4.2 Multi-Messenger Astronomy
The technology used in LIGO is very delicate and complex.
- LIGO uses high-powered solid-state lasers for interferometry. Advanced LIGO included an upgrade to even more powerful lasers. The laser crystals require extreme care, and technicians must wear static-free, lint-free “bunny suits” to work with them.
- The mirrors are 40kg masses that hang from a series of four pendulums in order to significantly reduce seismic noise. The pendulums in turn hang from an active seismic isolation system. All of this is necessary to ensure that ambient vibrations are not confused for gravitational waves.
- The laser paths between the sources and mirrors are 4km long vacuum tubes. The beam tubes are one meter in diameter and are actually the largest vacuum systems by volume in the world.
Multi-messenger astronomy combines multiple different observational techniques to provide a more complete view of our universe.
- Gravitational astronomy alone will allow us to study black holes, supernovas, and neutron stars in exciting new detail.
- But by combining gravitational astronomy techniques with detectors searching for other signals, like electromagnetic waves, neutrinos, and cosmic rays, we will greatly increase our ability to learn about the universe.
- Multi-messenger astronomy is already the standard. Individual telescopes looking for specific types of light or particles each provide one unique window onto astronomical events. By combining them we can get a more complete view of these events.
- Gravitational wave detectors will provide an entirely new window into the universe that will help to fill some of the holes in our understanding.
Gamma ray bursts are exciting multi-messengers ripe for study with gravitational astronomy
- There are two types of gamma ray bursts (GRBs), short (<2 seconds) and long (>2 seconds). Long GRBs are theorized to come from very powerful supernovas that often leave behind black holes or neutron stars. Short GRBs are believed to be produced by the merger of binary systems in which at least one of the objects is a neutron star.
- As their name suggests, GRBs are best detected in the gamma ray spectrum. Now that we have been studying them for a long time, however, we see that these events also produce other types of electromagnetic signals, as well as cosmic rays.
- With gravitational astronomy we will be able to measure the macroscopic quantities of GRB-producing systems, like mass and spin, that we cannot get from electromagnetic signals.
In order to combine gravitational astronomy with other techniques we must be able to locate the sources of gravitational waves.
- Telescopes can only measure signals from the direction they are pointed in. LIGO is more like a microphone – it can measure gravitational waves coming from almost any direction.
- If we want to determine where a gravitational wave came from, we need to use at least three detectors in order to triangulate the source.
- LIGO’s two detectors are not enough, but current collaboration with the VIRGO detector in Italy provides enough detectors to roughly determine the source of waves.
- The localization of gravitational wave sources is still quite poor by astronomical standards, but the idea is that once a general source region has been determined we can look at that area with other types of telescopes. Then we can find coincidences.
- Future gravitational observatories, like KAGRA in Japan and LIGO-India, will significantly reduce uncertainty in gravitational wave source localization.