initial LIGO construction - LIGO Caltech

Project Description
Advanced LIGO: Context and Overview
Advanced LIGO
Gravitational waves offer a remarkable opportunity to see the universe from
a new perspective, providing access to astrophysical insights that are
available in no other way. The initial LIGO gravitational wave detectors
have started observations, and are already yielding data that are being
interpreted to establish new upper limits on gravitational-wave flux. The sensitivity of the initial LIGO instruments is such that it is
perfectly possible that discoveries will be made. If they succeed, there
will be a strong demand from the community to improve the sensitivity
allowing more astrophysical information to be recovered from the signals.
If no discovery is made, there will be no lesser urgency to improve the
sensitivity of the instrument to the point where there is a general
consensus that gravitational waves will be detected often and with a good
signal-to-noise ratio. The development of the next generation of instrument
must be pursued aggressively to make the transition from the initial to the
Advanced detector in a timely way - after the complete science run of the
initial detector, but as quickly as possible thereafter. The Advanced LIGO detector upgrade meets these requirements for an
instrument that will establish gravitational-wave astronomy. It is more
than ten times more sensitive, and over a much broader frequency band, than
initial LIGO. It can see a volume of space more than a thousand times
greater than initial LIGO, and extends the range of compact masses that can
be observed at a fixed signal strength by a factor of four or more. This proposal to build Advanced LIGO has grown out of the LIGO Scientific
Collaboration and has broad support both nationally and internationally
from that community. A closely coordinated community R&D program, exploring
the instrument science and building and testing prototype subsystem
elements, has brought the design to a highly refined state. The LIGO
Laboratory will lead and coordinate the fabrication and construction of the
instruments, with the continued strong participation of the community. Advanced LIGO can lead the gravitational-wave field to maturity. The LIGO Mission
From its outset, LIGO has been approved by the National Science Foundation
to directly observe gravitational waves from cosmic sources, and to open
the field of gravitational wave astronomy. The program and mission of the
LIGO Laboratory is to: . observe gravitational wave sources,
. develop advanced detectors that approach and exploit the facility
limits on interferometer performance,
. operate the LIGO facilities to support the national and international
scientific community,
. provide data archiving for the LIGO data and contribute computational
resources for the analysis of data,
. develop the software infrastructure for data analysis and participate
in the search and analysis,
. and support scientific education and public outreach related to
gravitational wave astronomy. LIGO is envisioned as a new capability contained in a set of facilities and
not as a single experiment. The LIGO construction project has provided the
facilities that support the scientific instrumentation, and the initial set
of laser interferometers to be used in the first LIGO scientific
observation periods. The facilities include the buildings and vacuum systems at the two
observatory sites. The two observatories are located at Hanford, Washington
and Livingston, Louisiana. The performance requirements on the LIGO
facilities were intended to accommodate the initial interferometers and
future interferometer upgrades and replacements, and possible additional
interferometers with complementary capabilities. The requirements on the
LIGO facilities were intended to permit future interferometers to reach
levels of sensitivity approaching the ultimate limits of ground-based
interferometers, limited by reasonable practical constraints on a large
facility at a specific site. This proposal is for the second generation of instruments to be installed
in the LIGO infrastructure, and is expected to bring the science of
gravitational radiation from a discovery mode to a mode of astrophysical
observation.
LIGO Detector Scientific Goals
The scientific program for LIGO is both to test relativistic gravitation
and to open the field of gravitational wave astrophysics. More precise
tests of General Relativity (and competing theories) will be made. LIGO
will enable the establishment of a brand new field of astronomy, using a
completely new information carrier: the gravitational field. Initial LIGO represents an advance over all previous searches of two or
three orders of magnitude in sensitivity and in bandwidth. Its reach is
such that, for the first time, foreseeable signals due to neutron-star
binary "inspirals" from the Virgo Cluster (15 Mpc distant) would be
detectable. At this level of sensitivity, it is plausible, though not
certain, that the first observations of gravitational waves will be made.
