UFO'S of UAP'S, ASTRONOMIE, RUIMTEVAART, ARCHEOLOGIE, OUDHEIDKUNDE, SF-SNUFJES EN ANDERE ESOTERISCHE WETENSCHAPPEN

What is dark matter? New detector results have left physicists in the dark.

Credit: sakkmesterke / Shutterstock.com

Don Lincoln is a senior scientist at the U.S. Department of Energy's Fermilab, America's largest Large Hadron Collider research institution. He also writes about science for the public, including his recent "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Things That Will Blow Your Mind" (Johns Hopkins University Press, 2014). You can follow him onFacebook. Lincoln contributed this article to Live Science's Expert Voices: Op-Ed & Insights.

They say that love makes the world go around and that may well be true. But when you look at things on a much larger scale — say the size of galaxies — love just isn't enough. And, for that matter, neither are the stars of the galaxies themselves. In fact, what makes galaxies go around is a kind of matter that has never been directly observed. That undiscovered "stuff" is called dark matter, and an amazing new measurement was recently announced that is causing the scientific world to rethink long-held thoughts.

The most recent contribution to our knowledge of dark matter was made by the Large Underground Xenon (LUX) collaboration. LUX is a vessel consisting of a third of a ton of liquid xenon and it is the most powerful dark-matter detector ever constructed. Located at the Sanford Underground Research Facility (SURF), this dark-matter laboratory sits nearly a mile under the Black Hills, near Lead, South Dakota. It is designed to occasionally detect the vaporous wind of dark matter that is thought to waft through the solar system. [6 Cool Underground Science Labs]

And the thing is, it didn't detect anything. That non-discovery is what's causing physicists to rethink how they think about dark matter.

Holding up galaxies

Dark matter is an answer to a nearly century-old problem. In the early 1930s, shortly after astronomers realized that the universe consisted of countless galaxies, scientists turned their attention to understanding the dynamics of how stars orbited within the galaxies — essentially, how galaxies rotate. Dutch astronomer Jan Oort applied Newton's laws of motion and gravity to the observed matter in our own Milky Way and found that our galaxy rotated faster than he had calculated. It appeared that the Milky Way had twice the mass that astronomers had estimated. Of course, this was in an era in which precise galactic astronomy was coming into existence and a disagreement between calculation and measurement of only a factor of two was considered excellent agreement.

A violent collision of galaxy clusters formed the Abell 520 galaxy cluster. False-color maps superimposed on the image reveal the highest concentration of mass in the cluster (blue), which scientists say is dominated by dark matter.
**Credit: ASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University)**

However, shortly after Oort's measurement, Bulgarian-Swiss astronomer Fritz Zwicky was studying the Coma Cluster, a large group of more than a thousand galaxies that were bound together by their mutual gravity in an enormous structure. When he measured the velocity of the galaxies, he found that they, too, were moving far too fast for gravity's gentle tug to hold them together. By all rights, the cluster should have torn itself apart. But it didn't. He calculated that the cluster held 400 times more matter than could be seen by ordinary telescopes. Modern measurements have reduced that number, but the discrepancy still is thought to be a factor of 100. Zwicky proposed that there was a type of unseen matter holding together the cluster that he called "dunkle Materie," or dark matter. [TED-Ex Talk: How Does Dark Matter Explain a Star's Speed?]

In the 1970s, astronomer Vera Rubin was trying to find a noncontroversial topic to study when she turned her attention to the rotation curves of galaxies. This is a measure of the orbital speed of stars in galaxies as a function of their distance from the center. She found that the measurements agreed with predictions very well at the center of the galaxy and even approaching the periphery. But, in the very outskirts of the galaxy, stars were orbiting far faster than could be accommodated by the known laws of physics and the observed matter. It looked like Zwicky's dark matter might be showing up in the interiors of galaxies as well. (Clearly, Rubin failed epically in her attempt to find a noncontroversial topic.)

Over the decades, several ideas have been proposed to explain an extensive array of astronomical mysteries, from the possibility that Newton's laws of motion might not apply when accelerations become small, to the idea that both Newton and Einstein were wrong aboutgravity. These hypotheses have not survived rigorous testing. Another idea was that perhaps there exist types of matter in the universe that do not emit electromagnetic energy … this was Zwicky's dark matter.

