Gravity Wave Observatory Could Spot One Black-hole Merger per Hour by 2024

The National Science Foundation (NSF) is awarding Caltech and MIT $20.4 million to upgrade the Laser Interferometer Gravitational-wave Observatory (LIGO) into the Advanced LIGO Plus. Advanced LIGO Plus is expected to commence operations in 2024 and to increase the volume of deep space the observatory can survey by as much as seven times.

The observatory made its first detection — the gravitational waves from the merger of two black holes — in September that year. It has now bagged ten black-hole mergers, plus one merger of two neutron stars. LIGO has been undergoing periodic improvements, and is now about to reopen after an upgrade designed to increase its sensitivity by 50%.

Souped-up system

The ALIGO+ upgrades will be able to detect neutron-star mergers that occur within 325 megaparsecs (around 1 billion light years) of Earth, says Ken Strain, a physicist at the University of Glasgow. That would nearly double the design sensitivity of 173 megaparsecs that LIGO expects to reach before the ALIGO+ upgrade.

LIGO is already able to spot black holes billions of parsecs away. By 2022, it should detect about one such event per day, and the subsequent ALIGO+ upgrade should push that to one event every few hours.

Reducing noise will enable researchers to tell how the black holes were spinning before they merged, which can provide clues to their history.

Future of Gravitational Astronomy

What Comes Next for LIGO? Planning for the post-detection era in gravitational-wave detectors and astrophysics. (31-page report,Dec 2016)

Once constructed and installed, the Advanced LIGO interferometers were rapidly commissioned to sensitivities well beyond those obtained with the initial LIGO interferometers, achieving ∼70-80 Mpc BNS inspiral range. In 2018, they were aiming for 150 million parsec range detections.

The A+ design incorporates many of the elements identified in the first DAWN white paper, to produce a design with strain sensitivity corresponding to a BNS/BBH range of ∼ 350/2240 Mpc. This would lead to an increase in range of 1.6X and 1.8X respectively for BNS and 20Msun BBH mergers, or alternatively a detection rate increase of 6.4X (BNS) and 4.4X (BBH) with respect to Advanced LIGO.

The A+ design improves the sensitivity in all frequency bands. At the heart of A+ are frequency-dependent squeezing and improved test mass coatings. Squeezed light will be used to reduce the quantum noise at low frequencies through the reduction of radiation pressure on the test masses and at high frequencies (over 500 Hz) through the reduction of shot noise. Improvements at low and mid frequencies also rely on a reduction of coating thermal noise by a factor of two over current Advanced LIGO mirror coatings. Balanced homodyne detection is planned, to provide a lower loss, higher fidelity readout of the gravitational wave channel over DC readout.

The concepts behind many of the planned A+ upgrades have already been experimentally demonstrated, either on large scale interferometers or at laboratory scale. At this point, many of the primary challenges are on developing engineered solutions which can be deployed in A+. The one notable exception is the production of optical coatings with reduced thermal noise; this remains a vigorous area of R&D in the LIGO Scientific Collaboration Coatings Working Group.

LIGO, Virgo, and KAGRA will be in observing mode and making frequent detections during the period when LIGO-India is planned to become operational in 2024, at about the time A+ will come online.

The LIGO Voyager Concept – Beyond the Advanced Ligo Plus

LIGO Voyager would have three times the detection range to a BNS range (to 1100 Mpc). This would have a low-frequency cutoff down to 10 Hz.

While A+ will dramatically increase the rates of certain classes of gravitational-wave sources and enable the first detections of others, the fundamental facilities limits of the LIGO Observatories can accommodate interferometers with even greater sensitivities. A factor of two to three can be gained in sensitivity over A+ in a broad frequency range with the current LIGO observatory infrastructure based on designs using our current level of understanding of other limits to sensitivity.

The Voyager design aims to reach the extant LIGO facilities limits through a major upgrade that incorporates several new ideas and technologies. Key to the design is the use of 200 kg crystalline silicon test masses using amorphous Si (α-Si) and SiO2 coatings and operated at 123 K (where the thermal expansion coefficient of Si crosses zero).

A change from the Nd:Yag 1 µm laser wavelength to 1.55-2.1 µm is needed to adapt to the transmission band of Si test masses. Stable high power lasers operating in this longer wavelength range will be needed, and frequency-dependent squeezing at these longer wavelengths will be required to achieve the desired quantum noise performance at low and high frequencies. This mandates the need for replacing the laser and optical components as well as high quantum efficiency photodetectors operating at the design wavelength. Newtonian noise cancellation will be needed over that needed for A+.

Plans are to convert the Caltech 40 meter interferometer into a Voyager testbed, including long wavelength lasers, silicon optics, and cryogenic suspension.

Meeting the Challenges of Building Improved Detectors

Realization of A+ and future interferometers such as Voyager will require not only engineering squeezed light, but also sustained R&D programs on optical coatings to better understand and reduce coating thermal noise, to enable ‘cold’ interferometer operation, to manufacture and coat test mass substrates made of new materials (silicon), and to cool and maintain low temperature test masses without introducing displacement noise. Among all these advances, the one domain which has the most immediate need and presents the most complex challenges is reducing coating thermal noise.

Beyond LIGO Voyager is LIGO Cosmic Explorer

There was a white paper that looked at LIGO Voyager and beyond.

Researchers envisage potentially three detector epochs post Advanced LIGO baseline over the next 25 years with working titles A+, LIGO Voyager and LIGO Cosmic Explorer. The funds required to implement the upgrades are classified as: modest, less than $10M to $20M; medium, $50M to $150M; major, greater than $200M.

The detection of gravity waves has opened up a new era of astronomy. We can push detection of blackhole and neutron star collisions out into the deepest reaches of space.

Once observations of gravitational waves with aLIGO, A+ and LIGO Voyager have established gravitational-wave astronomy, it will be timely to make a significant investment in a new Observatory capable of detecting binary neutron stars from the peak of star formation, binary black holes from throughout the Universe. They refer to this new facility as LIGO Cosmic Explorer with operations to commence post-2035, possibly in concert with LIGO Voyager.

One relatively straight forward way to achieve the strawman design sensitivity is to adapt relevant A+ and Voyager technology for a much longer interferometer, for example, 40 km in length. Alternatively, shorter baseline designs with breakthroughs in Newtonian noise removal, other cancelation techniques and mirror, coating and mechanical system engineering, may exploit Quantum Non Demolition interferometry (e.g. speed-meters) to reach similar sensitivities. Another approach may be to use the Xylophone strategy with a series of interferometers targeting limited frequency spans. The European Einstein Telescope (ET) forecasts technologies which overlap LIGO Voyager and LIGO Cosmic Explorer.

To some extent, the facility infrastructure can be decoupled from the instrument which will be first installed. The facility should be designed to have a long lifetime a 50 year (say) so capable of housing instruments starting with sensitivities 10 times aLIGO allowing upgrades to at least 100 times aLIGO. Such a facility will by necessity be much longer that 4 km.

SOURCES- Instrument Science White Paper from LIGO Scientific Collaboration, What Comes next for LIGO, LIGO

Written By Brian Wang.