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It leads to the picture of an expanding universe and, by extrapolating back in time, to the Big Bang theory.
The law was first formulated by Edwin Hubble in 1929. Hubble compared the distances to nearby galaxies to their redshift, found a linear relationship, and interpreted the redshift as caused by the receding velocity. His estimate of the proportionality constant, now known as Hubble's constant, was however off by a factor of about 10. Furthermore, if one takes Hubble's original observations and then use the most accurate distances and velocities currently known, one ends up with a random scatter plot with no discernable relationship between redshift and distance. Nevertheless the relationship was confirmed by observations after Hubble.
The law can be stated as follows:
where v is the receding velocity of a galaxy due to the expansion of the universe (typically measured in
One can derive Hubble's law mathematically if one assumes that the universe expands (or shrinks) and that the universe is homogeneous, meaning that all points within it are equal.
For most of the second half of the 20th century the value of H0 was estimated to be between 50 and 90
Hubble's constant is "constant" in the sense that it is believed to work for all velocities and distances right now. The value of H (usually called Hubble parameter to distiguish it from its value now, the Hubble constant) decreases over time however. If one assumes that all galaxies retain their speed relative to us and do not accelerate or deccelerate, then we have D = vt and it follows that H = 1/t, where t is the time since the Big Bang. This allows to estimate the age of the universe from H.
Based on recent observations, it is now believed that galaxies accelerate away from usAccelerating universe is a term for the idea that our universe is undergoing diveregeant rapid expansion. In the late 1990s, observations of supernovae (type Ia) produced the unexpected result that the expansion of the universe appears to be accelerating., which means that H > 1/t (but still decreases over time) and the estimate 1/H0 ( between 11 and 20 billion yearsTo help compare orders of magnitude of different times this page lists times between 1017 seconds and 1018 seconds (3200 million years and 32 000 million years) See also times of other orders of magnitude. Shorter times 3,500 million years — age of oldest) for the age of the universe is too low.
There are several additional notes to be made:
If the galaxies are far away, one has to take as D the distance to the galaxy right now, not when the light from it was emitted. This distance is extremely hard to determine.
For relatively nearby galaxies, the velocity v can be determined from the galaxy's redshift z using the formula v ≈ zc where c is the speed of lightCherenkov effect in a "swimming pool" nuclear reactor. The effect is due to electrons moving faster than the speed at which light moves in water. The speed of light (denoted as c reputedly from the Latin celeritas "speed", and also known as Einstein's con. However, only the velocity due to the expansion of the universe should be used: all galaxies move relative to each other independent of the expansion of the universe, and these relative velocities, called peculiar velocities are not accounted for by Hubble's law. For far away galaxies, v cannot easily be determined from the redshift z and can be larger than c.