IOP PUBLISHING REPORTS ON PROGRESS IN PHYSICS
Rep. Prog. Phys. 72 (2009) 076901 (25pp) doi:10.1088/0034-4885/72/7/076901
LIGO: the Laser Interferometer
Gravitational-Wave Observatory
The LIGO Scientific Collaboration, http://www.ligo.org
B P Abbott
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, R Abbott
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, P Ajith
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, B Allen
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S B Anderson
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Rep. Prog. Phys. 72 (2009) 076901 B P Abbott et al
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and J Zweizig
1
1
LIGO—California Institute of Technology, Pasadena, CA 91125, USA
2
Albert-Einstein-Institut, Max-Planck-Institut f
¨
ur Gravitationsphysik, D-30167 Hannover, Germany
3
Department of Physics, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA
4
Ginzton, E.L. Laboratory, Stanford University, Stanford, CA 94305, USA
5
Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA
6
Department of Physics, University of Florida, Gainesville, FL 32611, USA
2
Rep. Prog. Phys. 72 (2009) 076901 B P Abbott et al
7
School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK
8
Institut fur Gravitationsphysik, Leibniz Universit
¨
at Hannover, D-30167 Hannover, Germany
9
Albert-Einstein-Institut, Max-Planck-Institut f
¨
ur Gravitationsphysik, D-14476 Golm, Germany
10
Department of Physics, Montana State University, Bozeman, MT 59717, USA
11
LIGO—Hanford Observatory, Richland, WA 99352, USA
12
Department of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK
13
School of Physics, University of Western Australia, Crawley, WA 6009, Australia
14
LIGO—Massachusetts Institute of Technology, Cambridge, MA 02139, USA
15
Department of Physics, Columbia University, New York, NY 10027, USA
16
Department of Physics, The University of Texas at Brownsville and Texas Southmost College, Brownsville, TX
78520, USA
17
Department of Physics and Astronomy, San Jose State University, San Jose, CA 95192, USA
18
Relativity Group, Moscow State University, Moscow, 119992, Russia
19
LIGO—Livingston Observatory, Livingston, LA 70754, USA
20
Department of Physics and Astronomy, Washington State University, Pullman, WA 99164, USA
21
Department of Physics, University of Oregon, Eugene, OR 97403, USA
22
Department of Physics, Syracuse University, Syracuse, NY 13244, USA
23
Rutherford Appleton Laboratory, HSIC, Chilton, Didcot, Oxon OX11 0QX, UK
24
Department of Physics, University of Maryland, College Park, MD 20742, USA
25
Department of Physics, University of Massachusetts - Amherst, MA 01003, USA
26
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
27
Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA
28
Department of Engineering, University of Sannio at Benevento, I-82100 Benevento, Italy
29
Department of Physics and Astronomy, The University of Mississippi, University, MS 38677, USA
30
Research Scool of Physics and Engineering, Charles Sturt University, Wagga Wagga, NSW 2678, Australia
31
Caltech-CaRT, Pasadena, CA 91125, USA
32
Department of Physics and Astronomy, Carleton College, Northfield, MN 55057, USA
33
School of Physics, The University of Melbourne, Parkville VIC 3010, Australia
34
School of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA, UK
35
Institute of Physics, E
¨
otv
¨
os University, ELTE 1053 Budapest, Hungary
36
Department of Physics, University of Salerno, 84084 Fisciano (Salerno), Italy
37
Department of Physics and Astronomy, The University of Sheffield, Sheffield S10 2TN, UK
38
Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802,
USA
39
Inter-University Centre for Astronomy and Astrophysics, Pune - 411007, India
40
Department of Physics, Southern University and A&M College, Baton Rouge, LA 70813, USA
41
California Institute of Technology, Pasadena, CA 91125, USA
42
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
43
Department of Physics, The University of Texas at Austin, Austin, TX 78712, USA
44
Research School of Physics and Engineering, Australian National University, Canberra, 0200, Australia
45
Department of Physics, Embry-Riddle Aeronautical University, Prescott, AZ 86301, USA
46
Department of Physics, University of Minnesota, Minneapolis, MN 55455, USA
47
School of Chemistry and Physics, University of Adelaide, Adelaide, SA 5005, Australia
48
School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
49
Department of Astronomy and Astrophysics, Northwestern University, Evanston, IL 60208, USA
50
National Astronomical Observatory of Japan, Tokyo 181-8588, Japan
51
Institute of Applied Physics, Nizhny Novgorod, 603950, Russia
52
Department of Physics, University of Strathclyde, Glasgow, G1 1XQ, UK
53
Department of Physics, Loyola University, New Orleans, LA 70118, USA
54
Hobart and William Smith Colleges, Geneva, NY 14456, USA
55
Department of Physics, Louisiana Tech University, Ruston, LA 71272, USA
56
Department of Physics, Andrews University, Berrien Springs, MI 49104, USA
57
Relativity and Gravitation Group, Universitat de les Illes Balears, E-07122 Palma de Mallorca, Spain
58
Department of Physics and Astronomy, Sonoma State University, Rohnert Park, CA 94928, USA
59
Department of Physics and Astronomy, Trinity University, San Antonio, TX 78212, USA
60
Rochester Institute of Technology, Rochester, NY 14623, USA
61
Southeastern Louisiana University, Hammond, LA 70402, USA
E-mail: peter.fritschel@ligo.org
Received 2 March 2009, in final form 1 May 2009
Published 30 June 2009
Online at
stacks.iop.org/RoPP/72/076901
3
Rep. Prog. Phys. 72 (2009) 076901 B P Abbott et al
Abstract
The goal of the Laser Interferometric Gravitational-Wave Observatory (LIGO) is to detect and study
gravitational waves (GWs) of astrophysical origin. Direct detection of GWs holds the promise of
testing general relativity in the strong-field regime, of providing a new probe of exotic objects such as
black holes and neutron stars and of uncovering unanticipated new astrophysics. LIGO, a joint
Caltech–MIT project supported by the National Science Foundation, operates three multi-kilometer
interferometers at two widely separated sites in the United States. These detectors are the result of
decades of worldwide technology development, design, construction and commissioning. They are
now operating at their design sensitivity, and are sensitive to gravitational wave strains smaller than
one part in 10
21
. With this unprecedented sensitivity, the data are being analyzed to detect or place
limits on GWs from a variety of potential astrophysical sources.
