You know something?
Science is pretty amazing.
For thousands of years, people have wondered
about the ultimate building blocks of the
cosmos.
And, in the 20th and 21st century, we’ve
made some incredible progress.
For instance, if you take a bunch of quarks-
maestro, some quarks please- that’s good.
And- you take some leptons- leptons, please-
that’s good- oh, wait a minute, a little
bit more- perfect.
You take these quarks and leptons, and you
shake them up, you can make atoms.
Atoms like the ones you see here make up all
of ordinary matter, from me, to you, to the
most distant galaxies.
In fact, our best estimates tell us that there
are about 10 to the 80, that’s a one, followed
by eighty zeros, of atoms in the universe.
They’re all basically the same and we understand
them very well, using chemistry and nuclear
physics.
However, I have some humbling news for you.
If you add up all of the matter tied up in
stars and planets, it only amounts to half
a percent of the matter and energy in the
universe.
Even if you include the hydrogen gas between
the stars and galaxies that is invisible to
ordinary light, you still have only about
five percent.
Five percent!
So what is the other 95% and how is it that
we’ve missed finding it?
Scientists think that the universe is composed
of three different substances.
5% is ordinary matter, 27% is a substance
called dark matter and 68% is called dark
energy.
In another video, I talked about why we believe
in this substance called dark matter.
In this video, I want to tell you about dark
energy.
First, I should tell you that even though
dark matter and dark energy have similar names,
they are really very different things.
Scientists have long known for a long time
that the universe is expanding and have called
that process the Big Bang.
From what we understood about gravity, it
seemed clear that the expansion of the universe
should be slowing down.
After all, gravity is an attractive force.
If I throw this ball up, gravity will pull
the ball back downwards.
Its motion will slow down.
What we didn’t know was how gravity would
determine the ultimate fate of the universe.
Was the universe going to expand forever without
ever stopping?
Expand and stop at some unfathomable distant
time in the future?
Or expand for a while, then have the expansion
overcome by gravity and have the universe
collapse in some sort of big crunch?
Nobody knew the answer and the debate raged.
In order to resolve the question, we needed
to understand the expansion history of the
universe.
And to do that, astronomers used a class of
supernovae, which is the explosion of a dying
star.
This kind of supernovae was very well understood.
If you saw one, you knew the brightness of
the explosion.
However, just like a distant candle appears
dimmer than a close one, so too it is with
exploding stars.
By knowing the intrinsic brightness of the
explosion and how bright the star appeared
in the telescope, you could work out how far
away it was.
That’s the first measurement.
The second measurement of distance uses the
same idea as a train whistle.
That same drop in pitch that you hear as the
train passes and begins to move away from
you also shows up in the study of distant
stars and galaxies.
Stars moving towards you appear bluer than
the same star when stationary, while stars
moving away from you appear redder.
The faster the star moves away from you, the
redder it appears.
Since distant galaxies appear reddish, they
are moving away from us.
In addition, we can relate their distance
and their velocity.
If galaxies a certain distance away are moving
with a particular velocity, galaxies twice
as far away are moving with double the velocity,
and so on.
In this way, we can measure the color of the
distant galaxies and determine their velocity
and then work out their distance.
It’s all a bit tricky, but these two methods
of measuring distance are not very controversial
and they should agree.
So, in 1998, two experiments applied these
techniques and looked at the most distant
supernovae ever.
And they found that supernovae were dimmer
than predicted by the expansion of the universe.
This means that the stars were farther away
than expected, but that meant something even
more shocking.
It meant that the expansion of the universe
wasn’t slowing down, it was speeding up!
That was really a mind-blowing observation.
From what we knew about gravity, the expansion
of the universe should have been slowing down.
We didn’t know the details, but the slowing
down seemed assured.
And yet that’s not what the data said.
In physics, data is king.
If an idea disagrees with an accurate measurement,
the idea is wrong.
So this meant that in order for the expansion
of the universe to be getting faster, there
had to be some form of gravity that was repulsive.
It turns out that Einstein once postulated
a repulsive form of gravity in his equations.
This was because he knew that ordinary gravity
would make the universe contract and he needed
some kind of repulsive gravity to overcome
the attraction.
However, when the universe was found to be
expanding in the 1920s, Einstein took that
extra form of gravity out of his equations.
He even called it his biggest blunder.
Imagine if he hadn’t done that…
I mean, the dude could have been famous!
Now- it seems that we need a repulsive kind
of gravity to explain our observations.
So what is the source of this new form of
gravity?
It turns out that if the universe has an energy
field of the right kind, it can make the expansion
of the universe accelerate.
This form of energy is now called dark energy.
Now, to be honest, we aren’t 100% sure about
this dark energy hypothesis, although there
are now many measurements that support the
idea.
Even though dark energy is the most popular
explanation for the expansion mystery, other
suggestions have been made.
While dark energy is a constant energy density,
another idea called quintessence is also a
contender.
Quintessence is an energy field that varies
in time, and there are several ideas in contention.
We need to understand that the observation
of the accelerating expansion of the universe
is only about 15 years old.
It took a little while to assimilate the discovery
and then many years to design and build new
facilities to study it better.
Over the next couple of years, several new
observatories will begin operations to explore
this surprising discovery.
I don’t know what the final answer will
be, but I do know that any time you don’t
understand 95% of something, that somebody
will figure it out.
I won’t ask you to be patient, because,
well, it’s hard for me too.
But a better understanding of this incredible
mystery is just only a matter of time.
