When the New Horizons probe reached the outer dark of the Solar System, out past Pluto, its instruments picked up something strange.
An artist's impression of dark matter. (Mark Garlick/Science
Photo Library/Getty) |
Very, very faintly, the space between the stars was glowing
with optical light. This in itself was not unexpected; this light is called the
cosmic optical background, a faint luminescence from all the light sources in
the Universe outside our galaxy.
The strange part was the amount of light. There was
significantly more than scientists thought there should be – twice as much, in
fact.
Now, in a new paper, scientists lay out a possible
explanation for the optical light excess: a by-product of an otherwise
undetectable interaction of dark matter.
"The results of this work," write a team ofresearchers led by astrophysicist José Luis Bernal of Johns Hopkins University,
"provide a potential explanation for the cosmic optical background excess
that is allowed by independent observational constraints, and that may answer
one of the most long-standing unknowns in cosmology: the nature of dark
matter."
We have many questions about the Universe, but dark matter
is among the most vexing. It's the name we give to a mysterious mass in the
Universe responsible for providing far more gravity in concentrated spots than
there ought to be.
Galaxies, for instance, rotate faster than they should under
the gravity generated by the mass of visible matter.
The curvature of space-time around massive objects is
greater than it should be if we calculated the warping of space based only on
the amount of glowing material.
But whatever it is creating this effect, we can't detect it
directly. The only way we know it's there is that we just can't account for
this extra gravity.
And there's a lot of it. Roughly 80 percent of the matter in
the Universe is dark matter.
There are some hypotheses about what it might be. One of the
candidates is the axion, which belongs to a hypothetical class of particles
first conceptualized in the 1970s to resolve the question of why strong atomic
forces follow something called charge-parity symmetry when most models say they
don't need to.
As it turns out, axions in a specific mass range should also
behave exactly like we expect dark matter to. And there might be a way to
detect them – because theoretically, axions are expected to decay into pairs of
photons in the presence of a strong magnetic field.
Several experiments are searching for sources of these
photons, but they should also be streaming through space in excess numbers.
The difficulty is in separating them from all the other
sources of light in the Universe, and this is where the cosmic optical
background comes in.
The background is itself very difficult to detect since it's
so faint. The Long Range Reconnaissance Imager (LORRI) aboard the New Horizons
is possibly the best tool for the job yet. It's far from Earth and the Sun, and
LORRI is far more sensitive than instruments attached to the more distant
Voyager probes that launched 40 years earlier.
Scientists have presumed the excess detected by New Horizons
to be the product attributed to stars and galaxies that we can't see. And that
option is still very much on the table. The work of Bernal and his team was to
assess whether axion-like dark matter could possibly be responsible for the
extra light.
They conducted mathematical modeling and determined that
axions with masses between 8 and 20 electronvolts could produce the observed
signal under certain conditions.
That's incredibly light for a particle, which tends to be
measured in megaelectronvolts. But with recent estimates putting the
hypothetical piece of matter at a fraction of a single electronvolt, these
numbers would demand axions to be relatively beefy.
It's impossible to tell which explanation is correct based
solely on the current data. However, by narrowing down the masses of the axions
that could be responsible for the excess, the researchers have laid the
foundations for future searches for these enigmatic particles.
"If the excess arises from dark-matter decay to a
photon line, there will be a significant signal in forthcoming line-intensity
mapping measurements," the researchers write.
"Moreover, the ultraviolet instrument aboard New
Horizons (which will have better sensitivity and probe a different range of the
spectrum) and future studies of very high-energy gamma-ray attenuation will
also test this hypothesis and expand the search for dark matter to a wider
range of frequencies."
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