Thomas Lacroix, Joseph Silk

Could there be a large population of intermediate mass black holes (IMBHs) formed in the early universe? Whether primordial or formed in Population III, these are likely to be very subdominant compared to the dark matter density, but could seed early dwarf galaxy/globular cluster and supermassive black hole formation. Via survival of dark matter density spikes, we show here that a centrally concentrated relic population of IMBHs, along with ambient dark matter, could account for the Fermi gamma-ray "excess" in the Galactic center because of dark matter particle annihilations.

Chandrachur Chakraborty, China), Sudip Bhattacharyya, India)

The gravitomagnetic monopole is the proposed gravitational analogue of Dirac's magnetic monopole, and if detected, a new area of fundamental physics will be opened. Here we find the first observational evidence of such a monopole in the following way. Three primary methods to measure the black hole spin, based on the relativistic precession model of quasi-periodic oscillations in X-rays, broad relativistic X-ray spectral line and X-ray continuum spectrum, give significantly different spin values for the accreting black hole in GRO J1655--40. We demonstrate that the inclusion of just one additional parameter, that is the so-called NUT parameter or the gravitomagnetic monopole, can not only make the spin and other parameter values inferred from these three independent methods consistent with each other, but also make the inferred black hole mass consistent with an independently measured mass value. We argue that this evidence of the gravitomagnetic monopole in nature is not a result of fine-tuning, and hence is robust within our paradigm. Finally, our results tentatively suggest that the collapsed object in GRO J1655--40 could be a naked singularity.

Richard I. Anderson, Adam G. Riess

State-of-the art photometric measurements of extragalactic Cepheids account for the mean additional light due to chance superposition of Cepheids on crowded backgrounds through the use of artificial star measurements. However, light from stars physically associated with Cepheids may bias relative distance measurements if the changing spatial resolution along the distance ladder significantly alters the amount of associated blending. We have identified two regimes where this phenomenon may occur: Cepheids in wide binaries and open clusters. We estimate stellar association bias using the photometric passbands and reddening-free Wesenheit magnitudes used to set up the Riess et al. (2016) distance scale. For wide binaries, we rely on Geneva stellar evolution models in conjunction with detailed statistics on intermediate-mass binary stars. For the impact of cluster stars, we have compiled information on the frequency of Cepheids occurring in clusters and measured the typical cluster contribution in M31 via deep HST imaging provided by the PHAT project. We find that the dominant effect on the distance scale comes from Cepheids in clusters, despite cluster Cepheids being a relatively rare phenomenon. Wide binaries have a negligible effect on $H_0$ that is on the order of $0.004\%$ for long-period Cepheids observed in the near-infrared or when considering Wesenheit magnitudes. We estimate that blending due to cluster populations has previously resulted in an overestimate of $H_0$ by approximately $0.2\%$. Correcting for this bias, we obtain $H_0 = 73.06 \pm 1.76\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$, which remains in $3.3\sigma$ tension with the Planck value. We conclude that stellar association bias does not constitute a limit for measuring $H_0$ with an accuracy of $1\%$.

Gautam Satishchandran, Robert M. Wald

We investigate the gravitational memory effect for linearized perturbations off of Minkowski space in odd spacetime dimension $d$ by examining the effects of gravitational radiation from point particle scattering. We also investigate analogous memory effects for electromagnetic and scalar radiation. We find that there is no gravitational memory effect in all odd dimensions. For scalar and electromagnetic fields, there is no memory effect for $d\geq 7$; for $d=3$ there is an infinite momentum memory effect, whereas for $d=5$ there is no momentum memory effect but the displacement of a test particle will grow unboundedly with time. Our results are further elucidated by analyzing the memory effect for any slowly moving source of compact spatial support in odd dimensions.

Chunshan Lin, Jerome Quintin, Robert H. Brandenberger

We consider a modified gravity model with a massive graviton, but which nevertheless only propagates two gravitational degrees of freedom and which is free of ghosts. We show that non-singular bouncing cosmological background solutions can be generated. In addition, the mass term for the graviton prevents anisotropies from blowing up in the contracting phase and also suppresses the spectrum of gravitational waves compared to that of the scalar cosmological perturbations. This addresses two of the main problems of the matter bounce scenario.