If signals are not observed with initial LIGO, we will have set challenging
upper limits on gravitational wave flux, far beyond the capability of any
previously existing technology. The Advanced LIGO interferometers proposed here promise an improvement over
initial LIGO in the limiting sensitivity by more than a factor of 10 over
the entire initial LIGO frequency band. It also increases the bandwidth of
the instrument to lower frequencies (from ~40 Hz to ~10 Hz) and allows high-
frequency operation due to its tunability. This translates into an enhanced
physics reach that during its first several hours of operation will exceed
the integrated observations of the 1 year LIGO Science Run. These
improvements will enable the next generation of interferometers to study
sources not accessible to initial LIGO, and to extract detailed
astrophysical information. For example, the Advanced LIGO detectors will be
able to see inspiraling binaries made up of two 1.4 M neutron stars to a
distance of 300 Mpc, some 15x further than the initial LIGO, and giving an
event rate some 3000x greater. Neutron star - black hole (BH) binaries will
be visible to 650 Mpc; and coalescing BH+BH systems will be visible to
cosmological distance, to z=0.4. The existence of gravitational waves is a crucial prediction of the General
Theory of Relativity, so far unverified by direct observation. Although the
existence of gravitational radiation is not a unique property of General
Relativity, that theory makes a number of unambiguous predictions about the
character of gravitational radiation. These can be verified by observations
with LIGO. These include probes of strong-field gravity associated with
black holes, high-order post-Newtonian effects in inspiraling binaries, the
spin character of the radiation field, and the wave propagation speed. The gravitational wave "sky" is entirely unexplored. Since many prospective
gravitational wave sources have no corresponding electromagnetic signature
(e.g., black hole interactions), there are good reasons to believe that the
gravitational-wave sky will be substantially different from the
electromagnetic one. Mapping the gravitational-wave sky will provide an
understanding of the universe in a way that electromagnetic observations
cannot. As a new field of astrophysics it is quite likely that
gravitational wave observations will uncover new classes of sources not
anticipated in our current thinking.
Detector Design Fundamentals
The effect of a propagating gravitational wave is to deform space in a
quadrupolar form. The effect alternately elongates space in one direction
while compressing space in an orthogonal direction and vice versa, with the
frequency of the gravitational wave. A Michelson interferometer operating
between freely suspended masses is ideally suited to detect these
antisymmetric distortions of space induced by the gravitational waves; the
strains are converted into changes in light intensity and consequently to
electrical signals via photodetectors. Limitations to the sensitivity come from two sources: extraneous forces on
the test masses, and a limited ability to sense the response of the masses
to the gravitational wave strain. The thermally excited motion of the test
mass and the suspension is a fundamental limitation, intrinsic to the way
in which the measurement is performed; this influence is managed through
the selection of low-mechanical-loss materials and designs which capitalize
on them. Seismic motion causes forces on the mirrors due to the direct
coupling through the isolation and suspension system, a technical noise
source which is minimized through design; and due to the time-varying mass
distribution near the mass (the Newtonian background). Sensing limitations arise most fundamentally due to the statistical nature
of the laser light used in the interferometry, and the momentum transferred
to the test masses by the photons (linking the sensing and stochastic noise
limitations to sensitivity). Technical noise sources that limit the ability
to sense include frequency noise and intensity fluctuations in the laser
light. Scattered light, which adds random phase fluctuations to the light,
can also mask gravitational signals. In the limit, valid for LIGO, that the instrument is short compared with
the gravitational wavelength, longer arms give larger signals. In contrast,
most competing noise sources remain constant with length; this motivates
the 4km baseline of the Observatories. More generally, the scientific
capability of LIGO is defined within the limits imposed by the physical
settings of the interferometers and by the facility design, by the design
of the initial detectors and ultimately by future interferometers designed
to progressively exploit the facility capabilities. Although the rates for gravitatio