Title: Detecting primordial gravitational waves with circular polarization of the redshifted 21 cm line: I. Formalism Author: Christopher M. Hirata, Abhilash Mishra, Tejaswi Venumadhav

We propose a new method to measure the tensor-to-scalar ratio r using the circular polarization of the 21 cm radiation from the pre-reionization epoch. Our method relies on the splitting of the F=1 hyperfine level of neutral hydrogen due to the quadrupole moment of the CMB. We show that unlike the Zeeman effect, where MF=±1 have opposite energy shifts, the CMB quadrupole shifts MF=±1 together relative to MF=0. This splitting leads to a small circular polarization of the emitted 21 cm radiation. In this paper (Paper I in a series on this effect), we present calculations on the microphysics behind this effect, accounting for all processes that affect the hyperfine transition. We conclude with an analytic formula for the circular polarization from the Dark Ages as a function of pre-reionization parameters and the value of the remote quadrupole of the CMB. We also calculate the splitting of the F=1 hyperfine level due to other anisotropic radiation sources and show that they are not dominant. In a companion paper (Paper II) we make forecasts for measuring the tensor-to-scalar ratio r using future radio arrays.

Title: Detecting primordial gravitational waves with circular polarization of the redshifted 21 cm line: II. Forecasts Author: Abhilash Mishra, Christopher M. Hirata

In the first paper of this series, we showed that the CMB quadrupole at high redshifts results in a small circular polarization of the emitted 21 cm radiation. In this paper we forecast the sensitivity of future radio experiments to measure the CMB quadrupole during the era of first cosmic light (z~20). The tomographic measurement of 21 cm circular polarization allows us to construct a 3D remote quadrupole field. Measuring the B-mode component of this remote quadrupole field can be used to put bounds on the tensor-to-scalar ratio r. We make Fisher forecasts for a future Fast Fourier Transform Telescope (FFTT), consisting of an array of dipole antennas in a compact grid configuration, as a function of array size and observation time. We find that a FFTT with a side length of 100 km can achieve sigma(r)\sim 4 x 10^-3 after ten years of observation and with a sky coverage f_sky~0.7. The forecasts are dependent on the evolution of the Lyman- flux in the pre-reionization era, that remains observationally unconstrained. Finally, we calculate the typical order of magnitudes for circular polarization foregrounds and comment on their mitigation strategies. We conclude that detection of primordial gravitational waves with 21 cm observations is in principle possible, so long as the primordial magnetic field amplitude is small, but would require a very futuristic experiment with corresponding advances in calibration and foreground suppression techniques.

Title: Astronomical Catalogues for Locating Gravitational-wave Events Author: Kunyang Li, Roy Williams

Gravitational wave transients are caused by some of the most energetic events in the Universe, and a precise location would allow deep examination of the counterpart by electromagnetic waves (telescopes collecting light), the combination of GW and EM resulting in very much improved science return (multi-messenger Astronomy). Since the GW detectors do not provide good localization on the sky, the faint counterpart will be very difficult to find. One strategy to help the search is to look first where mass is concentrated and thus the prior probability of GW events is highest. In the first part of this paper, we present methods used to estimate stellar masses and metallicities of galaxies and galaxy clusters in different catalogues. In the second part of the paper, we test our estimation accuracy by comparing our results with stellar masses given in Stripe 82 Massive Galaxy Catalogue (S82-MGC). The relation between stellar mass we found and that from S82-MGC is provided for GWGC, 2MASS-GLADE, and WISExSCOS catalogue in the last part of the paper. Our results are used in an interactive web-based tool (Skymap Viewer) for astronomers to decide where to look first in EM follow-up observations of GW events in the future.

Ripples arrived six years late for gravitational wager

Last week's historic announcement about gravitational waves came too late for a bet, placed by two physicists, that the discovery would happen before 2010. But Prof Sheila Rowan and Prof Jim Hough said the seminal findings were "worth much more" than any winnings. Both scientists are part of the Ligo collaboration that detected the waves. Read more

A century ago, Albert Einstein predicted gravitational waves -- ripples in the fabric of space-time that result from the universe's most violent phenomena. NSF-funded researchers, using one of the most precise instruments ever made -- the Laser Interferometer Gravitational-wave Observatory (LIGO) -- have detected gravitational waves that emanate from 1.3 billion years ago. Read more

Title: Detection and localization of single-source gravitational waves with pulsar timing arrays Author: Xing-Jiang Zhu, Linqing Wen, George Hobbs, Yilin Zhang, Yan Wang, Dustin R. Madison, Richard N. Manchester, Matthew Kerr, Pablo A. Rosado, Jingbo Wang

