The Expanding Universe: Models
EXPANDING UNIVERSE: FRIEDMANN’S ASSUMPTIONS
Universe is not in rest position. It is expanding from the big
bang. But what’s its fate? We can explain expanding universe in terms of some
models, according to the Russian physicist and mathematician Alexander
Friedmann. There are actually three different kinds of models of Expanding Universe,
which obey Friedmann’s two fundamental assumptions.
MODEL 1: The universe is expanding sufficiently slowly that
the gravitational attraction between the different galaxies causes the
expansion to slow down and eventually to stop. The galaxies then start to move
toward each other and the universe contracts.
Model 1 shows how the distance between two neighboring
galaxies changes as time increases. It starts at zero, increases to a maximum,
and then decreases to zero again.
MODEL 2: The universe is expanding so rapidly that the
gravitational attraction can never stop it, though it does slow it down a bit.
Model 2 shows the separation between neighboring galaxies
starts at zero and eventually the galaxies are moving apart at a steady speed.
It never decreases to zero again.
MODEL 3: The universe is expanding only just fast enough to
avoid re-collapse.
Model 3 shows the separation starts at zero and increases
for ever. However, the speed at which the galaxies are moving apart gets
smaller and smaller, although it never quite reaches zero.
Feature of Model 1:
A remarkable feature of the Friedmann’s
Model 1 is that in it the universe is not infinite in space, but neither does
space have any boundary. Gravity is so strong that space is bent round on to
itself, making it rather like the surface of the earth.
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