My research focuses on cosmology and related issues of theoretical physics. I have done extensive work on the theory of the cosmic microwave background radiation and the ways in which it constrains our models of the universe. Current microwave observations, combined with optical observations of the large-scale galaxy distribution, cosmic abundances of light elements, and the supernova-1a Hubble diagram, combine to give tight constraints on the properties of the universe. The resulting "standard model" fits most observations well, but is troubling theoretically: our best guess says that only 4 percent of the universe's energy density is in the form of ordinary matter, 26 percent is made of as-yet undetected dark matter (which does not interact either via the strong or electromagnetic forces), and the remaining 70 percent is in an even stranger "dark energy", evenly distributed in space and having a negative effective pressure. Theorists have a number of good candidates for the dark matter particles, which are currently being pursued by many experimental groups, including the high energy experiment group at Pitt. Current ideas as to the nature of dark energy are all highly speculative.

We have a variety of observations to test our model of cosmology; these include the temperature and polarization fluctuations in the microwave background radiation, gravitational lensing, dynamics of galaxies and clusters of galaxies, and the large-scale distribution and velocities of galaxies and galaxy clusters. I am interested in possible extensions or modifications to the standard cosmological model, and observational tests which can distinguish particular alternatives from the standard cosmology. As an example, the "dark energy" may actually be telling us that the usual equations describing the expansion of the universe, based on general relativity, are not valid; in other words, we could be observing not the result of a mysterious form of energy density but rather the breakdown of our basic theory of gravitation. I also think about observations which are, to some extent, unexpected in the standard cosmological model, particularly "anomalies" in the microwave sky. All current anomalies, particularly the power spectrum asymmetry and the suppressed correlation function at large scales, can be probed to much higher precision using future polarization maps of the microwave sky (Ph.D. thesis topic of Simone Aiola 2016).

The standard model of cosmology strongly suggests that the universe underwent a period of exponential expansion, known as inflation, in its first moments. Inflation naturally explains why the universe is spatially flat, highly isotropic, and lacking in magnetic monopoles from grand-unified symmetry breaking to the standard model of particle physics. It also provides a mechanism for generating small density perturbations in the universe, which have gaussian random statistics, a power spectrum nearly but not quite scale invariant, and with equal fractional perturbations in all matter components. To a very good approximation, this is what our universe looks like. However, we have no realistic physical theory of inflation, and phenomenological models based on a classical scalar field with some specified potential lack a clear physical interpretation and require finely-tuned initial conditions. I am interested in constructing more realistic models, based on known properties of quantum fields in curved spacetimes, which explain how inflation began, the mechanism fueling the inflationary expansion, and the dynamical evolution of the universe throughout the inflationary epoch (Ph.D. thesis topic of Fernando Zago 2019). Future detection of a gravitational radiation background produced by inflation can put strong constraints on the expansion history of the universe during inflation (Ph.D. thesis topic of Jerod Caligiuri 2016).

On the observational side, I am a member (and current Collaboration Spokesperson) of the Simons Observatory. This international collaboration, in which Pitt is an institutional member, is fielding a set of new custom-designed telescopes with large arrays of superconducting bolometric detectors to observe the microwave sky from the Atacama Desert in the Chilean Andes. Simons Observatory has a wide range of science goals including detecting the inflationary gravitational background through the "B-mode" (parity-odd) signal it induces in the microwave background polarization; neutrino masses via gravitational lensing; polarization rotation via cosmic birefringence or cosmic magnetic fields (Ph.D. thesis topic of Yilun Guan 2021); large-scale motions via the kinematic Sunyaev-Zeldovich Effect (Ph.D. thesis topic of Hongbo Cai 2022) and the moving lens effect (Ph.D. thesis topic of Ali Beheshti 2026); microwave transients from stars, novae, and black holes (Ph.D. thesis topic of Emily Biermann 2024); and astrophysics of our own galaxy. Science observations are commencing during 2024 and 2025.

I have a long-standing interest in a stochastic background of gravitational waves from possible phase transitions in the early universe. This topic has generated widespread interest since the discovery of a stochastic background of gravitational waves via pulsar timing. Recent work with my collaborators resulted in the first direct numerical simulations of gravitational waves from turbulent plasma in the early universe, a key step in understanding the details of this potential signal for the Laser Interferometer Space Antenna, a joint ESA-NASA mission scheduled to launch in the mid-2030's.

 

Awards

 

• Fulbright US Scholar to Chile, 2024
• Fellow of the American Physical Society, 2014
• Outstanding Referee of the American Physical Society, 2009
• Cottrell Scholar of the Research Corporation, 2000
• Phi Beta Kappa, 1988
• Admiral of the Great Navy of the State of Nebraska, 1985