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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ORBITAL PERIODS OF THE PLANETS

For orbital period generally we refer to the sidereal period, that is the temporal cycle that it takes an object to make a full orbit, relative to the stars. This is the orbital period in an inertial (non-rotating) frame of reference (365,25 days for the earth).

CONSTRAINTS ON THE LOCATION OF A POSSIBLE 9TH PLANET

Image: The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. Such an orbital alignment can only be maintained by some outside force, Batygin and Brown say. Their paper argues that a planet with 10 times the mass of the earth in a distant eccentric orbit anti-aligned with the other six objects (orange) is required to maintain this configuration. Credit: Caltech The astronomers have noticed some of the dwarf planets and other small, icy objects tend to follow orbits that cluster together. To explain the unusual distribution of these Kuiper Belt objects, several authors have advocated the existence of a superEarth planet in the outer solar system ( planet Nine or planet X ).

RADIATIVE CLEARING OF PROTOPLANETARY DISCS

Image: protoplanetary disc surrounding the young star HL Tauri, a very young T Tauri star in the constellation Taurus, approximately 450 light-years (140 pc) from Earth in the Taurus Molecular Cloud. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system. Credit: ALMA (ESO/NAOJ/NRAO) T Tauri stars are pre-main-sequence stars in the process of contracting to the main sequence. Their central temperatures are too low for hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract, while moving towards the main sequence, which they reach after about 100 million years. Roughly half of T Tauri stars have circumstellar disks, which in this case are called protoplanetary discs because they are probably the progenitors of planetary systems like the Solar System.

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