But, even here, there were many possibilities. The most plausible option was that the universe was home to a menagerie of black holes, brown dwarfs, rogue planets and other dark objects that consisted of the same kind of ordinary matter that makes up the visible component of the universe. These objects are massive and compact and cold enough that they don’t emit light like stars. Some objects like these were found, but not enough to solve the mystery. And so, astronomical surveys in the 1990s completely ruled out this idea, too. Taking a page from Sherlock Holmes in “The Sign of Four,” in which he said, “When you have ruled out the impossible, whatever remains, however improbable, must be the truth,” scientists have been forced to conclude that a new form of invisible dark matter permeates the universe. Perhaps even more surprising, there appear to be five times more dark matter than ordinary matter.

The properties of dark matter

We have never directly observed dark matter, but we know a great deal about what it must be: It must be massive (because it affects the rotation of galaxies); it must be electrically neutral (because we can't see it); it must be different from ordinary matter (because we see no evidence for it interacting with matter in the usual ways); and it must be stable (because it has existed since the dawn of the universe). These properties are unequivocal.

However, we don't know exactly what it is. In the most popular generic theory, the dark-matter particle is called a WIMP, for weakly interacting massive particle. WIMPs are kind of like heavy neutrons (but definitely not neutrons), with a mass of 10 to 100 times heavier than a proton. They were created in great quantities during the Big Bang and a small relic remainder persists to this day.

When cosmologists add the WIMP idea to their ideas of the Big Bang, they can calculate how it would interact. They find that in the earliest stages of the universe, WIMPs were a dominant form of matter, but, as the universe expands and cools and the interaction rate drops to zero, the relic amount of WIMPs is about five times as massive as ordinary matter. Combined with the fact that the WIMP idea can also explain a pressing theoretical mystery of why the Higgs boson particle has such a small mass, scientists call this the "WIMP miracle," in view of the fact that the WIMP idea seems to answer so many questions.

A dark non-discovery

So this brings us back to the LUX experiment. It is simply the latest and most powerful experiment designed to detect WIMPs. The idea is that the detector will sit underground for a long time, say a year or more, and a rare WIMP will bump into a xenon atom and be detected. (The detector is buried underground to protect it from cosmic rays, which would bump into xenon atoms and fake a dark matter detection. Being half a mile or a mile underground stops nearly all cosmic rays.)

The above volume, part of the Large Underground Xenon collaboration, will be filled with liquid xenon and any interaction with the xenon can be detected, possibly revealing the identify of dark matter.
**Credit: Photo by C.H. Faham**

On Aug. 27, the LUX researchers announced their results. They saw no evidence for dark-matter WIMPs.

So that's not actually that surprising. There have been dozens of experiments that have looked for dark matter and found nothing. In this sense, LUX has joined a venerable group. But LUX is also far more capable. It was expected to have a better shot at finding WIMPs than any other apparatus.

But it didn't.

So does that mean that the WIMP idea is dead? No, not really. Dark matter detectors are optimized to be most sensitive to certain masses, kind of like how a particular microphone might pick up tenor voices better than they hear soprano or bass ones. LUX was optimized to find a WIMP with a mass of a few tens of times heavier than a proton. In fact, it ruled out a WIMP having a mass 50 times of a proton with impressive rejection. However, LUX doesn't do so well at detecting WIMPs with a mass below 10 times that of a proton. And, if WIMPs exist and have a mass of 1,000 times heavier than a proton, this is also not LUX's optimum range for investigation. So there remains a range of masses where a WIMP might exist.

Axions, dark life and dark charge

Even so, WIMPs were simply the most popular idea for dark matter. There are other hypotheses. One hypothesis points to a particle called a sterile neutrino, which is a cousin of the more familiar neutrino generated in nuclear reactions. In fact, neutrinos from the biggest nearby nuclear reactor (the sun) are constantly barraging the Earth. Unlike regular neutrinos, which are ultralight and interact via the weak nuclear force, sterile neutrinos are heavy and don't experience the weak force. It is the heavy mass and non-interacting qualities that make the sterile neutrino an ideal dark-matter candidate.

