John Ruhl

Connecticut Professor

Rockefeller Building

Other Information

Degree: B.S., Univ. of Michigan (1987)
Ph.D., Princeton (1993)

John Ruhl’s Site


Experimental Cosmology


The Cosmic Microwave Background Radiation (CMB) carries an enormous amount of information about the universe at a redshift z = 1,000, a few hundred thousand years after the big bang. We can also use the CMB as a “backlight” to learn more about the lower redshift universe. Studies of the CMB can answer many fundamental questions about the nature of the universe.

The CMB is almost uniform in its brightness across the sky; its brightness at frequencies from roughly 0.5 mm to 10 cm is very close to that of a 2.7 K Planck blackbody. However, there are small variations in the brightness, at the level of tens of microKelvin. It is also slightly polarized. By studying these temperature variations and the polarization, we have learned many interesting things (including the global curvature of spacetime in the universe, the amount of normal matter, and the amount and nature of the dark matter and “dark energy” in the universe), and will learn even more in the future.

We are currently working on two projects to explore the properties of the CMB.

One is the South Pole Telescope (SPT), a 10m diameter telescope located at the South Pole. We are currently observing with a camera (“SPT-3G”) that is optimized to look for clusters of galaxies via the Sunyaev-Zeldovich effect (using the CMB as a “backlight”), and to characterize the temperature and polarization variations of the CMB on very small angular scales (down to 1 arcminute). These observations tell us about Dark Matter, Dark Energy, and the history of structure formation in the universe.

CMB-S4 is a new experiment that is currently in the “design” phase, but which will ultimately use telescopes in Chile and at the South Pole to improve on the science targeted by SPT (above), and also search for the unique pattern of polarization that may have been imprinted by gravitational waves created during Cosmic Inflation, when the universe was only 10-34 (or so!) seconds old.


See full list of publications at ORCID (Open Researcher and Contributor ID)