Black holes, dark stars and gravitational waves: Ulrich Sperhake and Seppe Staelens

Welcome to Voices of Mathematics, a podcast from the Mathematics Faculty at the University of Cambridge. From number theory and geometry to cosmology and quantum physics, join us to explore topics across pure and applied mathematics, mathematical statistics and theoretical physics. In this episode we talk to two members of the Stephen Hawking Centre for Theoretical Cosmology in the Department of Applied Mathematics and Theoretical Physics. Ulrich Sperhake, Professor of Theoretical Physics, and graduate student Seppe Staelens discuss their fascinating research. Gravitational waves, whose discovery was announced ten years ago, have provided a wealth of information about what physicists believe are black holes. But could other objects be hiding in this data too?  "Gravitational wave signals are like fingerprints," says Sperhake. "You and I have similar fingerprints, but they are not the same. Similarly, subtle differences allow us to tell from an observed gravitational wave the properties of the black holes, or neutron stars, that emitted it." Because gravitational waves carry the imprint of the objects that caused them, they have given us a new tool for observing the Universe. And in the ten years since the first detection we have learnt a lot. Gravitational waves have given us the first observational evidence for the existence of black hole binaries, for example, and they have helped to survey the population of black holes that are out there. But at the same time, physicists have been caught in a kind of trap. When they identify an object or event on the basis of its gravitational wave fingerprint, it's like the police matching a fingerprint found at a crime scene to one found in the police database.  However, the person who left their fingerprint at the crime scene might not be on police record, so a close match in the police database would point to the wrong culprit. Similarly, a close match in the gravitational wave database would only ever point to a black hole or neutron star merger, when in reality the fingerprint might have come from something entirely different. The true culprit would never be discovered. When you have a hammer, everything looks like a nail For this reason physicists, including Sperhake and his colleagues, have sought to extend their database of fingerprints. To do this they reached for made-up objects we don't know really exist: boson stars. We talk to Ulrich Sperhake and Seppe Staelens to find out more about gravitational waves, and how imagining hypothetical objects can help physicists explore the Universe further.   The podcast is hosted by Marianne Freiberger and Rachel Thomas, Editors of Plus, from the communications and outreach team at the Mathematics Faculty.   To find out more about topics mentioned in this podcast see: Our article Dark stars: new theories shed light on black holes Our article Boson stars: beyond vanilla Discover more about the University of Cambridge Faculty of Mathematics at maths.cam.ac.uk.    Image showing the first depiction of hot plasma around what is believed to be a black hole, captured by the Event Horizon Telescope. Image credit EHT collaboration, CC BY 4.0.