Chapter 28 Stars and the Universe

730 CHAPTER 28: STARS AND THE UNIVERSE
the main sequence for billions of years. Collisions with large
objects from space, such as those thought to mark the ends of
past geologic eras, are possible. Fortunately, these events are
becoming less likely as the age of the solar system increases.
However, the very-long-term future of the universe is not
clear.
Three Possible Futures
The universe seems to have three possible futures. Some scientists
propose that the expansion of the universe may be
slowing due to gravity. However, it is possible that there is
not enough gravity to stop the expansion. In this case, the
universe will continue to expand without limit. Other scientists
propose that there may be enough gravity to just stop
the expansion, leading to a steady state. If the universe has
enough gravity to reverse its expanding phase, it could fall
back together in a very distant event that some astronomers
call the “big crunch.”
A good way to understand this is to consider a baseball
thrown straight up. Gravity brings the ball back to the
ground. However, if you could propel the ball fast enough, it
would continue upward into space and never return to Earth.
In recent years, astronomers have found evidence that the
universe is not only expanding, but that it is expanding at an
increasing rate. What force could work against the force of
gravity to cause this? It is as if you threw a ball up into the
air and it did not fall back to Earth. In fact, it is as if the ball
flew upward faster and faster with time. This would be surprising,
indeed. Astronomers find these observations just as
surprising.
Astronomers have named the mysterious cause of this accelerating
expansion “dark energy.” However, they cannot explain
it. Nor can they explain the source of gravitational force
that holds the rapidly spinning galaxies from breaking apart.
This force is attributed to the gravitational attraction of
“dark matter,” which astronomers think makes up about 90
percent of the matter in the universe. Dark matter and dark
energy, the mysteries of science just keep coming.