Gravitational-wave astronomy is an emerging
branch of observational astronomy which aims
to use gravitational waves (minute distortions
of spacetime predicted by Einstein's theory
of general relativity) to collect observational
data about objects such as neutron stars and
black holes, events such as supernovae, and
processes including those of the early universe
shortly after the Big Bang.
Gravitational waves have a solid theoretical
basis, founded upon the theory of relativity.
They were first predicted by Einstein in 1916;
although a specific consequence of general
relativity, they are a common feature of all
theories of gravity that obey special relativity..
However, after 1916 there was a long debate
whether the waves were actually physical,
or artefacts of coordinate freedom in general
relativity; this was not fully resolved until
the 1950s. Indirect observational evidence
for their existence first came in the late
1980s, from monitoring of the Hulse–Taylor
binary pulsar (discovered 1974); the pulsar
orbit was found to evolve exactly as would
be expected for gravitational wave emission.
Hulse and Taylor were awarded the 1993 Nobel
Prize in Physics for this discovery.
On 11 February 2016 it was announced that
the LIGO collaboration had directly observed
gravitational waves for the first time in
September 2015. The second observation of
gravitational waves was made on 26 December
2015 and announced on 15 June 2016. Barry
Barish, Kip Thorne and Rainer Weiss were awarded
the 2017 Nobel Prize in Physics for leading
this work.
== Observations ==
Ordinary gravitational waves frequencies are
very low and much harder to detect, while
higher frequencies occur in more dramatic
events and thus have become the first to be
observed.
In addition to a merger of black holes, a
binary neutron star merger has been directly
detected: a gamma-ray burst (GRB) was detected
by the orbiting Fermi gamma-ray burst monitor
on 2017 August 17 12:41:06 UTC, triggering
an automated notice worldwide. Six minutes
later a single detector at Hanford LIGO, a
gravitational-wave observatory, registered
a gravitational-wave candidate occurring 2
seconds before the gamma-ray burst. This set
of observations is consistent with a binary
neutron star merger, as evidenced by a multi-messenger
transient event which was signalled by gravitational-wave,
and electromagnetic (gamma-ray burst, optical,
and infrared)-spectrum sightings.
=== High frequency ===
In 2015, the LIGO project was the first to
directly observe gravitational waves using
laser interferometers. The LIGO detectors
observed gravitational waves from the merger
of two stellar-mass black holes, matching
predictions of general relativity. These observations
demonstrated the existence of binary stellar-mass
black hole systems, and were the first direct
detection of gravitational waves and the first
observation of a binary black hole merger.
This finding has been characterized as revolutionary
to science, because of the verification of
our ability to use gravitational-wave astronomy
to progress in our search and exploration
of dark matter and the big bang.
There are several current scientific collaborations
for observing gravitational waves. There is
a worldwide network of ground-based detectors,
these are kilometre-scale laser interferometers
including: the Laser Interferometer Gravitational-Wave
Observatory (LIGO), a joint project between
MIT, Caltech and the scientists of the LIGO
Scientific Collaboration with detectors in
Livingston, Louisiana and Hanford, Washington;
Virgo, at the European Gravitational Observatory,
Cascina, Italy; GEO600 in Sarstedt, Germany,
and the Kamioka Gravitational Wave Detector
(KAGRA), operated by the University of Tokyo
in the Kamioka Observatory, Japan. LIGO and
Virgo are currently being upgraded to their
advanced configurations. Advanced LIGO began
observations in 2015, detecting gravitational
waves even though not having reached its design
sensitivity yet; Advanced Virgo is expected
to start observing in 2016. The more advanced
KAGRA is scheduled for 2018. GEO600 is currently
operational, but its sensitivity makes it
unlikely to make an observation; its primary
purpose is to trial technology.
