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The goal of the workshop is to foster interdisciplinary ideas and new collaborations by bringing together experts from across gravitational wave subfields to discuss the state of the field and what we can learn using gravitational waves as a tool, now and in the coming years. Topics will include primordial backgrounds, compact binaries, pulsar timing arrays, and next generation experiments.
Confirmed Speakers:
Stephon Alexander (Brown)
Valerio De Luca (UPenn)
Amanda Farah (University of Chicago)
Josh Foster (UW Madison)
Francesco Iacovelli (Johns Hopkins)
Aurora Ireland (Stanford)
Max Isi (CCA)
Vicky Kalogera (Northwestern)
Marc Kamionkowski (Johns Hopkins)
Meng-Xiang Lin (UPenn)
Jan Schutte-Engel (UC Berkeley)
Stephen Taylor (Vanderbilt)
Helvi Witek (UIUC)
Nico Yunes (UIUC)
Bei Zhou (Fermilab & UChicago)
Compact object binaries merge at cosmological distances and their emitted gravitational-wave (GW) signals are understood from first principles. These GWs propagate through the Universe, unimpeded by intervening matter except for through the effects of gravitational lensing. Furthermore, these signals carry information about both the luminosity distance and redshift to their sources. These properties make GWs from compact binary mergers clean probes of both the Universe's expansion history and of gravitational lensing. In this talk, I will discuss how the mass distribution of binary black holes can be used to extract this information from the gravitational-wave dataset, even without theoretical predictions for the mass distribution's morphology.
The detection of gravitational waves from colliding black holes has opened a new frontier in the study of gravity — the eXtreme gravity regime, where spacetime dynamics are both violently strong and relentlessly dynamical. These signals carry a wealth of information about the fundamental laws governing gravity, and we are only now beginning to decode them. In this talk, I will focus on the ringdown phase, the final act of a black hole merger, when the remnant rings like a cosmic bell before settling down. I will discuss how to model this regime within effective field theory extensions of general relativity, and how recent analyses are already beginning to leverage ringdown observations to probe — and potentially reveal — deviations from Einstein’s theory. The question remains: will the ringdown continue to confirm relativity, or will it expose cracks in its foundation?
Gravitational wave (GW) astronomy offers unrivaled discovery opportunities in the coming decades, with the potential to shed light on physics ranging from the formation and evolution of black holes to fundamental aspects of the early Universe. However, a comprehensive examination of the GW frequency spectrum is hampered by presently limited prospects for GW detection at uHz frequencies. In this talk, I describe how the resonant energy transfer between GWs and binary systems may produce observable perturbations to orbital dynamics, paving the way toward future searches for uHz GW with lunar laser ranging and pulsar timing array datasets.
Primordial sources of gravitational waves (GWs) have traditionally been probed through their contribution to the stochastic GW background, detectable via pulsar timing arrays and ground-based laser interferometers. However, these same tensor perturbations can also leave an imprint on the cosmic microwave background (CMB) in the form of B-mode polarization. While a detection of primordial B-modes has long been regarded as a smoking gun for inflation, recent work has shown that non-inflationary sources in the early universe can also produce B-mode polarization on observable scales with amplitudes potentially within reach of upcoming CMB experiments.
In this talk, we examine two such sources: 1) first-order cosmological phase transitions occurring in the late, but pre-recombination, universe, and 2) scalar-induced tensor perturbations, generated at second order in cosmological perturbation theory from enhanced small-scale curvature perturbations. For each, we compute the angular spectrum of B-mode polarization and compare to that from inflation assuming tensor-to-scalar ratios targeted by next-generation CMB experiments. We find that in both cases, there exists viable parameter space yielding signals rivaling those from inflation. The scenarios can in principle be distinguished by making observations at multiple angular scales, since the spectral shapes are distinct from the inflationary case, featuring more power on smaller scales.
These results motivate broader observational strategies for uncovering primordial sources of GWs beyond inflation.
