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What Is Dark Matter?

CAS physicist Alex Sushkov searches for the elusive axion


Alex Sushkov is trying to catch a ghost. Or perhaps, the shadow of a ghost.

Sushkov is searching for axions—particles first postulated in 1977 but never detected. If they exist, say theorists, axions will be smaller than neutrinos, which are so tiny that they have nearly no mass. The quest to find them seems quixotic, but, if Sushkov succeeds, he may answer a cosmic question that has baffled scientists for decades: what is dark matter?

“Dark matter is a huge mystery—we know it’s there, but we don’t know much about it,” says Sushkov, a College of Arts & Sciences assistant professor of physics, a 2016 Sloan Research Fellow, whose work is also funded by the Heising-Simons Foundation and the Simons Foundation.

The idea of dark matter was born in the 1970s, when scientists—in particular, the American astronomer Vera Rubin—studied the movement of stars around galactic centers and found something peculiar: the stars were moving much faster than expected, given their mass. Something was propelling them, or pulling them, but what? “It was as if there is something there that no telescope could see,” says Sushkov. “And that thing is dark matter.”

Scientists call dark matter “dark” because it doesn’t emit electromagnetic radiation, including light, so they can’t see it; they can only infer its existence by its gravitational pull. And while there is wide scientific consensus that dark matter exists, and that it’s everywhere, says Sushkov, there are differing theories on what it actually is. The three leading contenders: axions, black holes, and weakly interacting massive particles—better known as WIMPs. For years, physicists have been betting on WIMPs, building ever-more-massive experiments to find them. The latest, the LUX-ZEPLIN (LZ) experiment being built in an underground mine in South Dakota, will contain seven tons of liquid xenon and will be able to detect as little as one WIMP a year. “If they don’t find anything, the WIMP is very likely done for,” says Steven Ahlen, a CAS professor of physics. “If they don’t find it in the next couple of years, there’s no hope. That makes it a very exciting time in physics.”

axion particle detectors built by physicist Alex Sushkov

Sushkov’s detector contains tiny cylinders, each holding a few grams of lead titanate. The lead atoms will, hopefully, respond to a wave of axions sweeping by, offering clues to the character of dark matter.

Sushkov is bypassing the WIMPs and betting on axions, and he’s building an experiment to do what no one has done before: detect their quiver through the universe.

Axions are so small, says Sushkov, that they act more like a fluid or a wave, rather than a particle. And so, like a radio operator listening for distant broadcasts, he needs tiny antennae to perceive the axions’ presence. He’s building a detector, about the size of a refrigerator, in a basement lab of BU’s Metcalf Science Center. The detector contains three tiny cylinders, only 5 mm long, each holding a few grams of a yellow powder called lead titanate. Those are the antennae. Lead titanate is “not a fancy-schmancy material,” says Sushkov, but it has the right character to respond to axions: it’s ferroelectric, meaning its atoms can be spun around by an external electric field. And although the spin of each lead atom is oriented randomly, the right type of signal can make all of them act in unison.

Theory predicts that a wave of axions sweeping through the detector would make the spins gyrate or “precess” in harmony, like a troupe of well-trained hula dancers. Magnetic field detectors in the device, which scientists can tune with an external magnetic field, will sense the gyrations and signal the presence of axions. Sushkov calls his detector the Cosmic Axion Spin Precession Experiment, or CASPEr.

“It’s kind of like looking for intelligent life in other parts of the universe—you have to look at a lot of different frequencies. If it’s not on one channel, it might be on another,” says Ahlen. “It’s a long-shot experiment, but if it succeeds, the results would be incredible.”

One of the challenges of the experiment is to isolate the lead titanate so it will detect axions only, not trucks rumbling past or grad students yakking on their cell phones. To block all unwanted signals and make the detector more sensitive, the lead titanate samples sit within liquid helium at a frigid 4 degrees Kelvin—that’s -452 degrees Fahrenheit—in a container isolated from vibrations and from electromagnetic interference.

Several other groups of scientists around the world are searching for axions, but Sushkov and his team are the first to use this method. They are still finishing and troubleshooting the detector, which should start gathering data in spring 2017.

“Day to day, as with any job, you’re dealing with problems, like something is broken, or there’s some power-line interference,” says Sushkov. “But every now and then it’s exciting to sit back and think about the cosmic implications of the work. It’s inspiring to listen to the universe.”

Barbara Moran, Senior Science Writer
Barbara Moran

Barbara Moran can be reached at bmoran@bu.edu.

