A study co-led by physicists at UC Riverside and UC Irvine has found that dark matter halos of ultra-diffuse galaxies are very odd, raising questions about physicists’ understanding of galaxy formation and the structure of the universe.

Ultra-diffuse galaxies are so called because of their extremely low luminosity. The distribution of baryons—gas and stars—is much more spread out in ultra-diffuse galaxies compared to “normal” galaxies with similar masses.

In the following Q&A, Hai-Bo Yu, an associate professor of physics and astronomy at UCRhis thoughts on the findings he and UCI’s Manoj Kaplinghat, his long-term collaborator, have published in The Astrophysical Journal about newly discovered ultra-diffuse galaxies and their dark matter halos.

Black holes are astronomical objects with extremely strong gravitational pulls from which not even light can escape. While the idea of bodies that would trap light has been around since the 18th century, the first direct observation of black holes took place in 2015.

Since then, physicists have conducted countless theoretical and experimental studies aimed at better understanding these fascinating cosmological objects. This had led to many discoveries and theories about the unique characteristics, properties, and dynamics of black holes.

Researchers at Ludwig-Maximilians-Universität and Max-Planck-Institut für Physik have recently carried out a theoretical study exploring the possible existence of vortices in black holes. Their paper, published in Physical Review Letters, shows that black holes should theoretically be able to admit vortex structures.

With the upgraded detectors at the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its sister facility Virgo, researchers can now measure significantly finer details of the gravitational-wave signals released from black hole mergers. This progress opens tantalizing prospects for black hole spectroscopy, a technique that involves analyzing the signal-frequency spectra of gravitational waves and that could be used to test the limits of the general theory of relativity. In 2019, an analysis of the first detected gravitational-wave signal (GW150914) indicated that it contained multiple tones, or “overtones” (see Synopsis: Hunting for Hair on Coalescing Black Holes), a finding that could lead to novel spectroscopy approaches. Now a new analysis of GW150914 by Roberto Cotesta of Johns Hopkins University in Baltimore and colleagues challenges that previous claim. Cotesta and his colleagues find that the suspected overtones could be caused by noise [1].

The overtones presented in the 2019 study were extracted from the “ringdown” phase of the merger, when the remnant black hole shakes like a struck bell. Cotesta and his colleagues wanted to test whether that 2019 conclusion was robust to the input assumptions used for the extraction. These assumptions include the time at which the gravitational-wave signal peaks and the noise that contributes to the measured signal. The team finds that the procedure is not robust and that some noise patterns—such as fluctuations occurring right around the signal peak—produce artifacts in the data that resemble overtones.

Theoretical physicist Swetha Bhagwat at the University of Birmingham, UK, who wasn’t involved in either study, says that while neither analysis has obvious faults, the fact that slight differences in the parameters used by the two teams lead to opposing conclusions highlights the need for further scrutiny. The detection of overtones has exciting implications for black hole spectroscopy, so it’s very important that the community debates this issue, she says.