was found by the
BICEP2 radio telescope. The microscopic examination of the
focal plane of the BICEP2 detector is shown here.
Indirect detection Although the waves from the Earth–Sun system are minuscule, astronomers can point to other sources for which the radiation should be substantial. One important example is the
Hulse–Taylor binary a pair of stars, one of which is a
pulsar. The characteristics of their orbit can be deduced from the
Doppler shifting of radio signals given off by the pulsar. Each of the stars is about and the size of their orbits is about 1/75 of the
Earth–Sun orbit, just a few times larger than the diameter of our own Sun. The combination of greater masses and smaller separation means that the energy given off by the Hulse–Taylor binary will be far greater than the energy given off by the Earth–Sun system roughly 1022 times as much. The information about the orbit can be used to predict how much energy (and angular momentum) would be radiated in the form of gravitational waves. As the binary system loses energy, the stars gradually draw closer to each other, and the orbital period decreases. The resulting trajectory of each star is an inspiral, a spiral with decreasing radius. General relativity precisely describes these trajectories; in particular, the energy radiated in gravitational waves determines the rate of decrease in the period, defined as the time interval between successive periastrons (points of closest approach of the two stars). For the Hulse–Taylor pulsar, the predicted current change in radius is about 3 mm per orbit, and the change in the 7.75 hr period is about 2 seconds per year. Following a preliminary observation showing an orbital energy loss consistent with gravitational waves, In 1993, spurred in part by this indirect detection of gravitational waves, the Nobel Committee awarded the Nobel Prize in Physics to Hulse and Taylor for "the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation". The lifetime of this binary system, from the present to merger is estimated to be a few hundred million years. Inspirals are very important sources of gravitational waves. Any time two compact objects (white dwarfs, neutron stars, or
black holes) are in close orbits, they send out intense gravitational waves. As they spiral closer to each other, these waves become more intense. At some point they should become so intense that direct detection by their effect on objects on Earth or in space is possible. This direct detection is the goal of several large-scale experiments. The only difficulty is that most systems like the Hulse–Taylor binary are so far away. The amplitude of waves given off by the Hulse–Taylor binary at Earth would be roughly
h ≈ 10−26. There are some sources, however, that astrophysicists expect to find that produce much greater amplitudes of
h ≈ 10−20. At least eight other binary pulsars have been discovered.
Difficulties Gravitational waves are not easily detectable. When they reach the Earth, they have a small amplitude with strain approximately 10−21, meaning that an extremely sensitive detector is needed, and that other sources of noise can overwhelm the signal. Gravitational waves are expected to have frequencies 10−16 Hz 4 Hz.
Ground-based detectors Though the Hulse–Taylor observations were very important, they give only
indirect evidence for gravitational waves. A more conclusive observation would be a
direct measurement of the effect of a passing gravitational wave, which could also provide more information about the system that generated it. Any such direct detection is complicated by the
extraordinarily small effect the waves would produce on a detector. The amplitude of a spherical wave will fall off as the inverse of the distance from the source (the 1/
R term in the formulas for
h above). Thus, even waves from extreme systems like merging binary black holes die out to very small amplitudes by the time they reach the Earth. Astrophysicists expect that some gravitational waves passing the Earth may be as large as
h ≈ 10−20, but generally no bigger.
Resonant antennas A simple device theorised to detect the expected wave motion is called a
Weber bar a large, solid bar of metal isolated from outside vibrations. This type of instrument was the first type of gravitational wave detector. Strains in space due to an incident gravitational wave excite the bar's
resonant frequency and could thus be amplified to detectable levels. Conceivably, a nearby supernova might be strong enough to be seen without resonant amplification. With this instrument,
Joseph Weber claimed to have detected daily signals of gravitational waves. His results, however, were contested in 1974 by physicists
Richard Garwin and
David Douglass. Modern forms of the Weber bar are still operated,
cryogenically cooled, with
superconducting quantum interference devices to detect vibration. Weber bars are not sensitive enough to detect anything but extremely powerful gravitational waves.