3 Comments on What Is Dark Matter?

  • JG Duker on 12.03.2016 at 11:47 pm

    Dark matter lives. JG D

  • Yehiel Gotkis on 01.13.2017 at 1:00 am

    After I ran my observations with a liquid vortex I came to a solid conclusion that the well-known effects lead to introduction of the Dark matter and Dark energy, could be interpreted without introduction of these vague entities. Here are my brief summary on this:

    Observing a gravity-driven liquid vortex – a generic analog of a cosmic black hole. What can we learn from it?
    Vortices are all the way around us. Assuming different kinds of vortices possess common generics, we can get a great insight in understanding the hardly accessible and/or observable vortices, for instance, cosmic black holes, by observing the easily accessible ones, for example, liquid vortices. Observing liquid vortices, also, allows to run the research with reasonable resources, in reasonable time, and under large variety of desired conditions, and, what is especially important, to observe the developments taking place beneath the funnel bottom, which in a way could be considered as analogous to the black hole event horizon.
    The liquid vortex I’ve observed was arranged to confine the vortex developments in the thin surface volume thus making it to act as a two-dimensional rather than a tri-dimensional one and to consider them as reasonably generic ones likely related to the black hole developments. The collection of videos of a liquid vortex located in the middle of a reasonably large water pool I recorded under a variety of different conditions could be accessed at YouTube under my name YehielGotkis .
    Recently, a lot of new findings about the black holes was made and published. Surprisingly, or maybe not, many of the effects discovered were also observed occurring with our liquid vortex. Thorough scrutinizing of the liquid vortex observations produced a shocking grasp even questioning the widely-assumed existence of the dark matter and dark energy.
    The liquid vortex was, of course, sucking the upper layer of the liquid and together with it whatever was floating over the surface, the foam, dry leaves other floating debris. With regard to the black hole case, the upper liquid surface of the water pool could be related to the spacetime, the floating stuff could be thought as matter resemblances.
    For the matter/spacetime duo the interaction between the material object and the spacetime causes appearance of an envelopping depression, or a “dent” over the spacetime at the location of the material object. The depression follows the object when the object is moving.
    The spacetime and the matter appear to be inherently adhered to each other both statically (as stiction) and dynamically (friction) via gravity interactions forcing them to follow each other’s motion: where the spacetime there the matter. And vice versa. This mutual matter/spacetime interaction could be guessed to cause effects analogous to hydrodynamic drag and friction – gravitational drag and gravitational friction in this case. If these or similar relations develop at the spinning black holes, then the black holes should be thought of pulling-in not only the surrounding matter but also the spacetime itself whirling around the black hole in the same manner as water whirls flowing into the liquid vortex funnel.
    Intriguingly, this rationale allows to explain the well-known paradoxical observations, the galaxy rotation curves anomaly and the Universe accelerated expansion, with no necessity to introduce the two famous but still challenging to prove, hypotheses:
    • The existence of dark matter – our liquid vortex model allows to interpret the known observables as due to the whirling spacetime additive contribution to the rotation of the visible matter in the black hole proximity,

    • The existence of dark energy – the vortex analysis-based interpretation: at the galactical periphery, where the pulling-in force diminishes, the keeping on centrifugal force induced by the spinning spacetime, accelerates the conventional matter away from the galaxy center.
    Another typical occurrence I would like to attract your attention is associated with the development of a spiral galaxy-like shape when a handful of floating shredded dry leaves was spread around the vortex (video at https://www.youtube.com/watch?v=l8HBO6E8LAg). Deep significance of this development stirred my mind and I would like to share it with you here:
    • Shredded leaves were always arranging in a spiral galaxy-like shape. So, what forced the initially completely disarranged flock of shredded intellect-less dry leaves to organize themselves in a spiral shape? How did they know about the spiral geometry to follow?
    The answer looks obviously simple to me now… The whirling water drove them to form the galaxy-like shape. The complex spiral structure could never be established without being governed by the vortex whirling flows. Thus, if in a system of interest we observe a spiral shape, we must postulate that there is a vortex whirling media the system of interest is resting on, which drives the spiral formation.
    This important deduction leads us to an unambiguous conclusion about what causes formation of the galaxy spiral geometries:
    There must be an actuating vortex and a whirling media driving the galaxy matter to form the spiral arrangement. And what could be the nature of the vortex and the whirling media the “floating” galaxy matter is resting upon? The only possible candidate is the spacetime whirlingly flowing into the black hole vortex funnel.

  • Yehiel Gotkis on 01.23.2017 at 10:54 pm

    Please, consider the following correction note to my comment
    Existence of the dark matter
    As per the BHSSR, the pulled-in by the BH whirling spacetime, as a spinning elastic thin film, can modify the radial and angular distributions of the shear stress and the actual force defining the (regular) mass rotational velocity. Which will depend on the spacetime media “fluidity” in the same way as it depends on the water fluidity for the LV. Obviously, the spacetime “deformation” depending on the BH activity, may extend far beyond the disc of the observable regular mass and appear as an external halo influencing the galactic dynamics.

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