It has been claimed that black holes naturally grow more massive as the universe expands. We take this surprising proposal at face value and compute its implications for the merger rates of the black hole binaries formed when pairs of massive stars collapse.
Bigger black holes merge more quickly than smaller ones. A “cosmological coupling” would boost predictions for the number of observable mergers by a factor of 1,000 or more, far beyond the observed number. In addition, most mergers would involve black holes that have grown significantly since their “birth” leaving them with masses 100s times larger than the Sun and no such systems have even been detected.
Consequently the cosmological coupling hypothesis is falsified.
Abstract
It was recently suggested that “cosmologically coupled” black holes with masses that increase in proportion to the volume of the Universe might constitute the physical basis of dark energy. We take this claim at face value and discuss its potential astrophysical implications. We show that the gravitational wave emission in binary systems would be significantly enhanced so that the number of black hole mergers would exceed the observed rate by orders of magnitude, with typical masses much larger than those seen by the LIGO-Virgo-KAGRA network. Separately, if the mass growth happens at fixed angular momentum, the supermassive black holes in matter-deficient elliptical galaxies should be slowly rotating. Finally, cosmological coupling would stabilize small black holes against Hawking radiation-induced evaporation.
- Ghodla, Easther, Briel and Eldridge
- Observational implications of cosmologically coupled black holes
- The Open Journal of Astrophysics 6 or ArXiV:2306.08199
Predicted verses observed merger rates in LIGO/Virgo/KAGRA. The blue (k=0) line corresponds to regular uncoupled black holes. This actually under-predicts the observed population of merger events but including cosmological coupling (for two different versions of the proposals) leads to at least 100 times more mergers than actually observed.