Xavier Siemens

Xavier Siemens is a Spanish-American astrophysicist and professor of physics at Oregon State University. He is co-director of the NANOGrav Physics Frontiers Center and is known for his contributions to gravitational wave astronomy, particularly in pulsar timing array science and the calibration of LIGO data. In 2023, he co-led the NANOGrav collaboration's announcement of evidence for a gravitational wave background at nanohertz frequencies.

Early life and education Siemens was born in Madrid, Spain. He received his BSc and MSc in physics from Imperial College London in 1995. He completed his PhD in physics at Tufts University in 2002 under the supervision of Alexander Vilenkin, with his doctoral research focusing on cosmic strings and their gravitational wave signatures.

Career After completing his PhD, Siemens held a postdoctoral position at the University of Wisconsin–Milwaukee (UWM) from 2002 to 2006, where he joined the LIGO Scientific Collaboration and became co-chair of its Calibration Team. He then spent a year as a Senior Postdoctoral Scholar at the California Institute of Technology. His American Physical Society Fellowship citation recognized this work alongside his pulsar timing array contributions. He also led LIGO's first search for gravitational-wave bursts from cosmic strings.

Awards and honours 2024 – International Congress of Basic Science Frontiers of Science Award in Astrophysics and Cosmology, shared with the NANOGrav collaboration for the 15-year gravitational-wave background paper 2024 – Fellow of the American Physical Society, for "foundational contributions to low-frequency gravitational wave detection using pulsar arrays and his pioneering work in developing methods to calibrate the strain for ground-based interferometers"

Selected publications