Leptogenesis is arguably the best-motivated theory of baryogenesis, given the discovery of finite neutrino masses. However, its experimental test remains elusive due to the high energy scales involved. In this talk, I will discuss gravitational waves (GWs) produced via graviton bremsstrahlung from right-handed neutrino decays during leptogenesis. Detecting such a GW signal would provide strong evidence for leptogenesis and for the existence of heavy right-handed neutrinos. I will also discuss how an era of early matter domination, which can be induced by the presence of right-handed neutrinos, can mimic the effects of additional relativistic degrees of freedom and a higher reheating temperature. Information from the graviton bremsstrahlung GW spectrum can break this degeneracy.
Since strong evidence for low-frequency gravitational waves (GWs) was delivered by pulsar-timing array experiments two years ago, its origin and source properties remain unknown. The amplitude of the stochastic GW background may be in mild tension with some black-hole population predictions in the literature, which can be resolved by a combination of a modified galaxy stellar mass function, improved pulsar noise modeling, or even additional contributions from cosmological signals. Anisotropy signatures in the GW background remain undetected and its polarization structure untapped. Resolved signals from individual supermassive black-hole binary systems are still elusive, and targeted GW searches on periodic quasar candidates have not yet yielded compelling evidence. The opportunities for major progress in all these areas remain wide open, and should be accelerated by new facilities projected to come online within the next 5-10 years. I will discuss the direction in which this field is headed, and its connections to LISA through similar source classes and analysis approaches.
Gravitational waves induce correlations in pulse-arrival times characterized by the well-known Hellings-Downs curve. If, however, there are spin-0 or spin-1 gravitational-wave modes, which may arise in alternative-gravity theories, and/or anisotropies in the gravitational-wave background, there may be different patterns of correlations. I will describe an elegant (at least I think so) mathematical formalism for characterizing and rapidly calculating these correlations. The formalism also generalizes to angular-deflection correlations sought in astrometric searches for gravitational waves.
An important next step for pulsar timing arrays (PTAs) is to measure anisotropies in the stochastic gravitational wave background (SGWB) at $\sim$ nano-Hz frequencies. We present simple analytical estimates of the anisotropy signals in PTA, considering contributions from both shot-noise and large-scale structure (LSS), under the assumption that PTA signals primarily originate from the inspiral phase of supermassive black hole binaries (SMBHBs). We find that shot-noise-induced signal dominates over the LSS-induced one and exhibits a steep frequency dependence compared to the isotropic background signals, with $C_{\ell>0,h^2}^{\rm SN} \propto f^{8/3}$. At high frequencies, shot-noise anisotropies can be close to the background and even naively surpass the current NANOGrav constraints. Given the potentially large amplitude of these signals, near-future PTA experiments may be able to detect shot-noise anisotropies, or at least place tighter upper limits, which would already be informative for distinguishing between different SMBHB source configurations and possible source origins. While the correlation between PTA anisotropies and galaxy surveys could, in principle, offer a novel probe of SMBHB evolution, such studies are likely to remain challenging in the near term, as PTA sensitivity is currently limited to large angular scales.
We study the vacuum amplification of chiral gravitational waves in dynamical Chern–Simons gravity. Graviton production arises from the parity-violating Pontryagin coupling, leaving distinct signatures in the stochastic gravitational wave background. We analyze four scenarios for the pseudoscalar evolution—rapid velocity transitions, finite constant velocity, and effective fluid-like behavior—and show that each can produce spectra dominated by parity violation. We compare these predictions with the sensitivity of current and upcoming detectors, highlighting observational windows to test chiral gravitational waves.
The search for modifications to Einstein’s theory of general relativity has become increasingly intriguing as recent and upcoming gravitational wave experiments open a new window to probe gravity with compact objects such as black holes. One natural extension to general relativity arises from the four-dimensional string-inspired effective field theory of gravity which introduces a nonminimally coupled scalar field. In this talk, I will take this a step further and consider scalar-tensor theories of gravity which introduce two scalar fields coupled to curvature as well as each other, as such interactions are not ruled out by solar system tests. To consider the effect of these interactions in strong gravity environments, I will present numerical relativity simulations of two scalar fields evolving around single and binary black holes, demonstrating the dynamical formation of the black-hole hairs.
