Claudia de Rham, Andrew J. Tolley, Shuang-Yong Zhou

Lorentz-invariant massive gravity is usually associated with a strong coupling scale $\Lambda_3$. By including non-trivial effects from the Stueckelberg modes, we show that about these vacua, one can push the strong coupling scale to higher values and evade the linear vDVZ-discontinuity. For generic parameters of the theory and generic vacua for the Stueckelberg fields, the $\Lambda_2$-decoupling limit of the theory is well-behaved and free of any ghost or gradient-like instabilities. We also discuss the implications for nonlinear sigma models with Lorentzian target spaces.

LIGO Scientific Collaboration, Virgo Collaboration

The LIGO detection of GW150914 provides an unprecedented opportunity to study the two-body motion of a compact-object binary in the large velocity, highly nonlinear regime, and to witness the final merger of the binary and the excitation of uniquely relativistic modes of the gravitational field. We carry out several investigations to determine whether GW150914 is consistent with a binary black-hole merger in general relativity. We find that the final-remnant's mass and spin, determined from the inspiral and post-inspiral phases of the signal, are mutually consistent with the binary black-hole solution in general relativity. The data following the peak of GW150914 are consistent with the least-damped quasi-normal-mode inferred from the mass and spin of the remnant black hole. By using waveform models that allow for parameterized general-relativity violations during the inspiral and merger phases, we perform quantitative tests on the gravitational-wave phase in the dynamical regime and, bound, for the first time several high-order post-Newtonian coefficients. We constrain the graviton Compton wavelength in a hypothetical theory of gravity in which the graviton is massive and place a $90\%$-confidence lower bound of $10^{13}$ km. Within our statistical uncertainties, we find no evidence for violations of general relativity in the genuinely strong-field regime of gravity.

Diego Blas, Mikhail M. Ivanov, Ignacy Sawicki, Sergey Sibiryakov

We point out that the observed time delay between the detection of the signal at the Hanford and Livingston LIGO sites from the gravitational wave event GW150914 places an upper bound on the speed of propagation of gravitational waves, $c_{gw}\lesssim 1.7$ in the units of speed of light. Combined with the lower bound from the absence of gravitational Cherenkov losses by cosmic rays that rules out most of subluminal velocities, this gives a model-independent double-sided constraint $1\lesssim c_{gw}\lesssim 1.7$. We compare this result to model-specific constraints from pulsar timing and cosmology.

Erminia Calabrese, Nicholas Battaglia, David N. Spergel

We show that the gravitational wave source counts distribution can test how gravitational radiation propagates on cosmological scales. This test does not require obtaining redshifts for the sources. If the signal-to-noise from a gravitational wave source is proportional to the strain then it falls as $R^{-1}$, thus we expect the source counts to follow $dN/dS \propto S^{-4}$. However, if gravitational waves decay as they propagate or can propagate into other dimensions, then there can be deviations from this generic prediction. We consider the possibility that the signal-to-noise falls as $R^{-\gamma}$, where $\gamma=1$ recovers the expected predictions in a Euclidean uniformly-filled universe. We forecast the sensitivity of future observations in constraining gravitational wave physics using this method by simulating sources distributed over a finite range of signal-to-noise. We first consider the case of few objects, 7 sources, with a signal-to-noise from 8 to 24, and impose a lower limit on $\gamma$, finding $\gamma>0.33$ at 95% confidence level. The distribution of our simulated sample is very consistent with the distribution of the candidate black holes binary systems observed by Advanced LIGO. We then consider the improvement coming from further detections, simulating 100 observations spanning a wider range of signal-to-noise and measure $\gamma$ with $\sigma(\gamma)\sim 0.15$, percent level precision will be possible with 10000 objects. We generalize the formalism to account for a range of chirp masses and the possibility that the signal falls as $\exp(-R/R_0)/R^\gamma$.

J. P. W. Verbiest, L. Lentati, G. Hobbs, R. van Haasteren, P. B. Demorest, G. H. Janssen, J.-B. Wang, G. Desvignes, R. N. Caballero, M. J. Keith, D. J. Champion, Z. Arzoumanian, S. Babak, C. G. Bassa, N. D. R. Bhat, A. Brazier, P. Brem, M. Burgay, S. Burke-Spolaor, S. J. Chamberlin, S. Chatterjee, B. Christy, I. Cognard, J. M. Cordes, S. Dai, T. Dolch, J. A. Ellis, R. D. Ferdman, E. Fonseca, J. R. Gair, N. E. Garver-Daniels, P. Gentile, M. E. Gonzalez, E. Graikou, L. Guillemot, J. W. T. Hessels, G. Jones, R. Karuppusamy, M. Kerr, M. Kramer, M. T. Lam, P. D. Lasky, A. Lassus, P. Lazarus, T. J. W. Lazio, K. J. Lee, L. Levin, K. Liu, R. S. Lynch, A. G. Lyne, J. Mckee, M. A. McLaughlin, S. T. McWilliams, D. R. Madison, R. N. Manchester, C. M. F. Mingarelli, D. J. Nice, S. Oslowski, N. T. Palliyaguru, et al. (33 additional authors not shown)

