Space-based Gravity Wave Telescopes and Black Hole Science in the 2030s

Brian Wang |
April 12, 2019 |

Around 2030, a space-based gravitational array will join other ground and space-based telescopes in exploring the secrets of the universe like the recently imaged black hole.

LISA (Laser Interferometer Space Antenna) will be a Gravitational Wave Observatory with an arm-length of 2.5 million km, compared to the few km’s of the ground-based observatories. LISA will enable us to discover the parts of the universe that are invisible by other means, such as black holes, the Big Bang, and other, as yet unknown, objects. LISA will enhance our knowledge about the beginning, evolution and structure of our universe.

The ESA satellite mission LISA Pathfinder has already successfully demonstrated the technology for a gravitational wave observatory in space such as LISA.

Ground-based gravity detectors are limited to objects with masses a few 10’s that of the Sun, which produce high-frequency signals. Larger masses, such as the mergers of massive black holes at the centers of galaxies, produce signals at much lower frequencies, undetectable on Earth.

An ideal instrument for measuring gravitational waves over a broad band of low frequencies is a laser interferometer with an arm length as large as possible and long integration times, the primary impetus for a space-borne detector. LISA can be thought of as a high precision Michelson interferometer in space with an arm length of 2.5 mil­lion km. The arm length has been carefully chosen to allow observation of most of the interesting sources of gravitational waves in the target frequency band.

LISA will work with regular telescopes based on the electromagnetic spectrum (for example, observations from visible light, infra-red or x-rays). It will open the gravitational wave window in space and measure gravitational radiation over a broad band of frequencies, from about 0.1 mHz to 100 mHz, a band where the Universe is richly populated by strong sources of gravitational waves.

Massive black hole mergers detected by LISA out to modest redshifts could well be visible to SKA and LSST as transients in the same region of the sky. The identification of 5-10 counterparts during the a 2-year LISA mission would not be surprising.

Gravity Wave Detection in the 2030s

By 2028, gravitational wave astronomy will be well-established through ground-based observations operating at 10 Hz and above, and through pulsar timing arrays (PTA) at nHz frequencies. The huge frequency gap between them will be completely unexplored until LISA is launched.

The ground-based network of advanced interferometric detectors (three LIGO detectors, VIRGO, and the Kamioka Gravitational Wave Detector (KAGRA)) will have observed inspiralling binaries up to around 100 solar masses and measured their population statistics. Some, or all, of these detectors will have been further enhanced in sensitivity. It is possible that the third-generation Einstein Telescope (ET) will have come into operation by 2028, further extending the volume of space in which these signals can be detected.

At the other end of the mass spectrum, Pulsar Timing Arrays will have detected a stochastic background due to many overlapping signals from supermassive black hole binaries with masses over 1 billion solar masses, and they may have identified a few individual merger events. The background will help determine the mass function of supermassive black holes at the high-mass end, but it will not constrain the mass function for the much more common 1 million solar mass black holes that inhabit the centers of typical galaxies and are accessible to LISA.

By 2028, theoretical advances and predictable improvements in computer power will have made it possible to compute the complex waveforms expected from EMRIs and supermassive black hole binaries with high precision. This will allow searches in LISA data to approach the optimum sensitivity of matched filtering, and it will make tests of general relativity using these signals optimally sensitive.

SOURCES – LISA project

Written By Brian Wang,

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