Dark Matter, Dark Energy And The Shadow Universe

Mar 6, 2012
Originally published on March 6, 2012 5:24 pm

I have bad news for you. You don't matter. Not very much. Not really at all. The darkness, that's what matters.

Many folks have heard of Dark Matter and Dark Energy. Most folks, however, can't tell you anything about them. They're dark. They're lurking out there. That's about it.

They're too important to leave it at that. So, let's look at the "whys" and "wherefores" of the Dark Duo. With today's post, I'm going to begin this exploration with a simple fact and its cosmic (literally) interpretation.

Let's start with a very important distinction. Dark Matter and Dark Energy have nothing (as far as we know) to do with each other. The only thing they have in common is that evocative adjective "dark," which, for astrophysicists, simply means we can see an effect but we can't see the cause.

Astronomers see things moving when they look out at the universe. Using the laws of physics, they try to infer what drives those motions. There are only four known forces in the universe: gravity, electromagnetism and two forms of nuclear force. These are the only ways we know for stuff to push (or pull) other stuff.

On astronomical scales, gravity is the main actor. Ever since Newton and Einstein we have known how to relate gravity to the quantities of matter and energy present in any given environment.

Over the last few decades something remarkable has been observed. The matter and energy we can see is moving too fast to be driven by its own gravitational force. Based purely on the motions of galaxies, astronomers have been forced to infer the presence of matter that's undetectable with light-gathering telescopes.

Likewise, based on the motions of the universe as a whole (and its structure), astronomers have been forced to infer the presence of energy that cannot be directly detected.

Dark Matter is a form of mass (some new kind of particle) that simply does not emit light. Dark Energy is some kind of cosmos-filling field capable of doing work that is also invisible. But Dark Matter and Dark Energy are very different.

They were "discovered" in very different ways. The only connection they share is their ability to move stuff we can see around in detectable ways, while remaining resolutely in the shadows themselves.

They're the definition of mysterious.

Now for the important part (as if what I just described wasn't important enough).

The "dark universe" — the sum of Dark Matter and Dark Energy — is pretty much THE universe. Observations put the dark universe at about 95 percent of the total. That means our kind of matter and energy — the stuff you see, touch and experience every day — is a mere 1/20th of the cosmos.

If this were an election, we would have no say in the congress of creation. Remember that next time you see an image of a beautiful spiral galaxy. What you see is not the galaxy at all. What you see is some luminous chaff floating at the center of a vast, dark ocean. So it is not that the universe is big and we are small that robs us of significance. We are barely part of the universe at all. That is why we don't matter.

And yet ...

As far as we can tell, the dark part of the universe is immune to "clumping" on our kinds of scales (i.e. stars, planets, people). Left to its own devices, it seems the dark universe would never be able to make something as remarkable as life. Now that is truly something worth a moment's reflection.

Perhaps the dark universe makes us matter more than we ever realized.

You can keep up with more of what Adam Frank is thinking on Facebook and Twitter. His new book is About Time: Cosmology and Culture at the Twilight of the Big Bang.

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