40 | New Scientist | 12 October 2019
vast numbers of tiny particles, each possessing
mass but lacking the capacity to interact with
ordinary matter. Yet although these weakly
interacting massive particles, or WIMPs,
remain the theoretical front runners, they
have yet to show up in experiments. “That’s
why primordial black holes are getting more
interest lately,” says Green.
If WIMPs don’t make up dark matter,
there are a host of rivals waiting to take
their place. On the other end of the spectrum,
appropriately enough, are MACHOs: massive
compact halo objects. These are large objects
that float freely through space and emit little
if any radiation, which would explain why we
haven’t seen them. Neutron stars and starless
planets have been proposed as MACHOs, as
have primordial black holes.
“Primordial black holes are my favourite
explanation for dark matter,” says Vidotto.
Astronomical observations, however, have
concluded that they are unlikely to account for
all of dark matter, which means there must be
something else out there to pick up the slack.
If WIMPs made up the other part, we would
expect them to surround every primordial
black hole, drawn in by its gravitational pull.
That higher density of WIMPs would increase
the probability of WIMP-WIMP collisions,
generating a distinctive shower of gamma
rays that has never been seen.
“If one day we discovered even a few
primordial black holes, you just have to
concede that whatever dark matter is, not
all of it is made of WIMPs,” says Dan Hooper,
head of the theoretical astrophysics group
at Fermilab in Illinois.
Another, more intriguing, option is that the
primordial black holes could be creating the
dark matter particles themselves through the
medium of Hawking radiation. Calculations
predict that the bigger one of these black holes
is, the lower its temperature, meaning it emits
fewer and lighter particles. As it shrinks, it
heats up, radiating more and more energy.
That means small primordial black holes
can spew more massive, complex particles.
“The kinds of particles that are generated by
Hawking radiation don’t depend on the stuff
that falls into a black hole,” says Hooper. “The
black hole doesn’t care what kind of particle
you are, you’re just as likely to be made. That
includes dark matter and everything else.”
Whatever particles exist, whether they
are predicted by the standard model of
particle physics or not, should be emitted
by primordial black holes as they evaporate.
That includes massive dark matter particles
that are too big for us to create in the LargeSeeking a theory of everything?
Head for the Cosmos stage at New Scientist Live on Sunday
newscientistlive.comHadron Collider or any experiments planned
on Earth. “If we can find primordial black holes
and observe them in their last few seconds as
they get to those high temperatures, it gives
us access to physics that we’d never otherwise
be able to do,” says Jane MacGibbon at the
University of North Florida. If those massive
particles do exist, they could turn the standard
model on its head.
Primordial black holes could clear up
other cosmological conundrums too. One
of the biggest puzzles in the universe today
is the mystery surrounding its present rate
of expansion. At the moment, we have two
ways of measuring it: one involves flashing
a speed gun at nearby objects to detect
their acceleration away from us, the other
extrapolates data from ancient light to
work out a current value.
The trouble is, the two methods produce
conflicting results – they are in tension. “One
thing that is known to relax this tension is if
there was some extra ‘dark’ radiation earlier in
the universe’s history,” says Hooper. So-called
dark radiation – particles that move close
to the speed of light and can travel straight
through matter without stopping – could
be emitted by primordial black holes as they
evaporate away. If enough of these black
holes existed in the early universe, that extraLIGO may have
spotted the
universe’s first
black holes“ Ancient black
holes would
give us access
to physics we
would never
otherwise be
able to do”