I'm Thad Szabo, and I teach Physics and Astronomy
at Cerritos College.
If we look at the expansion of the universe,
at first it was thought that, as things are
expanding while objects have mass, the mass
is going to be attracted to other mass, and
that should slow the expansion. Then, in the
late 1990's, you have the supernova surveys
that are looking deeper into space than we've
ever looked before, and measuring distances
accurately to greater distances than we've
ever seen before. Something really surprising
came out, and that was what we'll now use
"dark energy" now to explain, and that is
that the acceleration is not actually slowing
down - it's not even stopped. It's actually
getting faster, and if you look at the most
distant objects, they're actually moving away
from us and the acceleration is increasing
the acceleration of expansion. This is actually
a huge result.
One of the ideas of trying to explain it is
to use the "cosmological constant," which
is something that Einstein actually introduced
to his field equations to try to keep the
universe the same size. He didn't like the
idea of a universe changing, so he just kind
of cooked up this term and threw it into the
equations to say, alright, well if it isn't
supposed to expand or contract, if I make
this little mathematical adjustment, it stays
the same size.
Hubble comes along about ten years later,
and is observing galaxies and measuring their
red shifts and their distances, and says wait
a minute - no the universe is expanding. And
actually we should really credit that to Georges Lemaître, who was able to interpret Hubble's
data to come up with the idea of what we now
call the Big Bang.
So, the expansion's happening - wait, it's
getting faster. And now the attempt is to
try to understand how dark energy works. Right
now, most of the evidence points to this idea
that the expansion will continue in the space
between galaxies. That the forces of gravity,
and especially magnetism and the strong nuclear
force that holds protons and neutrons together
in the center of an atom, would be strong
enough that dark energy is never going to
be able to pull those objects apart.
However, there's a possibility that it doesn't
work like that. There's actually a little
bit of experimental evidence right now that,
although it's not well-established, that there's
a little bit of a bias with certain experiments
that dark energy may get stronger over time.
And, if it does so, the distances won't matter
- that any object will be pulled apart. So
first, you will see all galaxies recede from
each other, as space starts to grow bigger
and bigger, faster and faster. Then the galaxies
will start to be pulled apart. Then star systems,
then planets from their stars, then stars
themselves, and then other objects that would
typically be held together by the much stronger
forces, the electromagnetic force objects
held by that will be pulled apart, and then
eventually, nuclei in atoms.
So if dark energy behaves so that it gets
stronger and stronger over time, it will eventually
overcome everything, and you'll have a universe
with nothing left. That's the 'Big Rip' - if
dark energy gets stronger and stronger over
time, it will eventually overcome any forces
of attraction, and then everything is torn
apart.
