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Excuse Our Coups! (Betty Bowers)

It's been a bit over two weeks since the debacle at the Capitol happened, and I must confess that I…See More
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14:00 19 January 2013
by Lisa Grossman

Earth is constantly crashing through huge walls of dark matter, and we already have the tools to detect them. That's the conclusion of physicists who say the universe may be filled with a patchwork quilt of force fields created shortly after the big bang.

Observations of how mass clumps in space suggest that about 86 per cent of all matter is invisible dark matter, which interacts with ordinary matter mainly through gravity. The most popular theory is that dark matter is made of weakly interacting massive particles.

WIMPs should also interact with ordinary matter via the weak nuclear force, and their presence should have slight but measurable effects. However, years of searches for WIMPs have been coming up empty.

"So far nothing is found, and I feel like it's time to broaden the scope of our search," says Maxim Pospelov of the University of Victoria in Canada. "What we propose is to look for some other signatures."

Bubbly cosmos

Pospelov and colleagues have been examining a theory that at least some of the universe's dark matter is tied up in structures called domain walls, akin to the boundaries between tightly packed bubbles. The idea is that the hot early universe was full of an exotic force field that varied randomly. As the universe expanded and cooled, the field froze, leaving a patchwork of domains, each with its own distinct value for the field.

Having different fields sit next to each other requires energy to be stored within the domain walls. Mass and energy are interchangeable, so on a large scale a network of domain walls can look like concentrations of mass – that is, like dark matter, says Pospelov.

If the grid of domain walls is packed tightly enough – say, if the width of the domains is several hundred times the distance between Earth and the sun – Earth should pass through a domain wall once every few years. "As a human, you wouldn't feel a thing," says Pospelov. "You will go through the wall without noticing." But magnetometers – devices that, as the name suggests, measure magnetic fields – could detect the walls, say Pospelov and colleagues in a new study. Although the field inside a domain would not affect a magnetometer, the device would sense the change when Earth passes through a domain wall.

Dark matter walls have not been detected yet because anyone using a single magnetometer would find the readings swamped by noise, Pospelov says. "You'd never be able to say if it's because the Earth went through a bizarre magnetic field or if a grad student dropped their iPhone or something," he says.

Network needed

Finding the walls will require a network of at least five detectors spread around the world, Pospelov suggests. Colleagues in Poland and California have already built one magnetometer each and have shown that they are sensitive enough for the scheme to work.

Domain walls wouldn't account for all the dark matter in the universe, but they could explain why finding particles of the stuff has been such a challenge, says Pospelov.

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From starts with a bang, good graphs showing how dark matter is important to explaining the gravitational observations

The leading candidate for the first scenario is the addition of some type of dark matter to the Universe, while the second scenario requires MOND, MOG, the relativistic TeVeS, or some similar type of modification. I’ve written about these possibilities manymanymanymany timesbefore, but there’s one very simple test that you can apply to tell which of these two possibilities are consistent with our actual Universe. A test that — spoiler — the advocates of #2are terrified of bringing up in their own papers.

Image credit: Mark Subbarao, Dinoj Surendran, and Randy Landsberg for the SDSS team.

You look at the Universe on the largest scales. Not on the scale of stars, nor at individual galaxies, not even at clusters or supercluster of galaxies, but at the entirety of the visible Universe. Those scales, the largest possible scales.

Because on those scales, there’s no denying that gravitational forces dominate, and the other forces are all but insignificant. If you can accurately measure how the Universe clusters on the largest scales, you can compare the predictions of a general relativity + dark matter-dominated Universe with what you observe, as well as a no-dark-matter + modified gravity Universe, and see what you get. Below is the prediction of the standard ΛCDM cosmological model (GR + dark matter).

Image credit: Michael Kuhlen, Mark Vogelsberger, and Raul Angulo.

In particular, it’s the largest scales — all the way on the left — that are the best and most robust test of these two scenarios. While many other variables enter into play (and the uncertainty rises) the farther to the right you’re willing to go, the largest scales are the simplest and most straightforward test of which of these possibilities is correct. Why’s that?

On these scales, simulations are not required, and the way the largest-scale structures in the entire Universe are distributed/correlated (which is what the Power Spectrum measures) is known, exactly. So what do we see when we look out at the Universe on these largest scales, and compare with the predictions of these different scenarios?

Courtesy of Scott Dodelson, I present to you the one graph that incontrovertibly settles the matter, at least for the time being.

Image credit: Scott Dodelson, from http://arxiv.org/abs/1112.1320.

Those red points (with error bars, as shown) are the observations — the data — from our own Universe. (Courtesy of the Sloan Digital Sky Survey.) The black line is the prediction of our standard ΛCDM cosmology, with normal matter, dark matter (in six times the amount of normal matter), dark energy, and general relativity as the law governing it. Note the small wiggles in it and how well — how amazingly well – the predictions match up to the data.

Now look at the blue curves: these are models with no dark matter. The dotted blue curve is what you’d get in a no-dark-matter Universe that abided by general relativity. The “wiggles” you get are far too large in amplitude, and the spectrum fails to rise on progressively smaller scales as required by our Universe. The solid blue curve is what TeVeS — the relativistic version of MOND — predicts. It can raise the overall amplitude of the Universe’s power to an appropriate level at a few select points, but the spectrum is all wrong. Ruinously wrong. It’s not even close to viable.

And until those in favor of modifying gravity can successfully predict the large-scale structure of the Universe the way that a Universe full of dark matter does, it’s not worth paying any mind to as a serious competitor. You cannot ignore physical cosmology in your attempts to decipher the cosmos, and the predictions of large-scale structure are some of the most basic and important predictions that come out of physical cosmology. And that’s why the Universe needs dark matter — and not MOND, MOG, TeVeS, or any other dark-matter-free alternative — in one all-important graph!

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