>>MORDECAI-MARK MAC LOW: My name is Mordecai-Mark Mac Low,
and my position is curator of astrophysics.
In the last century, we have learned that
the universe is expanding and that the stuff
that it is made of is primarily not normal
matter. It's primarily something that we have
called "dark matter" and something else that
we are calling "dark energy." What do we know
about dark energy? So, the entire universe
is expanding.
[WALTZ MUSIC]
MAC LOW: There's a lot of mass in it.
If the density is above some threshold, the
universe should go out and come back in and
the universe will end in fire. Conversely,
if the density is under that threshold, the
universe will keep going out ever decelerating
and expanding and it will end in ice. So people
have tried to determine what the behavior
of the universe actually was. Was it decelerating
slowly, or so quickly that it would turn around
and crunch? By the 1990s, by looking out to
great distances and measuring the velocities
of the galaxies they found there, people had
found a way of measuring the deceleration
and they did this by using a certain kind
of supernovae called "Type 1A Supernovae."
These Type 1A Supernovae always have the same
brightness. Or at any rate, can be calibrated
to measure their brightness. So, looking for
these supernovae out across the universe allowed
a measurement of how fast the universe was decelerating.
And a graduate student whose-
this project was his dissertation, finally said,
"Let's put them together and find
out how fast the universe is decelerating."
So he takes all his measurements, calculates them,
measures the rate of deceleration, and must've got something wrong.
Because he gets a negative deceleration,
otherwise known as acceleration. Which nobody
asked for, nobody expected, what's going on
here? Well, obviously the first answer was,
"Oh, I made a mistake in my calculations."
So, he goes and shows it to his thesis adviser
and the whole group,
and they all busily repeat the calculations, say,
"There, there. We'll take care of this." But
of course it doesn't go away and eventually
they have to say, "Look, the universe isn't
decelerating. It's accelerating." And it turns
out that the equations of general relativity
naturally admit an accelerating universe filled
with energy, and that energy got a label stuck
on it, called "dark energy." There are about
six different measurements that all independently
measure the strength of the dark energy. So,
we know pretty well that it does exist and
now we're trying to measure was the strength
of the dark energy always the same as the
universe expanded? Because if it changed over
time, that would be a big indicator of what
kind of thing it might be. Satellites like
WFirst in the U.S. and Euclid in Europe, ground-based
telescopes are going to measure that fairly
precisely. Then we'll step back and see what
we've got.
[BUZZ, MUSIC]
