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GENERAL ARTICLE

Gravitational-Wave Astronomy

A New Window to the Universe

P Ajith and K G Arun

Keywords

General relativity, gravitational

waves, astrophysics, interferom-

etry.

P Ajith is a postdoctoral

scholar at the California

Institute of Technology. His

research interests include

gravitational-wave theory,

data analysis and astro-

physics. He also likes to

take pictures like the one of

his friend Arun against the

backdrop of the Pacific

ocean.

K G Arun is Assistant

Professor of Physics at the

Chennai Mathematical

Institute. His research

areas are theoretical and

astrophysical aspects of

gravitational waves. His

current project outside

physics is to learn to

identify various Ragas by

listening to Carnatic music.

We present a broad overview of the emergingeld of gravitational-wave astronomy. Althoughgravitational waves have not been directly de-tected yet, the worldwide scientic communityis engaged in an exciting search for these elusivewaves. Once detected, they will open up a newobservational window to the Universe.

Four hundred years after Galileo's telescope launchedoptical astronomy, a major revolution in astronomy us-ing gravitational-wave telescopes is expected to occurin the very near future. Every new step in astronomythat allowed us to observe the Universe in a dierentwavelength of the electromagnetic spectrum has had im-mense and immediate impact on our science. WhileGalileo's observations challenged the prevalent world-view of the times and paved the way to the Enlighten-ment and the Scientic Revolution, the expansion of as-tronomy into other wavelengths of the electromagneticspectrum { radio, microwave, infrared, ultraviolet, X-ray, gamma-ray wavelengths { revolutionized our un-derstanding of the Cosmos. Some of the most impor-tant observations include the discovery of new planets,new galaxies, extragalactic supernovae, quasars, pulsars,gamma-ray bursts, expansion of the Universe, cosmicmicrowave background, and evidence of astrophysicalblack holes.

The late 1980s witnessed the emergence of a new astron-omy based on neutrino detectors. Neutrinos are sub-atomic particles produced by the decay of radioactive el-ements. Astrophysical neutrinos are typically producedin nuclear reactions that take place in the interior of

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GENERAL ARTICLE

stars, in contrast with electromagnetic waves, which areproduced at the stellar surfaces. Hence, the observationof neutrinos enables us to probe the core of the astro-nomical source, thus complementing the electromagneticobservations. Neutrino detectors opened up the possi-bility of probing the Universe using observations otherthan that of electromagnetic waves. Scientists have nowrecognized the potential of `multi-messenger' astronomy,in which dierent astronomical observations of the samephenomenon are combined to produce a more completepicture of the phenomenon.

1. Gravitational-Wave Astronomy

The existence of gravitational waves is one of the mostintriguing predictions of the General Theory of Rela-tivity proposed by Albert Einstein in 1915. GeneralRelativity { the most accurate theory of gravity avail-able { describes gravity as the curvature of the space-time, produced by mass-energy concentrations in thespacetime. Whenever these mass concentrations changeshape, they produce distortions in the spacetime geome-try that propagate with the speed of light { called gravi-tational waves. The generation of gravitational waves isanalogous to the generation of electromagnetic waves ina radio transmitter or a mobile phone. While changes inthe electric eld produce electromagnetic waves, changesin the gravitational `eld' produce gravitational waves.According to General Relativity, gravitational waves alsohave two independent polarization states and propagateat the speed of light.

Although any accelerated motion of masses can pro-duce gravitational waves, those produced by the mo-tion of terrestrial sources are too weak to be detectableby any conceivable technology. Thus, unlike the caseof electromagnetic waves, constructing a gravitational-wave `generator' is not feasible in the foreseeable future.But a number of astronomical sources can produce grav-

Four hundred years

after Galileos

telescope launched

optical astronomy, a

major revolution in

astronomy using

gravitational-wave

telescopes is

expected to happen

in the very near

future.

