Type Ia supernova constraints on compact object dark matter

The nature of dark matter (DM) is a central question in cosmology today. While elementary particles could explain DM, compact astrophysical objects such as black holes formed in the early universe offer a theoretically appealing alternate route. Here, we constrain the fraction of DM that can be made up of primordial black holes (PBHs) with masses M ≳ 0.01 M⊙, with Type Ia supernovae. Utilizing the Dyer-Roeder distance relation, we find a maximum fractional amount of DM in compact objects (fp) of 0.50 at 95% confidence level (C.L.), in the flat ΛCDM model and 0.49 when marginalising over a constant dark energy equation of state or spatial curvature, demonstrating robustness to the cosmological model. With a prior on the homogeneity parameter, η, including values >1, we derive η = 1.08 ± 0.17, hence, fp < 0.32 at 95% C.L., showing that the prior assumption of η ≤ 1 gives a conservative upper limit on fp. The Hubble constant we infer is consistent with the homogeneous case, showing that inhomogeneities like compact dark matter cannot account for the observed Hubble tension. In conclusion, we can exclude stellar masses PBHs as comprising all of the observed dark matter.