About
Professor Savvas M. Koushiappas is a faculty member at Brown University, serving as a Professor of Physics. His research primarily focuses on the interface between cosmology and particle physics, with a predominant emphasis on dark matter physics. He investigates the nature of dark matter particles from an experimentally-motivated theoretical perspective, exploring how their properties such as mass and couplings influence the large-scale structure of the universe. His work also involves disentangling diffuse high-energy astrophysical backgrounds in the context of dark matter searches and developing new statistical approaches to various problems in the field. In addition to his core research in dark matter, Professor Koushiappas maintains interests in classical astrophysical problems, contributing to fields such as galaxy formation, gravitational lensing, and gravitational waves. He joined Brown University in 2008 after postdoctoral positions at Los Alamos National Laboratory and ETH-Zurich. He earned his Ph.D. from The Ohio State University in 2004. His scholarly work includes numerous publications in high-impact journals, and he has been recognized with several awards and honors from organizations such as the Department of Energy, the National Science Foundation, and NASA.
Research topics
- Physics
- Theoretical physics
- Astrophysics
- Astronomy
- Computer Science
- Quantum mechanics
Selected publications
Satellite-borne $γ$-ray astrophysics from coherent interactions in oriented crystals
arXiv (Cornell University) · 2026-01-07
preprintOpen accessHigh-density and high-Z crystals are key elements of most space-borne $γ$-ray telescopes operating at gigaelectronvolt energies (such as Fermi-LAT). The lattice structure is usually neglected in the development of a crystalline detector, although its effects on the energy deposit development should be taken into account, since the interactions of a high-energy ($\sim$ 10~GeV) photon or e$^\pm$ impinging along the axis of an oriented crystal are different than those observed in a fully isotropic medium. Specifically, if the angle between a photon (e$^\pm$) trajectory, and the crystal axis is smaller than $\sim$ 0.1$^\circ$, a large enhancement of the pair production (bremsstrahlung) cross section is observed. Consequently, a photon-induced shower inside an oriented crystal develops within a much more compact region than in an amorphous medium. Moreover, for photon energies above a few gigaelectronvolt and incidence angles up to several degrees, the pair production cross section exhibits a pronounced dependence on the angle between the crystal axis and the photon polarization vector. In this work we show that these effects could be exploited to develop a novel class of light-weight pointing space-borne $γ$-ray telescopes, capable of achieving an improved sensitivity and resolution, thanks to a better shower containment in a smaller volume, with respect to non-oriented crystalline detectors. We also show that an oriented tracker-converter system could be used to measure the polarization of a $γ$-ray source above few gigaelectronvolts, in a regime that remains unexplorable through any other detection technique. This novel detector concept could open new pathways in the study of the physics of extreme astrophysical environments and potentially improve the detector sensitivity for indirect dark matter searches in space
A Cosmological Uncertainty Relation and Late-Universe Acceleration
ArXiv.org · 2026-04-30
articleOpen access1st authorCorrespondingWe propose that the size of the universe and its rate of expansion cannot be simultaneously specified with arbitrary precision, a quantum mechanical statement encoded in a deformed commutation relation for the scale factor. The deformation modifies the Friedmann equation by adding a geometric correction to the expansion rate, and the sign and magnitude of a single free exponent determine the cosmological behavior. When the exponent is positive, the model predicts late-time dark energy with $w > -1$, testable with current and next-generation surveys. When the exponent is sufficiently negative, the same deformation produces a non-singular classical bounce that resolves the Big Bang singularity. The model introduces no new particles or fields and preserves a scale-invariant primordial power spectrum. The deformation has a natural interpretation as a horizon-scale phenomenon, with the cosmological horizon, and not the Planck length, setting its characteristic scale. The late-universe regime is then its generic application, with the expansion history as the primary observable signature. Cosmic acceleration may be the macroscopic imprint of quantum gravity at the cosmological horizon.