MiniGRAIL is a spherical gravitational wave antenna using this principle. It is based at
Leiden University, consisting of an exactingly machined 1,150 kg sphere cryogenically cooled to 20 millikelvins. The spherical configuration allows for equal sensitivity in all directions, and is somewhat experimentally simpler than larger linear devices requiring high vacuum. Events are detected by measuring
deformation of the detector sphere. MiniGRAIL is highly sensitive in the 2–4 kHz range, suitable for detecting gravitational waves from rotating neutron star instabilities or small black hole mergers. There are currently two detectors focused on the higher end of the gravitational wave spectrum (10−7 to 105 Hz): one at
University of Birmingham, England, and the other at
INFN Genoa, Italy. A third is under development at
Chongqing University, China. The Birmingham detector measures changes in the polarization state of a
microwave beam circulating in a closed loop about one meter across. Both detectors are expected to be sensitive to periodic spacetime strains of
h ~ , given as an
amplitude spectral density. The INFN Genoa detector is a resonant antenna consisting of two coupled spherical
superconducting harmonic oscillators a few centimeters in diameter. The oscillators are designed to have (when uncoupled) almost equal resonant frequencies. The system is currently expected to have a sensitivity to periodic spacetime strains of
h ~ , with an expectation to reach a sensitivity of
h ~ . The Chongqing University detector is planned to detect relic high-frequency gravitational waves with the predicted typical parameters ≈1011 Hz (100 GHz) and
h ≈10−30 to 10−32.
Interferometers A more sensitive class of detector uses a laser
Michelson interferometer to measure gravitational-wave induced motion between separated 'free' masses. This allows the masses to be separated by large distances (increasing the signal size); a further advantage is that it is sensitive to a wide range of frequencies (not just those near a resonance as is the case for Weber bars). After years of development ground-based interferometers made the first detection of gravitational waves in 2015. Currently, the most sensitive is
LIGO the Laser Interferometer Gravitational Wave Observatory. LIGO has three detectors: one in
Livingston, Louisiana, one at the
Hanford site in
Richland, Washington and a third (formerly installed as a second detector at Hanford) that is planned to be moved to
India. Each observatory has two
light storage arms that are 4 kilometers in length. These are at 90 degree angles to each other, with the light passing through 1 m diameter vacuum tubes running the entire 4 kilometers. A passing gravitational wave will slightly stretch one arm as it shortens the other. This is the motion to which an interferometer is most sensitive. Even with such long arms, the strongest gravitational waves will only change the distance between the ends of the arms by at most roughly 10−18 m. LIGO should be able to detect gravitational waves as small as
h ~ . Upgrades to LIGO and
Virgo should increase the sensitivity still further. Another highly sensitive interferometer,
KAGRA, which is located in the
Kamioka Observatory in Japan, is in operation since February 2020. A key point is that a tenfold increase in sensitivity (radius of 'reach') increases the volume of space accessible to the instrument by one thousand times. This increases the rate at which detectable signals might be seen from one per tens of years of observation, to tens per year. Interferometric detectors are limited at high frequencies by
shot noise, which occurs because the lasers produce photons randomly; one analogy is to rainfall the rate of rainfall, like the laser intensity, is measurable, but the raindrops, like photons, fall at random times, causing fluctuations around the average value. This leads to noise at the output of the detector, much like radio static. In addition, for sufficiently high laser power, the random momentum transferred to the test masses by the laser photons shakes the mirrors, masking signals of low frequencies. Thermal noise (e.g.,
Brownian motion) is another limit to sensitivity. In addition to these 'stationary' (constant) noise sources, all ground-based detectors are also limited at low frequencies by
seismic noise and other forms of environmental vibration, and other 'non-stationary' noise sources; creaks in mechanical structures, lightning or other large electrical disturbances, etc. may also create noise masking an event or may even imitate an event. All of these must be taken into account and excluded by analysis before detection may be considered a true gravitational wave event.
Einstein@Home The simplest gravitational waves are those with constant frequency. The waves given off by a spinning, non-axisymmetric neutron star would be approximately
monochromatic: a
pure tone in
acoustics. Unlike signals from supernovae or binary black holes, these signals evolve little in amplitude or frequency over the period it would be observed by ground-based detectors. However, there would be some change in the measured signal, because of
Doppler shifting caused by the motion of the Earth. Despite the signals being simple, detection is extremely computationally expensive, because of the long stretches of data that must be analysed. The
Einstein@Home project is a
distributed computing project similar to
SETI@home intended to detect this type of gravitational wave. By taking data from LIGO and GEO, and sending it out in little pieces to thousands of volunteers for parallel analysis on their home computers, Einstein@Home can sift through the data far more quickly than would be possible otherwise.
Space-based interferometers Space-based interferometers, such as
LISA and
DECIGO, are also being developed. LISA's design calls for three test masses forming an equilateral triangle, with lasers from each spacecraft to each other spacecraft forming two independent interferometers. LISA is planned to occupy a solar orbit trailing the Earth, with each arm of the triangle being 2.5 million kilometers. This puts the detector in an excellent vacuum far from Earth-based sources of noise, though it will still be susceptible to heat,
shot noise, and artifacts caused by
cosmic rays and
solar wind.