The highly stable spin of neutron stars can be exploited for a variety of (astro-)physical investigations. In particular arrays of pulsars with rotational periods of the order of milliseconds can be used to detect correlated signals such as those caused by gravitational waves. Three such "Pulsar Timing Arrays" (PTAs) have been set up around the world over the past decades and collectively form the "International" PTA (IPTA). In this paper, we describe the first joint analysis of the data from the three regional PTAs, i.e. of the first IPTA data set. We describe the available PTA data, the approach presently followed for its combination and suggest improvements for future PTA research. Particular attention is paid to subtle details (such as underestimation of measurement uncertainty and long-period noise) that have often been ignored but which become important in this unprecedentedly large and inhomogeneous data set. We identify and describe in detail several factors that complicate IPTA research and provide recommendations for future pulsar timing efforts. The first IPTA data release presented here (and available online) is used to demonstrate the IPTA's potential of improving upon gravitational-wave limits placed by individual PTAs by a factor of ~2 and provides a 2-sigma limit on the dimensionless amplitude of a stochastic GWB of 1.7x10^{-15} at a frequency of 1 yr^{-1}. This is 1.7 times less constraining than the limit placed by (Shannon et al. 2015), due mostly to the more recent, high-quality data they used.

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100 years after Einstein predicted the existence of gravitational waves, the National Science Foundation gathers scientists from Caltech, MIT and the LIGO Scientific Collaboration to update the scientific community on efforts to detect them.

Press release page

Youtube link  (Backup/alt link)

Observation of Gravitational Waves from a Binary Black Hole Merger

Tests of general relativity with GW150914

V. Connaughton, E. Burns, A. Goldstein, L. Blackburn, M. S. Briggs, B.-B. Zhang, C. M. Hui, P. Jenke, J. Racusin, C. A. Wilson-Hodge, P. N. Bhat, W. Cleveland, G. Fitzpatrick, M. M. Giles, M. H. Gibby, J. Greiner, A. von Kienlin, R. M. Kippen, S. McBreen, B. Mailyan, C. A. Meegan, W. S. Paciesas, R. D. Preece, O. Roberts, L. Sparke, M. Stanbro, K. Toelge, P. Veres, H.-F. Yu, other authors

With an instantaneous view of 70% of the sky, the Fermi Gamma-ray Burst Monitor (GBM) is an excellent partner in the search for electromagnetic counterparts to gravitational wave (GW) events. GBM observations at the time of the Laser Interferometer Gravitational-wave Observatory (LIGO)event GW150914 reveal the presence of a weak transient source above 50 keV, 0.4~s after the GW event was detected, with a false alarm probability of 0.0022. This weak transient lasting 1 s does not appear connected with other previously known astrophysical, solar, terrestrial, or magnetospheric activity. Its localization is ill-constrained but consistent with the direction of GW150914. The duration and spectrum of the transient event suggest it is a weak short Gamma-Ray Burst arriving at a large angle to the direction in which Fermi was pointing, where the GBM detector response is not optimal. If the GBM transient is associated with GW150914, this electromagnetic signal from a stellar mass black hole binary merger is unexpected. From our measurement of the fluence seen by GBM, we calculate a luminosity in hard X-ray emission between 1~keV and 10~MeV of $1.8^{+1.5}_{-1.0} \times 10^{49}$ erg s$^{-1}$. The observation by Fermi GBM encompasses 75% of the probability map associated with the LIGO GW event localization at the time the GW event was detected. Assuming the two events have a common origin, the combined LIGO and GBM observations can reduce the 90% confidence interval on sky location from 601 to 199 square degrees. Future joint observations of GW events by LIGO/Virgo and Fermi GBM could reveal whether the weak transient reported here is a plausible counterpart to the GW event GW150914 or a chance coincidence, and will further probe the connection between compact binary mergers and short Gamma-Ray Bursts.

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https://www.youtube.com/watch?time_continue=13&v=ThQ8P1KSFiw

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https://www.youtube.com/watch?v=e8JK3qVGuUg

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https://www.youtube.com/watch?v=5EhSk_QRbQc

P.A. Evans, J.A. Kennea, S.D. Barthelmy, D. N. Burrows, S. Campana, S.B. Cenko, N. Gehrels, P. Giommi, F. E. Marshall, J. A. Nousek, P. T. O'Brien, J. P. Osborne, D.M. Palmer, M. Perri, J. L. Racusin, M.H. Siegel, G. Tagliaferri

The Advanced LIGO observatory recently reported the first direct detection of gravitational waves. We report on observations taken with the Swift satellite two days after the GW trigger. No new X-ray, optical, UV or hard X-ray sources were detected in our observations, which were focussed on nearby galaxies in the gravitational wave error region and we discuss the implications of this.