[time-nuts] Hunting dark matter with GPS data

Bill Hawkins bill.iaxs at pobox.com
Sat Mar 25 02:09:33 EDT 2017

Before you go looking for flaws in space, read the comments to the
sciencemag article at the link.

Thomas Lee Elifritz is informative.

It's amazing what you can do with math if you make a few simplifying

Bill Hawkins

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From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of André
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To: Discussion of precise time and frequency measurement
Subject: [time-nuts] Hunting dark matter with GPS data

People in the list may be interested in replicating the work...




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

“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

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, too.

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

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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