In cosmology, the Hubble expansion is the motion of galaxies away from each other, due to the expansion of the universe. This motion results in measureable redshifts. Cosmology, from the Greek: κοσμολογία (cosmologia, κόσμος (cosmos) world + λογια (logia) discourse) is the study of the universe in its totality and by extension mans place in it. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant. ... Redshift describes a change in the wavelength of light, in which the wavelength is longer, or redder, than when it was emitted at the source. ...
According to Hubble's Law, the apparent relative velocity of a galaxy with respect to the Earth can be found by multiplying its distance from us by the Hubble constant. (This relation is approximate and begins to fail at very large distances.) In practice, this law (derived from observations of galaxies for which independent estimates of distance are available) is turned around and used to estimate distance from redshift. For example, it was first realized that quasars are very distant (and hence, very ancient) objects when it was discovered that they have enormous redshifts. Hubbles law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance. ... Hubbles law is the statement in astronomy that the redshift in light coming from distant galaxies is proportional to their distance. ... This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. ...
Hubble flow is the relative motion of non-interacting particles due to this universal expansion. Deviations from Hubble flow are observed on smaller scales where gravitational forces becomes important (such as the Local group). Map of the local group The Local Group is the group of galaxies that includes our galaxy, the Milky Way. ...
Hubble's law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance.
Though there was considerable scatter (now known to be due to peculiar velocities), Hubble was able to plot a trend line from the 46 galaxies he studied and obtained a value for the Hubble constant of 500 km/s/Mpc, which is much higher than the currently accepted value due to errors in his distance calibrations.
The value of the Hubble constant was the topic of a long and rather bitter controversy between Gérard de Vaucouleurs who claimed the value was 80 and Allan Sandage who claimed the value was 40.
That is, we should be in the Hubbleflow for at least this number of collisions and above.
According to this tired light theory the Hubble constant H can be thought of as a measure of the electron density of intergalactic space, n.
To show an expanding universe, galaxies have to be in the Hubbleflow and it is known that this starts at about 6 or 7 million light year away (some say 50 million light years away).
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