Can Cosmologically Coupled Mass Growth of Black Holes Solve the Mass Gap Problem?

Abstract

Observations of elliptical galaxies suggest that black holes (BHs) might serve as dark energy candidates, coupled to the expansion of the Universe. According to this hypothesis, the mass of a BH could increase as the Universe expands. BH low-mass X-ray binaries (LMXBs) in the Galactic disk were born several gigayears ago, making the coupling effect potentially significant. In this work, we calculate the evolution of BH binaries with a binary population synthesis method to examine the possible influence of cosmologically coupled growth of BHs, if it really exists. The measured masses of the compact objects in LMXBs show a gap around ∼2.5–5 M ⊙, separating the most-massive neutron stars from the least-massive BHs. Our calculated results indicate that considering the mass growth seems to (partially) account for the mass gap and the formation of compact BH LMXBs, alleviating the challenges in modeling the formation and evolution of BH LMXBs with traditional theory. However, critical observational evidence like the detection of intermediate-mass BH binaries is required to test this hypothesis.

Constraints on Cosmological Coupling from the Accretion History of Supermassive Black Holes

“…On the other hand, Lei et al (2024) argue, based on a much smaller sample of AGN at z > 4.5 with much lower stellar masses, that k = 3 can be ruled out at ≈2σ. For stellar mass BHs, Gao & Li (2023) argue that cosmological coupling of BHs can explain the distribution of compact object masses in low mass X-ray binaries, in particular the "mass gap" between the highest-mass neutron stars and lowest-mass BHs. However, Rodriguez (2023) disfavor coupling based on observations of a BH binary in the globular cluster NGC 3201 and Andrae & El-Badry (2023) argue that two BH binaries found by Gaia are likely to have formed with BH masses that were too small if k = 3.…”

Cosmological coupling of nonsingular black holes

“…[1] that BHs could be responsible for the observed accelerated expansion of the universe (see, however, refs. [8][9][10][11][12][13][14][15][16][17]).…”