(Some figures in this article are in colour only in the electronic version)
This article was invited by B Berger.
Contents
1. Introduction 4
2. Gravitational waves 5
3. LIGO and the worldwide detector network 5
4. Detector description 6
4.1. Interferometer configuration 6
4.2. Laser and optics 6
4.3. Suspensions and vibration isolation 8
4.4. Sensing and controls 8
4.5. Thermal effects 10
4.6. Interferometer response and calibration 10
4.7. Environmental monitors 10
5. Instrument performance 10
5.1. Strain noise spectra 10
5.2. Sensing noise sources 11
5.3. Seismic and thermal noise 12
5.4. Auxiliary degree-of-freedom noise 13
5.5. Actuation noise 13
5.6. Additional noise sources 14
5.7. Other performance figures-of-merit 14
6. Data analysis infrastructure 14
7. Astrophysical reach and search results 15
7.1. Compact binary coalescences 16
7.2. GW bursts 17
7.3. Continuous wave sources 19
7.4. Stochastic GW background 20
8. Future directions 22
Acknowledgments 22
References 23
1. Introduction
The prediction of gravitational waves (GWs), oscillations in
the space–time metric that propagate at the speed of light, is one
of the most profound differences between Einstein’s general
theory of relativity and the Newtonian theory of gravity that it
replaced. GWs remained a theoretical prediction for more than
50 yearsuntilthefirstobservational evidencefortheirexistence
came with the discovery and subsequent observations of the
binary pulsar PSR 1913 + 16, by Russell Hulse and Joseph
Taylor. This is a system of two neutron stars (NSs) that orbit
each other with a period of 7.75 h. Precise timing of radio
pulses emitted by one of the NSs, monitored now over several
decades, shows that their orbital period is slowly decreasing at
just the rate predicted for the general-relativistic emission of
GWs [1]. Hulse and Taylor were awarded the Nobel Prize in
Physics for this work in 1993.
In about 300 million years, the PSR 1913 + 16 orbit
will decrease to the point where the pair coalesces into a
single compact object, a process that will produce directly
detectable GWs. In the meantime, the direct detection of
GWs will require similarly strong sources—extremely large
masses moving with large accelerations in strong gravitational
fields. The goal of LIGO, the Laser Interferometer
Gravitational-Wave Observatory [2], is just that: to detect and
study GWs of astrophysical origin. Achieving this goal will
mark the opening of a new window on the universe, with
the promise of new physics and astrophysics. In physics,
GW detection could provide information about strong-field
gravitation, the untested domain of strongly curved space
where Newtonian gravitation is no longer even a poor
approximation. In astrophysics, the sources of GWs that
LIGO might detect include binary NSs (like PSR 1913 + 16
but much later in their evolution); binary systems where a
black hole (BH) replaces one or both of the NSs; a stellar
core collapse which triggers a type II supernova; rapidly
rotating, non-axisymmetric NSs; and possibly processes in
the early universe that produce a stochastic background of
GWs [3].
In the past few yearsthe field has reached amilestone, with
decades-old plans to build and operate large interferometric
GW detectors now realized in several locations worldwide.
This paper focuses on LIGO, which operates the most
sensitive detectors yet built. We aim to describe the LIGO
detectors and how they operate, explain how they have
achieved their remarkable sensitivity and review how their
data can be used to learn about a variety of astrophysical
phenomena.
