George Johnson

We study how the constraints on the primordial black hole density arising from the extragalactic photon background are modified in the scenario that there exist extra large spatial dimensions. We find that though the overall magnitude of the constraints is not substantially different, the mass ranges to which they apply are, and for some choices of mass it is possible for the black holes to constitute the entirety of the dark matter.

P. J. E. Peebles

Observations of the nearby large galaxies that can be examined in particularly close detail suggest that many have small stellar luminosity fractions in bulges and haloes. Simulations of galaxy formation tend to produce considerably larger fractions of the star particles in model bulges, stellar haloes, and more generally in orbits seriously different from circular. The situation might be improved by a prescription for non-Gaussian initial conditions on the scale of galaxies.

Daniel Baumann, Carlos Duaso Pueyo, Austin Joyce, Hayden Lee, Guilherme L. Pimentel

We extend the cosmological bootstrap to correlators involving massless particles with spin. In de Sitter space, these correlators are constrained both by symmetries and by locality. In particular, the de Sitter isometries become conformal symmetries on the future boundary of the spacetime, which are reflected in a set of Ward identities that the boundary correlators must satisfy. We solve these Ward identities by acting with weight-shifting operators on scalar seed solutions. Using this weight-shifting approach, we derive three- and four-point correlators of massless spin-1 and spin-2 fields with conformally coupled scalars. Four-point functions arising from tree-level exchange are singular in particular kinematic configurations, and the coefficients of these singularities satisfy certain factorization properties. We show that in many cases these factorization limits fix the structure of the correlators uniquely, without having to solve the conformal Ward identities. The additional constraint of locality for massless spinning particles manifests itself as current conservation on the boundary. We find that the four-point functions only satisfy current conservation if the s, t, and u-channels are related to each other, leading to nontrivial constraints on the couplings between the conserved currents and other operators in the theory. For spin-1 currents this implies charge conservation, while for spin-2 currents we recover the equivalence principle from a purely boundary perspective. For multiple spin-1 fields, we recover the structure of Yang-Mills theory. Finally, we apply our methods to slow-roll inflation and derive a few phenomenologically relevant scalar-tensor three-point functions.

Srikanta Panda, Pavan K. Aluri, Pramoda Kumar Samal, Pranati K. Rath

In the standard model of cosmology, Cosmic Microwave Background (CMB) sky is expected to show no symmetry preferences. Following our previous studies, we explore the presence of any particular parity preference in the latest full-mission CMB temperature maps from ESA's Planck probe. Specifically, in this work, we will probe (a)symmetry in power between even and odd multipoles of CMB via it's angular power spectrum from Planck 2015 data. Further we also assess any specific preference for mirror parity (a)symmetry, by analysing the power contained in $l+m$=even or odd mode combinations.

Katsuki Aoki, Mohammad Ali Gorji, Shinji Mukohyama

In a very recent paper [1], we have proposed a novel $4$-dimensional gravitational theory with two dynamical degrees of freedom, which serves as a consistent realization of $D\to4$ Einstein-Gauss-Bonnet gravity with the rescaled Gauss-Bonnet coupling constant $\tilde{\alpha}$. In the present paper, we study cosmological implications of the theory in the presence of a perfect fluid and clarify the similarities and differences between the results obtained from the consistent $4$-dimensional theory and those from the previously considered, naive (and inconsistent) $D\rightarrow 4$ limit. Studying the linear perturbations, we explicitly show that the theory only has tensorial gravitational degrees of freedom (besides the matter degree) and that for $\tilde{\alpha}>0$ and $\dot{H}<0$, perturbations are free of any pathologies so that we can implement the setup to construct early and/or late time cosmological models. Interestingly, a $k^4$ term appears in the dispersion relation of tensor modes which plays significant roles at small scales and makes the theory different than not only general relativity but also many other modified gravity theories as well as the naive (and inconsistent) $D\to 4$ limit. Taking into account the $k^4$ term, the observational constraint on the propagation of gravitational waves yields the bound $\tilde{\alpha} \lesssim {\cal O}(1)\,{\rm eV}^{-2}$. This is the first bound on the only parameter (besides the Newton's constant and the choice of a constraint that stems from a temporal gauge fixing) in the consistent theory of $D\to 4$ Einstein-Gauss-Bonnet gravity.

Nikita Moriakov, Ashwin Samudre, Michela Negro, Fabian Gieseke, Sydney Otten, Luc Hendriks

We investigate the use of deep learning in the context of X-ray polarization detection from astrophysical sources as will be observed by the Imaging X-ray Polarimetry Explorer (IXPE), a future NASA selected space-based mission expected to be operative in 2021. In particular, we propose two models that can be used to estimate the impact point as well as the polarization direction of the incoming radiation. The results obtained show that data-driven approaches depict a promising alternative to the existing analytical approaches. We also discuss problems and challenges to be addressed in the near future.

Yoshiaki Sofue

We show that the Earth acts as a high-efficiency gravitational collector of low-velocity flow of dark matter (DM). The focal point appears on the Earth's surface, when the DM flow speed is about 17 km s$^{-1}$ with respect to the geo-center. We discuss diurnal modulation of the local DM density influenced by the Earth's gravity. We also touch upon similar effects on galactic and solar system objects.

Golam Mortuza Hossain, Susobhan Mandal

In the study of spherical degenerate stars such as neutron stars, general relativistic effects are incorporated by using Tolman-Oppenheimer-Volkoff equations to describe their interior spacetime. However, the equation of states employed in such studies are invariably computed in flat spacetime. We show that the equation of states computed in the curved spacetime of these stars depend explicitly on the metric function. Further, we show that ignoring such metric-dependent gravitational time dilation effect leads one to grossly underestimate the mass limits of these compact stars. In turns, it provides a natural way to alleviate the so-called hyperon puzzle of the neutron stars.