APOD: LIGO-Virgo GW170814 Skymap (2017 Sep 28)

[heehaw]

It seems surprising that all the detections so far have seemed so similar. Could something be amiss?

[hamilton1]

quote ...

Thanks for replies, looks like I jumped the gun there.

[Chris Peterson]

It seems surprising that all the detections so far have seemed so similar. Could something be amiss?

Like what? LIGO has a fairly narrow frequency range over which it detects gravitational waves, and the mergers of these midsized black holes is squarely in the middle of it. And black holes are really very simple things- almost everything about them is distilled down to just a few parameters, which are mostly similar. So we'd expect black hole mergers to be extremely simple, to all happen the same way. And that's what LIGO is showing us.

[neufer]

<<The Kamioka Gravitational Wave Detector (KAGRA) is a project of the gravitational wave studies group at the Institute for Cosmic Ray Research (ICRR) of the University of Tokyo. KAGRA has two arms, 3 km long, which form a laser interferometric gravitational wave detector. It is built in the Kamioka Observatory near the neutrino physics experiments. The excavation phase of tunnels was completed on 31 March 2014. KAGRA has suffered numerous delays. Early planning had hoped to begin construction in 2005 and observation in 2009 but is now likely to enter operation in 2018. Excess water in the tunnels caused significant delays in 2014 and 2015.

KAGRA will detect chirp waves from binary neutron star coalescence at 240 Mpc away with a signal to noise ratio of 10. The expected number of detectable events in a year is two or three. To achieve the required sensitivity, the existing state of the art techniques as used by LIGO and VIRGO (low-frequency vibration-isolation system, high-power laser system, Fabry-Pérot cavities, resonant side band extraction method, and so on) will be extended with an underground location, cryogenic mirrors, and a suspension point interferometer.>>

<<A third-generation detector at the existing LIGO sites is being planned under the name "LIGO Voyager" to improve the sensitivity by an additional factor of two, and halve the low-frequency cutoff to 10 Hz. Plans call for the glass mirrors and 1064 nm lasers to be replaced by even larger 160 kg silicon test masses, cooled to 123 K (a temperature achievable with liquid nitrogen), and a change to a longer laser wavelength in the 1500–2200 nm range at which silicon is transparent. (Many documents assume a wavelength of 1550 nm, but this is not final.)

A design for a larger facility with longer arms is called "Cosmic Explorer", and would improve on the European Einstein Telescope proposal. This is based on the LIGO Voyager technology, but expanded to an ET-like triangular configuration with 40 km arms.>>

<<Einstein Telescope (ET) or Einstein Observatory, is a proposed third-generation ground-based gravitational wave detector, currently under study by some institutions in the European Union. It will be located underground to reduce seismic noise and "gravity gradient noise" caused by nearby moving objects.

The arms will be 10 km long (compared to 4 km for LIGO, and 3 km for Virgo and KAGRA), and like LISA, there will be three arms in an equilateral triangle, with two detectors in each corner.

In order to measure the polarization of incoming gravitational waves and avoid having an orientation to which the detector is insensitive, a minimum of two detectors are required. While this could be done with two 90° interferometers at 45° to each other, the triangular form allows the arms to be shared. The 60° arm angle reduces the interferometer's sensitivity, but that is made up for by the third detector, and the additional redundancy provides a useful cross-check.

Each of the three detectors would be composed of two interferometers, one optimized for operation below 30 Hz and one optimized for operation at higher frequencies.

The low-frequency interferometers (1 to 250 Hz) will use optics cooled to 10 K (−441.7 °F; −263.1 °C), with a beam power of about 18 kW in each arm cavity. The high-frequency ones (10 Hz to 10 kHz) will use room-temperature optics and a much higher recirculating beam power of 3 MW.>>

[Case]

It seems surprising that all the detections so far have seemed so similar. Could something be amiss?

Like what? LIGO has a fairly narrow frequency range over which it detects gravitational waves, and the mergers of these midsized black holes is squarely in the middle of it.

The wavelength of the gravitational waves relates to the mass. The length of the ‘arms’ of the detector determine what wavelengths can be observed. To detect mergers of e.g. supermassive black holes, if those happen frequently enough in a mission time frame, we’d need multiple detectors much larger than the Earth itself. See the ESA-NASA LISA mission (Laser Interferometer Space Antenna). Launch is expected in 2034.