Posts Tagged ‘Dark Matter’


Dark matter is normally thought to form a spherical halo (illustrated in blue) around galaxies like the Milky Way. Two physicists suggest that dark matter could collapse into more complex structures.

BY EMILY CONOVER

Clumps of dark matter may be sailing through the Milky Way and other galaxies.

Typically thought to form featureless blobs surrounding entire galaxies, dark matter could also collapse into smaller clumps — similar to normal matter condensing into stars and planets — a new study proposes. Thousands of collapsed dark clumps could constitute 10 percent of the Milky Way’s dark matter, researchers from Rutgers University in Piscataway, N.J., report in a paper accepted in Physical Review Letters.

Dark matter is necessary to explain the motions of stars in galaxies. Without an extra source of mass, astronomers can’t explain why stars move at the speeds they do. Such observations suggest that a spherical “halo” of invisible, unidentified massive particles surrounds each galaxy.

But the halo might be only part of the story. “We don’t really know what dark matter at smaller scales is doing,” says theoretical physicist Matthew Buckley, who coauthored the study with physicist Anthony DiFranzo. More complex structures might be hiding within the halo.

To collapse, dark matter would need a way to lose energy, slowing particles as gravity pulls them into the center of the clump, so they can glom on to one another rather than zipping right through. In normal matter, this energy loss occurs via electromagnetic interactions. But the most commonly proposed type of dark matter particles, weakly interacting massive particles, or WIMPs, have no such way to lose energy.

Buckley and DiFranzo imagined what might happen if an analogous “dark electromagnetism” allowed dark matter particles to interact and radiate energy. The researchers considered how dark matter would behave if it were like a pared-down version of normal matter, composed of two types of charged particles — a dark proton and a dark electron. Those particles could interact — forming dark atoms, for example — and radiate energy in the form of dark photons, a dark matter analog to particles of light.

The researchers found that small clouds of such dark matter could collapse, but larger clouds, the mass of the Milky Way, for example, couldn’t — they have too much energy to get rid of. This finding means that the Milky Way could harbor a vast halo, with a sprinkling of dark matter clumps within. By picking particular masses for the hypothetical particles, the researchers were able to calculate the number and sizes of clumps that could be floating through the Milky Way. Varying the choice of masses led to different levels of clumpiness.

In Buckley and DiFranzo’s scenario, the dark matter can’t squish down to the size of a star. Before the clumps get that small, they reach a point where they can’t lose any more energy. So a single clump might be hundreds of light-years across.

The result, says theoretical astrophysicist Dan Hooper of Fermilab in Batavia, Ill., is “interesting and novel … but it also leaves a lot of open questions.” Without knowing more about dark matter, it’s hard to predict what kind of clumps it might actually form.

Scientists have looked for the gravitational effects of unidentified, star-sized objects, which could be made either of normal matter or dark matter, known as massive compact halo objects, or MACHOs. But such objects turned out to be too rare to make up a significant fraction of dark matter. On the other hand, says Hooper, “what if these things collapse to solar system‒sized objects?” Such larger clumps haven’t have been ruled out yet.

By looking for the effects of unexplained gravitational tugs on stars, scientists may be able to determine whether galaxies are littered with dark matter clumps. “Because we didn’t think these things were a possibility, I don’t think people have looked,” Buckley says. “It was a blind spot.”

Up until now, most scientists have focused on WIMPs. But after decades of searching in sophisticated detectors, there’s no sign of the particles (SN: 11/12/16, p. 14). As a result, says theoretical physicist Hai-Bo Yu of the University of California, Riverside, “there’s a movement in the community.” Scientists are now exploring new ideas for what dark matter might be.

M.R. Buckley and A. DiFranzo. Collapsed dark matter structures. Physical Review Letters, in press, 2018.

https://www.sciencenews.org/article/clumps-dark-matter-could-be-lurking-undetected-our-galaxy

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Take a look around you, and in your mind’s eye, randomly wipe out all but a small fraction of what you can see. Pretend the vast rest of reality is there but invisible.

You’d probably like a device that helps you see much more of it.

Scientists working at CERN, the European Organization for Nuclear Research, have made some progress in that direction with the Alpha Magnetic Spectrometer (AMS), which has been riding aboard the International Space Station since 2011.

Physicists believe that mental exercise in blindness reflects the reality of our universe, only about 4% of which manifests as the kind of matter and energy we can perceive.

More than 70% consists of so-called dark energy, physicists say, and more than 20% is dark matter, neither of which humans can directly detect so far.

But scientists feel certain it must exist, partly because of the gravity it exerts on the visible universe.

This week, CERN scientists published an analysis of data from the AMS, which detects subatomic particles constantly bombarding Earth. They include exceedingly rare antimatter particles that can result from the breakdown of dark matter.
They are called positrons, also known as anti-electrons. They have the same mass as electrons, but electrons have a negative charge, and positrons have a positive charge.

Scientists believe dark matter collides, splitting into pairs of electrons and positrons, so the ability to examine positrons in detail could help in proving the existence of dark matter.

Positrons are produced in minute quantities in our corner of the universe, and mostly come flying our way from its far reaches, bundled up with gangs of other subatomic particles, mainly protons and electrons.

The flying particles bear the name “cosmic rays,” a misnomer given to them at a time when they were not as well understood.

The AMS project has analyzed 41 billion cosmic ray particles, and determined 10 million of them to be made of electrons and positrons.
There have been fluctuations in the number of positrons in the mix, and thanks to the orbiting spectrometer, for the first time in a half-century of cosmic ray research scientists have been able to measure an important peak in positrons.

“AMS now unveiled data that no other experiment could ever record,” said CERN spokesman Arnaud Marsollier.

The data hint at the existence of dark matter. But CERN scientists are not completely sure yet that dark matter is the true source of the positrons.

“It may come from high-energy phenomena somewhere in our universe: But what?” Marsollier asks. “Pulsars? Supernovas?”

Pulsars are stars similar to black holes that spray particles and light through the universe. Supernovas are exploded former stars.

Because it detects particles as opposed to light, the way a telescope would, AMS may also be able to see other cosmic phenomena a telescope cannot.

The data released this week need more study, but at first glance, CERN says, what they have seen so far looks “tantalizingly consistent with dark matter particles.”

If that’s the case, the AMS may have begun to remove humanity’s greatest blindfold.

http://www.cnn.com/2014/09/20/tech/innovation/dark-matter-experiment-data/index.html?hpt=hp_t2