A Cosmological Uncertainty Relation and Late-Universe Acceleration
arXiv (Cornell University) · 2026-04-30
preprintOpen access1st authorCorrespondingWe propose that the size of the universe and its rate of expansion cannot be simultaneously specified with arbitrary precision, a quantum mechanical statement encoded in a deformed commutation relation for the scale factor. The deformation modifies the Friedmann equation by adding a geometric correction to the expansion rate, and the sign and magnitude of a single free exponent determine the cosmological behavior. When the exponent is positive, the model predicts late-time dark energy with $w > -1$, testable with current and next-generation surveys. When the exponent is sufficiently negative, the same deformation produces a non-singular classical bounce that resolves the Big Bang singularity. The model introduces no new particles or fields and preserves a scale-invariant primordial power spectrum. The deformation has a natural interpretation as a horizon-scale phenomenon, with the cosmological horizon, and not the Planck length, setting its characteristic scale. The late-universe regime is then its generic application, with the expansion history as the primary observable signature. Cosmic acceleration may be the macroscopic imprint of quantum gravity at the cosmological horizon.
Satellite-borne γ-ray astrophysics from coherent interactions in oriented crystals
ArXiv.org · 2026-01-07
articleOpen accessHigh-density and high-Z crystals are a key element of most space-borne $γ$-ray telescopes operating at GeV energies (such as Fermi-LAT). The lattice structure is usually neglected in the development of a crystalline detector, although its effects on the energy deposit development should be taken into account, since the interactions of a high energy ($\sim$~GeV) photon or e$^\pm$ impinging along the axis of an oriented crystal are different than the ones observed in a fully isotropic medium. Specifically, if the angle between a photon (e$^\pm$) trajectory and the crystal axis is smaller than $\sim$ 0.1$^\circ$, a large enhancement of the pair production (bremsstrahlung) cross-section is observed. Consequently, a photon-induced shower inside an oriented crystal develops within a much more compact region than in an amorphous medium. Moreover, for photon energies above a few GeV and incidence angles up to several degrees, the pair-production cross-section exhibits a pronounced dependence on the angle between the crystal axis and the photon polarization vector. \\ In this work we show that these effects could be exploited to develop a novel class of light-weight pointing space-borne $γ$-ray telescopes, capable of achieving an improved sensitivity and resolution, thanks to a better shower containment in a smaller volume with respect to non-oriented crystalline detectors. We also show that an oriented tracker-converter system could be used to measure the polarization of a $γ$-ray source above few GeV, in a regime that remains unexplorable through any other detection technique. This novel detector concept could open new pathways in the study of the physics of extreme astrophysical environments and potentially improve the detector sensitivity for indirect Dark Matter searches in space.
Field equations in Chern-Simons-Gauss-Bonnet gravity
Physical review. D/Physical review. D. · 2025-03-03 · 1 citations
articleSenior authorWe investigate the effects of Chern-Simons-Gauss-Bonnet gravity on fundamental metrics. This theory involves perturbative corrections to general relativity, as well as two scalar fields, the axion and the dilaton, which arise from Chern-Simons and Gauss-Bonnet gravity modifications respectively. The combined Chern-Simons-Gauss-Bonnet gravity is motivated by a wide range of theoretical and phenomenological perspectives, including particle physics, string theory, and parity violation in the gravitational sector. In this work, we provide the complete set of field equations and equations of motion of the Chern-Simons-Gauss-Bonnet modified gravity theory for a suite of fundamental metrics (Friedmann-Lema\^{\i}tre-Robertson-Walker, Schwarzschild, spherically symmetric, and perturbed Minkowski), under no prior assumptions on the behavior of the fields. The full set of field equations and equations of motion can be numerically solved and applied to specific observables under certain assumptions, and can be used to place constraints on the Chern-Simons-Gauss-Bonnet modified gravity theory.
Kinetically Coupled Dark Matter Condensates
ArXiv.org · 2025-06-09
preprintOpen accessDark matter consisting of ultralight bosons can form a macroscopic Bose-Einstein condensate with distinctive observational signatures. While this possibility has been extensively studied for axions and axion-like particles $-$ pseudoscalars with masses protected by shift symmetry $-$ realistic models from string theory and other higher-dimensional theories predict more complex structures. Here we investigate a two-field generalization where an axion couples to a moduli field through its kinetic term, representing the phase and radial modes of a complex scalar field. We demonstrate that when this system forms a gravitationally bound Bose-Einstein condensate, the kinetic coupling produces dramatic modifications to cosmological evolution compared to the canonical single-field case. Most notably, the axion Jeans scale becomes dynamically dependent on the moduli field's evolution, fundamentally altering structure formation. By mapping existing observational constraints from canonical axion models to our two-field scenario, we identify regions of parameter space that are already excluded by current observations. In particular, consistency with observations requires that the moduli field must take on small field values, $χ/M_{\rm pl} \ll 1$, throughout most of cosmic history for this class of axions to remain a viable description of all dark matter.