Another possible dark-matter particle, the axion, was proposed in 1977 as a way to ensure that the strong nuclear force treated matter and antimatter on equal footing (so as agree with observations). The axion is a very light, but still massive, hypothetical particle. The LUX detector is not designed to study axions.

Then, of course, there is the even more creative hypothesis, which suggests that dark matter isn't a single, neutral, non-interacting particle. After all, ordinary matter is pretty complicated. At the quantum scale, we have quarks and leptons and four forces. At the macro scale, we have you and me and sugar and stars and volcanoes and all the various ways they interact. Ordinary matter has all sorts of interactions and constituents. Why not dark matter? [7 Strange Facts About Quarks]

Under this hypothesis, dark matter might have a "dark charge," or dark matter's form of electrical charge. In the same way that dark matter doesn't experience dark charge, maybe ordinary matter doesn't experience dark charge. This dark charge would interact with dark photons, or the dark twins of ordinary light particles.

And, even more daring, perhaps dark matter experiences many forces and there is an entire complex dark sector, with dark atoms and dark molecules and perhaps even dark life. We are able to set constraints on these possible dark interactions; for instance, we know enough to rule out dark stars and planets, but dark asteroids are possible. Again, LUX is silent on these new ideas.

It's not fair to say that the LUX measurement leads to a crisis in particle physics and cosmology. But it certainly gives scientists pause and suggests that maybe we should take another look at this WIMP thing. Maybe other ideas need to be revisited. On the other hand, scientists who want to continue to pursue the WIMP idea still have something to look forward to as technology advances. LUX uses a third of a ton of liquid xenon. In 10 or 15 years, scientists are planning to build detectors that might contain 100 tons, providing even more chances of capturing that rare WIMP interaction. These are heady times to be a dark-matter scientist.

But, in the end, we still don't know. We just know that the capabilities of LUX are good enough that maybe it's time to broaden our thinking. In the words of the rock band Buffalo Springfield, "There's something happening here. What it is ain't exactly clear..."

Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Live Science.

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Visit to 'Armageddon Asteroid' Could Save Future Earth from Impact — **An artist's impression of asteroid Bennu, the target of NASA's OSIRIS-REx sample return mission.**
**Credit: NASA's Goddard Space Flight Center, Greenbelt, Maryland**

CAPE CANAVERAL, Fla. — The asteroid-sample-return mission that launched into space yesterday (Sept. 8) may help protect Earth from future life-threatening impacts by rogue space rocks.

The target of the OSIRIS-REx mission is an asteroid known as Bennu, which circles the sun just inside the orbit of Mars (occasionally crossing Earth's orbital path). Although Bennu is unlikely to collide with the planet, understanding its composition could help scientists figure out how to deflect other rocky bodies that may be on a crash course with Earth.

"I think that's the whole reason we have missions like OSIRIS-REx — to go out and see what we really know about asteroids," Lindley Johnson, director of NASA's Planetary Defense Coordination Office, said at a news conference Wednesday (Sept. 7). [NASA's Mission to Asteroid Bennu: OSIRIS-REx Complete Coverage]

Defending Earth

After the sun formed, leftover dust and rock circled the newborn star in a disk. The material smashed together, building planetary embryos. Some bits of debris hurled down on the young planets, while some material was cast out of the solar system forever. Over time, the fiery rain slowed as the population of dust and rock dwindled.

As life evolved on Earth, bits of remaining debris occasionally crashed down — at times, wiping out plants and animal species. While most of the largest rocks have already collided with planets or have left the solar system, enough leftover material remains to pepper our planet.

"Over a hundred tons of material fall to Earth every day," Johnson said.

Most of that is dust, but occasionally, a solid chunk of rock makes it to the ground (a meteorite). Those meteorites can be chunks that have broken off of asteroids, but they also may be debris that was once part of a planet or the moon and was blown off in an early collision. For example, Earth is covered in meteorites that were once part of the moon and Mars — places humans have been able to study up close with various probes, rovers and (in the case of the moon) human missions.

