Arthur Kosowsky

  • Professor and Department Chairperson
315 Allen Hall


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 alternatives 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 (current research topic of Fernando Zago). 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 of the Atacama Cosmology Telescope (ACT) project, which built a custom-designed 6-meter microwave telescope with superconducting bolometric detectors to observe the microwave sky from the Atacama Desert in the Chilean Andes. ACT has produced microwave maps with arcminute angular resolution in three frequency bands. Notable ACT achievements have included the first direct detection of gravitational lensing of the cosmic microwave background radiation, the first detection of seven acoustic peaks in the microwave background power spectrum, improved constraints on the parameters describing the standard cosmology, the first detection of galaxy cluster motions via their imprint on the microwave background radiation, and detection of one of the earliest and largest galaxy clusters. The second-generation receiver with polarization capability, known as ACTPol, was installed on the telescope in 2013, and the third season of ACTPol observations finished in January 2016. Former Pitt graduate student Simone Aiola has been one of the primary mapmakers for these data, and the collaboration is now working on a series of papers characterizing the signals in the new maps.  The third-generation Advanced ACT polarized receiver, which will eventually map half of the sky in 5 frequency bands, completed its first observing season in December 2016, using a new dichroic detector array and rotating half-wave plates to control systematic errors at large angular scales. The ACTPol maps have significant overlap with various optical surveys, particularly the Sloan Digital Sky Survey, and Advanced ACT intends to cover much of the sky region which will be observed by the Large Synoptic Survey Telescope (LSST) beginning after 2020. The combination of microwave maps and optical maps leads to numerous probes of fundamental physics and astrophysics. Primary science goals for Advanced ACT include detection of inflationary gravitational waves via B-mode polarization, neutrino masses via gravitational lensing, growth of structure via the kinematic Sunyaev-Zeldovich effect (Ph.D. thesis topic of Suman Bhattacharya 2008),  tests of dark energy versus modified gravity, constraints on reionization of the universe, and probes of quasar feedback in galaxies (Ph.D. thesis topic of Suchetana Chatterjee 2009).

I am also involved in the Simons Observatory, initiated in March 2016, which plans to build new microwave telescopes and detector arrays at the ACT site. So far, this is a joint effort of the ACT and Polarbear collaborations, with major financial support from the Simons Foundation. Pitt has joined the collaboration as an institutional member, and I serve on the Science Board. We anticipate this project will serve as a bridge to the ultimate ground-based microwave experiments known as S4.

I am currently Divisional Associate Editor for Physical Review Letters.


  •  Fellow of the American Physical Society, 2014

Selected Publications

  • "Evidence for the Thermal Sunyaev-Zeldovich Effect Associated with Quasar Feedback,'' D. Crichton et al. (24 authors including A. Kosowsky), Mon. Not. R. Ast. Soc. 458, 1478 (2016).
  • "Microwave Background Correlations from Dipole Anisotropy Modulation,'' S. Aiola, B. Wang, A. Kosowsky, T. Kahniashvili, and H. Firouzjahi, Phys. Rev. D 92, 063008 (2015).
  • "Microwave Background Polarization as a Probe of Large-Angle Correlations,'' A. Yoho, S. Aiola, C.J. Copi, A. Kosowsky, and G.D. Starkman, Phys. Rev. D 91, 123504 (2015).
  • "The Atacama Cosmology Telescope: Lensing of the Microwave Background by Dark Matter Haloes,'' M. Madhavacheril et al. (42 authors including A. Kosowsky), Phys. Rev. Lett. 114, 151302 (2015) (Editor's Suggestion).
  •  "The History of Inflation from Microwave Polarimetry and Laser Interferometry,'' J. Caligiuri, A. Kosowsky, W. Kinney, and N. Seto, Phys. Rev. D 91, 103529 (2015).
  • "Dynamics of Gauge Field Inflation,'' S. Alexander, D. Jyoti, A. Kosowsky, and A. Marciano, J. Cosm. Astropart. Phys. 1505, 005 (2015).
  • "Gaussian Approximation of Peak Values in the Integrated Sachs-Wolfe Effect,'' S. Aiola, A. Kosowsky, and B. Wang, Phys. Rev. D 91, 043510 (2015).
  • "Precision Epoch of Reionization Studies with Next-Generation CMB Experiments,'' E. Calabrese et al. (24 authors including A. Kosowsky), J. Cosm. Astropart. Phys. 1408, 010 (2014).
  • “The Atacama Cosmology Telescope: Cosmological Parameters from Three Seasons of Data,” J. Sievers et al. (93 authors including A. Kosowsky), Journal of Cosmology and Astroparticle Physics 10 (2013), 060.
  • “The Atacama Cosmology Telescope: Sunyaev-Zeldovich Selected Galaxy Clusters at 148 GHz on the Celestial Equator,” M. Hasselfield et al. (40 authors including A. Kosowsky), Journal of Cosmology and Astroparticle Physics 07 (2013), 008.
  • “Detection of Galaxy Cluster Motions with the Kinematic Sunyaev-Zel'dovich Effect,” N. Hand et al. (58 authors including A. Kosowsky, primary author), Physical Review Letters 109, 041101 (2012).
  • “The Atacama Cosmology Telescope: ACT-CL J0102-4215 `El Gordo,' A Massive Merging Cluster at Redshift 0.87,” F. Menanteau et al. (24 authors including A. Kosowsky), Astrophysical Journal 748, 7 (2012).
  • “Detection of the Power Spectrum of Cosmic Microwave Background Lensing by the Atacama Cosmology Telescope,” S. Das et al. (41 authors including A. Kosowsky), Physical Review Letters 107, 021301 (2011).

Graduate Advisor

Yilun Guan
Fernando Salviatto Zago
Hongbo Cai