Gravitational Wave
Astronomy: TheDawn Has Arrived
Misao Sasaki, Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto, Japanoday I would like to talk about this really waves, which are essentially dipolar in nature, withbrand new topic of astrophysics and cosmol- the electric field pointing towards the transverse diogy known as gravitational wave astronomy. rection, and the magnetic field perpendicular to theDuring my talk please do not hesitate to interrupt me, electric field, this will be dipolar because you have ablame me, complain, or ask questions. Let me first vector which is propagating. In this case physicists
explain what gravitational waves (GWs) are. Many call it spin-1. Now in the case of quadrupolar flucof you may know, but in case some people dont know, tuations if the propagation waves are going towards
GWs are so-called ripples of space-time; fluctuations you, they become oblate and prolate and are knownof space-time itself. Why does space-time fluctuate? as spin-2. In this case you have the so-called crossThis is because of the famous theory of Einsteins mode pattern and plus mode pattern. In total, there
known as General Relativity, which was discovered are two polarization patterns. So these waves will be
one hundred years ago in 1915. The next year after produced whenever there is some dynamical motion.
discovering General Relativity, Einstein discovered For example, even me, or you, when we are walkthese GWs will propagate at the speed of light. How- ing around like this then our quadrupole moment isever, at the time he never thought these waves would changing but the amplitude is so tiny that nobody
actually be detected due to their incredibly small notices it; but when it comes to all these astronomiamplitude, but at least theoretically it was predicted. cal bodies, things can change dramatically.

T

Gravitational waves are produced by any system
In fact, compact astronomical bodies are one ofthat has mass-energy and motion. Once you have the most promising candidates in gravitational wave
a system with mass-energy undergoing some type astronomy. The most promising being neutron starof motion, this produces a so-called quadrupole mo- (NS) or black hole (BH) binaries. These objects arement in space-time. These quadrupole moments are very compact and are the densest astronomical obsimilar to an elongated ball such as a rugby ball and jects we know of in the Universe. In the case of BHs
if you rotate this rugby ball your quadrupole moment for example, if the sun (which is about 100 times
is changing in time. In this case you are producing bigger than the diameter of the Earth) collapses tosome GWs and because of this quadrupole nature about a few kilometers then it forms a BH and justthey are referred to as spin-2, but maybe I shouldnt before collapse to a BH you have an incredibly dense
say that yet. Anyway, compared to electromagnetic star called a neutron star. The star itself is so dense

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it is essentially one huge nucleus. If you have two ofthese things orbiting each other then they are changing a huge quadruple moment and emit potentiallymeasurable GWs. If you have some slowly changing
orbits at first, then because of the gravitational wave
emission the system loses energy, the two bodies approach, and the rotation period becomes shorter andshorter. The frequency becomes higher and higher
and the amplitude becomes larger and larger. Thisis called the chirp signal. In the end, the two bodies
coalesce and probably form a black hole.
This was the expectation and there have been
many theoretical studies into this phenomena. In
fact, we had indirect evidence of this occurring in
1974 with the observation of the Hulse-Taylor pulsar
system whose period was changing with time. The
observation matched the theory so well that it led tothe Nobel Prize being awarded for this in 1993. After
that, many observational astronomers found many
similar systems; but this is still all in- direct evidence
because we had not seen any actual gravitational
waves, just the result of the wave emission. This allchanged in Feb 2016 when LIGO (The Laser Interferometer Gravitational-Wave Observatory) announced
that they had actually detected gravitational waves
with their detector. The waves came from a blackhole binary system- two black holes orbiting, theninspiraling into each other, and merging into a single,
larger, black hole. The mass of each progenitor blackhole was much bigger than expected, about 30 solar
masses each, located roughly 1 billion light years
from Earth. It was also a really lucky observation
because LIGO was planned a long time ago, took
about 20 years to build a nice detector, and then,on the second day during a test run, they found thesignal!

