Dark energy, dark matter

(Photo by Lauro Roger McAllister)

We’ve known since 1998 that 96 percent of the cosmos is made of unknown stuff. Not merely unknown as in “We know nothing about virtually all of the 100 billion planets in our galaxy”; this ignorance goes much deeper. Essentially, it’s that 96 percent of the universe seems composed of entities that are neither some form of light, nor any of the 92 elements or their constituent particles.

A quick inventory of the known composition of the cosmos reveals the most common items to be photons: bits of light of various kinds, from simple particles of orange to exotic powerful gamma rays. The next most abundant item is the tiny particle called the neutrino. A trillion of them pass through each of your fingernails each second. (They don’t actually have a preference for fingernails; you know that.)

Neutrinos weigh almost nothing, and they don’t clump together to form anything. Since they don’t interact with matter in the normal way, they simply fly through stuff as if it isn’t there. Neutrinos keep passing through the entire Earth – it takes them each about 1/20th of a second to do this –then continue on as if nothing happened. They actually seem pretty useless.

Now we get to more substantial subatomic particles like electrons and quarks: Electrons don’t combine with anything, not even their fellow electrons. But quarks do, and always come in groups of three. They form all the stuff in every atom’s nucleus. All together, the observable universe contains a lot of these particles: a one followed by 80 zeroes.

The three quarks forming a proton can be orbited by a single electron, and now you have the most common atom in the cosmos: hydrogen. This makes up most of your body and two-thirds of each glass of water. The second-most-common element is helium, the most renowned loner in existence. It combines with nothing. It plays no role in life.

The third most common element is oxygen, which bonds with just about anything. Naturally, it combines with hydrogen to form the universe’s most common compound by far: water. Since oxygen is rather heavy, seven-eighths of each glass of water are oxygen alone, by weight. The Moon is mostly oxygen, too – as is every golden retriever.

All these known entities comprise just four percent of the cosmos. The rest is mysterious stuff. First there’s dark matter, which is invisible, but has gravity. We don’t know what it is, but it makes our galaxy spin in a strange way and glues groups of galaxies together. It’s possible that it doesn’t exist, if (and only if) gravity behaves differently at great distances, when it’s weak, than it does close-in, when it’s strong. Some think that’s likely, but the majority of astronomers brush that off as far-fetched.

Finally we come to the universe’s main component: dark energy. That’s our label, our term, for a total mystery. We know only one thing about it: It’s repulsive. There’s a joke there, but I’ll resist. What this means is that it’s an anti-gravity force. It’s making the cosmos expand faster and faster.

This underlying energy also seems responsible for nonstop particles and anti-particles springing briefly into existence in every tiny piece of space, then vanishing again. It probably caused the Big Bang. No one knows what it is. Will it ever reverse itself, so that the cosmos will then start contracting? Does the universe oscillate, and breathe in and out like the old Hindu legend of the breaths of Brahma? No one knows.

That’s the bottom line when it comes to the universe: It’s mostly unknown substances, overwhelmingly dark matter and dark energy. And we didn’t even get started with the stuff “over the horizon,” the things receding faster than light. Those represent ten thousand times more material than the stuff we see, or else infinitely more.

Does all this make you feel small? It doesn’t do that to me. I don’t feel small – just stupid.


Want to know more? To read Bob’s previous “Night Sky” columns, visit our website at www.hudsonvalleyalmanacweekly.com/category/columns/night-sky/.


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