The KBC Void (or Local Hole) is an immense, comparatively empty region of space, named after astronomers Ryan Keenan,
Amy Barger, and
Lennox Cowie, who studied it in 2013.[1] The existence of a local underdensity has been the subject of many pieces of literature and research articles.[2][3][4]
The underdensity is proposed to be roughly spherical, approximately 2 billion
light-years (600
megaparsecs, Mpc) in diameter. As with other
voids, it is not completely empty but contains the
Milky Way, the
Local Group, and the larger part of the
Laniakea Supercluster. The Milky Way is within a few hundred million light-years of the void's center.[5]
It is debated whether the existence of the KBC void is consistent with the
ΛCDM model. While Haslbauer et al. say that voids as large as the KBC void are inconsistent with ΛCDM,[6] Sahlén et al. argue that the existence of supervoids such as the KBC void is consistent with ΛCDM.[7] Galaxies inside a void experience a gravitational pull from outside the void, which yields a larger local value for the
Hubble constant, a cosmological measure of how fast the universe expands. Some authors have proposed the structure as the cause of the
discrepancy between measurements of the Hubble constant using galactic
supernovae and
Cepheid variables (72–75 km/s/Mpc) and from the
cosmic microwave background and
baryon acoustic oscillation data (67–68 km/s/Mpc).[8]
Other work has found no evidence for this in observations, finding the scale of the claimed underdensity to be incompatible with observations which extend beyond its radius.[9] Important deficiencies were subsequently pointed out in this analysis, leaving open the possibility that the Hubble tension is indeed caused by outflow from the KBC void, albeit in the context of
MOND gravity rather than
general relativity.[6] It was later discovered that this outflow model successfully predicted the bulk flow curve, an important measure of the velocity field in the local Universe.[10]