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GENERAL ARTICLE

itational waves that are detectable using the currentcutting-edge technology. These include violent astro-physical phenomena such as colliding black holes, col-lapse of massive stars resulting in supernovae, rapidly ro-tating neutron stars, etc., and various energetic processesthat might have happened in the early Universe.

Unlike astronomical electromagnetic waves, which areproduced by accelerating electrons or atoms and henceare of microscopic origin, gravitational waves are pro-duced by coherent bulk motions of large amounts ofmass-energy (except for the stochastic waves producedin the early Universe) and are of macroscopic origin.Thus, gravitational waves carry dierent informationabout their source, hence complementing the electro-magnetic observations. Also, gravitational waves arethe only means of directly observing certain sources,such as binaries of black holes, which are `dark' in theelectromagnetic spectrum. Furthermore, the interactionof gravitational waves with matter is extremely weak,which is a great advantage for astronomy. This meansthat these waves arrive at an observer nearly unaectedby any intervening matter, thus carrying `uncorrupted'information about their sources.

The weak coupling to matter also makes the detectionof gravitational waves an enormous experimental chal-lenge. Although these elusive waves have not been di-rectly detected yet, there are strong indirect evidencessupporting their existence. General Relativity predictsthat when two compact stars orbit a common centerof mass, the gravitational waves would carry away theorbital energy and would cause the two stars to drawcloser, and eventually to merge with each other. Morethan 30 years of radio observations of the binary pul-sar system PSR B1913+16 showed that the decay ofits orbital period agrees precisely with the prediction ofGeneral Relativity. (A pulsar is a rotating neutron starthat emits a beam of electromagnetic radiation, so that

The existence of

gravitational waves

is one of the most

intriguing

predictions of the

General Theory of

Relativity proposed

by Albert Einstein

in 1915.

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GENERAL ARTICLE

Gravitational-wave

astronomy is like

listening to the

Universe. By

decoding the emitted

gravitational-wave

signal, it is possible to

extract the physical

properties of the

source, such as the

component masses,

spins, distance and

energetics.

observers from any xed direction will see highly regularpulses). Russell Hulse and Joseph Taylor were awardedthe Nobel Prize in 1993 for their discovery of this binary.In later years, more such binaries have been discovered,which further conrmed the prediction of General Rel-ativity.

Neutron stars and black holes are highly dense objects:A neutron star with mass equal to that of the Sun willhave a radius of around 15 km, while a black hole withthe same mass will have a radius of 3 km (recall thatthe Sun's radius is around 700,000 km). In a neutronstar, matter takes exotic forms, which is a major puzzlefor modern nuclear physics. In black holes, the matteris converted into an extreme form of spacetime curva-ture, such that a black hole is completely described byits mass, angular momentum and electric charge (pop-ularly described as `black holes have no hair!'). Themerger of such compact objects are among the most en-ergetic events in the Universe, where a small percentageof the mass of the objects is converted into gravitationalenergy according to Einstein's famous formula E = mc2.For example, the energy released by the merger of twosolar-mass black holes ( 1046 J) is several hundredtimes larger than the electromagnetic energy releasedby the Sun over its entire lifetime! In the nal stages ofthe coalescence of stellar-mass-black-hole/neutron-starbinaries, the orbital frequency sweeps from around 10 Hzto a few kHz, which is the frequency band of audio sig-nals that the human ear is sensitive to. Since the fre-quency of the emitted gravitational waves is twice theorbital frequency, it is possible to convert such signalsinto audio signals and `listen' to them. In this sense,gravitational-wave astronomy is like `listening' to theUniverse. By decoding the emitted gravitational-wavesignal, it is possible to extract the physical propertiesof the source, such as the component masses, spins, dis-tance and energetics.

