotic region if the background field is switched off. The energy density of the particles decreases the vacuum energy and the Hubble expansion rate.

The sensitivity to infrared physics of this decay rate requires that gravitational backreaction be taken into account on the horizon scale. A Linear Response analysis shows the relevance of the scalar degree of freedom of the conformal anomaly to de Sitter instability, backreaction and screening of vacuum energy. The conformalon scalar gives rise to cosmological horizon modes that can generate the primordial density fluctuations observed in the CMB anisotropy, and lead to dynamical, spacetime dependent dark energy. Possible observational tests of the conformal quantum origin of primordial density fluctuations is suggested by the prediction of equality of scalar and tensor weights spectral indices, and the bispectral shape function of non-Gaussian correlations in the CMB.

Introduction of the torsion tensor on the space-time manifold  leads to a Weyl invariant geometry, in which t e torsion trace acts as a "U(1)" compensating field for the conformal transformations. Such  symmetry can be extended to the whole matter sector included in the  standard model by coupling the Higgs scalar (and all other possible  fundamental scalars) to the torsion. A relevant question is then whether such framework can be used to describe cosmic inflation, as a spontaneous symmetry breaking phenomenon. It turns out that this is indeed possible because the Weyl invariance imposes a distinct field space geometry, which introduces a slow roll plateau in the otherwise  too steep quartic potential dictated by Weyl symmetry. The model can  have observable consequences that distinguish it from other inflationary scenarios, at least under some choices of initial conditions.