Dark Matter Matters, Especially When You Can't Find It


There is a phantom in the machinery of the universe, and it evades even the best “ghost hunting” physicists.

Known for nearly 80 years, and for lack of a better terminology simply called “dark matter,” it still eludes detection to this day. Even our biggest particle accelerator, the Large Hadron Collider (LHC), apparently hasn’t been able to manufacture it.

Researchers at this week’s meeting of the American Association for the Advancement of Science (AAAS) reported that their Alpha Magnetic Spectrometer (AMS) on the International Space Station may yield new clues as to what dark matter is. This $1.5 billion “bug trap” experiment collects all kinds of exotic high-energy particles from deep space. Any unusual peaks in the count of particles monitored by the AMS might be the result of dark matter behavior. The AMS team are expected to release their results next month.

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What’s frustrating is, that whatever it is, dark matter is a lynchpin in modern astrophysics. It is the ghostly link between the infinitely small and infinitely large: particle physics and cosmology.

But unlike ghosts, dark matter is unequivocally real — like the character Griffin in The Invisible Man written in the 1897 by H.G. Wells. People see Griffin’s actions as he actually moves things around but never the body (at least until his death).

Dark matter’s gravitational pull is the glue of the universe, holding together galaxies and clusters of galaxies. Dark matter warps space like a funhouse mirror, stretching images of far-flung galaxies. Dark matter is also imprinted on the acoustic waves frozen in our snapshot of the 13.7 billion year-old universe, cosmic microwave background radiation.

Therefore, dark matter is not an illusion based on some lack of understanding of gravity, as some have suggested. “If solving dark matter calls for a modified theory of gravity I’ll eat my PowerPoint slides — and my laptop computer!” said astrophysicist Mike Turner, of the University of Chicago.

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In fact, “dark” is a misnomer says Harvard University theoretical physicist Lisa Randall because it does not interact with radiation. A dark object still absorbs or reflects a little light. A black cat eating licorice in a coal bin is dark matter.

So, let’s call the glue of the universe invisible matter, with a tip of the hat to 19th century poet Hughes Mearns:

“Yesterday, upon the stair; I met a man who wasn’t there; He wasn’t there again today; I wish, I wish he’d go away…”

Invisible matter particles were presumably cooked up when the newborn universe was a “quark soup,” 1/100,000th of a second after the Big Bang. The seething hot universe was essentially a nuclear reactor for making particles more massive than quarks.

One idea is that invisible matter was born out of a phase transition — like water vapor turning into droplets. It may have decoupled from normal matter to make such hypothetical particles as axions or WIMPs (Weakly Interacting Massive Particles). Normal matter and invisible matter may have transferred properties between each other in the primeval cauldron. Both forms of matter store the same amount of energy in the same volume, so they must have a kinship.

Today there is an ongoing do-loop between theories and experiments for invisible matter. One theory is that invisible matter particles occasionally collide with each other to make specific flavors of cosmic rays, anti-protons or gamma rays. And, all of this must be sorted out from the cacophony of high-energy radiation saturating the universe.

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Rather than waiting for Mother Nature, physicists are eagerly trying to home cook their own invisible matter in the LHC. This allows for the invisible matter egg-hunt to be done under controlled conditions.

But, like our Invisible Man, invisible matter particles can only be inferred from the behavior of observable particles created deep in the bowels of the LHC’s building-sized detector arrays.

Head-on collisions of clusters of protons accelerated to near the speed of light yield cascades of subatomic particles meticulously recorded by concentric rings of detectors. The subatomic shrapnel from the collision is inventoried by massive data processing. A deficit in the tally could mean that an invisible matter particle was created and promptly zipped through all the layered detectors without a trace.

But is evidence of absence really absence of something tangible? The same effect could be caused by an escaping neutrino (which in fact may account for some fraction of invisible matter) or simply bad data. Or maybe there is an unintended observer bias when looking at the data.

What’s more, only a fraction of the collision data deemed “most interesting” can be stored and the rest is thrown away. “I stay awake every night worrying we might be throwing out the baby with the bathwater,” said U.C. Santa Barbra physicist Claudio Campagnari.

And, can some conclusions be gleaned about invisible matter from a null result?

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The conventional wisdom is that a heavy “mother particle” breaks down into a cascade of smaller particles that include invisible matter. But this may require a big boost in power for the LHC, which, after all, is trying in inch a littler closer to the mind bogging energies that existed around the time of the Big Bang. By 2020 the LHC may be doubled in energy, and a brawny Super-LHC could come online after 2030. Fabricating invisible matter may have to wait until then, or it may never be achievable — at least to satisfy hard core skeptics.

Astrophysicists hold out the hope that invisible matter may coagulate inside stars, or supermassive black holes. Perhaps invisible matter collapsed into a dense disk in the heart of our Milky Way and aligned to the galactic plane. Such hypothetical objects would provide a “rich signal” for invisible matter, Randall pointed out.

So the ghost hunt goes on, and predictions among astrophysicists for nailing invisible matter range from having a solution within a few years to “someday.”

Most humbling of all is that invisible matter is not even strange because the atoms stars, planets and people are made of account for only 0.5 percent of the energy-mass budget of the universe. Therefore we are the oddballs if you take a holistic view of the cosmos.

In a sense we are the shadow universe. We’re like the tiny LED light bulbs strung out on a Christmas tree, and we are left wondering what the tree is made of. “You have to ask the question of why everything we see should be normal matter,” said Randall.

Image credit: NASA, CERN, J. Read & O. Agertz

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