TheKBC Void (orLocal Hole) is an immense, comparatively empty region of space, named after astronomersRyan Keenan,Amy Barger, andLennox 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 billionlight-years (600megaparsecs, Mpc) in diameter. As with othervoids, it is not completely empty; it contains theMilky Way, theLocal Group, and the larger part of theLaniakea 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 theΛCDM model. While Haslbauer et al. say that voids as large as the KBC void are inconsistent with ΛCDM,[6]Sahlén [sv] 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 theHubble constant, a cosmological measure of how fast the universe expands. Some authors have proposed the structure as the cause of thediscrepancy between measurements of the Hubble constant using galacticsupernovae andCepheid variables (72–75 km/s/Mpc) and from thecosmic microwave background andbaryon acoustic oscillation (BAO) data (67–68 km/s/Mpc).[8] BAO data presented at NAM 2025 showed that a void model is ~ 1 x 10^8 times more likely than a void-free model consistent with Planck cosmology.[9]
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.[10] 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 ofMOND gravity rather thangeneral 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.[11]
