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)

Comments

A Sapphire Super-Earth

Twenty-one light years away, in the constellation Cassiopeia, a planet by the name of HD219134 b orbits its star with a year that is just three days long. With a mass almost five times that of Earth, it is what is known as a super-Earth. Unlike our planet, however, these super-Earths were formed at high temperatures close to their host star and contain high quantities of calcium, aluminum and their oxides – including sapphire and ruby. HD219134 b is one of three candidates likely to belong to a new, exotic class of exoplanets. These objects are completely different from the majority of Earth-like planets. They have 10 to 20 percent lower densities than Earth. Researchers looked at different scenarios to explain the observed densities. For example, a thick atmosphere could lead to a lower overall density. But two of the exoplanets studied, 55 Cancri e and WASP-47 e, orbit their star so closely that their surface temperature is almost 3,000 degrees and they would have lost this ...

A Galaxy on the Edge

Image: This colourful stripe of stars, gas, and dust is actually a spiral galaxy named NGC 1055. Captured here by ESO’s Very Large Telescope (VLT), this big galaxy is thought to be up to 15 percent larger in diameter than the Milky Way. NGC 1055 appears to lack the whirling arms characteristic of a spiral, as it is seen edge-on. However, it displays odd twists in its structure that were probably caused by an interaction with a large neighbouring galaxy. Credit: ESO Spiral galaxies throughout the Universe take on all manner of orientations with respect to Earth. We see some from above (as it were) or “face-on” — a good example of this being the whirlpool-shaped galaxy NGC 1232. Image: This spectacular image of the large spiral galaxy NGC 1232 was obtained on September 21, 1998, during a period of good observing conditions. It is based on three exposures in ultra-violet, blue and red light, respectively. The colours of the different regions are well visible : the cent...

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

SpaceTime

Search This Blog