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Gravitational-wave science holds the potential to ad-dress some of the key questions in fundamental physics,astrophysics and cosmology. For instance, the observedexpansion rate of the Universe is inconsistent with theprediction of General Relativity based upon the mass-energy content of the Universe inferred from electromag-netic observations. This means that either General Rel-ativity needs to be modied at large scales or that theUniverse contains enormous amount of mass-energy thatis not visible in electromagnetic observations, termed`dark energy'. Combined gravitational-wave and elec-tromagnetic observations can be used to map the expan-sion history of the Universe, which is crucial in under-standing the nature of dark energy. Gravitational-waveobservations will also facilitate unique precision tests ofGeneral Relativity.

Gravitational waves from the merger of binaries involv-ing neutron stars will carry information about the in-ternal structure of the neutron star, and might revealthe central engine of certain types of gamma-ray bursts.(Although gamma-ray bursts are the brightest astro-nomical events in the electromagnetic spectrum, notmuch is known about their central engines). In the caseof a small compact object inspiralling into a much largerblack hole, a `map' of the spacetime geometry aroundthe larger object will be encoded in the gravitational-wave signal. Decoding the signal will enable us to inferthe nature of the massive object, and to test whetherit is indeed a black hole as described by General Rel-ativity. Gravitational-wave signals from core-collapsesupernovae will provide valuable information on the in-ternal processes that take place during the explosion.Detecting the stochastic background of cosmic gravita-tional waves can help us to trace the Universe back to atime when it was as young as 1030 seconds, which canbe probed by no other astrophysical means.

Gravitational-wave

science holds the

potential to

address some of

the key questions

in fundamental

physics,

astrophysics and

cosmology.

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2. Detection of Gravitational Waves: A MajorExperimental Challenge

When gravitational waves pass through the Earth, theydistort the geometry of the spacetime. Observing thetiny distortions in the spacetime geometry is the key tothe detection of gravitational waves. For example, as-sume that we arrange a ring of `test masses' in a perfectcircle. If a gravitational wave passes perpendicular tothe plane of the circle, the spacetime will get distortedin such a way that the circle will get deformed into el-lipses (see Figure 1). But, even if produced by someof the most energetic events in the Universe, the space-time distortions produced by gravitational waves on theEarth is extremely small. For example, a core-collapsesupernova in our own galaxy will produce a deformationof the order of a billionth of a trillionth (1022 1020)of the radius of the circle!

Laser interferometry provides a precise way of measur-ing such small deformations. In a laser interferometer, acoherent laser beam is split by a beam splitter and sentin two orthogonal directions. These beams are reectedback by two mirrors, which are in turn recombined toproduce an interference pattern. Gravitational wavesinduce a relative length change between the two orthog-onal arms of an interferometer, which produces a changein the interference pattern (see Figure 2). Figure 1. Effect of gravita-

tional waves on a ring of

testmasses.Whena gravi-

tational wave passes per-

pendicular to the plane of

the ring, it deforms the cir-

cular ring into ellipses. In

this figure, the deformation

is greatly exaggerated; the

actual distortion is minute,

but it can be detected using

a high precision laser inter-

ferometer.

Whengravitational

waves pass through

the Earth, they distort

the geometry of the

spacetime. Observing

the tiny distortions in

the spacetime

geometry is the key to

the detection of

gravitational waves.

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GENERAL ARTICLE

Figure 2. Schematic dia-

gram of a Michelson Inter-

ferometer.

Figure 3. An aerial view of

the 4-km LIGO observatory

in Livingston, USA.

Courtesy: LIGO Laboratory.

Since the relative length change produced by a gravita-tional wave is proportional to the length of the arm, thelonger the arm of an interferometer, the more sensitiveit is. Thus, modern gravitational-wave telescopes areinterferometers with arms several kilometers long. (Inorder to further increase the round-trip time, the lightis reected back and forth many times in each arm bycreating resonant cavities). The LIGO (Laser Interfer-ometer Gravitational-Wave Observatory) observatoriesin USA consist of two interferometers with 4 km arms(see Figure 3). The French{Italian Virgo observatory

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Although a direct

detection of

gravitational waves is

yet to be made, the

non-detection is

consistent with the

astrophysical

expectations of the

event rates within the

volume of the

Universe accessible

to the initial detectors.

has 3 km arms, while the British{German GEO600 de-tector has 600 m long arms and the Japanese TAMA 300detector has 300 m arms. These interferometers aim todetect gravitational waves in the frequency band of 10 Hz to 10 kHz.

