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The Dark Universe

Dark matter ring. i i

hide captionA dark matter ring in a galaxy cluster.

M.J. Jee/Johns Hopkins University/NASA
Dark matter ring.

A dark matter ring in a galaxy cluster.

M.J. Jee/Johns Hopkins University/NASA

We are living through golden times. At least when it comes to cosmology and particle physics. Nothing is more exciting in science as when new technologies allow for the testing and advancement of theories. Sometimes, decades pass before new machines are capable of probing into new realms. But eventually, if the ideas are testable, their day comes. And then it’s either glory or the garbage can.

The history of cosmology during the past 100 years is a great example. Einstein was the first to propose a model for the cosmos, based on his freshly-minted theory of general relativity that attributes gravity to the curvature of space around a massive body. The year was 1917, and he had no reason to suppose that the universe was changing in time. So, he chose the simplest possible geometry for the cosmos, static and spherical.

Between 1917 and 1929, the year Hubble discovered that galaxies were moving apart at a rate that grew with their distance (the farther away the fastest they receded), many cosmological models were proposed, suggesting all kinds of alternative behaviors: In 1922, the Russian Alexander Friedmann proposed a time-varying universe, that could either expand forever or reach a maximum size and begin contracting until it reached a singular point. This alternation of expansion and contraction could, in principle, go on forever.

Einstein wasn’t very enthusiastic about Friedmann’s solutions. Only in 1931, when he visited Hubble at the Mount Wilson observatory, he finally conceded. He needed hard data to change his mind.

The next big episode happened in the late forties. In England, a trio of physicists had proposed a model where the universe didn’t really change in time: it was eternal. Their model was a reaction to the obvious corollary of Hubble’s discovery: an expanding universe was smaller in the past. Go back far enough and you reach a point when all matter was squeezed into a very small volume. Thus, you could argue that the universe had a starting point, a history. Echoes of genesis were too strong for many. So, the British trio proposed the “Steady State” model, where more matter is created in order to compensate for the dilution caused by the expansion. In the balance, the universe remains the same, in a steady state.

Meanwhile, George Gamow, a US-based Russian expatriate who studied with Friedmann, proposed what became known as the Big Bang model: the universe does have a history, and it started at a singularity in the past. Gamow wasn’t too interested in the metaphysical implications of his model. Together with Ralph Alpher and Robert Herman, he computed the approximate history of the universe, proposing that chemical elements where made in the primordial furnace moments after the bang: the cosmos as the ultimate alchemist. They also proposed that a hot universe should have left behind a sea of radiation, the remnants of the initial formation of atoms. (It was latter shown that only the lightest elements were made in primordial times. Stars are the true alchemists.)

In 1965, the radiation proposed by Gamow and collaborators was discovered. The Steady State model, a contender until then, had to be discarded. Once again, data decided which theory to pick, precisely as it should be in science.

Cut to the present time. We have two mysterious observations, still unexplained. First, that galaxies are shrouded in dark matter, a kind of material that doesn’t produce its own light, interacting only gravitationally (or very weakly otherwise) with ordinary matter (that is, matter made of normal stuff like protons and electrons). Second, that the universe is not only expanding, but doing so at an accelerated rate. The culprit has a name, “dark energy."

Dark matter, not being made of quarks and electrons, must be something new.  Because it contributes about 23 percent of the stuff in the cosmos, even neutrinos are not enough to do it. Dark energy, well, we really don’t know. All we do know is that it contributes some 73 percent of the total cosmic recipe. My bet is that it has some deep relation to the very nature of the vacuum.

Yes, it’s possible that the dominant component of our universe comes from emptiness.

In quantum theory, there is no such thing as empty space. Energy fluctuations capable of creating matter pop out of “nothing” and go back to nothing. This weirdness can be traced back to the uncertainty principle, which essentially says that nothing stands still; there is always some agitation. The world of the very small is in a perpetual dance of creation and destruction.

There are explanations for both dark matter and dark energy involving modifications of Einstein’s theory. At this point, they don’t seem very likely. And there are explanations that are related to the existence of theories of everything, to extra dimensions of space, and to a symmetry called “supersymmetry,” that mixes particles of matter and particles that transmit forces. Or, it could be something completely different and unexpected.

In science, it’s exciting not to know.

We need data to decide. And data is coming soon. Both the Large Hadron Collider, the giant particle accelerator in Switzerland, and a slew of new cosmological missions will help us see the way.

After all, experiments without theories are lame and theories without experiments are blind. I think Einstein would have agreed.

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