Research

I am currently pursuing several research projects in the topics of gravitational wave astrophysics and the origin and multimessenger emission of massive black holes.

You can find a brief summary of some of my projects below. If you have any questions or suggestions, do not hesitate to contact me at lorenz.zwick96@gmail.com!

Probing accretion disc physics with GW based observations.

Toghether with my frequent collaborators Mudit Garg, Andrea Derdzinski and Pedro R. Capelo, I am trying to determine whether GWs could offer new ways to probe the physics of gaseous systems, in particular accretion discs.
The presence of gas can cause a back-reaction on the motion of an embedded black hole binary, by means of both viscous and gravitational forces. Since GW emission is uniquely determined by the binary's orbit, any deviation from the vacuum expectation will be reflected in its GW emission. In a recent publication, we found that GWs originating from gas embedded sources will be more dirty than expected: Flow variability, turbulence and stochastic accretion can induce fluctuations in the GWs produced by massive binaries. In some cases, these fluctuations could be detectable by LISA, and could be used to place unprecedented constraints on the physics at play in accretion discs.

GWs can propagate information without being significantly extinguished or reprocessed by interstellar and intergalactic media. They are a one-to-one tracer of their source’s orbital motion, which typically occurrs on scales of km to AU, far beyond the fundamental resolution limits of electromagnetic instruments at cosmological distances. Moreover, GWs are now routinely detected by ground based instruments, and will be observed with SNRs exceeding the thousands in the near future. These properties make them an incredibly promising messenger in comparison to light, neutrinos or other particles, and have convinced me of the great potential of GW based observations.

Merger driven direct collapse of black holes.

The formation and growth of high redshift quasars remains one of the major open questions in modern astronomy. Direct collapse models propose mechanisms to rapidly form black hole seeds of a few tens of thousand solar masses , which then grow by accretion to become the billion solar mass quasars we see at redshifts beyond 7. In a recently submitted paper, my collaborators and I have proposed an alternative mechanism, in which the cores of a specific type of proto-galactic disc collapse via the general relativistic instability. These "supermassive discs" form rather late (redshift of ten) as a result of major galaxy mergers, and show very unusual hydrodynamical and thermodynamical properties. In this scenario, the direct collapse of their cores can result in a black holes of hundreds of million solar masses.

As the core collapses, it produces very specific electromagnetic, neutrino and GW counterparts, which can provide a smoking gun signature of the model. While this mechanism is rare by construction, it would provide a natural explanation for the truly monstrous quasars (such as TON618), which tend to reside in elliptical early-type galaxies, the natural product of major mergers.

Priorites in waveform templates: Vacuum or environment?

Recent years have seen significant advances in analytical approximations to GR, which have increased the accuracy of waveform templates to high post-Newtonian orders. Many environmental effects, such as viscous torques or the presence of a third massive body, are likely to perturb GW sources situated in realistic astrophysical environments. While these perturbations are small, so are the high order terms that are yet un-modelled in vacuum waveforms. In a recent article, we performed a systematic study of LISA sources, determining whether further developement of vacuum waveforms is required, despite the likely presence of environmental perturbations.

In many cases, and in particular for lighter sources, the largest contributions that are 1) detectable and 2) un-modelled are environmental. Our analysis shows that (arguably) the systematic inclusion of environmental perturbations should become a priority in the waveform modelling effort.

Non-planetary science with a ranging mission to Uranus.

NASA has recently announced the intent of constructing a flagship mission to one of the most underexplored planets in our solar system, Uranus. During its expected interplanetary cruise of ten years, the satellite will be in constant communication with the Earth through its radio link. Small Doppler shifts in the frequency of the link can be used to reconstruct the trajectory of the spacecraft with great precision.
Both gravitational waves and the influence of dark matter can modify the spacecraft's trajectory, producing small fluctuations in the radio link. A sufficiently precise link can track these fluctuations, turning the Earth-spacecraft system into a detector with a billion km arm.

These types of measurements have been attempted in the late nineties with radio data from the Cassini mission, but failed due to noise constraints. My collaborators (Deniz Soyuer, Daniel D'Orazio, Jozef Bucko, Prasenjit Saha) and I have shown that implementing state-of-the-art ranging technology on the Uranus mission could lead to adetection of GWs and a determination of the local dark matter content in the Solar system. We have recently won a small grant in order to pursue this research further. You can find our team listed on the ISSI website, under Team 551.