Together with Chris Gordon and Shreyas Tiruvaskar, from the University of Canterbury, Auckland cosmologists Richard Easther and Russell Boey have shown that measurements of ultralow frequency gravitational waves will lead to stringent tests of a novel theory of dark matter.
The physical nature of dark matter is one of the biggest open questions in science. Hundreds of theories have been proposed, ranging from black holes formed in the aftermath of the Big Bang through to particles with an intrinsic mass a trillion, trillion times smaller than an electron. This idea, known as Ultralight Dark Matter or UDLM, gives dark matter that is “wave like” inside galaxies but acts “normally” on larger scales.
A face-on spiral galaxy, NGC 3982. Similar to the Milky Way it almost certainly has a large black hole at its centre.
We can test ULDM models by looking for evidence of this wavelike behaviour inside galaxies. One key prediction is that some of the ULDM “pools” at the centre of a galaxy, in a structure called a soliton. The centres of galaxies are also home to supermassive black holes, so it makes sense to look for interactions between the soliton and supermassive black holes.
In particular, big galaxies (like our Milky Way) from via the mergers of smaller galaxies and when galaxies merge, their central supermassive black holes and solitons will also merge. However, the black holes do not merge immediately, and billions of years can pass between their final coalescence and the mergers of their parent galaxies. However, if they orbit each other inside a ULDM soliton they will feel “drag”, like a cyclist riding on a windy day. This reduces their orbital energy and they will migrate towards the centre of the soliton, accelerating their merger.
The final stages of a black hole merger is marked by gravitational radiation, generated as the orbiting black holes stir up the fabric of space itself. These “waves” are a key prediction of Einstein’s General Relativity, but it took almost 100 years for its first observational confirmation by LIGO in 2015. However, LIGO “heard” the gravitational waves from the merger of two black holes that formed at the end of the lives of a pair of large stars. Galactic black holes are much bigger and their gravitational waves thus have a much lower frequency – a single wave crest may take years to pass the Earth, as opposed to the hundreds that go by every second in the signals seen by LIGO.
These ultra-low frequency gravitational waves are detected via observations of pulsars. The background rumble of gravitational waves produced by merging supermassive black holes throughout the universe leads to these ultra-stable clocks moving in and out of sync with themselves.
Artist’s interpretation of an array of pulsars being affected by gravitational ripples produced by a supermassive black hole binary in a distant galaxy. Credit: Aurore Simonnet for the NANOGrav Collaboration
It turns out that drag from a ULDM soliton would remove energy from the binary black holes that would otherwise be radiated as gravitational waves, particularly at the very lowest frequencies. Intriguingly, current data suggests that there are fewer low-frequency gravitational waves than we would naively expect. The statistical strength of this discrepancy is not overwhelming, so it is far too early to draw strong conclusions about ULDM on the basis of this deficit.
The theoretical calculations show that this effect remains strong even if ULDM does not account for all of the dark matter. The pulsar timing signal was first detected in 2023, after 15 years of observations. However, the quality of the data will improve dramatically in the near future as more data is gathered and new radio telescopes are coming online.
Consequently, pulsar timing tests of gravitational waves – which are fascinating in their own right – will lead to stringent tests of ultralight dark in the next few years.
Abstract
We investigate the impact of ultralight dark matter (ULDM) on the mergers of supermassive black holes (SMBH) and the resulting stochastic gravitational wave background. ULDM is based on exceptionally light particles and yields galactic halos with dense cental solitons. This increases the drag experienced by binary SMBH, decreasing merger times and potentially suppressing gravitational radiation from the binary at low frequencies. We develop semi-analytic models for the decay of SMBH binaries in ULDM halos and use current pulsar timing array (PTA) measurements to constrain the ULDM particle mass and its fractional contribution to the dark matter content of the universe. We find a median ULDM particle mass of 7. x 10-22 eV and show that scaling relations suggest that the drag remains effective at relatively low ULDM fractions which are consistent with all other constraints on the model. Consequently, future pulsar timing measurements will be a sensitive probe of any ULDM contribution to the overall dark matter content of the universe.
- Tiruvaskar, Boey, Easther and Gordon
- Ultralight Dark Matter Constraints from NanoHertz Gravitational Waves
- ArXiV:2512.XXXX
Constraints on the particle mass and the fraction of ultalight dark matter from current pulsar timing data.