The cosmic microwave background (CMB) is the photon remnant from the big bang. By tracing fluctuations in light intensity, fluctuations in the matter density of the universe at roughly 100,000 years after the big bang can be inferred and used to predict the present distribution of matter in the universe. The anisotropy in the CMB provides a key test of models of structure formation. Knowing the sizes of fluctuations on different angular scales may allow the measurements of cosmological parameters such as the mass density of the universe, the expansion rate, and the baryon density, etc. These tell us about the universe today, its past, and future evolution.

Mark Devlin is shown with graduate student David Mestre constructing part of the Mobile Anisotropy Telescope.

Devlin's work involves measurements of the CMB on angular scales of several degrees down to about 0.1 degree. His group builds state-of-the-art receivers to observe in the millimeter portion of the spectrum. These receivers generally operate at temperatures from 4.2 K down to 0.085 K in order to make high-sensitivity measurements. This requires us to build and operate the telescopes in unusual places, such as suspended from a Helium balloon at 120,000 ft or on a 17,000 ft plateau in Chile.

In addition to the MAT project, Penn also has an active radio astronomy group that is using ground-based radiotelescopes with larger aperture diameters to measure anisotropies on arcminute angular scales. Myers is leading this effort, and has an ongoing collaboration with astronomers at the California Institute of Technology in Pasadena, CA to carry out large surveys using instruments at their Owens Valley Radio Observatory (OVRO) located in the California desert near Bishop, CA.

The 5-meter telescope at Owens Valley, CA, used to measure CMB anisotropies at an angular scale of around 10'.

The CMB research projects carried out by the group also include using the cosmic microwave background as a screen against which to observe Compton scattering by hot gas in massive clusters of galaxies. This process, known as the Sunyaev-Zeldovich effect, allows the determination of the gas content of the cluster, and hence a measurement of the fraction of mass in the cluster made up by baryons. This is an important measurement for understanding of the cosmology of the universe we live in. When combined with X-ray observations of these galaxy clusters using Earth-orbiting satellites, a value for the Hubble constant can be determined. This work forms the basis for Brian Mason's PhD thesis, and early results show the Hubble constant to be 54 ± 14 km/s/Mpc, which implies a long distance scale, and thus an older universe.

Professor Steven Myers with graduate student Brian Mason.

Myers's group is also involved with a new instrument, the Cosmic Background Imager (CBI). This instrument is now under construction at Caltech, and is a collaboration between Caltech, Penn and U.Chicago scientists. Our goal of the CBI project is to construct an 13-element interferometer with a total diameter of around 6 meters to image the CMB on angular scales from 3 to 30 arcminutes.

The Cosmic Lens All-Sky Survey (CLASS) is now underway using the Very Large Array (VLA), an interferometric array of 27 radiotelescopes located in New Mexico. Originally started in 1994, the CLASS survey aims to observe over 10000 target radio sources in order to find instances of gravitational lensing. A large sample is needed, since lensing of a background radio source by a foreground galaxy is rare (1 in 500 to 1 in 1000). Myers and students working with him are leading the observations that form the basis of this radio lens survey. CLASS is a collaboration between scientists at U.Penn, Caltech, Jodrell Bank (UK) and Westerbork (Netherlands).

In CLASS, we are searching for new gravitational lenses suitable for use in the measurement of the Hubble constant. We have imaged over 7700 target radio sources in the Spring 1994 and Summer 1995. From these first phase of 3271 targets were observed in the Spring 1994 and mapping of all sources was completed in July 1994. Two new gravitational lenses have so far been found from the survey, and many other candidates are currently being followed up with Palomar, Keck, MERLIN and the VLBA. The first lens we found, 1608+656, is a quadruple lens, and has been observed to show time variability, and we are likely to be able to use time delays between the components to measure the Hubble constant. We think this is the best lens yet discovered for this measuement.

An image of the gravitational lens 1608+656 from the Hubble Space Telescope, taken in the 8140 Å band. The four images of the core of the background galaxy, a partial Einstein ring, and the forground lensing galaxy are easily seen.

For a synopsis of this work, see the Penn CLASS Home Page.

The prevalence and character of extra-solar planetary systems is of key importance to an unfolding picture of our place in the cosmos. Do biologically hospitable settings like the Earth abound, or does our terrestrial habitat constitute a rare and unique setting in violation of a generalized "Copernican Principle?" We are now poised at the threshold of a new age of discovery relevant to this question, ushered in by advances in astronomical techniques. This era has been heralded by the first successful observations of potentially proto-planetary disks around young stars and is spurred on by recent detections of extra-solar planets. The latter observations support the notion that planetary systems are common but are currently limited to the detection of planets of Jupiter-mass and greater in orbits very close to the star; the detection of Earth-like planets is not yet feasible. Studies of the detailed physical and chemical properties of proto-planetary disks thus remain one of the best vantage points from which to estimate the full range of planetary systems around stars other than our Sun.

Koerner and his group employ the world's most powerful astronomical observatories to obtain multi-wavelength images of forming solar systems at the highest available angular resolution. Instruments used in this effort are sensitive to radiation across a broad spectrum of wavelengths, from visible to radio, and include the Hubble Space Telescope, the 10m segmented-mirror telescope at the Keck Observatory on Mauna Kea, the JCMT-CSO sub-millimeter interferometer, the Caltech millimeter array at Owens Valley Radio Observatory, the BIMA Hat Creek millimeter array, and the Very Large Array of the National Radio Astronomy Observatory. Theoretical simulations of the radiation from dust and gas are used to analyze images with a view to ascertaining the range of properties - e.g., size, mass, temperature, density, kinematics, and chemical composition - in the circumstellar disks from which planets form. These simulations are fit simultaneously to multi-wavelength images targeted at young circumstellar environments in various stages of evolutionary development in order to discover the sequence of steps which links disks with the origin of planetary systems.

Caltech Millimeter Array.