Here's some relevant background: https://physicsworld.com/a/cosmological-coupling-is-making-b...

And the two papers mentioned in the OP:

https://iopscience.iop.org/article/10.3847/2041-8213/acb704

https://iopscience.iop.org/article/10.3847/1538-4357/acac2e

I extracted a summary using a large language model.

The two papers are related and explore the growth of supermassive black holes (SMBHs) in elliptical galaxies over cosmic time. The first paper, published in The Astrophysical Journal, focuses on the assembly of SMBH and stellar mass in elliptical galaxies and their relative positions on the M BH−M * plane, where M BH is the SMBH mass and M * is the stellar mass. The authors construct three samples of elliptical galaxies, one at z ∼ 0 and two at 0.7 ≲ z ≲ 2.5, and 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 7 between z ∼ 1 and z ∼ 0. 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. The authors 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.

The second paper, published in The Astrophysical Journal Letters, explores the implications of cosmological coupling for the growth of black holes and the origin of dark energy. The authors consider the growth of SMBHs in elliptical galaxies over 0 < z ≲ 2.5 and find evidence for cosmologically coupled mass growth among these black holes, with zero cosmological coupling excluded at 99.98% confidence. Cosmological coupling refers to the idea that the 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. 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. The authors propose that stellar remnant black holes are the astrophysical origin of dark energy, explaining the onset of accelerating expansion at z ∼ 0.7.

Taken together, the two papers suggest that there is a physical mechanism that preferentially grows SMBHs in elliptical galaxies at z ≲ 2, and that the growth of black holes is cosmologically coupled, which could have implications for the origin of dark energy. The idea of cosmological coupling is important because it implies that black holes can contribute to the energy density of the universe in a way that is not dependent on accretion or mergers, and that the growth of black holes is not isolated from the expansion of the universe. This means that black holes could be a potential source of dark energy, which is the mysterious force that is causing the accelerated expansion of the universe.