What if you threw a baseball up in the air
and it never stopped?
And then, not only did it defy gravity, it
started racing away faster and faster with
nothing propelling it.
That's exactly how astronomers felt in 1998,
except, instead of a ball, they found the
entire universe was racing apart faster and
faster by the instant.
So, why would the universe do that?
Our best theory is that there's something
invisible all around you--and even though
you've never seen it, it makes up 70% of the
universe.
It's called dark energy.
So, 20 years ago, by looking at supernovae,
astronomers noticed that the universe wasn't
just expanding outward--it was actually accelerating
faster and faster outward.
And they began to think, maybe space itself
is expanding.
But saying it just is special and expands
is kind of random.
It's like having magnets and seeing them attract
and just saying, "OK, magnets attract," and
not worrying about electromagnetism or the
fields involved.
Now, remember, in Einstein's view, spacetime
is basically this mesh that just sits there
unless energy or matter bends it.
And you might have seen videos or images of
the bending of that spacetime mesh that causes
things to attract.
But maybe there's a special kind of substance,
something invisible and undetectable that
pushes the spacetime mesh apart.
OK: Imagine that space is full of bubbles
that repel each other.
And whenever there's enough space for a new
bubble to form, it does.
And that bubble repels all the other bubbles.
And then bubbles keep popping into existence,
pushing each other apart, and pushing space
with it.
So, space and time probably isn't made of
bubbles, but on really small scales, you actually
do have particles popping in and out of existence.
They're called virtual particles, and they
live for a very short time before they annihilate
each other.
So maybe the energy of these particles is
what's causing the universe to expand.
But if you calculate the amount of pushing
from all the virtual particles that exist
in the standard model of particle physics,
you get too much pushing.
You actually get a trillion trillion trillion
trillion trillion trillion trillion trillion
trillion times too much pushing.
So, our current quantum theories of particle
physics can't explain observations of dark
energy.
And some people think we just need to add
something onto those quantum theories--something
like supersymmetry.
But other people think that we actually need
to change our theory of gravity--change general
relativity so, say, space itself expands without
worrying about these particles.
Either way, dark energy is even hard to imagine.
Even in our shows here at NOVA, we show some
beautiful things, but they're not what dark
energy would look like.
If you make dark energy, say, green, so you
could see it, it would actually look like
this.
It has the same density everywhere.
And if it started to expand, it would actually
do this.
You see it expanding?
Dark energy doesn't even get diluted because
it's embedded in space.
As space expands, you get more space, but
you get more dark energy with it, and it looks
the same.
For now, all we know is that the bigger the
universe gets, the faster it flies apart.
And one day, the distance between our Milky
Way and the neighboring galaxies will be growing
faster than the speed of light.
When that happens, even our best telescopes
won't be able to see those galaxies.
I say, let's get to know our neighbors while
we still can.
