This major upgrade will increase the sensitivity of the LIGO instruments by a factor of 10, giving a one thousand-fold increase in the number of astrophysical candidates for gravitational wave signals.

"We anticipate that this new instrument will see gravitational wave sources possibly on a daily basis, with excellent signal strengths, allowing details of the waveforms to be observed and compared with theories of neutron stars, black holes, and other astrophysical objects moving near the speed of light," says Jay Marx of the California Institute of Technology, executive director of the LIGO Laboratory.

Gravitational waves are ripples in the fabric of space and time produced by violent events in the distant universe--for example, by the collision of two black holes or by the cores of supernova explosions. Gravitational waves are emitted by accelerating masses much in the same way as radio waves are produced by accelerating charges-- such as electrons in antennas.

David Reitze of the University of Florida, spokesperson for the LIGO Scientific Collaboration, adds that "these ripples in the space-time fabric travel to Earth, bringing with them information about their violent origins and about the nature of gravity that cannot be obtained by other astronomical tools."

Albert Einstein predicted the existence of these gravitational waves in 1916 in his general theory of relativity, but only since the 1990s has technology become powerful enough to permit detecting them and harnessing them for science.

Although they have not yet been detected directly, the influence of gravitational waves on a binary pulsar system (two neutron stars orbiting each other) has been measured accurately and is in excellent agreement with the predictions. Scientists therefore have great confidence that gravitational waves exist. But a direct detection will confirm Einstein's vision of the waves, and allow a fascinating and unique view of cataclysms in the cosmos.

The Advanced LIGO detector, to be installed at the LIGO Observatories in Hanford, Washington, and Livingston, Louisiana, using the existing infrastructure, will replace the present detector, and will transform gravitational wave science into a real observational tool. David Shoemaker of MIT, the project leader for Advanced LIGO, says the "the improvement of sensitivity will allow the data set generated after one year of initial operations to be equaled in just several hours."

The change of more than a factor of 10 in sensitivity comes also with a significant increase in the sensitive frequency range, and the ability to tune the instrument for specific astrophysical sources. This will allow Advanced LIGO to look at the last minutes of life of pairs of massive black holes as they spiral closer, coalesce into one larger black hole, and then vibrate much like two soap bubbles becoming one.

It will also allow the instrument to pinpoint periodic signals from the many known pulsars that radiate in the range from 500 to 1000 Hertz (frequencies which correspond to high notes on an organ). Recent results from the Wilkinson Microwave Anisotropy Probe have shown the rich information that comes from looking at the photon, or infrared cosmic background, which originated some 400,000 years after the Big Bang. Advanced LIGO can be optimized for the search for the gravitational cosmic background--allowing tests of theories about the development of the universe only 10 to the minus 35 seconds after the Big Bang.

The LIGO Observatories were planned at the outset to support the continuing development of this new science, and the significant infrastructure of buildings and vacuum systems is left unchanged. The upgrade calls for changes in the lasers (180 watt highly stabilized systems), optics (40 kg fused silica "test mass" mirrors suspended by fused silica fibers), seismic isolation systems (using inertial sensing and feedback), and in how the microscopic motion (in the range of 10 to the minus 20 meters) of the test masses is detected.

Several of these technologies are significant advances in their fields, and have promise for application in a wide range of precision measurement, state-of-the-art optics, and controls systems. A program of testing and practice installation will allow the new detectors to be brought online with a minimum of interruption in observation. The instruments will be ready to start scientific operation in 2014.

The design of the instrument has come from scientists throughout the 50-institution, 600-person LIGO Scientific Collaboration, an international group that carries out both instrument development and scientific data analysis for LIGO. In the United States, these efforts (and in particular the LIGO Laboratory) are supported by the National Science Foundation (NSF).

"Advanced LIGO will be one of the most important scientific instruments of the 21st century. For the first time, it will let us listen in on the sounds of the universe, as unseen explosions, collisions, and whirlpools shake the fabric of space-time and send out the ripples that Advanced LIGO will measure. We in the German-British GEO project are excited that our long-standing partnership with LIGO allows us to contribute to Advanced LIGO some of the key technologies we have developed and tested in our GEO600 instrument," says Bernard F. Schutz, director of the Albert Einstein Institute in Germany.

The NSF funds the project through the Major Research Equipment and Facilities Construction (MREFC) budget account. The Caltech-MIT LIGO Laboratory will carry out the project.

Several international partners have already approved funding for significant contributions of equipment, labor, and expertise:

The UK contribution is the suspension assembly and some optics for the mirrors whose movements register the passage of the gravitational waves; this has been funded via Britain's Science and Technology Facilities Council (STFC).

The German contribution is the high-power, high-stability laser whose light measures the actual movements of the mirrors; this has been funded via the Max Planck Society in Munich.

The University of Florida and Columbia University are taking on specific responsibilities in the design and construction of Advanced LIGO.

Other members of the LIGO Scientific Collaboration (LSC), with NSF or other funding, will participate in all phases of the effort.

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