Cosmology has undergone wholesale changes during the past few years, as new
astronomical instruments have captured a flood of new information about the
cosmos. Seven decades ago, astronomers deduced that the entire universe is
expanding, and that this expansion began in a "big bang" everywhere in space
about 15 billion years ago.  This raises a crucial cosmological question:
Will the universe expand forever, or will the gravitational forces among
different objects in the universe someday reverse the expansion to produce a
cosmic contraction, leading to a "big crunch"?  Amazing though it may seem,
astronomers have now apparently answered this question with an entirely
unanticipated conclusion.  Not only will the universe expand eternally, but
this expansion will itself accelerate, so that different regions of space
will move apart from one another ever more rapidly as time goes on.<p>

The driving force behind this cosmic acceleration is the unanticipated
existence of "dark energy" throughout the cosmos.  This dark energy makes
space tend to expand, in opposition to gravity's tendency to make space
contract.  However, because the amount of matter in the universe will not
change with time, whereas new dark energy comes into existence continuously
as the universe expands, the struggle between the two effects can have only
one outcome.  Dark energy will triumph, and the universe will eventually
become immensely larger, more rarefied, and darker than it is today, as stars
and galaxies burn themselves out with no hope of replacement.<p>

How reliable is this conclusion?  I will review the two independent lines of
evidence that point to it, along with the prospects for additional
observational information in the near future.