Using pulsar timing arrays Pulsars are highly magnetized, rapidly rotating neutron stars. A pulsar emits beams of radio waves that, like lighthouse beams, sweep through the sky as the pulsar rotates. The signal from a pulsar can be detected by radio telescopes as a series of regularly spaced pulses, essentially like the ticks of a clock. GWs affect the time it takes the pulses to travel from the pulsar to a telescope on Earth. A
pulsar timing array uses
millisecond pulsars to seek out perturbations due to GWs in measurements of the time of arrival of pulses to a telescope, in other words, to look for deviations in the clock ticks. To detect GWs, pulsar timing arrays search for a
distinct quadrupolar pattern of correlation and anti-correlation between the time of arrival of pulses from different pulsar pairs as a function of their angular separation in the sky. Although pulsar pulses travel through space for hundreds or thousands of years to reach us, pulsar timing arrays are sensitive to perturbations in their travel time of much less than a millionth of a second. The most likely source of GWs to which pulsar timing arrays are sensitive are supermassive black hole binaries, which form from the collision of galaxies. In addition to individual binary systems, pulsar timing arrays are sensitive to a stochastic background of GWs made from the sum of GWs from many galaxy mergers. Other potential signal sources include
cosmic strings and the primordial background of GWs from
cosmic inflation. Globally there are seven active pulsar timing array projects. The
North American Nanohertz Observatory for Gravitational Waves (NANOGrav) uses data collected by the
Arecibo Radio Telescope,
Green Bank Telescope,
Very Large Array, and the
Canadian Hydrogen Intensity Mapping Experiment. The Australian
Parkes Pulsar Timing Array (PPTA) uses data from the
Parkes radio-telescope. The
European Pulsar Timing Array (EPTA) uses data from the four largest telescopes in Europe: the
Lovell Telescope, the
Westerbork Synthesis Radio Telescope, the
Effelsberg Telescope and the
Nancay Radio Telescope. The Indian Pulsar Timing Array (InPTA) uses data from the
Giant Metrewave Radio Telescope, and the MeerKAT Pulsar Timing Array (MPTA) uses data from the
MeerKAT radio telescope. With the African Pulsar Timing (APT) group, these collaborations also collaborate under the title of the
International Pulsar Timing Array project. Additionally, the
Chinese Pulsar Timing Array (CPTA) uses data from the
Five-hundred-meter Aperture Spherical Telescope. In June 2023, NANOGrav, EPTA, InPTA, PPTA, and CPTA published the first evidence for a stochastic
gravitational wave background. In particular, they announced evidence for the
Hellings-Downs curve, the tell-tale sign of the gravitational wave origin of the observed background. In December 2024, MPTA also published evidence for the gravitational wave background.
Primordial gravitational wave Primordial gravitational waves are gravitational waves observed in the
cosmic microwave background. They were allegedly detected by the
BICEP2 instrument, an announcement made on 17 March 2014, which was withdrawn on 30 January 2015 ("the signal can be entirely attributed to
dust in the Milky Way" The signal increased in frequency from 35 to 250 Hz over 10 cycles (5 orbits) as it rose in strength for a period of 0.2 second. Since then LIGO and Virgo have reported more
gravitational wave observations from merging black hole binaries. On 16 October 2017, the LIGO and Virgo collaborations announced the first-ever detection of gravitational waves originating from the coalescence of a binary neutron star system. The observation of the
GW170817 transient, which occurred on 17 August 2017, allowed for constraining the masses of the neutron stars involved between 0.86 and 2.26 solar masses. Further analysis allowed a greater restriction of the mass values to the interval 1.17–1.60 solar masses, with the total system mass measured to be 2.73–2.78 solar masses. The inclusion of the Virgo detector in the observation effort allowed for an improvement of the localization of the source by a factor of 10. This in turn facilitated the electromagnetic follow-up of the event. The signal lasted about 100 seconds, much longer than the few seconds measured from binary black holes. Also in contrast to the case of binary black hole mergers, binary neutron star mergers were expected to yield an electromagnetic counterpart, that is, a light signal associated with the event. A gamma-ray burst (
GRB 170817A) was detected by the
Fermi Gamma-ray Space Telescope, occurring 1.7 seconds after the gravitational wave transient. The signal, originating near the galaxy
NGC 4993, was associated with the neutron star merger. This was corroborated by the electromagnetic follow-up of the event (
AT 2017gfo), involving 70 telescopes and observatories and yielding observations over a large region of the electromagnetic spectrum which further confirmed the neutron star nature of the merged objects and the associated
kilonova. In 2021, the detection of the first two neutron star-black hole binaries by the LIGO and VIRGO detectors was published in the Astrophysical Journal Letters, allowing to first set bounds on the quantity of such systems. No neutron star-black hole binary had ever been observed using conventional means before the gravitational observation. == Microscopic sources ==