Hubble bubble (astronomy)

In cosmology, a Hubble bubble refers to a possible departure of the locally measured value of the Hubble constant, which describes the rate of cosmic expansion, from its true value averaged over the whole universe, appearing as a local monopole in the peculiar velocity field that could be caused by Earth sitting inside a large, comparatively low density region of space. The Copernican principle holds that the rate of cosmic expansion should be the same everywhere, but if the Milky Way lies within such an underdense void, matter there would exert less gravitational pull, so the local expansion rate would appear faster than the true cosmic average and objects within the void would seem to recede more quickly than expected. The hypothesis offers a possible alternative explanation for observations otherwise attributed to dark energy, and it has also been proposed as one possible contributor to the discrepancy between different measurements of the Hubble constant known as the Hubble tension.