Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy (2302.07878v1)

Abstract: 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 $0<z\lesssim2.5$. 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 $z\lesssim7$, 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 $\Omega_{\Lambda}$ 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 $z \sim 0.7$.

Observational Evidence for Cosmological Coupling of Black Holes and its Implications

The paper authored by Farrah et al. examines the concept of cosmological coupling of black holes (BHs) and presents observational data suggesting that such a phenomenon might contribute to dark energy. By invoking the cosmological coupling hypothesis, the authors explore the idea that the mass of black holes could grow in proportion with the scale factor of the universe, independent of accretion or mergers, which challenges traditional scenarios depicted by the Kerr black hole solution.

Cosmological Coupling and Observational Findings

The authors test cosmological coupling by studying the growth of supermassive black holes (SMBHs) in elliptical galaxies over the redshift range $0 < z \lesssim 2.5$. They find that the BHs in red-sequence ellipticals exhibit a significant mass increase over time, which is consistent with the cosmological coupling hypothesis where the mass grows in proportion with a power of the scale factor $a$. The hypothesis is supported by rejecting zero cosmological coupling with a confidence level of 99.98%.

The paper utilizes observational samples in elliptical galaxies at various redshifts to compute the cosmological coupling strength $k$, yielding $k = 3.11^{+1.19}_{-1.33}$ at 90% confidence, which is consistent with BHs having vacuum energy interiors. This value is in line with the BHs contributing an effectively constant cosmological energy density and participating in the Friedmann equations as a form of vacuum energy.

Theoretical Implications and Dark Energy

By positing that these cosmologically coupled BHs could serve as a source of dark energy, the authors speculate that stellar remnant black holes form an astrophysical origin of dark energy. They propose that this mechanism could explain the observed value of $\Omega_{\Lambda}$, the dark energy density parameter, consistently with observations of compact objects and the cosmic star formation history.

The authors support this claim by modeling various scenarios of the cosmic star formation rate density and its dependence on the initial mass function (IMF) and BH accretion history. They integrate these models with cosmological coupling to show consistency with the measured dark energy density. Additionally, their results interface with constraints from massive compact halo objects (MACHOs) and suggest that non-singular $k=3$ BHs are compatible with these constraints.

Future Tests and Theoretical Challenges

Several paths for future experimental and observational tests are proposed. These include:

• Investigating the cosmic microwave background (CMB) for signatures related to $k=3$ BHs,
• Analyzing gravitational lensing data of gamma-ray bursts which could probe BH densities,
• Exploring BH-BH merger rates influenced by cosmological coupling,
• Assessing the implications of coupling on binary inspiral time decay,
• Developing specific predictions for quasar observations and the evolution of the SMBH mass function.

Additionally, challenges remain within theoretical constructs to reconcile observed behaviors under the framework of general relativity (GR) and in developing a cosmological black hole model with all the identified properties.

Conclusion

The paper by Farrah and colleagues indicates a potential shift in understanding the cosmological role of black holes, proposing a novel account for the astrophysical source of dark energy. By exploring cosmological coupling, they lay a foundation for addressing the accelerating expansion of the universe as an outcome of stellar remnant black holes. Future observational validations are crucial to firmly establish these conclusions.