Investigations in Gravitational Radiation

Ground based gravitational wave detectors such as LIGO have been extremely successful, providing the first direct detections of black hole and neutron star binaries, indicating new physics through the unearthing of an unexpected population of binary black holes, and in solving longstanding astronomical mysteries such as the origins of short hard gamma ray bursts, the production mechanism for elements heavier than iron, and the physics mechanisms underlying kilonovae. To detect new physics beyond the signals emitted by compact binaries, upgraded sensitivity will be necessary. The required sensitivity will be achieved using a combination of increased laser power and squeezed states.

Increased laser power presents some technical problems for the interferometers which we will help to address through the research proposed here. First, high power laser light incident on coated optical surfaces in the LIGO vacuum envelope appears to lead to the appearance of a distribution of point scatterers on the optics. These scatterers lead to nonuniform heating of the optic, resulting in degraded optical performance. These problems currently limit the ability of LIGO to increase the circulating optical power, and thereby place a practical limit on the sensitivity levels achievable in ground based instruments. Furthermore, anomalous scattering of light by these impurity centres will also affect the squeezing that can be achieved in LIGO. Tega Edo, whilst a PDRA at Sheffield developed an optical imaging system for large optics. This system has been tested on optics loaned from the Glasgow lab. Working with Tega (who is moving to Caltech as a postdoc) and Garilynn Billingsley (Caltech), we will continue to develop this surface imaging system using initial LIGO optics on loan from Caltech. This work is done with a view to improved understanding of the character and origins of the surface inclusions, with a view to assisting in the development of future advanced coatings, and also working towards an in-situ version of the surface imaging apparatus that could be deployed in the advanced LIGO interferometers themselves, as a diagnostic.

Second, the compound suspensions developed at Glasgow and the body modes of the suspended optics lead to many sources of narrowband sinusoidal background (lines) which can (a) cause the interferometers to lose lock when they result in instabilities; and (b) result in inaccesible bands of frequencies where sensitivity to gravitational waves is decreased. Using a Sheffield developed algorithm, IWAVE, deployed on the front end machines at the LIGO sites, we have shown that we can prevent power build-up in optical body modes (parametric instabilities) and characterise and greatly attenuate the build-up of violin mode oscillations in the interferometers. We will continue to develop the IWAVE based front end systems to solve these problems.

Third, the IWAVE algorithm is a candidate for close-to-real-time identification of post binary merger ringing in gravitational wave data, for the detection and diagnosis of background oscillations to other continuous wave gravitational-wave searches, and as a candidate for a future close to real time continuous gravitational wave search algorithm. We have formed a working group, in collaboration with Lilli Sun (faculty member, ANU) to and Max Fays (Liege) for the further study of the applications of the IWAVE real time wave tracking algorithm to continuous wave gravitational wave data analysis.

Finally, the group has been involved through the work of Timesh Mistry (Ph. D. student) in the development of a Newtonian calibrator installed at the Hanford site. We will continue to support this work, in particular by researching the use of class A/B drivers for the motor mechanism to reduce ambient noise caused by the calibrator.

We request a PDRA over 3 years to support our activities in these areas.