Daniel King's List: Dark Matter Research

• A scientific consensus has emerged that dark matter makes up more than 80 per cent of all the matter
• dark matter might not keep its identity secret much longer.
• What it's actually made of, nobody knows.
• Whoever wins the race will crack one of nature's most exasperating cryptic codes and plug some gaping holes in our grasp of how the universe evolved.
• "It's tremendous to have a competing theory because the best scientific work takes place when it's driven by competition,"
• There are now so many completely independent arguments that all point to dark matter - it's just overwhelming."
• So what is it? Frankly, nobody knows.
• One popular bet, though, is that dark matter is made of "weakly interacting massive particles" or WIMPs.
• Experiments at accelerator laboratories such as CERN on the French-Swiss border should already have manufactured them if they were lighter than 50 protons.
• "What we would dearly like is some indication that the theory is right by actually detecting some of this family of supersymmetric particles,"
• how on earth can you trap something as slippery as a WIMP?
• scientists might be able to sense a WIMP passing through chilled ultrapure wafers of the element germanium, already developed for the semiconductor industry.
• occasionally, though sheer luck, a WIMP might bump a germanium nucleus, making it rattle around. This would liberate a tiny amount of heat that cutting-edge sensors could pick up on the germanium's surface.
• The logical solution was to relocate the entire experiment deep underground, where overlying rock would block most cosmic rays but let the slippery WIMPs pass through.
• "If even a flake of your skin or a hair gets inside the detector, that would be bad."
• "Eat a banana and you've just taken in a lot of potassium,"
• cools the detectors down to an operating temperature of just one-twentieth of a degree above absolute zero. That makes the atoms in the crystals as still and motionless as possible.
• . Computers analyse the signals to distinguish an exotic WIMP from run-of-the-mill background particles,
• the Gran Sasso laboratory sits beneath around a mile of rock. This cuts down the interfering cosmic rays by a factor of about a million.
• "Gran Sasso is probably the best laboratory in the world for dark matter experiments,"
• XENON100
• is a cylinder just 30cm in width and height, containing xenon liquid and gas at -100°C. It's shrouded by copper, lead and polyethylene to block as many normal background particles as possible.
• Nonetheless, they plan to upgrade from 11lbs of target to 220lbs.
• they hope to up sticks to a lab in Sudbury mine in Canada, an active nickel mine more than a mile underground
• the Large Hadron Collider at CERN. It might actually create dark matter as it slams protons together at nearly the speed of light.
• If dark matter particles are born in its collisions, the telltale sign will be that some energy goes "missing"
• That would give the dark matter hunters some clues as to the mass of particles they are looking for.
• "It will be enormously surprising if these critical particles escape.
• My natural tendency is to say we'll find them within five years.
• Every month, just 20 of these would react with neutrinos to leave just 20 telltale argon atoms as evidence that Davis would have to find.
• The Nobel Foundation described his experiments as "considerably more difficult than finding a particular grain of sand in the whole of the Sahara desert".
• So the prime suspect for the dark matter is something like a neutrino, but it must be much heavier and much more sluggish.
• The dark matter hunters have no such guarantee. It's possible that the dark matter particles feel no force at all except gravity - in which case physicists have no chance of catching them at all.
• There is certainly no financial incentive. True, the technologies used to hunt dark matter might eventually have useful spinoffs for medical scanners or suchlike. But no one can think of any direct benefit of dark matter itself.
• "When I'm on a plane, sometimes people ask me - what's the point of discovering dark matter? And I'm speechless, because I can't tell him or her it's going to help find a cure for cancer,"
• . "To most people it probably sounds crazy. But I just think it's important to understand what nature is doing."
• They all agree that rivalry can only go so far. If any team detects a WIMP, they'll rely on their competitors as expert witnesses.
• the DAMA/LIBRA
• announced that they had discovered dark matter in 1998. But to this day, they have had trouble convincing people.
• The upshot is that the Earth's total motion through space varies over the year, being about 30 per cent faster in June than December.
• Calculations suggest that roughly 10 per cent more dark matter particles should pass through DAMA's detector in summer than in winter.
• the reason why the community gets a little bit upset with them is that they don't show all of their data,
• "If they're going to make a claim that what they're seeing is WIMPs, they have a responsibility to show us everything, warts and all. They don't. We would love to be able to prove once and for all whether DAMA is right, but we can't without their data."
• Still, deep underground, the search for dark matter will go on until there's no hiding place left.

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• Only recently has the existence of dark matter over much larger scales than even galaxy clusters been confirmed.
• Comparing this density with actual counts of stars, one finds that the number of observed stars falls short, by perhaps as much as a factor of 2
• There is at most an amount of dark matter in the disk equal to the amount of luminous matter. A more conservative estimate might place the amount of dark matter at about 25 percent of the amount of luminous matter.
• The dark matter in the disk most likely consists of very dim stars, such as white and even black dwarfs.
• The first real surprise emerges in the outermost parts of galaxies, known as galaxy halos.
• Here, there is negligible luminosity, yet there are occasional orbiting gas clouds
• which allow one to measure rotation velocities and distances.
• The rotation velocity is found not to decrease with increasing distance from the galactic center. This constancy of velocity implies that the galaxy's cumulative mass must continue to increase with the radial distance from the center of the galaxy, even though the cumulative amount of light levels off.
• over a length scale of about 1 megaparsec. Over this scale, 95 percent of the measured mass is dark.
• On the very largest scales, there are no longer any gravitationally bound objects. Yet the galaxies are not distributed perfectly uniformly: there remain small density fluctuations that have persisted since the very earliest epochs of the universe.
• One can even pinpoint the sources of the flows, since there are vast concentrations of matter that must be responsible. The nearest one has been dubbed the Great Attractor
• If real, it must consist of more galaxies than would be found in a dozen rich galaxy clusters.
• the density of matter in the universe should just equal the critical density at which the universe is closed.
• Alternatively, if we adopt the mass-to-light ratio of 300 measured on large scales as being a universal value, we would conclude that Omega is 0.2, far less than the value predicted by inflation.
• We have two choices. Either the dark matter consists of ordinary, baryonic matter, or else it consists of some more exotic form of matter.
• If the universe is at critical density, 90 percent of the matter in the universe must be nonbaryonic.
• There are so many neutrinos left over from the big bang that a neutrino mass of even 50 eV, or one ten-thousandth the mass of an electron, would suffice to close the universe.
• In our low-energy universe today, the lightest supersymmetric particle should still survive. It is expected to be the partner, in the sense of having a complementary spin, of the photon, and is therefore known as a photino. Its mass is expected to be 10 to 100 times that of the proton. The photino is uncharged and interacts very weakly with matter.
• the amount of dark baryonic matter is still most likely several times that in luminous baryonic matter
• One's first expectation might be that baryonic dark matter consists of burnt-out stars in the galactic halo, yet other forms, such as planets and black holes, are also possible
• It is most unlikely that baryonic dark matter can account for the closure density
• baryonic dark matter is a serious candidate for dark matter at least in galaxy halos, if not on larger scales.
• Two experiments reported in 1993 have found strong evidence for the existence of MACHOs. The technique used is gravitational microlensing.

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• the mysterious stuff comprising 85% of the universe's matter.