Question

There is a theory that gravity could pull galaxies back together, causing a reverse big bang.

Which observation makes this theory less likely than the idea that the universe may continue to expand?

Answer 1

In fact, recent observations of the universe have shown it is expanding at an increasing speed. In other words, the universe is still growing and all the stars, planets and galaxies are separating from each other. To put it simply, in the movie Harry Potter Harry Potter and the Prisoner of Azkaban, Harry's aunt inflates like a balloon, which make her clothes to rip and stretch. The Universe is growing like Harry's aunt, and the galaxies are going away from each other, like the buttons of her dress. Thus, the right answer is: The expansion of the universe is accelerating.

Answer 2

The expanding universe is finite in both time and space. The reason that the universe did not collapse, as Newton's and Einstein's equations said it might, is that it had been expanding from the moment of its creation. The universe is in a constant state of change. The expanding universe, a new idea based on modern physics, laid to rest the paradoxes that troubled astronomers from ancient times until the early 20th Century.
Edwin Hubble
In 1929 Edwin Hubble, working at the Carnegie Observatories in Pasadena, California, measured the redshifts of a number of distant galaxies. He also measured their relative distances by measuring the apparent brightness of a class of variable stars called Cepheids in each galaxy. When he plotted redshift against relative distance, he found that the redshift of distant galaxies increased as a linear function of their distance. The only explanation for this observation is that the universe was expanding.

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