Pulsar timing arrays (PTAs) can be used to search for very low frequency (10^{-9}--10^{-7} Hz) gravitational waves (GWs). In this paper we present a general method for the detection and localization of single-source GWs using PTAs. We demonstrate the effectiveness of this new method for three types of signals: monochromatic waves as expected from individual supermassive binary black holes in circular orbits, GWs from eccentric binaries and GW bursts. We also test its implementation in realistic data sets that include effects such as uneven sampling and heterogeneous data spans and measurement precision. It is shown that our method, which works in the frequency domain, performs as well as published time-domain methods. In particular, we find it equivalent to the F_{e-}statistic proposed in Ellis et al. (2012) for monochromatic waves. We also discuss the construction of null streams -- data streams that have null response to GWs, and the prospect of using null streams as a consistency check in the case of detected GW signals. Finally, we present sensitivities to individual supermassive binary black holes in eccentric orbits. We find that a monochromatic search that is designed for circular binaries can efficiently detect eccentric binaries with both high and low eccentricities, while a harmonic summing technique provides greater sensitivities only for binaries with moderate eccentricities.

BICEP2 and Planck joint study: Gravitational waves remain elusive

A new joint analysis of data from two South Pole-based experiments--the BICEP2 telescope and the Keck Array, both supported by the National Science Foundation--and the European Space Agency's Planck satellite, has found no conclusive evidence of primordial gravitational waves, despite earlier reports of a possible detection. Read more

Title: The Universe as a Cosmic String Author: Florian Niedermann, Robert Schneider, Stefan Hofmann, Justin Khoury

The cosmology of brane induced gravity in six infinite dimensions is investigated. It is shown that a brane with FRW symmetries necessarily acts as a source of cylindrically symmetric gravitational waves, so called Einstein-Rosen waves. Their existence essentially distinguishes this model from its codimension-one counterpart and necessitates to solve the non-linear system of bulk and brane-matching equations. A numerical analysis is performed and two qualitatively different and dynamically separated classes of cosmologies are derived: degravitating solutions for which the Hubble parameter settles to zero despite the presence of a non-vanishing energy density on the brane and super-accelerating solutions for which Hubble grows unbounded. The parameter space of both the stable and unstable regime is derived and observational consequences are discussed: It is argued that the degravitating regime does not allow for a phenomenologically viable cosmology. On the other hand, the super-accelerating solutions are potentially viable, however, their unstable behaviour questions their physical relevance.

Title: An all-sky search for continuous gravitational waves in the Parkes Pulsar Timing Array data set Author: X.-J. Zhu, G. Hobbs, L. Wen, W. A. Coles, J.-B. Wang, R. M. Shannon, R. N. Manchester, M. Bailes, N. D. R. Bhat, S. Burke-Spolaor, S. Dai, M. J. Keith, M. Kerr, Y. Levin, D. R. Madison, S. Oslowski, V. Ravi, L. Toomey, W. van Straten

We present results of an all-sky search in the Parkes Pulsar Timing Array (PPTA) Data Release 1 data set for continuous gravitational waves (GWs) in the frequency range from 5 x 10^-9 to 2 x 10^-7 Hz. Such signals could be produced by individual supermassive binary black hole systems in the early stage of coalescence. We phase up the pulsar timing array data set to form, for each position on the sky, two data streams that correspond to the two GW polarizations and then carry out an optimal search for GW signals on these data streams. Since no statistically significant GWs were detected, we place upper limits on the intrinsic GW strain amplitude h0 for a range of GW frequencies. For example, at 10^-8 Hz our analysis has excluded with 95% confidence the presence of signals with h01.7 x 10^-14. Our new limits are about a factor of four more stringent than those of Yardley et al. (2010) based on an earlier PPTA data set and a factor of two better than those reported in the recent Arzoumanian et al. (2014) paper. We also present PPTA directional sensitivity curves and find that for the most sensitive region on the sky, the current data set is sensitive to GWs from circular supermassive binary black holes with chirp masses of 10^9 solar masses out to a luminosity distance of about 100 Mpc. Finally, we set an upper limit of 4 x 10^-3Mpc^-3Gyr^-1 at 95% confidence on the coalescence rate of nearby (z<0.1) supermassive binary black holes in circular orbits with chirp masses of 10^10 solar masses.

The discovery of three closely orbiting supermassive black holes in a galaxy more than four billion light years away could help astronomers in the search for gravitational waves: the 'ripples in spacetime' predicted by Einstein. Read more