Ssohrab Borhanian obtained his PhD from Penn State in 2021. After his PhD he joined the Jena group as a postdoctoral fellow.

During his PhD Borhanian developed an open-source Python package, GWBENCH, building on the core concepts of the Fisher information formalism, to benchmark the phenomenology of large gravitational-wave source populations for arbitrary detector network configurations. He performed a large-scale, comparative study of the relative performance of plausible candidates for global detector networks of the next decades, examining (i) detection efficiencies and rates, (ii) signal visibility and measurement qualities of binary parameters, and (iii) the potential for multi-messenger astronomy in synergy with electromagnetic telescopes.

GWBENCH helped him to demonstrate that there is an observable population of golden dark sirens—compact binary coalescences without electromagnetic counterparts—that could facilitate the resolution of the Hubble-Lemaître tension in the next five years and would enable per-event, high-precision cosmology. He further contributed to an investigation of multi-banding with future-generation, ground- and space-based detectors as a tool to constrain parametric deviations from general relativity and illustrated, using GWBENCH, the computational feasibility of multi-banding via archival searches in LISA’s data. Finally, he examined the phenomenology of binary black hole mergers and demonstrated a deeper connection of the GWs emitted by the inspiralling binary system and the ringing remnant black hole, tracing post-Newtonian-like signatures in the amplitudes of several gravitational-wave modes from the inspiral into the ringdown signal.

Arnab Dhani graduated in December 2022 and is currently a postdoc at the Max Planck Institute for Gravitational Physics in Germany. He has diverse research interests in gravitational-wave physics, including black hole quasi-normal modes, neutron star postmerger, gravitational-wave lensing, tests of general relativity, and cosmology. He is currently working on the impact of inaccurate waveform models on various GW science goals of ground- and space-based detectors. 

An exciting result from his study is that overtones and mirror modes of QNM are excited soon after the common horizon forms and including them in the modeling of the post-merger signal improves both their detectability, parameter estimation and black hole no-hair tests.  Examining numerical relativity simulations of black hole mergers he found the first overtone is most important in the leading mode for a given pair of spherical harmonic indices and decreases for sub-dominant modes.

Becca Ewing worked on the analysis of gravitational-wave data from ground- and space-based detectors. Her work showed that high fidelity observations of stellar-mass binary black holes by Cosmic Explorer and Einstein Telescope will  help in the archival searches to dig tens of these signals out of LISA data each year, facilitating multiband gravitational-wave astronomy.

She built a test suite for the O4 observing run of LIGO and Virgo to assess the performance of on-line search pipelines and their sensitivity. She was on the paper writing team for the collaboration’s third gravitaitonal-wave transient catalog (GWTC-3) and managed the section on what the search pipelines found, significance of the events and so on.

Anuradha Gupta  was a postdoctoral fellow at Penn State from 2017-2020. She is currently an Assistant Professor of Physics at the University of Mississippi, Oxford.

A principal theme of Anuradha’s research is testing general relativity. While at Penn State Gupta she developed a new approach to test general relativity using the multipole structure of binary black holes. The idea is to express gravitational wave signals, in particular their phase evolution, directly in terms of parameters that carry the signature of the mass- and current-multipoles. These parameters have well-defined structure in different theories of gravity and by directly measuring them one can constrain alternative theories of gravity.

Following up on her PhD work, she examined ways to detect and characterize binaries that would have undergone spin-orbit resonances. Such resonances are a signature of strong-field dynamics in general relativity and their detection would validate a key prediction of the theory. Furthermore, Gupta found that from the measured distribution of the effective spin––a mass-weighted projection of the component spins along the orbital angular momentum––it is possible to determine the fraction of detections that come from different formation channels and also determine the maximum spin of component black holes.

She collaborated with colleagues at Penn State on a study to understand the spin distribution of primordial binary black holes formed at the time of the QCD phase transition and used the LIGO-Virgo catalog to constrain them–-a work that earned her the AAS Buchalter Cosmology Prize. In an exciting new line of investigation Gupta showed how distance measurements from gravitational-wave observations of binary neutron star coalescences could be used to calibrate type Ia supernovae. Gupta’s investigation has added a new rung to the cosmic distance ladder that is completely free from any modeling uncertainties and systematic biases.

