The astronomy, gravitation and cosmology group is among the nation’s largest and most active with twelve participating researchers. These include seven current faculty (Patrick Brady, Philip Chang, Jolien Creighton, Dawn Erb, David Kaplan, Xavier Siemens, and Alan Wiseman), two Distinguished Professors Emeriti (John Friedman and Leonard Parker), two Adjunct Professors (Bruce Allen and Luis Anchordorqui), and one Visiting Scientist (Maria Alessandra Papa). The researchers are joined in their endeavors by research scientists, postdoctoral research associates, as well as graduate and undergraduate students. More details can be obtained from web pages of individual group members and from other areas of the Physics Department website.

## Leonard E Parker Center for Gravitation, Cosmology and Astrophysics

At the **Leonard E Parker Center for Gravitation, Cosmology and Astrophysics**, we push the frontiers of astrophysics through the novel use of observation, theory, and computation. By bringing together expertise in gravitational physics, astrophysics, and computing, we can address scientific challenges in relativistic astrophysics, time-domain astrophysics, gravitational-wave/multi-messenger astronomy, galaxy formation/evolution, and cosmology. Members of the CGCA participate in numerous large-scale collaborative efforts including:

**The Laser Interferometer Gravitational-Wave Observatory**

**The North American Nanohertz Observatory for Gravitational Waves**

**The Murchison Widefield Array**

**Global Relay of Observatories Watching Things Happen**

**The Australian Square Kilometre Array Pathfinder**

**The Zwicky Transient Facility**

CGCA members were directory involved in the first-ever discovery of gravitational waves in 2015, and in the discovery and follow-up of gravitational waves from two neutron stars in 2017.

## Faculty Research

**Patrick Brady’s **research interests include the dynamics of gravitational collapse, black holes, numerical relativity including simulation of binary coalescence, the detection of gravitational waves using interferometric gravitational-wave detectors, and multi-messenger studies of gravitational wave sources. In broad terms, he is interested in theoretical and experimental aspects of gravitation and gravitational-wave astronomy.

*For more information:* **Brady’s Research Website**

**Philip Chang’s** research interests include supernovae, compact objects, disk dynamics, plasma astrophysics, and turbulent star formation. In general terms, he is interested in various aspects of theoretical astrophysics and cosmology.

*For more information:* **Chang’s Research Website**

**Jolien Creighton’s** research concerns the portion of Einstein’s theory of General Relativity, where gravity is described in terms of geometry: the distances between points in space and time. Gravitational waves—oscillating gravitational fields that affect the distances between nearby points—are produced by the most violent events in our universe including supernovae, collisions of black holes, and the big bang.

The Laser Interferometric Gravitational-Wave Observatory, LIGO, is a national facility to detect gravitational waves. Creighton participates in the LIGO Scientific Collaboration in an international effort that will search for gravitational waves from the distant universe.

*For more information:* **Creighton’s Research Website**

**Dawn Erb’s** research interests include galaxy formation and evolution, the kinematics, chemical evolution and stellar populations of galaxies at high redshift, feedback processes in starburst galaxies, and the evolution of the intergalactic medium at high redshift.

*For more information:* **Erb’s Research Website**

**John Friedman** has worked on a variety of problems in gravitational physics, involving neutron stars, black holes and gravitational collapse, the topology of spacetime, and topological questions in quantum gravity.

Dr. Friedman no longer supervises students.

**David Kaplan’s** research concerns multi-wavelength (radio, infrared, optical, X-ray) observations of a variety of types of neutron stars. He works with NANOGrav to detect low-frequency gravitational waves using a galactic-scale detector, and on multi-messenger followup of gravitational wave sources detected with LIGO. He is also working on detecting radio transients with the Murchison Widefield Array and the Australia Square Kilometre Array Pathfinder

*For more information:* **Kaplan’s Research Website**

**Leonard Parker** was the first to show that quantum field theory implies particle creation in curved spacetimes. This process creates particles and amplifies small perturbations in an expanding universe. The observed features of the cosmic microwave background (CMB) radiation and of the large-scale structure of the present universe confirm that this particle creation process in an in inflationary universe was responsible for the CMB temperature anisotropies and for the clumping of matter that gave rise to galaxies and galactic clusters.

Parker was first (with S. A. Fulling) to develop the method of adiabatic regularization in curved spacetime. He was also first to calculate the perturbations of the spectra of one-electron atoms in arbitrary gravitational fields in terms of the Riemann tensor.

Dr. Parker no longer supervises students

**Xavier Siemens’** research is focused mostly in gravity wave astrophysics, early universe cosmology, especially cosmic strings, and biophysics. He leads the NANOGrav Physics Frontiers Center at UWM. He also works on calibration of LIGO data.

*For more information:* **Siemens’ Research Website**

**Alan Wiseman’s** research is in the area of general relativity and gravitation with a special emphasis in gravitational-wave generation and detection. He has published a number of papers involving computations of the expected gravitational-wave signals from inspiraling binary star systems using post-Newtonian theory. He also works in the area of black hole perturbation theory and the radiation reaction problem in curved space time.

*For more information:* **Wiseman’s Research Website**