16 November 2019 | New Scientist | 37the first fraction of a second, the window in
which dark matter is thought to have formed.
One possibility my colleagues Hooman
Davoudiasl and Sam McDermott and I have
investigated is that our universe experienced
a brief period of hyper-fast expansion during
this era. We already think it did something
similar right at the beginning, in an event
known as cosmic inflation.
Another – somewhat less explosive – burst
of expansion may have occurred somewhat
later as well, still within the first fraction of
a second of cosmic history. It would have
diluted the amount of dark matter in the
early universe, and thereby changed our
expectations for how strongly this substance
should interact – and how difficult it should
be for us to detect.
Alternatively, there may have been a
population of particles that decayed at some
point in the early universe, disappearing and
creating dark matter. Dark matter particles
created in this way could be extremely feebly
interacting, explaining why they have gone
undetected for so long.
A third possibility is that our universe
went through an abrupt change during its
first moments – not merely a steady cooling,
but a total phase transition. We already know
of two such transitions in which the nature of
particles and their interactions changed within
the universe’s first second, what are known as
the QCD and electroweak phase transitions.
But there may have been others. A phase
transition in dark matter interactions could
have influenced how dark matter formed
in the early universe, again altering our
expectations for the kinds of experiments
that might detect it today.
It is too early to say whether the right answer
is one, some or none of the above possibilities.
Perhaps an experimental breakthrough will
change the game yet again. But the stubborn
elusiveness of dark matter has left many
physicists and cosmologists surprised and
confused. In droves, we are returning to our
chalkboards, revisiting and revising our
assumptions – and with bruised egos and a
bit more humility, desperately attempting to
find new ways to make sense of a very dark
and hidden universe. ❚universe. Maybe we haven’t seen the particles
because dark matter is different from what
we had expected – or perhaps because the
universe’s first moments were.
The amount of dark matter that was created
in and survived the big bang depends on how
our universe evolved during its hot and volatile
youth. We know a great deal about most of our
universe’s 13.8-billion-year history, but we have
no direct observations that enable us to studyof dark matter, without leading to any
appreciable interactions with ordinary matter.
The hidden-sector particles might become
bound to each other, forming dark nuclei or
dark atoms. One day, we could even discover
something like a periodic table of the hidden
sector elements. For that reason, of all the
plausible ideas about dark matter that have
grown in popularity in recent years, this is
perhaps my favourite.
Many of these alternative dark matter
candidates call for experiments very unlike
those designed to hunt for WIMPs. One
example is the Axion Dark Matter Experiment,
ADMX, based at the University of Washington
in Seattle and managed by scientists at my
institute, Fermilab. It uses powerful magnetic
fields to try to convert one hypothesised type
of ultra-light dark matter particle, axions,
into photons.
Some physicists are trying to produce
dark matter using particle beams originally
designed to study neutrinos. Others are
designing tunable electronic circuits that could
pick up signals of dark matter waves, much
like a radio picks up electromagnetic waves
consisting of photons. There are even ideas
involving gravitational wave detectors. These
ideas may not seem to have much in common,
but they are all motivated by testing previously
overlooked possibilities for dark matter.
There is an even more dramatic possibility
that many cosmologists are considering. Our
surprise at dark matter’s no-show is based on
our current understanding of the earlyDan Hooper is head of
theoretical astrophysics at
the Fermi National Accelerator
Laboratory (Fermilab) near
Chicago and a professor of
astronomy and astrophysics at
the University of Chicago. He is
the author of At the Edge of
Time: Exploring the mysteries
of our universe’s first secondsXE
NO
N
Huge purpose-built
detectors such as
XENON1T have failed
to find dark matter