36 | New Scientist | 16 November 2019Rather like the temperature of Goldilocks’s
porridge, the strength of the weak force seems
just right to explain how dark matter came to
be formed in the heat of the big bang.
But that story now seems rather a fairy
tale. If dark matter does consist of WIMPs,
we can estimate how much it should
interact with ordinary atomic matter via
the weak force, and so design experiments
to detect it. These experiments, housed in
deep underground laboratories to avoid the
constant bombardment of cosmic radiation,
started out small, deploying detectors of only
a few kilograms of crystalline materials such
as germanium, calcium tungstate or sodium
iodide, sensitive to the light, heat and electric
charge that would be produced in collisions
of WIMPs with normal matter.
Over the past two decades, the size and
sophistication of these experiments has
hugely increased. The latest iterations are
enormous, deploying anything up to tonnes
of liquid xenon as their detectors. These
experiments – XENON1T under the Gran
Sasso mountain in Italy (pictured, right), LUX
in South Dakota and PandaX-II in Sichuan,
China – are each roughly 10,000 times as
sensitive as the most sophisticated dark
matter detectors operating in 2006.A disturbance in the force
But they too have failed to turn up WIMPs.
The only experiment that even claims to have
detected anything resembling dark matter
goes by the name of DAMA. Most researchers
think the signal it picked up is almost certainly
produced by something else: a long list of
other experiments have searched for the
kinds of WIMPs that could have made it,
but have seen nothing.
The only other possible piece of evidence we
have for WIMPs comes in the form of a strange
gamma-ray signal seen emanating from the
centre of the Milky Way. My collaborators and
I spotted this signal in data from NASA’s Fermi
space telescope more than a decade ago. It took
years for us to convince most people that it
was real. We continue to debate whether these
gamma rays are produced by dark matter, or by
something else, such as a group of thousands
of rapidly spinning neutron stars. At the
moment, we just can’t be sure.
Whatever the resolution of that argument,
the longer we go without directly detecting
WIMPs, the more we are forced to confront
the uncomfortable possibility that they
might not be there. And yet dark matter
must exist – alternative explanations, such as
modifying gravity to produce the same sort of
effects, don’t seem to work (see “A disturbance
in the force”, left). If not WIMPs, then what?A whole new world
One possibility is that dark matter could
interact with other forms of matter and energy
even less than we had imagined – perhaps only
through gravity or some force so feeble that we
haven’t even discovered it yet. Such a particle
would be even more difficult to detect in
underground experiments or to produce
with particle accelerators.
The problem is that such non-interacting
particles would probably survive the big
bang in vast numbers, and wildly exceed the
abundance of dark matter in our universe
today. But if they interact rarely enough,
perhaps these particles were never produced
in great quantities in the first place, instead
building up an appreciable abundance only
gradually over the first fraction of a second
of cosmic history.
It could be, too, that dark matter is just one
of several kinds of particles that almost never
interact with any known forms of matter and
energy. This “hidden sector” of particles would
involve forces and interactions that we have
never observed, and that allow dark matter
to evolve in a rich variety of ways. These
interactions may have depleted the amountDespite dark matter’s long-standing
refusal to reveal itself (see main
story), most physicists remain
confident that it exists – the evidence
in its favour is just too great. A few,
however, champion a very different
possibility. Rather than explaining
the motions of stars around galaxies
with new forms of matter, they
speculate that a different conception
of gravity may be the answer.
These ideas fall under the
general umbrella of modified
Newtonian dynamics, or MOND.
This postulates that gravity works
ordinarily here on Earth and in our
solar system, but differently in the
low-acceleration environments
experienced by stars throughout the
Milky Way and other galaxies.
In these circumstances, the force
of gravity is effectively strongerthan Newton or Einstein thought.
This strengthening of gravity creates
the illusion that unseen dark matter
must be present.
Many versions of MOND have been
proposed over the past few decades,
but they have suffered from a range
of problems, both observational and
theoretical. Perhaps the single
biggest failure is MOND’s inability
to explain the temperature patterns
observed in the cosmic microwave
background, the relic radiation of
the big bang. Whereas dark matter
enables us to explain and understand
the observed features of this light in
incredible detail, no version of MOND
has ever remotely done the same.
Compounding these problems is
the fact that no version of MOND has
been able to explain the observed
dynamics of galaxy clusters.“ The longer we go
without finding
WIMPs, the
more we must
confront the
possibility they
aren’t there”