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Astronomy and particle physics race to replace Standard Model

A panel of Nobel Laureates discusses why astronomers and cosmologists are …

Data from the WMAP observatory confirmed predictions of inflationary cosmology

If energy issues seem to be attracting the attention of a lot of physicists, the Large Hadron Collider seems to be drawing the attention of many of the rest of them, including people in fields like cosmology, which deals with items on the opposite end of the size scale. In turn, the people working on the LHC and other particle detectors are carefully paying attention to the latest astronomy results, hoping they'll put limits on the properties and identities of the zoo of theoretical particles that need to be considered.

There are two reasons for this newfound unity in physics. If cosmology has become a part of elementary particle physics, as Nobel Laureate George Smoot put it at the Lindau Meeting, it's because we've found that "it's a continuum from quantum mechanics to clumps of matter to galaxies." The properties of the tiniest particles should dictate what the Universe looks like, but all the cosmological data is telling us there must be something in addition to what we know about, dark matter particles that we haven't yet identified. 

The second issue is that we know the Standard Model, which describes the properties of these particles, is wrong, but we're not sure what to replace it with yet, and it's entirely possible that astronomy and cosmology will provide key insights into this process.

The Lindau meeting featured an all-star panel that ran through some of the evidence that we could be on the verge of finding something big, in a discussion entitled "Dark Matter, Dark Energy, and the LHC." Smoot and his co-laureate John Mather, who won for the Cosmic Background Explorer, were joined by physicists David Gross, Carlo Rubbia, Gerard t'Hooft, and Martinus Veltman.

From particles to the Universe

The cosmologists kicked things off by discussing the evidence for dark matter. Smoot described how cosmologists can model a universe with dark matter, gas, photons, and neutrinos initially clumped, and observe how they spread. These models provide some specific predictions about things like the cosmic microwave background and other properties we can observe, with an accuracy of about one percent. And they give us a Universe that's only about 27 percent matter, most of it dark, along with lots of dark energy, which is pushing the Universe apart at an expanding pace

Nobody has a clue what dark energy might be, but dark matter is really coming in to its own. Saying, "I'm a measuring kind of guy," Mather discussed some of the ways that astronomers were pinning down the properties of dark matter. These include detailed measurements of some aspects of the cosmic microwave background left over from the Big Bang, and large-scale surveys of gravitational lensing, which will produce data on the quantity and distribution of dark matter. Mather also suggested that we can read the temperatures of X-ray emitting gas clouds, and this will let us weigh the galaxy clusters that hold the gas in place, providing an independent measure.

Initial attempts at most of these have already been done, which is what gives us a lot of our confidence in the existence of dark matter. But Mather also suggested it may be time to try a more exotic search, one for early-generation stars that lived unusually long because dark matter annihilations within the stars have prevented their gravitational collapse.

David Gross highlighted some confusing astronomical data that indicates an excess of gamma rays that may be the product of dark matter annihilations. Some studies haven't seen this, and the ones that have suggest very different dark matter properties. Nevertheless, Gross was optimistic we'd have that sorted out within a decade.

He was also convinced that there's something there to sort out, saying that astronomers had convinced him that not only does it exist, but it takes the form of weakly interacting massive particles, or WIMPs (Weakly Interacting Massive Particles).

The only one who appeared not to be convinced was Veltman, who called dark matter "bullsh*t," and said he thought highly of an alternative called MOND (Modified Newtonian Dynamics). As the rest of the panel tried to find out why, it became clear that Veltman had stopped paying attention to the field about five years ago, and wasn't up on the latest data. At this point, Gross described the bullet cluster as a clear demonstration of dark matter, while Rubbia referred to data related to the production of matter during the Big Bang; the panel eventually moved on.

What is this dark stuff, anyway?

So, if almost everyone is convinced that dark matter exists, what is it? The term WIMP could cover a lot of ground. Rubbia mentioned one possible WIMP: sterile neutrinos, a heavy version of the three familiar flavors of this particle. Rubbia said some of the recent results, like strange antineutrino masses and the confirmation of flavor oscillations have gotten a lot of people excited about the prospects of discovering a sterile neutrino.

The biggest problem is that there are no sterile neutrinos in the Standard Model. That in itself isn't so much of a problem; evidence has been piling up from these and other experiments that indicate that the Standard Model isn't a complete description of particle physics. But that also means that we've got no clear idea of what to replace it with, all of which makes predicting the precise properties of a sterile neutrino rather challenging. That, in turn, makes creating a detector to pick one up very challenging.

The other big alternative source of dark matter comes via supersymmetry, which postulates another complete set of particles that match the quarks, leptons, etc. that we already know about (it's a bit like having a full set of anti-particles, without the annihilation-upon-contact aspect). The symmetry works for particle identity, but breaks down for mass; the supersymmetric particles are much heavier than their regular counterparts, and that heft places the lightest supersymmetric particle right in the range expected for dark matter.

Some forms of supersymmetry could handle a number of other issues with the standard model and observational astronomy, but we'll go over those in more detail once we start our coverage of our visit to the LHC.

Fortunately, these heavy particles (assuming they are WIMPs) are well within the energy reached by the LHC, and they're obviously stable, since they seem to have lasted the lifetime of the Universe. So, if they exist, we should be able to detect them as they carry mass and energy away from collisions.

Taking odds on what we're going to see

With the possibilities laid out, the panel spent some time discussing what they felt were the likely outcomes from the experiments in the works. Gross said that he thinks we'll get a dark matter candidate particle before we know whether it's supersymmetric; if we somehow stumble onto supersymmetry first, then we'll have a dark matter candidate by default. t'Hooft mentioned it was possible that all the supersymmetric particles would require higher energies, but suggested the theorists who like the concept will go on quite happily, and wait for the next accelerator.

Veltman suggested the chances of seeing the Higgs boson, another target of the LHC, "are minimal" given the current data. Cryptically, he also suggested that we might see something that looks a lot like a Higgs, but isn't.

Rubbia felt the same way about the quark-gluon plasma that the LHC will produce by colliding lead ions. Scientists at Brookhaven's RHIC have claimed to produce a quark-gluon plasma that behaves as a perfect liquid and, although Rubbia called this work "beautiful," he felt that the material being observed had been misidentified. After RHIC's work, he now thinks that the LHC won't reach sufficient energies to make what he'd consider the real plasma.

Will the LHC spot evidence of extra dimensions required by string theory? Not according to Peter Gross, who termed it "an act of desperation" to need so many possible models before we can get them to spit out something that looks like our Universe, with all the fundamental physical constants having the right values. 

A specific constant—Einstein's cosmological one, which could explain dark energy—is also bugging a lot of people. The current estimates for the cosmological constant are all very small, almost (but not quite) zero. t'Hooft (and others) seems to think it would be more satisfying if it were simply zero, or a large enough number that it might be easier to find a relationship between it and some other constants.

In fact, t'Hooft suggested that the LHC will make our current list of fundamental constants much larger, and that it will take theorists many years to start reducing that list to a manageable number. But, he added, the more unexplained constants you have, the more interesting it should be.

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