one detector to the next: 7 ms. This observation tells
us that the waves are propagating in a particular
direction which we can now try to pin-point, albeit
not very accurately. It was concluded that the waves
came from the approximate direction of the Large
Magellanic Cloud.
This is like the time of Galileo; pointing a telescopeat the sky for the first time and finding somethingoutside the Earth which we can never see otherwise.
Besides this one particular observation of gravitational waves, there are many more events to come.
Just like light, gravitational waves are used as atool to explore the unknown Universe. Already weknow there are unexpectedly many black hole binary
systems radiating gravitational waves and we haveto explain why there are so many. This poses a very
interesting question for the current generation of astrophysicists and cosmologists and may be related tothe dark matter problem. This is also the first direct
detection of a strongly relativistic gravitational field.
All of our previous evidence of black holes has always been indirect; we never actually saw space-time
consisting of black holes, but in this case we did. In
the case of observing black holes, you also have thisso-called quasinormal ringdown tail because blackholes can also oscillate in addition to emitting gravitational waves, but the emitted oscillation damps
away. The damping rate and frequency is charactisitc to the nature of the black hole. Both of theseobservations were also consistent with the measured
ringdown signal.
Once you have confidence in the theory, you canthen use gravitational waves to see astronomical phenomenon. For example, we know how electromagnetic waves are generated and propagate, so we canuse them to observe and understand the Universe.
So this is only the beginning and we expect gravitational wave astronomy to become more developed in
the coming years. We have a new detector nearing
completion in Italy called Virgo, which will hopefully
be ready soon. In Japan we are building anotherdetector called KAGRA which will hopefully start by
2018. And it was also recently announced that there
will be a LIGO India; same as LIGO in America,
but in India. Hopefully soon we will eventually havethese five large scale detectors operating together andwe can then really do a lot of interesting astrophysics.

LIGO is not just one gravitational wave detector,
but actually consists of two detectors. One is locatedin Livingston, Louisiana and the other is locatedin Hanford, Washington and they are separated byabout 3000 km. Two detectors are necessary toconfirm whether or not a wave really truly passed
through the Earth or not, and that it was not justsome systematic error. The detectors have an arm
length of 4 km and an accuracy of 1021 . This is so
accurate that you can detect a slight change in thedistance between the arm lengths at one-hundredth
the size of an atomic nucleus. The observed signal
matched up incredibly well with the signal predictedby theory. In addition, with two detectors you could
In addition to ground based detectors, we also
actually measure the travel time of the wave from plan to go to space to observe gravitational waves.

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From space we can observe an even wider range ofgravitational phenomena. One such planned detector
designed by a Japanese group is called DECIGO andit will hopefully launch by 2030, but nobody knowsfor sure. In addition there is also the LISA project,from Europe and the U.S., which will also launcharound the same time.
Furthermore, there is also the so-called pulsar timing array method for observing very low frequency
gravitational waves. In this case you can use natural
astronomical objects as the detector; in this casepulsars (rapidly rotating neutron stars). Pulsars are
distributed all throughout the galaxy and act as very
accurate clocks with a rotational period often on theorder of milliseconds. Any changes due to gravitational wave emission will affect these clocks and you
can then use them to observe gravitational waves
with frequencies as low as 108 Hz.
Between ground based detectors, space based detectors, and pulsar timing arrays, we will soon beable to observe gravitational waves with the relative
frequency difference on the order of 1010 . This isvery similar to the electromagnetic case, where wehave low frequency radio astronomy, visual optical astronomy, high frequency ultraviolet electromagnetic
radiation, x-rays, and gamma rays; a whole bandof frequencies by which we can explore and understand the Universe better. In this case we hope todo the same with gravitational waves. By combining
gravitational waves with electromagnetic waves andneutrinos, we hope to understand the Universe very
deeply.
In summary, the LIGO detection is the first milestone in gravitational wave astronomy- the dawn has
arrived! The observed signal is fully consistent with
Einstein’s theory of general relativity. This was also
the first direct evidence of black holes existing in
the Universe. The LIGO observation also indicatesa need for a new theory for understanding stellar
populations that would give rise to such black holebinary systems. Future detector networks, both on
ground and in space, will accelerate progress in gravitational wave astrophysics. Gravitational waves arean essential tool for explaining the physics of theunknown Universe in this century! Thank you verymuch.

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