Physics XII Content
The galaxies we see in all direction are moving away from the earth, as evidenced by their red shifts. Hubble’s law describes this expansion. The fact that we see all other galaxies moving away from us does not imply that we are the centre of at the universe. All galaxies will see all other stars moving away from them in an expanding universe. IN the 1920s astomer Edwin Hubble discovered, that the universe wasn’t static rather it was expanding and finding that revealed the universe was apparently born in a Big Bang .
The observation obtained from Hubbles space telescope in 1998 revealed that a long time ago, the universe was expanding more slowly than it is today. The expansion of the universe was not slowing due to gravity, but instead inexplicably was accelerating. The name of the unknown force driving this accelerating expansion is dark energy and it remains one of the greatest mysteries in science.
As the action of gravity works against the expansion and if the density was large enough the expansion would stop. This is called a closed universe. If the density is small enough, the expansion whould continue forever ( an open universe) at a certain critical density, the universe would asymptotically approach zero expansion rate but never collapse. Remarkably all the present existing evidence indicates that the universe is very close to of that critical density .