LIGO-Virgo-KAGRA

05 Feb 2023

OSR blog post

Everyone who is slightly interested in astronomy has heard of gravity detectors or at least seen two kilometers of tubes that connect at the right angles.

The most famous is the one that was built first, in the early 2000s LIGO started working on the North American continent. A year later, Virgo in Italy joined, and since a year ago, this team has three members – the Japanese interferometer KAGRA has joined the team.

After more than two years of upgrades, maintenance and sensitivity improvements at two LIGO facilities 3,000 km apart, LLO (Livingston) and LHO (Hanford), LIGO predicts that their next observation  (O4 – fourth in line) will start in May 2023.

The long wait until the start of O4 will be worth it. The latest upgrades to the LIGO and Virgo instruments in particular will result in increased sensitivity of the detectors, capable of sensing “weaker” gravitational waves than before – which also means they detect more events than before.

LIGO, Virgo and KAGRA are working closely together to launch the O4 phase. There is always a chance that detection will be made during engineering work, so that if any events occur, they can be released to the scientific community and forwarded for further study.

Caltech/MIT/LIGO Lab, Public domain, via Wikimedia Commons

O4 will consist of up to 18 months of active observation. The official completion of work will likely be extended to 2025, and the extended work time will also provide additional time for the development of the O5 program. This change is motivated by the upgrade plans for the O5 observation.

The sensitivity improvement expected for the O5 phase comes primarily from the mirror coating which will provide a lower noise floor, and these changes are still in development and not expected to be available by the original O4 completion date. During this year and a half of work, there may be a break of a month or two, which will be necessary for equipment maintenance.

That gravitational waves exist was assumed earlier. In his general theory of relativity, Albert Einstein predicted the existence of gravitational waves a century ago, and scientists have been trying to discover them for some 50 years.

At first, their existence was confirmed by indirect methods. Until 2015, when the first direct detection took place, and two years later, the team that worked on the LIGO detector received the Nobel Prize in Physics for it.

However, there are still many unknowns regarding gravity. The topic has just been started from the scientific side. It remains to be seen whether future time will also confirm the existence of gravitons – elementary particles. In theories of quantum gravity, a graviton is a hypothetical quantum of gravity, an elementary particle that mediates the gravitational force.

If it exists, it is assumed that the graviton is massless, because gravitational forces act on very long ranges and propagate at the speed of light. For now, the status of the particle is hypothetical, but the gravitational wave was also hypothetical.