In the Dark

A couple of papers were published recently that attracted quite a lot of media interest so I thought I’d mention the work here.

The researchers detail the theory in two papers, published in The Astrophysical Journal and The Astrophysical Journal Letters, with both laying out different aspects of the cosmological connection and providing the first “astrophysical explanation of dark energy”. The lead author of both papers is Duncan Farrah of the University of Hawaii. Both are available on the arXiv, where all papers worth reading in astrophysics can be found.

The first paper, available on the arXiv here, entitled Preferential Growth Channel for Supermassive Black Holes in Elliptical Galaxies at z<2, and makes the argument that observations imply that supermassive black holes grow preferentially in elliptical galaxies:

The assembly of stellar and supermassive black hole (SMBH) mass in elliptical galaxies since z∼1 can help to diagnose the origins of locally-observed correlations between SMBH mass and stellar mass. We therefore construct three samples of elliptical galaxies, one at z∼0 and two at 0.7≲z≲2.5, and quantify their relative positions in the MBH−M∗ plane. Using a Bayesian analysis framework, we find evidence for translational offsets in both stellar mass and SMBH mass between the local sample and both higher redshift samples. The offsets in stellar mass are small, and consistent with measurement bias, but the offsets in SMBH mass are much larger, reaching a factor of seven between z∼1 and z∼0. The magnitude of the SMBH offset may also depend on redshift, reaching a factor of ∼20 at z∼2. The result is robust against variation in the high and low redshift samples and changes in the analysis approach. The magnitude and redshift evolution of the offset are challenging to explain in terms of selection and measurement biases. We conclude that either there is a physical mechanism that preferentially grows SMBHs in elliptical galaxies at z≲2, or that selection and measurement biases are both underestimated, and depend on redshift.

arXiv: 2212.06854

Note the important caveats at the end. I gather from people who work on this topic that it’s a rather controversial claim.

The second paper, entitled Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy and available on the arXiv here, discusses a mechanism by which it is claimed that the formation of black holes actually creates dark energy:

Observations have found black holes spanning ten orders of magnitude in mass across most of cosmic history. The Kerr black hole solution is however provisional as its behavior at infinity is incompatible with an expanding universe. Black hole models with realistic behavior at infinity predict that the gravitating mass of a black hole can increase with the expansion of the universe independently of accretion or mergers, in a manner that depends on the black hole’s interior solution. We test this prediction by considering the growth of supermassive black holes in elliptical galaxies over 0<z≲2.5. We find evidence for cosmologically coupled mass growth among these black holes, with zero cosmological coupling excluded at 99.98% confidence. The redshift dependence of the mass growth implies that, at z≲7, black holes contribute an effectively constant cosmological energy density to Friedmann’s equations. The continuity equation then requires that black holes contribute cosmologically as vacuum energy. We further show that black hole production from the cosmic star formation history gives the value of ΩΛ measured by Planck while being consistent with constraints from massive compact halo objects. We thus propose that stellar remnant black holes are the astrophysical origin of dark energy, explaining the onset of accelerating expansion at z∼0.7.

arXiv:2302.07878

The first I saw of these papers was in a shockingly poor write-up in the Guardian which is so garbled that I dismissed the story out of hand. I recently saw it taken up in Physics World though so maybe there is something in it. Having scanned it quickly it doesn’t look trivially wrong as I had feared it would be.

I haven’t had much time to read papers over the last few weeks but I’ve decided to present the second paper – the more theoretical one – next time I do our cosmology journal club at Maynooth, which means I’ll have to read it! I’ll add my summary after I’ve done the Journal club on Monday afternoon.

In the meantime I was wondering what the general reaction in the cosmological community is to these papers, especially the second one. If anyone has strong views please feel free to put them in the comments box!

UPDATE: There is a counter-argument on the arXiv today.