Rachael Huxford worked on the sensitivity of Cosmic Explorer and Einstein Telescope, as well as future upgrades of LIGO and Virgo, in measuring the mass-radius curves of neutron stars from the signature of tidal deformability in gravitational waves from binary neutron star mergers. Her work has shown while current observatories will be limited in inferring the radii of neutron stars to about a kilometer, Cosmic Explorer and Einstein Telescope will improve the accuracy to less than 100 meters. Such precision measurements will enable in-depth studies of the dynamics of dense matter in neutron star cores.

Rachael also worked on developing a detector characterization that has led to efforts have lead to increased sensitivity of the detectors, swifter validation of gravitational-wave candidates and improved tools used for data-quality products.

Rahul Kashyap has a wide range of interests from tests of general relativity to numerical simulations of binary neutron star (BNS) mergers and from exploiting gravitational wave observations to understanding the dense matter equation of state and probing the universe with kilonova as standard sirens. Recently, he showed how gravitational waves will carry the signature of prompt collapse of  the remnant from binary neutron star mergers to black holes, the imprint of QCD phase transitions in BNS post-merger gravitational-wave signals  and how quasi-universal relations in the effective tidal deformability of binary neutron stars could be used in inferring the equation of state of dense matter.

Shiksha Pandey  graduated with a master’s degree from Penn State in August 2024. Her research focused on the characterization of gravitational wave detector networks, particularly assessing the potential role of LIGO-India within the context of multi-messenger astrophysics. Her work explored how LIGO-India could contribute even as next-generation detectors like Cosmic Explorer and the Einstein Telescope become operational.

Surabhi Sachdev was an ECoS Fellow at Penn State during 2018-2021. After spending two years at University of Wisconsin, Milwaukee, WI, she will join the faculty of Georgia Institute of Technology in November 2022.

Sachdev led the LIGO-Virgo collaboration in the writing of the second binary neutron star (GW190425) discovery paper (arXiv: 2001.01761, Physical Review Letters). She is among a small team of researchers who develop and maintain the GSTLAL search software and analysis pipeline. GSTLAL is a flagship analysis pipeline for the collaboration that looks for events both in low-latency for astronomical follow-up but also analyzes the data offline with a greater efficiency. She has led a number of papers on the technical aspects of the pipeline and contributed to many others (arXiv: 1901.08580, 1912.07740, 1904.06020, 1901.02227). She is the chief architect of a new analysis pipeline called the Early Warning System (arXiv:2008.04288, Astrophysical Journal Letters), whose goal is to provide alerts to astronomers of an imminent binary neutron star merger so that events can be observed with electromagnetic telescopes right at the onset of coalescence.

The cosmological population of black hole mergers could create a background noise masquerading any primordial stochastic backgrounds that might be present in the data, potentially losing the ability to learn about the energy scale of electroweak and other phase transitions in the very early Universe. Sachdev showed that it is possible to detect and subtract every binary black hole signal from the data and look for a primordial background (arXiv: 2002.05365, Physical Review D). She also showed this might not hold good for the binary neutron star population if the merger rate is at the higher end of the current rate uncertainty. In a second paper, Sachdev worked on detecting stellar-mass binary black holes in the data from the Laser Interferometer Space Antenna (LISA), showing how archival searches for such systems would be greatly facilitated by the observation of such systems in ground-based detectors that could be operating at the same time as LISA (arXiv: 2011.03036, Physical Review D).

Divya Singh worked on understanding the nature of dark matter from gravitational-wave observations and online searches for gravitational waves. Her research focussed on constraining the properties of asymmetric dark matter candidates from gravitational-wave observation, in particular using tidal deformability to distinguish black hole binaries from neutron star binaries.

Her analysis of data from LIGO and Virgo searching for sub-solar mass binary black holes has placed stringent limits on the contribution of black holes in this mass range. She showed how properties of certain types of dark matter would affect the mass spectrum of stellar-mass black holes and their binaries.

She also worked on the reanalysis of GW170817 to better understand how best to measure the tidal deformability and the impact of various priors on the posterior distribution of tidal deformability.