Almost all these detectors have achieved the design sen-sitivity goal of their initial congurations, and have con-ducted several long data-taking runs. Although a directdetection of gravitational waves is yet to be made, thenon-detection is consistent with the astrophysical ex-pectations of the event rates (coalescence rates of com-pact binaries, galactic supernova rates, etc.) within thevolume of the Universe accessible to the initial detec-tors. Several astrophysically interesting upper limitshave been already derived based on this data. Advancedcongurations of these observatories will be operationalby 2015 with a factor-of-ten improvement in the sensi-tivity as compared to their initial congurations. Ad-vanced ground-based detectors are expected to observegravitational-wave signals at monthly or even weeklyrates. A space-based antenna called LISA is also ex-pected to be operational by the next decade, and a thirdgeneration ground-based interferometer called EinsteinTelescope is being designed in Europe. These advanceddetectors will provide powerful tools for precision cos-mology, astronomy and strong-eld tests of gravity.

Indeed, laser interferometry is not the only way of de-tecting gravitational waves. The experimental eort forthis was pioneered by the American physicist JosephWeber using resonant-bar detectors. The idea behindthese detectors is that as a gravitational wave passesthrough an object, it will get deformed. If the ob-ject is vibrating at a characteristic resonance, then thedeformation will appear as a deviation from its reso-nant ringing. Several resonant-bar detectors are opera-tional around the world, although with lower sensitivityand bandwidth compared to interferometers. Another

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Interferometric

gravitational-wave

detectors are nearly

omni-directional

instruments.

promising way of detecting gravitational waves in thevery-low frequency band (109 106 Hz) is by usingpulsar-timing arrays. The gravitational waves distortthe spacetime when they pass through the Earth re-sulting in a correlated delay in the arrival times of thepulses from several pulsars. Millisecond pulsars (pul-sars with rotational periods of a few milliseconds) pro-vide a set of accurate `reference clocks' which can beused to track such deformations. Several arrays of ra-dio telescopes in Australia, Europe and North Americaare searching for gravitational waves in this frequencyband. Also, experiments such as the recently launchedPlanck satellite will seek to detect ultra-low-frequency( 1016 Hz) gravitational waves produced in the earlyUniverse through their imprint on the polarization ofthe cosmic microwave background radiation.

3. Indian Participation

Interferometric gravitational-wave detectors are nearlyomni-directional instruments. Thus, it is dicult topoint the gravitational-wave detectors to a particularlocation in the sky or to identify the sky-location ofa source from a single-detector observation. The sky-localization of the source is achieved by combining datafrom multiple detectors located at dierent geographi-cal locations (similar to radio astronomy). Thus, it isimportant to have a worldwide detector network { bothfrom the point of establishing condence in our rst de-tections, as well as exploring exciting new astrophysicsfrom these sources. Several studies have pointed outthat the optimal location for another detector to aug-ment the sensitivity of the current global network is inthe Indian Ocean region, with Australia and India astwo potential choices.

The LIGO laboratory is currently investigating the pos-sibility of installing a third Advanced LIGO detectorin Australia or India with considerable international

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Box 1. LIGO-India

The current major IndIGO plans on gravitational-wave astronomy relate to the LIGO-India project. LIGO-India is a proposed advanced gravitational-wave detector to belocated in India as part of the worldwide network, whose concept proposal is now underactive consideration. LIGO-India is envisaged as a collaborative project between India,USA and other international partners. The design of the interferometric detector will besimilar to that of the Advanced LIGO detectors in USA. Adding a new detector in India,geographically well separated from the existing LIGO-Virgo detector array, will boostthe detection condence of the rst detections and will improve the sky coverage, sourcelocalization accuracies and event rates. This also presents the scientic community inIndia, including motivated students, a unique opportunity to join the worldwide hunt forthe rst gravitational-wave signal.