But asteroids are more challenging. The sample of Bennu that scientists are aiming to return in the OSIRIS-REx mission will help scientists better understand the composition of other asteroids and space debris if they were to prove threatening.

**The size of asteroid Bennu, which is 1,614 feet (492 meters) wide, is compared with the Empire State Building and Eiffel Tower in this NASA image.**
**Credit: NASA**

During the OSIRIS-REx mission, scientists also will study how the sun affects the orbital paths of small rocky bodies. As sunlight strikes a rotating asteroid, the radiating heat provides a tiny thrust that can change the orbit of the rock — a process known as the Yarkovsky effect.

"We can't just determine where an asteroid is going to go just by orbital dynamics," said Alex Young, a heliophysics scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We have to account for this Yarkovsky effect."

Observing how the sun ever so slightly redirects Bennu will help scientists to improve their models of other asteroids and their potential to impact Earth.

On top of all that, guiding OSIRIS-REx into orbit around Bennu will help NASA learn more about manipulating a spacecraft around small bodies.

"NASA's gotten very skilled at going to Mars, Saturn, through multibody systems," said NASA Chief Scientist Ellen Stofan. But smaller objects are different, and maneuvering OSIRIS-REx is a step toward landing humans on asteroids and the moons of Mars, she said.

Shifting orbits

NASA is sending a probe to the asteroid Bennu to collect samples from the space rock and return them to Earth. See how NASA's OSIRIS-REx mission to collect samples of the asteroid 1999 RQ36 will happen in this Space.com infographic.
**Credit: Karl Tate/SPACE.com**

Debris from the early solar system still floats between the planets. While most of the largest objects have been identified, NASA estimates that only half of the rocks the size of Bennu have been spotted.

"We've got to find them first before we can do anything about them," Johnson said.

For that, planetary defense researchers turn to NASA's NEOWISE, a repurposed telescope mission to identify near-Earth objects. The instrument has helped scientists to build a catalog of rocks near the planet, some passing closer than satellites.

Observation isn't the only tool for planetary defense. In partnership with the European Space Agency, NASA is working on the Double Asteroid Redirection Test (DART). The two agencies plan to send a spacecraft to rendezvous with an asteroid and its moon. By deliberately crashing into the tiny moonlet, DART will shift its course by "a fraction of a percent"according to NASA. The mission will test the ability of the space agencies to deflect an asteroid years before it collides with Earth.

Johnson described DART as "a kinetic impactor that would impact the moon of Didymos, which we affectionately call Didymoon."

Another upcoming project is planning to bring a boulder-size asteroid sample to the moon, where astronauts can remove samples to carry back to Earth. After traveling to an asteroid and selecting a hunk to take home, the Asteroid Redirect Mission (ARM) will attempt to change the path of the asteroid by using its mass as what Johnson called "a natural tug rope."

"Gravity will pull the asteroid off its natural orbit," he said.

Asteroid Basics:

Asteroids are fascinating for lots of reasons. They contain a variety of valuable resources and slam into our planet on a regular basis, occasionally snuffing out most of Earth's lifeforms. How much do you know about space rocks?

Of course, NASA doesn't have a mission ready to deflect incoming danger. Johnson told Space.com that it will take somewhere from five to 10 years to launch a mission that will effectively deflect a specific threat.

Earth-threatening asteroids aren't prolific in the solar system. Johnson said that serious impacts that might be worth deflecting happen only about once every couple of centuries.

"It's not something I worry about a lot on a regular basis," he said.

But that doesn't mean we should become complacent.

"Since we don't know everything that's out there, it's something that could happen almost any day," he said.

He suggested that the best way to avert disaster is to improve the ability to identify potentially hazardous objects. The sooner large, dangerous objects are identified, the more time humans on Earth have to take the necessary steps.

"We need to improve our capabilities to detect and find them further out in space so we have time to prepare," he said.

Follow Nola Taylor Redd on Twitter @NolaTRedd and Google+. Follow us at@Spacedotcom, Facebook or Google+. Originally published on Space.com.