Black holes are among the most intriguing predictions of general relativity, composed of the fabric of spacetime itself. Observations of black holes offer unique access to extreme gravity, and they enable us to address puzzles in fundamental physics ranging from dark matter to the very nature of gravity itself.
Among the compelling dark matter candidates are ultralight axion-like particles and dark photons. The latter are represented by Proca fields, and I will present new results of Proca hair around black holes. In the second half of the talk, I will present simulations of binary black holes surrounded by a scalar dark matter environment and the resulting gravitational wave signals.
Tidal Love numbers describe the conservative response of compact objects to external tidal perturbations. Remarkably, within General Relativity, they are found to vanish exactly for black holes in vacuum. We begin by revisiting this vanishing property from a symmetry-based perspective. We then explore how this feature is modified when black holes are embedded in external environments, assessing the prospects for detecting environmental-induced tidal effects with future gravitational wave observations. Finally, we quantify the systematic biases introduced when tidally deformed binaries in environments are incorrectly modelled as isolated systems.
Motivated by recent studies of light QCD axions in dense nuclear matter, in this talk, we will discuss the effects of the light QCD axion on gravitational observables of neutron stars: mass, radius, and tidal deformability. It has been shown that at sufficiently high baryon densities, finite density corrections to the axion potential source an axion field. We provide a numerical study of the dynamics of the light QCD axion in neutron stars, including the frictional effects of the gravitational self-interactions of the axion field. This analysis shows that the mass-radius relationship of the star can be affected at the ~3% level while the tidal deformability-compactness relationship can be affected at the ~10% level. We comment on the potential for axion studies with future gravitational wave observations of neutron star physics, possible electromagnetic probes of the light QCD axion, and future examination of direct effects of the light QCD axion on the baryonic equation of state in a NS.
With sensitivity improvements of next-generation (XG) gravitational-wave detectors, we expect to resolve >99% of binary black hole (BBH) mergers across the Universe and roughly half of all binary neutron star (BNS) mergers. Detecting a cosmological stochastic background in the presence of such astrophysical foregrounds is therefore challenging and calls for robust statistical methodology.
I will present a two-step framework: (i) identify and remove resolvable BBH signals in the time–frequency domain, and (ii) perform a joint analysis that simultaneously fits the remaining BNS foreground and the cosmological GW background (CGWB). In addition, I will quantify the impact of shot noise arising from finite observation time and occasional nearby loud events on the analysis, present a mitigation strategy, and estimate the resulting sensitivity reduction.
A yet elusive class of sources of GWs are stochastic backgrounds. Such signals can be originated either by the incoherent superimposition of unresolved signals of astrophysical origin, or by several processes in the very early Universe. Detecting the latter kind of signal would thus be of pivotal importance to understand the phenomena happening right after the Big Bang.
Third-generation detectors, Einstein Telescope and Cosmic Explorer, thanks to an increase of more than one order of magnitude in sensitivity and a larger bandwidth will have an outstanding potential in this direction. The subtraction of the astrophysical foreground to reveal an underlying cosmological stochastic background will anyway pose a significant challenge.
After showing a recent derivation of the spectral features of the astrophysical background generated by a finite population of sources, we will present a novel approach for characterizing this astrophysical 'confusion noise' and residuals from the subtraction of resolved ones, where we average the cross-correlated partially cleaned detectors’ outputs over many noise realizations.
Crucially, we stress that the masking effect of the astrophysical foreground depends on the specific cosmological search conducted through the employed filter function. We show that, in a network comprising Einstein Telescope and two Cosmic Explorers, black hole binaries do not compromise the sensitivity to cosmological searches; while the effect of neutron star binaries is more substantial.
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