Nagoya team finds way to detect hidden 'hair' on black holes

A Nagoya University team led by PhD student Ariadna Uxue Palomino Ylla has developed a method to detect extra matter or new physics around black holes—called 'hair'—by analysing the ringdown gravitational waves emitted after a merger. The approach, published in the Journal of Cosmology and Astroparticle Physics, could let European observatories like Virgo and the planned Einstein Telescope test general relativity in extreme conditions.

Bottom line — The method provides a template for future gravitational wave detectors to spot deviations from Einstein's black hole model.

Go deeper (7)

• When two black holes merge, the resulting object rings like a struck bell, radiating gravitational waves that fade in a pattern called ringdown, the team explained.
• According to general relativity, ringdown depends only on the black hole's mass and spin—the 'no-hair theorem'—but extra matter or modified gravity would alter the signal, said Palomino Ylla.
• The researchers modelled hair as a thin anisotropic fluid on standard black holes and used the link between light orbits and ringdown to calculate how hair shifts frequency and damping rate, the study in JCAP details.
• A key finding is that hair does not shift frequency and damping by the same amount; the gap between them depends on the local pressure of hidden matter at the photon orbit, Palomino Ylla noted.
• For spinning black holes, waves co-rotating and counter-rotating with the spin respond differently to hair, producing a signature pattern that varies with the type of matter, the team said.
• The method was tested on three known theoretical black holes and extended to spinning cases, providing a general framework instead of studying each possible hair type from scratch, according to the researchers.
• While the results are early estimates, the approach helps scientists know what to look for in real gravitational wave data from LIGO, Virgo, KAGRA, and future observatories like the Einstein Telescope and LISA, the authors stated.

Read the reporting