=== Low frequency ===
An alternative means of observation is using
pulsar timing arrays (PTAs). There are three
consortia, the European Pulsar Timing Array
(EPTA), the North American Nanohertz Observatory
for Gravitational Waves (NANOGrav), and the
Parkes Pulsar Timing Array (PPTA), which co-operate
as the International Pulsar Timing Array.
These use existing radio telescopes, but since
they are sensitive to frequencies in the nanohertz
range, many years of observation are needed
to detect a signal and detector sensitivity
improves gradually. Current bounds are approaching
those expected for astrophysical sources.
=== Intermediate frequencies ===
Further in the future, there is the possibility
of space-borne detectors. The European Space
Agency has selected a gravitational-wave mission
for its L3 mission, due to launch 2034, the
current concept is the evolved Laser Interferometer
Space Antenna (eLISA). Also in development
is the Japanese Deci-hertz Interferometer
Gravitational wave Observatory (DECIGO).
== Scientific value ==
Astronomy has traditionally relied on electromagnetic
radiation. Originating with the visible band,
as technology advanced, it became possible
to observe other parts of the electromagnetic
spectrum, from radio to gamma rays. Each new
frequency band gave a new perspective on the
Universe and heralded new discoveries. During
the 20th century, indirect and later direct
measurements of high-energy, massive, particles
provided an additional window into the cosmos.
Late in the 20th century, the detection of
solar neutrinos founded the field of neutrino
astronomy, giving an insight into previously
inaccessible phenomena, such as the inner
workings of the Sun. The observation of gravitational
waves provides a further means of making astrophysical
observations.
Russell Hulse and Joseph Taylor were awarded
the 1993 Nobel Prize in Physics for showing
that the orbital decay of a pair of neutron
stars, one of them a pulsar, fits general
relativity's predictions of gravitational
radiation. Subsequently, many other binary
pulsars (including one double pulsar system)
have been observed, all fitting gravitational-wave
predictions. In 2017, the Nobel Prize in Physics
was awarded to Rainer Weiss, Kip Thorne and
Barry Barish for their role in the first detection
of gravitational waves.Gravitational waves
provide complementary information to that
provided by other means. By combining observations
of a single event made using different means,
it is possible to gain a more complete understanding
of the source's properties. This is known
as multi-messenger astronomy. Gravitational
waves can also be used to observe systems
that are invisible (or almost impossible to
detect) to measure by any other means. For
example, they provide a unique method of measuring
the properties of black holes.
Gravitational waves can be emitted by many
systems, but, to produce detectable signals,
the source must consist of extremely massive
objects moving at a significant fraction of
the speed of light. The main source is a binary
of two compact objects. Example systems include:
Compact binaries made up of two closely orbiting
stellar-mass objects, such as white dwarfs,
neutron stars or black holes. Wider binaries,
which have lower orbital frequencies, are
a source for detectors like LISA. Closer binaries
produce a signal for ground-based detectors
like LIGO. Ground-based detectors could potentially
detect binaries containing an intermediate
mass black hole of several hundred solar masses.
Supermassive black hole binaries, consisting
of two black holes with masses of 105–109
solar masses. Supermassive black holes are
found at the centre of galaxies. When galaxies
merge, it is expected that their central supermassive
black holes merge too. These are potentially
the loudest gravitational-wave signals. The
most massive binaries are a source for PTAs.
Less massive binaries (about a million solar
masses) are a source for space-borne detectors
like LISA.
Extreme-mass-ratio systems of a stellar-mass
compact object orbiting a supermassive black
hole. These are sources for detectors like
LISA. Systems with highly eccentric orbits
produce a burst of gravitational radiation
as they pass through the point of closest
approach; systems with near-circular orbits,
which are expected towards the end of the
inspiral, emit continuously within LISA's
frequency band. Extreme-mass-ratio inspirals
can be observed over many orbits. This makes
them excellent probes of the background spacetime
geometry, allowing for precision tests of
general relativity.In addition to binaries,
there are other potential sources:
Supernovae generate high-frequency bursts
of gravitational waves that could be detected
with LIGO or Virgo.
