TESTING THE SPEED OF GRAVITATIONAL WAVES OVER COSMOLOGICAL DISTANCES

Image: Merging black holes ripple space and time in this artist's concept. Credit: Swinburne Astronomy Productions

On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) observed a transient gravitational-wave signal from a black hole-black hole binary (BHBH) inspiral.

The Fermi gamma ray burst monitor revealed the presence of a possible electromagnetic counterpart (Connaughton et al. 2016): a weak transient source above 50 keV, 0.4 s after the GW event was detected with a false alarm probability of 0.0022. The spatial location of both events are poorly determined but mutually consistent.

If the EM and GW signals are related to the same transient phenomenon this opens up a new physical window with which to test the relative speeds of light and gravitational waves.

Several of the alternative theories of gravity invoked to explain the accelerated expansion of the Universe predict c_GW ≠ c_light. In a recent paper (Collett & Bacon 2016) the authors assume that the EM and GW are emitted at the same instant and they obtain a constraint on the ratio of the speeds of light and gravitational waves at the level of 10⁻¹⁷.

The assumption that the electromagnetic and gravitational wave emissions are emitted at the same time is a strong one, so they suggest a method that does not make such an assumption using a strongly lensed GW event and EM counterpart. They show that a single strongly lensed GW event would produce robust constraints at the 10⁻⁷ level, if a high energy EM counterpart is observed within the field-of-view of an observing gamma ray burst monitor.

Collett & Bacon 2016 - Testing the speed of gravitational waves over cosmological distances with strong gravitational lensing (arXiv)
Connaughton et al. 2016 - Fermi GBM Observations of LIGO Gravitational Wave event GW150914 (arXiv)

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GAMMA-RAY EMISSION FROM THE SNR HB3

Image: At a distance of about 20,000 light years, G292.0+1.8 is one of only three supernova remnants in the Milky Way known to contain large amounts of oxygen. These oxygen-rich supernovas are of great interest to astronomers because they are one of the primary sources of the heavy elements (that is, everything other than hydrogen and helium) necessary to form planets and people. The X-ray image from Chandra shows a rapidly expanding, intricately structured, debris field that contains, along with oxygen (yellow and orange), other elements such as magnesium (green) and silicon and sulfur (blue) that were forged in the star before it exploded. Credit: NASA/CXC/SAO The processes of particles acceleration to very high energies from the supernova shock region and diffusion in the interstellar medium of such particles has not been well understood so far. Gamma-ray observations in the GeV regime are a powerful probe of these mechanisms

CONTAMINATION BY SUPERNOVAE IN GLOBULAR CLUSTERS

Credit: ALMA (ESO/NAOJ/NRAO)/Alexandra Angelich (NRAO/AUI/NSF) Only a small amount of the supernovae products remains trapped within globular clusters and this "catch" only occurs in the most massive cases (mass cluster ≥ 10^6 solar masses).

Boulevard of Broken Rings

Credit: ESO/Perrot This Picture illustrates the remarkable capabilities of SPHERE (the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument), a planet-hunting instrument mounted on ESO's Very Large Telescope (VLT) in Chile: It shows a series of broken rings of dust around a nearby star. These concentric rings are located in the inner region of the debris disc surrounding a young star named HD 141569A, which sits some 370 light-years away from us. In this image we see what is known as a transition disc, a short-lived stage between the protoplanetary phase, when planets have not yet formed, and a later time when planets have coalesced, leaving the disc populated only by any remaining - and predominantly dusty - debris. What we see here are structures formed of dust, revealed for the first time in near-infrared light by SPHERE - at a high enough resolution to capture remarkable detail! The area shown in this image has a diameter of just 200 times the Earth–Sun distan...

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