> Tue, 10 Apr 2001
> I am currently a junior at Northwestern State University in 
> Natchithches, Louisiana. I am taking a Physical Science II class, and 
> the professor has addressed us with the question "What is dark energy 
> and what does it have to do with the universe?" I would like to know 
> can you help me with this question? If you can supply me with the 
> answer to those questions, I would greatly appreciate it. Your help is very 
> much needed.
> 
> Thanks in advance,
> Saunya



Hi, Saunya, and thanks for the question.

We believe that the universe is expanding
due to the Big Bang which happened about 12 billion years ago.
Using Einstein's equations of general relativity we can write down
other equations explaining how the universe grows with time after this
Big Bang.

It turns out that there is room in the equations for
an extra parameter which was first just believed to be a
trick of the mathematics.  This extra parameter was called
the "cosmological constant".  If it should be real and not just
some mathematical trick, it would create negative gravity -- meaning
that it would cause the fabric of the universe to repel against itself.
Now, this would be opposite to the gravity we know of, which 
attracts, binding people to the earth and the earth to the sun
and the sun to the galaxy and all the galaxies together.

If there were no real "cosmological constant", the expansion
of the universe would slow down since all of the galaxies
are gravitational attracted to each other and will pull on each
other.  This would be like putting on the brakes and slowing
down a car.  But, if we add the effects of the supposed cosmological
constant and its repulsive gravity, things would change.
At some point the braking effect of all of the gravity of
the universe would start to be overcome by the pushing of the
cosmological constant, or Dark Energy.  This would be like trying
to stop a car with brakes, but at the same time having the
gas pedal pushed down a little bit.  Gravity of the galaxies 
tries to slow the expansion down, but the Dark Energy tries
to speed it back up.  At some point, as the gravitational
brakes wear down, the push of Dark Energy would take over!

Is this happening?
As we look into space, the further away we look, the further
back in time we are looking actually, since it takes a certain
amount of time for light from each star to propagate through space.
So, as we look at far away stars we are actually looking at the
history of the universe.  By using supernovas in distant space
as a standard measuring stick, we can look at the history of
the universe and see how fast things were expanding at different
points in time.

It turns out that when we examine the measurements of how fast
things were expanding throughout the history of the universe
using the supernovas, we find that we we can make the data match
the theory very well by making the cosmological constant or Dark Energy
a real part of nature, not just a mathematical trick!

In other words, there is some property of space and time which
is described by this cosmological Dark Energy which Einstein
discovered in his equations.  The expansion of the universe was
slowing down, but recently (over the last billion years) it
is starting to speed back up!

The universe keeps getting stranger...

Keep questioning!
Michael Scott Armel
CfPA