Rotating neutron stars are a source of continuous
high-frequency waves if they possess axial
asymmetry.
Early universe processes, such as inflation
or a phase transition.
Cosmic strings could also emit gravitational
radiation if they do exist. Discovery of these
gravitational waves would confirm the existence
of cosmic strings.Gravitational waves interact
only weakly with matter. This is what makes
them difficult to detect. It also means that
they can travel freely through the Universe,
and are not absorbed or scattered like electromagnetic
radiation. It is therefore possible to see
to the center of dense systems, like the cores
of supernovae or the Galactic Centre. It is
also possible to see further back in time
than with electromagnetic radiation, as the
early universe was opaque to light prior to
recombination, but transparent to gravitational
waves.
The ability of gravitational waves to move
freely through matter also means that gravitational-wave
detectors, unlike telescopes, are not pointed
to observe a single field of view but observe
the entire sky. Detectors are more sensitive
in some directions than others, which is one
reason why it is beneficial to have a network
of detectors.
=== In cosmic inflation ===
Cosmic inflation, a hypothesized period when
the universe rapidly expanded during the first
10−36 seconds after the Big Bang, would
have given rise to gravitational waves; that
would have left a characteristic imprint in
the polarization of the CMB radiation.It is
possible to calculate the properties of the
primordial gravitational waves from measurements
of the patterns in the microwave radiation,
and use those calculations to learn about
the early universe.
== Development ==
As a young area of research, gravitational-wave
astronomy is still in development; however,
there is consensus within the astrophysics
community that this field will evolve to become
an established component of 21st century multi-messenger
astronomy.Gravitational-wave observations
complement observations in the electromagnetic
spectrum. These waves also promise to yield
information in ways not possible via detection
and analysis of electromagnetic waves. Electromagnetic
waves can be absorbed and re-radiated in ways
that make extracting information about the
source difficult. Gravitational waves, however,
only interact weakly with matter, meaning
that they are not scattered or absorbed. This
should allow astronomers to view the center
of a supernova, stellar nebulae, and even
colliding galactic cores in new ways.
Ground-based detectors have yielded new information
about the inspiral phase and mergers of binary
systems of two stellar mass black holes, and
merger of two neutron stars. They could also
detect signals from core-collapse supernovae,
and from periodic sources such as pulsars
with small deformations. If there is truth
to speculation about certain kinds of phase
transitions or kink bursts from long cosmic
strings in the very early universe (at cosmic
times around 10−25 seconds), these could
also be detectable. Space-based detectors
like LISA should detect objects such as binaries
consisting of two white dwarfs, and AM CVn
stars (a white dwarf accreting matter from
its binary partner, a low-mass helium star),
and also observe the mergers of supermassive
black holes and the inspiral of smaller objects
(between one and a thousand solar masses)
into such black holes. LISA should also be
able to listen to the same kind of sources
from the early universe as ground-based detectors,
but at even lower frequencies and with greatly
increased sensitivity.Detecting emitted gravitational
waves is a difficult endeavor. It involves
ultra stable high quality lasers and detectors
calibrated with a sensitivity of at least
2·10−22 Hz−1/2 as shown at the ground-based
detector, GEO600. It has also been proposed
that even from large astronomical events,
such as supernova explosions, these waves
are likely to degrade to vibrations as small
as an atomic diameter.
== See also ==
Gravitational-wave observatory
List of gravitational wave observations
Matched filter
== References ==
== Further reading ==
== External links ==
LIGO Scientific Collaboration
AstroGravS: Astrophysical Gravitational-Wave
Sources Archive
Video (04:36) – Detecting a gravitational
wave, Dennis Overbye, NYT (11 February 2016).
Video (71:29) – Press Conference announcing
discovery: "LIGO detects gravitational waves",
National Science Foundation (11 February 2016).