Field Equations in Chern-Simons-Gauss-Bonnet Gravity
arXiv (Cornell University) · 2024-11-08
preprintOpen accessSenior authorWe investigate the effects of Chern-Simons-Gauss-Bonnet gravity on fundamental metrics. This theory involves perturbative corrections to general relativity, as well as two scalar fields, the axion and the dilaton, that arise from Chern-Simons and Gauss-Bonnet gravity modifications respectively. The combined Chern-Simons-Gauss-Bonnet gravity is motivated by a wide range of theoretical and phenomenological perspectives, including particle physics, string theory, and parity violation in the gravitational sector. In this work, we provide the complete set of field equations and equations of motion of the Chern-Simons-Gauss-Bonnet modified gravity theory for a suite of fundamental metrics (Friedmann-Lemaitre-Robertson-Walker, Schwarzschild, spherically symmetric, and perturbed Minkowski), under no prior assumptions on the behavior of the fields. The full set of field equations and equations of motion can be numerically solved and applied to specific observables under certain assumptions, and can be used to place constraints on the Chern-Simons-Gauss-Bonnet modified gravity theory.
Physical review. D/Physical review. D. · 2023-04-24 · 37 citations
articleOpen accessSenior authorAn only early or only late time alteration to $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ has been inadequate at resolving both the ${H}_{0}$ and ${S}_{8}$ tensions simultaneously; however, a combination of early and late time alterations to $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ can provide a solution to both tensions. As an illustration, we examine a combined early dark energy, decaying dark matter model. While early dark energy has the ability to resolve the ${H}_{0}$ tension, it leads to a discrepancy in ${S}_{8}$ measurements. We show that the addition of decaying dark matter helps resolve the ${S}_{8}$ discrepancy that would otherwise be enhanced in an early dark energy model, while the latter is able to relieve the ${H}_{0}$ disagreement to within the 95th percentile interval. Our results show a preference for the combined model over $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ with $\mathrm{\ensuremath{\Delta}}\mathrm{AIC}=\ensuremath{-}6.72$, hinting that both early and late Universe modifications may be necessary to address the cosmological tensions.
Quantum gravity signatures in the late Universe
Physical review. D/Physical review. D. · 2023-10-06 · 1 citations
articleWe calculate deviations in cosmological observables as a function of parameters in a class of connection-based models of quantum gravity. In this theory nontrivial modifications to the background cosmology can occur due to a distortion of the wave function of the Universe at the transition from matter to dark energy domination (which acts as a ``reflection'' in connection space). We are able to exclude some regions of parameter space and show with projected constraints that future experiments like DESI will be able to further constrain these models. An interesting feature of this theory is that there exists a region of parameter space that could naturally alleviate the ${S}_{8}$ tension.
Cosmology with the Laser Interferometer Space Antenna
Living Reviews in Relativity · 2023 · 318 citations
- Physics
- Theoretical physics
- Astrophysics
Abstract The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational-wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational-wave observations by LISA to probe the universe.
Recent grants
Imprints of New Physics on Cosmological Observations
NSF · $240k · 2020–2023
Energetic Neutrinos from the Sun and the Earth and the Granularity of the Dark Matter Distribution
NSF · $105k · 2010–2013
Connecting Particle Physics with Astrophysics
NSF · $210k · 2014–2017
Frequent coauthors
- 41 shared
Alex Geringer-Sameth
- 26 shared
Kyriakos Vattis
Massachusetts General Hospital
- 21 shared
Filippo Vernizzi
Centre National de la Recherche Scientifique
- 20 shared
Marc Kamionkowski
Johns Hopkins University
- 19 shared
J. García-Bellido
- 18 shared
Andrew R. Zentner
University of Pittsburgh
- 17 shared
Nicola Tamanini
Université Toulouse III - Paul Sabatier
- 17 shared
Abraham Loeb
Education
- 2004
Ph.D.
The Ohio State University
Other
ETH-Zurich (Swiss Federal Institute of Technology)
Other
Los Alamos National Laboratory
Awards & honors
- Department of Energy, Office of High Energy Physics, 2017-20…
- National Science Foundation, Division of Physics (Theoretial…
- NASA Fermi Guest Investigator, 2013-2015
- Department of Energy, Office of High Energy Physics, 2012-20…
- NASA EPSCoR-RID, 2012
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