Astronomers decode black holes’ ‘ringing’ to test Einstein’s theory

Think of black holes as bells from the cosmos. When they collide and coalesce, the resulting black hole “rings” like a bell, with vibrations known as quasinormal modes (gravitational-wave frequencies that decay in amplitude during the ‘ringdown’ phase).

These are not just ghostly echoes. They are unambiguous probes of black hole mass and spin. They also show whether Einstein’s theory, General Relativity, withstands the rigors of the universe at its most extreme.

A sweeping international review led by the University of Birmingham, Johns Hopkins University, and the Instituto Superior Técnico of Lisbon, together with the Institute of Physics, shows how black hole spectroscopy, once a theoretical idea, is now blossoming into experimental science.

Since the first gravitational-wave detection in 2015, the LIGO-Virgo-KAGRA collaboration has captured hundreds of black hole mergers, and dozens of ringdowns have already been measured. So far, every tone agrees with Einstein’s predictions. But the current generation of detectors is limited.

Even when black holes collide, their gravitational fields are so strong that they can’t be accurately recreated in a lab on Earth. Researchers mining LIGO data have found a complex’ ringdown’ hence confirmations of these mergers, including numerous overtones (similar to music harmonics), interactions between vibrational modes, dynamic excitations, lyric, unusual “exceptional points” where even methods combine, and long discharge “tails”, exaggerated in crowded cosmic places.

These characteristics allow one to use black hole ringdowns as a probe of physics beyond the Standard Model, particularly modifications to the gravity sector, dark matter, and Lorentz-invariance violations in regions near the horizon. In bringing together more than 70 experts from around the world, the review is an unprecedented assessment of this emerging field.

And future observatories, such as Europe’s Einstein Telescope, the US Cosmic Explorer, and the upcoming LISA mission in space, are sure to revolutionize the discipline. An expected high merger rate with frequent and multi-mode excitations will enable them to start probing features beyond the standard model: alternative inflationary (beyond-Einstein) gravity, DM interactions, and quantum mechanics near horizons.

As review co-lead Dr. Gregorio Carullo put it: “By listening to the ringing of newly formed black holes, we are turning gravitational waves into a tool for exploring some of the deepest questions in physics, from the nature of gravity itself to the possibility of discovering entirely new forms of matter and energy.”

Black holes ring: Physics to pay close attention

And looking ahead: “As gravitational-wave detectors become more sensitive, black hole spectroscopy promises to transform black holes from mysterious objects into precision laboratories to study challenging astrophysical processes and uncover new fundamental physics phenomena.”

Journal Reference:

1. Emanuele Berti, Vitor Cardoso, Gregorio Carullo*, Jahed Abedi, Niayesh Afshordi, Simone Albanesi, Vishal Baibhav, Swetha Bhagwat, José Luis Blázquez-Salcedo, Béatrice Bonga et al. Black hole spectroscopy: from theory to experiment. Classical and Quantum Gravity. DOI 10.1088/1361-6382/ae59e2