The expansion of the universe

The light year or unit of light is the unit we usually use to describe the distances between stars and galaxies. But given the size of the universe, the unit of light year is not enough. The unit used for this is Megaparsec. One Megaparsec is 32,60,000 Light years. This is a very large distance. The Milky Way galaxy is just 2.5 million light-years away from the Andromeda galaxy.

The rate of expansion of our universe is 67.3 km / s per Megaparsec.

The image shows a supernova at redshift z = 0.51 (corresponding to a distance of about 10,000 million light-years) is observed on five dates with the SUSI camera at the 3.6-m New Technology Telescope (NTT). The host galaxy is clearly visible and the supernova reaches its maximum brightness around 13 March 1997, after which it fades.

That is, if the distance between two galaxies is one megaparsec, the rate at which space expands between them is 67.3 km / s.
These galaxies have a velocity of 67.3 km / s, even though they have no velocity of their own. The reason for such separation is that the space between them expands. This is the expansion of the universe.

If the distance between two galaxies is 10 megaparsec, the distance between them will be 673 km / s. That is, each megaparsec distance between them expands to 67.3 km / s so that at 10 megaparsec it becomes 673 km / s.

Thus if the distance between two galaxies is 4500 megaparsec then the distance between them will be 3,03,120 km / s. As such, the relative speed between them is much faster than the speed of light. But this is not a violation of the Special Theory of Relativity.

This is because these galaxies have little relative velocity with the space around them. However, due to the expansion of space between the two galaxies, they have so much velocity to each other.

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