The IndIGO

collaboration aims to

bring about a

significant Indian

contribution in

building a

gravitational-wave

observatory in the

Asia-Pacific region as

a part of the global

network.

participation. This presents Indian science an excellentopportunity to launch a major initiative in a promisingexperimental research frontier well in time before it hasobviously blossomed. The Indian researchers workingin the eld have formed a scientic consortium, calledthe Indian Initiative in Gravitational-wave Observations(IndIGO). The consortium now includes ten premier In-dian institutions and about 30 researchers from Indiaand abroad. The IndIGO collaboration aims to bringabout a signicant Indian contribution in building agravitational-wave observatory in the Asia-Pacic regionas a part of the global network. To support the researchand development connected with this endeavour, a re-search group at the Tata Institute of Fundamental Re-search in Mumbai is currently building a 3-meter scaleadvanced interferometer prototype. The prototype willprovide an active research and development platform forexperimental gravitational-wave research in India (SeeBox 1).

With the ongoing upgrades of the ground-based inter-ferometers, we are on the eve of a new era in astronomy.Direct, routine observations of gravitational waves ex-pected by the middle of this decade will revolutionizeour understanding of the Cosmos. Future observatorieslike Einstein Telescope and LISA will take gravitational-wave astronomy to the forefront of precision astronomy.

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Suggested Reading

[1] For more information and updates on various Indian activities and

opportunities in education and research in gravitational-wave astronomy,

see http://www.gw-indigo.org.

[2] Participate in the search for gravitational waves: Einstein@Home dis-

tributed computing project http://einstein.phys.uwm.edu.

[3] IndIGO Mock Data Challenge for Students http://www.gw-indigo.org/

mdc2011, The Black-hole hunter game http://www.blackholehunter.org/

[4] Web sites of other gravitational-wave projects: http://www.ligo.org;

http://www.ego-gw.it; http://www.geo600.org/; http://gwcenter.icrr.u-

tokyo.ac.jp/en/.

[5] Introductory articles: B F Schutz, Gravitational Radiation, in Encyclo-

pedia of Astronomy and Astrophysics, http://arxiv.org/abs/gr-qc/0003069;

K S Thorne, Gravitational Waves: A new window onto the Universe,

http://arxiv.org/abs/gr-qc/9704042.

[6] Popular books: K S Thorne, Black Holes and Time Warps: Einsteins

Outrageous Legacy, W W Norton & Company, 1995; M Bartusiak,

Einsteins Unfinished Symphony: Listening to the Sounds of Space-Time,

Berkeley Trade, 2003.

[7] Textbooks: B F Schutz, A first course in General Relativity, Cambridge

University Press, 1985; P Saulson, Fundamentals of Interferometric

Gravitational Wave Detectors, World Scientific, Singapore, 1994.

[8] Review articles: B S Sathyaprakash and B F Schutz, Physics, astrophysics

and cosmology with gravitational waves, Living Rev. Relativity, Vol.12,

p.2, 2009. http://relativity.livingreviews.org/Articles/lrr-2009-2.

Address for Correspondence

P Ajith

LIGO Laboratory and

Theoretical Astrophysics

California Institute of

Technology

MS 18-34, Pasadena

CA 91125,USA

Email: ajith@caltech.edu

K G Arun

Chennai Mathematical

Institute

Plot H1, SIPCOT IT Park

Siruseri, Padur Post

Chennai 603 103, TN, India.

Email: kgarun@cmi.ac.in

There are plenty of ongoing activities in India contribut-ing to the development of various aspects of gravitational-wave astronomy, thus providing the motivated youngminds a unique opportunity to be part of this emergingand exciting research frontier.