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Some Of Universe’s Missing Mass Found

Tiffany Tsai |
May 30, 2011 | 7:04 p.m. PDT

Staff Writer
If you look out into space on one of the rare unpolluted nights in Los Angeles, you may see bright pinpoints punctuating an expansive shield of darkness.  Presumably, even in the darkness there are objects.  You cannot see these objects, however, because they’ve sidestepped the glow of other stars.

But imagine for a moment that you obtain a hypothetical, infinitely luminous flashlight, and you shine it into all the spaces between the stars.  Some of the hidden objects now come into sight as light bounces off them and back to your eye or telescope, but others let the light pass by as if they weren’t even there.  It’s like when Harry Potter dons his invisibility cloak and Snape can see through to the wall behind Harry.

Welcome to the world of dark matter.

One of astronomy’s deepest riddles, where dark matter hides, has now partially come to light.  On May 23, 2011, Monash University in Australia announced that an aerospace engineering and science student found dark matter tucked away in filaments of galaxies.  Amelia Fraser-McKelvie, 22, is an undergraduate working with physicists Jasmina Lazendic-Galloway and Kevin Pimbblet in a summer holiday internship.

The Sydney Morning Herald printed this statement by Fraser-McKelvie:

“If we're looking very, very long distances from Earth we're detecting mass, but if we're looking closer to Earth we only see about half the mass that we're expecting to see.  This is what is called the missing mass problem…People have theorised that this mass has settled in filaments that extend between clusters of galaxies, so we tested and confirmed this prediction by detecting it in the filaments.”

“Filament” might sound like scraggly radiative dandruff interspersed throughout a solar neighborhood, but galaxy filaments are actually the largest structures that astrophysicists have identified. Also known as great walls, filaments are strings of galaxies that connect larger galaxies.  If galaxy complexes were neurons in the brain of the universe, large galaxy clusters would be the sparse but heavy nuclei, and filaments the long, spindly neural axons trailing from cluster to cluster.

Fraser-McKelvie studied the X-ray counts emitted by different filaments, which she translated into the surface brightness and electron densities of the filaments.  Initially she thought that her data did not indicate any dark matter, but a reanalysis by Lazendic-Galloway, an X-ray astronomy expert, proved otherwise.

The Monash team hails from a relatively short intellectual family tree, one that begins with Fritz Zwicky’s experiments in the early 1930s.  Ever since Isaac Newton, scientists have known that two bodies, whether they are sun-sized plasma spheres or pint-sized Chihuahuas, exert a gravitational force on each other.  Increasing the mass of either body causes a proportional increase in their mutual attraction, while separating them makes their attraction flag.

This concept, when applied to planetary orbits, leads to a relationship between the orbital velocity of a galaxy’s elements and how massive the galaxy is.  Zwicky tried to calculate the mass of the Coma galaxy cluster in two ways: first, he applied the gravitational principle to deduce mass by orbital velocity, and then he added up the light output of the visible bodies to deduce mass by luminosity.

The answers didn’t match up.

Instead, the mass he calculated using the first method produced a number about ten times larger than the galaxy’s luminosity would suggest.  Thus, Fritz postulated a mysterious “dark matter” that was transparent to everything except gravity.  In the Harry Potter analogy, most wizards and ordinary people cannot see Harry in his cloak, but Mrs. Norris with her feline sharpness discerns him in the dark, and so does Mad-Eye Moody with his magic eye.  Mrs. Norris and Moody represent the gravitational force.

 UCLA Physics)
UCLA Physics)
In 1972, Vera Rubin presented conclusive evidence for the existence of dark matter.  Based on velocities of interstellar matter orbiting around galaxies, she showed that mass is more evenly spread out than photographs such as the one to the right suggest.

However, not all dark matter is equally dark.  The type investigated by Fraser-McKelvie and her mentors is baryonic dark matter, which light—or electromagnetic radiation, in physics parlance—“sticks” to.  Previously, scientists had speculated that baryons were all secreted away in Massive Compact Halo Objects (MACHOs) like brown dwarfs, while non-baryons existed in Weakly Interacting Massive Particles (WIMPs).  The filament theory is something of a breakthrough.

 “I cannot underscore enough what a terrific achievement this is. We will use this research as a science driver for future telescopes that are being planned, such as the Australian Square kilometre Array Pathfinder, which is being built in outback Western Australian,” Pimbblet told the university’s news service.

Australia, having elected a scientific-frontiers reputation over the cowboys-shooting-kangaroos image some time ago, pours the majority of its education money into the physical sciences, according to LSE.  Reuters ranked the country ninth internationally for scientific output during 1998-2008, just behind the Netherlands and in front of Taiwan.  Dark matter research is in its infancy in Australia and elsewhere, though, with models still dominating over actual observations. 

“What is needed now is an accurate determination of laments’ electron densities and plasma temperatures from spectral fitting to provide a solid comparison with the models, which will then provide more realistic predictions for the missing baryon problem,” the researchers wrote in their paper, which was published in Monthly Notices of the Royal Astronomical Society this April.


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