[time-nuts] Hunting dark matter with GPS data

André Esteves aifesteves at gmail.com
Fri Mar 24 16:29:31 EDT 2017

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Hunting dark matter with GPS data

By Adrian ChoJan. 30, 2017 , 2:30 PM

WASHINGTON, D.C.—A team of physicists has used data from GPS
satellites to hunt for dark matter, the mysterious stuff whose gravity
appears to hold galaxies together. They found no signs of a
hypothetical type of dark matter, which consists of flaws in the
fabric of space called topological defects, the researchers reported
here on Saturday at a meeting of the American Physical Society. But
the physicists say they have vastly narrowed the characteristics for
how the defects—if they exist—would interact with ordinary matter.
Their findings show how surprisingly innovative—and, in this case,
cheap—methods might be used to test new ideas of what dark matter
might be.

“It is so interesting and refreshing and exciting, and the cost is
basically zero,” says Dmitry Budker, an experimental physicist at the
Johannes Gutenberg University of Mainz in Germany, who was not
involved in the work. “It’s basically the cost of the students
analyzing the data.”

Astrophysicists think that dark matter makes up 85% of all the matter
in the universe. Yet so far they have inferred dark matter’s existence
only from its gravitational pull. For decades, many physicists have
tried to directly detect a promising candidate for particles of dark
matter, so-called weakly interacting massive particles, or WIMPs. But
enthusiasm is waning as ever-more-sensitive detectors have failed to
find the particles floating through our galaxy and passing through
Earth. So many physicists are thinking more broadly about what dark
matter might be.

For example, instead of a new subatomic particle, dark matter could be
something far bigger and weirder: macroscopic faults in the vacuum of
space called topological defects. Topological defects are best
explained with an analogy to magnetic materials such as nickel. Nickel
atoms act like little magnets themselves, and below a certain
temperature, neighboring atoms tend to point in the same direction, so
that their magnetic fields reinforce one another. But that orderly
alignment can suffer defects if, for example, atoms in different
regions point in different directions. When this happens, the regions
meet along a craggy surface called a “domain wall,” which is one type
of topological defect. There can be pointlike and linelike defects,

A similar thing might happen in space itself. Some theories predict
that empty space is filled with a quantum field. If that field
interacts with itself, then, as the infant universe expanded and
cooled, the field may have taken on a value or “phase,” which would be
a bit like the direction in which nickel atoms point. Regions of space
with different phases would then meet at domain walls. These domain
walls would have energy and, through Einstein’s famous equivalence,
E=mc2, mass. So they would generate gravity and could be dark matter.

Now, Benjamin Roberts and Andrei Derevianko, two physicists at the
University of Nevada in Reno, and their colleagues say they have
performed the most stringent search yet for topological dark matter,
using archival data from the constellation of 31 orbiting GPS
satellites. Each satellite  carries an atomic clock and broadcasts
timing signals. Receivers on Earth use the timing information from
multiple satellites to determine how far it is from each of them and,
hence, its location.

To use those data to search for dark matter, the researchers had to
invoke another bit of speculative physics. Theory suggests that within
a topological defect, the constants of nature will change. In
particular, the passing of a topological defect should fiddle with the
so-called fine structure constant, which determines the strength of
the electromagnetic force and the precise frequency of radiation that
an atom will absorb or emit as an electron in it jumps from one
quantized energy level to another. But an atomic clock works by
measuring just such a frequency. So were a GPS satellite to pass
through a topological defect, the defect should cause the satellite’s
atomic clock to skip a beat.

One jump in one atomic clock wouldn’t be proof enough for topological
defects. So the researchers looked for a stronger signal, the wave of
time shifts that would sweep across the whole 50,000-kilometer-wide
GPS network if Earth passed through a large domain wall as the galaxy
rotates in its cloud of dark matter. Combing 16 years of GPS data,
they found no evidence of a shift greater than half a nanosecond,
Roberts told the meeting. They placed limits on the number of such
topological defects and how strongly they interact with matter—limits
that are up to six orders of magnitude more stringent than ones set by
previous studies of supernova explosions. The researchers haven’t yet
reached the limitations set by the clocks’ noise, Roberts reported,
“so there’s a lot of room to improve.”

“It seems like a worthwhile study to pursue,” says Glennys Farrar, a
theorist at New York University in New York City. “The idea of how
you’d extract a signal was fun to think about.” Still, she says, the
particular model of dark matter that Roberts, Derevianko, and
colleagues test seems “rather narrow.” For example, she notes, they
have to arbitrarily assume that the domain wall isn’t much thicker
than Earth is wide. Budker agrees with that point. But he also notes
that the work is just one example of a raft of new ideas physicists
are hatching to look for different types of dark matter.

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