Shane’s Drivel

There’s a pretty slick uber cool series on The History Channel called The Universe. The show is actually in its second season and its available on iTunes and DVD. They do a good job of describing complex mysteries in language that’s easy to understand.

Tonight, I watched their episode on dark matter and dark energy. Dark Energy is perhaps one of the most shocking discoveries of the last century. Einstein first postulated that dark energy must exist, but then wrote off his idea as preposterous because this concept just seemed so bizarre. He called it his greatest blunder. But 80 years later, science proved Einstein was right.

Dark Energy

Imagine throwing a baseball into the air. The ball goes up, slows down, than falls to the ground. Imagine the baseball going up, perhaps slowing down and perhaps not, and then accelerating indefinitely into space. That would blow away everything you thought you thought understood about gravity. Well, this is what’s happening to the universe.

Trust me on this – techniques exist to look into space, see galaxies, and determine how fast they are moving towards us or away from us. Some galaxies out there are a lot farther away from us than others. When we see a galaxy a million light years away, that means we’re observing what that galaxy looked liked a million years ago (and we won’t know what it looks like right now until a million years from now).

In 1998, astronomers showed that galaxies closer to us appear to moving faster than galaxies farther away from us. And since galaxies closer to us are a much more recent picture of time than galaxies a lot farther away, this means galaxies must be accelerating. The same observations also showed that everything in the universe is moving farther and farther away from each other. So the big shocker is that the universe is expanding and it is doing so at a continuously accelerating pace.

Why? Nobody knows. Some form of energy must be doing it, and that energy has been named “dark energy” simply because we can’t see it and don’t understand much about it.

Dark Matter

Yeah, that’s kinda deep. And it might not make much sense if you can’t grasp that bit about proving that galaxies are accelerating away from each other. Well, the next topic in the show is a lot easier to understand.

Imagine swinging a ball on a string around in a circle. The more you pull in the string, the faster the ball moves around in a circle. The longer you make the string, the slower the ball moves around in a circle. Planets revolving around the sun behave the same way.

Gravity is that string. Gravity is what prevents the Earth moving around the Sun instead of hurling away into space. The harder gravity pulls, the faster planets will revolve around the Sun. Planets closer to the Sun make their orbits a lot faster than planets farther away from the Sun.

Another factor is mass. If two planets were the same distance from the Sun, the larger planet would orbit faster. But hold that thought, we’ll come back to it in a minute*.

OK, the big eureka moment on dark matter happened when we looked closely at other galaxies. A galaxy is similar to a solar system on a really big scale – there’s a huge mass in the center and stuff revolves around it. And as with a solar system, we’d expect stuff on the fringes to revolve slower than stuff closer to the center, right? Bzzzt! Wrong!

Observations show that stuff swirling around galaxies is revolving around the galactic center at a constant pace. Stuff on the edges moves the same speed as stuff in the middle. Whoa. So much for our understanding of gravity – unless the stuff on the outside is way, way more massive* than we thought.

Further observations showed the stuff on the edges didn’t “look” big enough to have the mass necessary to revolve at the speed that its clearly revolving at.  Stuff that size just can’t revolve at that speed at that distance. It just can’t.

Unless there’s more mass there than we think. And there is. Since we can’t see it, we call it “dark matter”. Around the edges of every galaxy, there must be a halo of hugely massive “stuff” that we call dark matter. That makes for a good theory, but guess what – the existence of such dark matter has actually been proven.

Gravity can bend light. Trust me, it can. When planet moves around the sun such that the sun should be blocking our view of the planet, we can still see it for a time – because the sun’s gravity has bent light rays traveling from that planet to us. Picture a beam of light bending partially around the sun so that we could see part way around the sun.

Same thing happens on a galactic scale. We can see stuff that should be blocked out by a another galaxy that’s in our way. And by observing how much the light bends we can conclude how much gravity was required to bend the light. And this concept as been used to prove there’s a halo of unseen mass around the edges of every galaxy.

Neat, huh?