Research Area Gravitational Waves

Detector research & development 

Gravitational wave observatories these days routinely measure a mirror displacement of less than a ten-thousandth of the size of a proton. This sensitivity is like measuring the distance to the center of our Galaxy to the precision of the width of a human hair. Researchers of the Quantum Universe are involved in international R&D efforts for detectors for both next generation ground-based gravitational wave observatories like the Einstein Telescope (ET) as well as space-based gravitational wave observatories like the Laser Interferometer Space Antenna (LISA).

Several cluster researchers have been part of research and development of the LISA mission proposal which has recently been formally adopted by the European Space Agency as an approved space mission.

A joint UHH-DESY team researches heat-extraction schemes for cold mirrors to solve the pressing problem of how to extract the heat out of GW detector test mass mirrors that are illuminated by tens of kilowatts of light power. Low-pressure helium gas cooling, use of squeezed light and parametric cooling of a pendulum-suspended mirror have been investigated. The ultra-high vacuum system for the Hamburg ET Shot-Noise Prototype has been installed and commissioned and a first low power interferometer operationalised. Low thermal noise silicon-based coatings which could be used with wavelength-doubled ultra-stable lasers have been studied. Experimental demonstration has been achieved for how ET could compensate for quantum radiation pressure noise without additional narrow-band filter cavities.

A ‘quantum expander’ concept has revealed that sensitivities at kHz frequencies can be greatly increased for conventional ground-based GW observatories. Could a storage ring can be turned into GW observatory via accurate timing measurements of a circulating test mass? This new proposal has been studied and is technologically limited by noise sources.

Seismic noise limits low-frequency GW detection. Work on novel compact displacement sensor design, control systems for test mass seismic isolation and distributed acoustic sensing promise to help open ground-based GW detection below 1 Hz. Seismic noise measurements using distributed fibre sensors have been carried out in collaboration with geophysicists from UHH excellence cluster CLICCS and with the DESY accelerator division. 

Astrophysical sources of gravitational waves 

Gravitational waves and their electromagnetic counterparts from the mergers of compact objects have been extensively studied. Measuring electromagnetic emission concomitant with GW detection enables the determination of the redshift of the source and a wide range of new and detailed physics of the merger of compact sources.

Numerical relativity simulations of compact binary mergers using innovative numerical techniques are employed and are crucial to follow merger dynamics and generation of gravitation waves. This has led to new and detailed predictions for heavy element production, electromagnetic transients and multi-messenger signals, ejecta as well as the GW from neutron star mergers. Brighter electromagnetic kilonovae have been predicted for mergers with one higher spin component. The uncertain equation of state of neutron stars can also be inferred and constrained by comparing models to observations. The parameter space for a possible first order phase transition to quarks, inside neutron stars, has been refined in the context constraints from tidal deformability and radius. of A unique Lagrangian relativity code has been developed which confers advantages in the numerical treatment of the vacuum, particle advection and neutron star surface.

Low radio-frequency follow-up observations were conducted using the European Low-Frequency Array (LOFAR) of the electromagnetic counterpart of the gravitational wave event GW170817, the first binary neutron star merger to be detected by Advanced LIGO–Virgo. Further observations related to expected GW sources like supermassive black holes have been carried out with the eROSITA X-ray telescope, LOFAR and the Australian SKA Pathfinder (ASKAP). University of Hamburg’s 1.2 meter robotic TIGRE telescope in Mexico  is being integrated into an international network for GW event follow-up and time-domain astronomy.

Precise theoretical predictions and reliable templates are critical for successful data analysis and interpretation of the gravitational-wave signals. High precision waveform models including tidal and finite-size effects in binary coalescences are needed to discern the neutron star equation of state, test general relativity and search for new particles in ‘gravitational colliders’. An effective field theory approach using the theory of scattering amplitudes has been employed to make progress in computing the dynamics of binary systems and their spin effects.

Cosmological sources of gravitational waves 

Changes of phase, like liquid water boiling to a vapour, but occurring on a cosmic scale  throughout the early Universe are expected to leave behind a cosmic hum i.e. an ever-present background of gravitational waves. The cluster has been involved in the recent NanoGRAV results which hint at the presence of a low frequency GW background. 

The prediction of GW signals of cosmological origin has been carried out, motivated by fundamental particle-physics questions, with focus on stochastic backgrounds from cosmological phase transitions and from the inflaton’s interactions with the Standard Model. Generic effects on axion dynamics have also been obtained and couplings from the particular structures of axions arising in string theory. 

Precise and more efficient calculations for the GW spectrum arising from first order cosmological phase transitions have been done for relativistic effects from high-speed bubble collisions, kinetic energy fraction controlling the amplitude of the stochastic GW signal and the effects of thermal plasma. Production of GW backgrounds from the decay of cosmic strings has also been studied for non-standard cosmologies and meta-stable cosmic string networks. The ability of future GW detectors, like ET and LISA, to probe beyond the standard model physics has been evaluated, along with pulsar timing constraints.

GW sources in an ultra-high frequency range (well beyond ET range) cannot consist of compact objects but instead are cosmological backgrounds. New generic sources of high frequency GW have been found such as axion inflaton oscillations and fragmentation and axion spinning. A workshop was organised and an international network established by cluster researchers for high frequency GW activities. Peaked GW spectra have also been studied arising from models of axion inflation in string theory.

Self excited peaks of GW production during axion inflation have been calculated which lead to enhanced production of primordial black holes. The formation of oscillons has also been found to source characteristic GW either via self-couplings or via axionic hybrid inflation. Furthermore, peaks in the spectrum of GW could also arise from evaporation of sufficiently light primordial black holes. 

The idea of using GW as a thermometer for the big bang has been worked out via the use of strong laser beams in strong magnetic fields converting the background GW into photons. Such conversion, called the Gertsenshtein effect, has also been employed in the new proposal to use radio telescopes to search for high frequency GW via spectral distortions they leave in the cosmic microwave background radiation.