
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).
Regards,
Padi Boyd, for the Ask an Astrophysicist
