16 November 2019 | New Scientist | 35>Delve deeper into
the dark universeJoin astrophysicists Chamkaur Ghag and
Kathy Romer for a fascinating evening
exploring the mysteries of dark matter
and dark energy on 27 November at
Congress Hall in London. To find out more,
see newscientist.com/science-events/the cosmic microwave background,
the radiation left over from the big bang.
Measurements of this radiation provide us
with a map of how matter was distributed
throughout our universe only a few hundred
thousand years after its beginning. This map
tells us that our universe was very uniform in
its youth, with only the smallest variations in
density. Without help from dark matter, there
is no way that these density variations could
have grown fast enough to form the galaxies
and other large structures of today’s universe.
A decade or more ago, many physicists,
including me, thought we knew what dark
matter was likely to consist of: weakly
interacting massive particles, or WIMPs. As
their name suggests, these are relatively heavy
particles that, besides gravity, only interact via
the weak nuclear force, which also governs sub-
atomic processes such as radioactive beta
decay. WIMPs seemed compelling because
we could understand how they would have
been created in the early universe.
During the first millionth of a second or
so after the big bang, all of space was filled
with a hot, dense plasma in which all sorts of
particles, from photons and electrons to top
quarks and Higgs bosons, were constantly
being created and destroyed. As space
expands, however, the temperature of the
plasma steadily drops. Eventually, it can’t
supply the energy required to make heavier
particles, and their production stops.
When this happens to a species of particle,
most are destroyed – annihilated – and
converted into other forms of energy. How
many survive depends on how and how
often the particles interact.
This leads us to a happy coincidence: for a
particle species to emerge from the big bang
with an abundance equal to that of dark matter
today, it must have interacted through a force
about as powerful as the weak nuclear force.
A stronger force would have caused too many
particles to be destroyed, while a feebler force
would have allowed too many to survive.