If the universe is sufficiently dense, gravity will someday pull it all back together in an event called sort of like the big bang in reverse.

Big Crunch.

Explanation:

Big Crunch is defined as the event which defines the universe's ultimate fate, in this process the universe expansion will reverse which causes the cosmic factor will reach to zero and this is followed by an event which causes the reformation of universe with another Big bang.

The Scenario of the Big Crunch hypothesis that the matter density throughout the universe is extremely high and can be say that it sufficiently dense by which, the attraction through gravity is too large which can overcome the universe's expansion and it can be say that it is the big bang in reverse.

b.transmitters send radio waves, and receivers capture radio waves.