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The Question

(Submitted June 13, 1997)

Can you tell me about the end of time?

The Answer

Thanks for your question about the end of time. In order to arrive at an answer, astronomers use their knowledge of gravity together with the Big Bang.

We observe all distant galaxies to be receding from us, and from this we conclude that the Universe is expanding uniformly. In fact, the current picture of the evolution of the cosmos is that since the birth of the Universe in the explosive Big Bang, the Universe has continued to expand.

We also observe that massive objects attract each other through the gravitational force. This force tends to contract matter locally (for example, a gas cloud condenses to form a star). On the large scale you can think of the expansion of the Universe acting to separate galaxies from one another, and the gravitational force acting to attract them toward one another.

The "end of time" depends on just how much mass there is in the Universe. We talk about this in terms of the density of the Universe, and compare densities to the critical density. If the density is greater than the critical density, then eventually gravity will overtake the expansion. The expansion will slow down and eventually reverse, so that the Universe will be contracting. Eventually it will end in a collapse (or a bounce) called the Big Crunch. If the density is less than the critical density then the Universe will continue to expand forever, with the gravitational force never overtaking the expansion. An ongoing area of research is to measure the density of the Universe. Currently, some observations (and some theories) indicate that the density of the Universe is very close to the critical density. In this case the expansion will slow down so that it is approaching zero expansion as time approaches infinity.

If you are curious about this topic, you might want to check out the book "Cosmic Questions: Galactic Halos, Cold Dark Matter, and the End of Time" by Richard Morris (1993).


Padi Boyd,
for the Ask an Astrophysicist

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