4
Rep. Prog. Phys. 72 (2009) 076901 B P Abbott et al
2. Gravitational waves
The essence of general relativity is that mass and energy
produce a curvature of four-dimensional space–time, and that
matter moves in response to this curvature. The Einstein
field equations prescribe the interaction between mass and
space–time curvature, much as Maxwell’s equations prescribe
the relationship between electric charge and electromagnetic
fields. Just as electromagnetic waves are time-dependent
vacuum solutions to Maxwell’s equations, GWs are time-
dependent vacuum solutions to the field equations. GWs are
oscillating perturbations to a flat, or Minkowski, space–time
metric, and can be thought of equivalently as an oscillating
strain in space–time or as an oscillating tidal force between
free test masses.
As with electromagnetic waves, GWs travel at the
speed of light and are transverse in character, i.e. the strain
oscillations occur in directions orthogonal to the direction
in which the wave is propagating. Whereas electromagnetic
waves are dipolar in nature, GWs are quadrupolar: the strain
pattern contracts space along one transverse dimension, while
expanding it along the orthogonal direction in the transverse
plane (see figure 1). Gravitational radiation is produced
by oscillating multipole moments of the mass distribution
of a system. The principle of mass conservation rules
out monopole radiation, and the principles of linear and
angular momentum conservation rule out gravitational dipole
radiation. Quadrupole radiation is the lowest allowed form
and is thus usually the dominant form. In this case, the GW
field strength is proportional to the second time derivative
of the quadrupole moment of the source, and it falls off in
amplitude inversely with distance from the source. The tensor
character of gravity—the hypothetical graviton is a spin-2
particle—means that the transverse strain field comes in two
orthogonal polarizations. These are commonly expressed in
a linear polarization basis as the ‘+’ polarization (depicted in
figure 1) and the ‘×’ polarization, reflecting the fact that they
are rotated 45
◦
relative to one another. An astrophysical GW
will, in general, be a mixture of both polarizations.
GWs differ from electromagnetic waves in that they
propagate essentially unperturbed through space, as they
interact only very weakly with matter. Furthermore, GWs
are intrinsically non-linear, because the wave energy density
itself generates additional curvature of space–time. This
phenomenon is only significant, however, very close to strong
sources of waves, where the wave amplitude is relatively
large. More usually, GWs distinguish themselves from
electromagnetic waves by the fact that they are very weak.
One cannot hope to detect any waves of terrestrial origin,
whether naturally occurring or manmade; instead one must
look for very massive compact astrophysical objects, moving
at relativistic velocities. For example, strong sources of GWs
that may exist in our galaxy or nearby galaxies are expected to
produce wave strengths on Earth that do not exceedstrainlevels
of one part in 10
21
. Finally, it is important to appreciate that
GW detectors respond directly to GW amplitude rather than
GW power; therefore the volume of space that is probed for
potential sources increases as the cube of the strain sensitivity.
time
h
Figure 1. A GW traveling perpendicular to the plane of the diagram
is characterized by a strain amplitude h. The wave distorts a ring of
test particles into an ellipse, elongated in one direction in one
half-cycle of the wave, and elongated in the orthogonal direction in
the next half-cycle. This oscillating distortion can be measured with
a Michelson interferometer oriented as shown. The length
oscillations modulate the phase shifts accrued by the light in each
arm, which are in turn observed as light intensity modulations at the
photodetector (green semi-circle). This depicts one of the linear
polarization modes of the GW.
3. LIGO and the worldwide detector network
As illustrated in figure 1, the oscillating quadrupolar strain
pattern of a GW is well matched by a Michelson interferometer,
which makes a very sensitive comparison of the lengths of
its two orthogonal arms. LIGO utilizes three specialized
Michelson interferometers, located at two sites (see figure 2):
an observatory on the Hanford site in Washington houses
two interferometers, the 4 km-long H1 and 2 km-long H2
detectors; and an observatory in Livingston Parish, Louisiana,
houses the 4 km-long L1 detector. Other than the shorter
length of H2, the three interferometers are essentially identical.
Multiple detectors at separated sites are crucial for rejecting
instrumental and environmental artifacts in the data, by
requiring coincident detections in the analysis. Also, because
the antenna pattern of an interferometer is quite wide,
source localization requires triangulation using three separated
detectors.
The initial LIGO detectors were designed to be sensitive
to GWs in the frequency band 40–7000 Hz, and capable of
detecting a GW strain amplitude as small as 10
−21
[2]. With
funding from the National Science Foundation, the LIGO sites
and detectors were designed by scientists and engineers from
the California Institute of Technology and the Massachusetts
Institute of Technology, constructed in the late 1990s, and
commissioned over the first 5 years of this decade. From
November 2005 to September 2007, they operated at their
design sensitivity in a continuous data-taking mode. The data
from this science run, known as S5, are being analyzed for
a variety of GW signals by a group of researchers known as
the LIGO Scientific Collaboration [4]. At the most sensitive
frequencies, the instrument root-mean-square (rms) strain
noise has reached an unprecedented level of 3 × 10
−22
in a
100 Hz band.
Although in principle LIGO can detect and study GWs
by itself, the potential to do astrophysics can be quantitatively
and qualitatively enhanced by operation in a more extensive
network. For example, the direction of travel of the GWs and
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