Advanced LIGO: Context and Overview
Advanced LIGO
Gravitational waves offer a remarkable opportunity to see the universe from a new perspective, giving access to astrophysical insights which are available in no other way. The initial LIGO gravitational wave detectors have started observations, and are already providing data which 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 high 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 a 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 by a factor of 4 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
LIGO was 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.
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 from a limited ability to sense the response of the masses to the gravitational wave strain. Seismic motion causes forces on the mirrors due to the direct coupling through the isolation and suspension system, which is minimized through design; and due to the time-varying mass distribution near the mass (the Newtonian background). The thermal motion of the test mass and the suspension also plays a role; this influence is managed through the selection of low-mechanical-loss materials and designs which capitalize on them.
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). Frequency noise and intensity fluctuations in the laser light, and 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 strain 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 gravitational wave sources have large uncertainty, a linear improvement in sensitivity linearly improves the distance searched for detectable sources. This increases the detection rate by the cube of the sensitivity improvement.
LIGO Initial 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. A brand new field of astronomy, based on observations using a completely new information carrier: the gravitational field, will be enabled.
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 guaranteed, 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 program for the initial detectors is designed to understand the detector and to execute searches for astrophysical sources of all types. The program is designed to make detections as well as to set upper limits.
There are no known gravitational wave sources whose “best-guess” rates and strengths are sufficiently large that detections are assured during the initial LIGO Science Run. There are great uncertainties associated with either or both the rates and strengths of all of the conjectured sources. However, the initial LIGO Science Run, with the initial detectors, will extend the sensitivity to gravitational wave sources in a new frequency regime by two to three decades in amplitude and bandwidth from all previous searches
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 providing there are sufficiently high signal to noise detections. These include probes of strong-field gravity associated with black holes, 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.
The initial LIGO detectors can measure:
· Compact binary inspirals of two 1.4 M neutron stars to a distance of 20 Mpc;
· Gravitational waves from gamma-ray bursts if the broadband RMS strain is ~10-18 m;
· Black hole formation and ringdown of two 20 M black holes to a distance of ~100 Mpc;
· Nonaxisymmetric supernovae to ~15 Mpc if centrifugal hangup forces the object into a triaxial shape which then emits a substantial fraction of its rotational energy as gravitational waves;
· Nascent neutron stars to a distance of ~1 Mpc;
· General gravitational wave bursts whose source or detailed character (i.e., waveform) is not known in advance for strains greater than ~10-18;
· Pulsars and rapidly rotating neutron stars as distant as 10 kpc for mass asymmetry parameters e>10-5 and for stellar rotation rates > 150 Hz (i.e., 300 Hz gravitational wave signals);
· Stochastic gravitational wave background if WGW~10-5.
Advanced LIGO Scientific Goals
· The Advanced LIGO interferometers proposed here promise an improvement over initial LIGO in the limiting sensitivity by more than a factor of 10 in the frequency band 100 Hz f 200 Hz 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 Advanced LIGO interferometers will also have a considerably greater sensitive frequency range. These improvements will enable the next generation of interferometers to study sources not accessible to initial LIGO, and to make detections with a signal to noise ratio allowing the extraction of detailed astrophysical information.. For example, t
he Advanced LIGO detectors will be able to see also be able to perform all observations described earlier at greater sensitivity and higher event rate:
Iinspiraling neutron star (NS) and black hole (BH) binaries made up of s: two 1.4 M neutron stars will be detectable atto a distance of 300 Mpc, some 15x further than the initial LIGO, and giving an event rate some 3000x greater. ; NS+BHNeutron star – black hole (BH) binaries will be visible to 650 Mpc; and coalescing BH+BH systems are 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 providing there are sufficiently high signal to noise detections. These include probes of strong-field gravity associated with black holes, 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.
· Tidal disruption of a NS by a BH: the coalescence of a NS+BH binary may be accompanied by the tidal disruption of the lighter star. Detection by Advanced LIGO would be able to provide observational data on the NS radius that may be compared with nuclear equation of state predictions.
· BH+BH mergers and ringdowns: gravitational radiation in the LIGO band from BH+BH interactions comes primarily from the final merger and ringdown phases of these coalescences. Advanced LIGO will be able to detect radiation from BH binaries having total mass as great as 2000 M to distances corresponding to z=1.
· Supernovae: empirical evidence suggests that neutron stars in type II supernovae receive kicks of magnitude as large as ~1000 km/s. These violent recoils imply that the supernova event may be strongly asymmetric. Further, numerical models suggest that the proto-neutron star will be unstable to convective overturn, and that gravitational radiation produced in the first second or so of its life may be detectable anywhere throughout our Galaxy and its orbiting companions (i.e., the Magellanic Clouds).
· Nascent neutron stars: the narrowband tunability of Advanced LIGO interferometers will be exploited to search with high sensitivity at higher frequencies for gravitational radiation arising from rotating compact stars, especially from Low-Mass X-Ray Binaries (LMXB). Neutron star rotation rates of ~100 revolutions per second may be detectable to distances encompassing the Virgo Cluster.
· General gravitational wave bursts: the Advanced LIGO reach will enable observational searches for gravitational wave bursts correlated with Gamma-Ray Bursts (GRBs) to much greater distances than will be possible with initial LIGO. The next generation of orbiting gamma-ray telescopes will be operational in the time frame of Advanced LIGO operations; thereby providing astrophysical triggers for GRBs. Independently verified triggers enable on-source/off-source observations of gravitational waves to be made.
· Stochastic Signals: the sensitivity improvement of Advanced LIGO, coupled with the decrease in lower frequency cutoff, means that an observational measurement of the stochastic gravitational wave background can be performed with limiting sensitivity after 1 year of observation of WGW~5x10-9.