Hubble bubble (astronomy)

In astronomy, a Hubble bubble would be "a departure of the local value of the Hubble constant from its globally averaged value," or, more technically, "a local monopole in the peculiar velocity field, perhaps caused by a local void in the mass density." The Hubble constant, named for astronomer Edwin Hubble, whose work made clear the expansion of the universe, measures the rate at which expansion occurs. In accordance with the Copernican principle that the Earth is not in a central, specially favored position, one would expect that measuring this constant at any point in the universe would yield the same value. If, on the other hand, Earth were at or near the center of a very low-density region of interstellar space (a relative void), the local expansion of space would be faster due to the lack of nearby mass to slow it down. Thus, stars inside such a "Hubble bubble" would accelerate away from Earth faster than the general expansion of the universe. This situation could provide an alternative to dark energy in explaining the apparent accelerating universe or contribute to explanations of the Hubble tension. In 1998, Zehavi et al. reported evidence in support of a Hubble bubble. The initial suggestion that local redshift velocities differ from those seen elsewhere in the universe was based on observations of Type 1a supernovae, often abbreviated "SNe Ia." Such stars have been used as standard candle distance markers for 20 years, and were key to the first observations of dark energy. Zehavi et al. studied the peculiar velocites of 44 SNe Ia to test for a local void, and reported that Earth seemed to be inside a relative void of roughly 20% underdensity, surrounded by a dense shell, a "bubble". In 2007, Conley et al. examined the SNe Ia color data comparisons while taking into account the effect of cosmic dust in external galaxies. They concluded that the data did not support the existence of a local Hubble bubble. In 2010, Moss et al.