SEARCHING FOR DARK MATTERSC
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When a s tronomer s look at the Univer se on the larges t scales , they see huge clus ter s of gala xies s trung out on
long f ilament s , which border enormous cosmic voids. They e xplain this dis tribution by sug ges ting dark matter
provides a ‘scaffold’ by drawing ordinar y matter together with it s gravit ational influence.Understand
dark matter with
these termsANNIHILATION
The process by which two dark
matter particles come together,
creating a cascade of new
particles. We’re attempting to
detect this with various
experiments around the world
and in space.GRAVITATIONAL
LENSING
A prediction of Einstein’s
General Theory of Relativity,
which says that mass bends
light. However, astronomers
often see more bending than
the amount of visible material
present would suggest.NEUTRINO
A small, almost massless
particle created by nuclear
reactions inside the Sun.
Additional neutrinos may be
created by dark matter
annihilations and detecting
them would be a big
breakthrough.STANDARD MODEL
The recipe book that par ticle
physicists use to explain a lot
of the subatomic world. It
contains rules regarding how
particles interact with forces
and light.SUPERSYMMETRY
An idea that goes beyond the
Standard Model and says
every ‘normal’ particle has a
supersymmetric partner
particle. The lightest of these
supersymmetric particles could
be responsible for dark matter.designed to boost the machine’s
power. Hopefully, by colliding
particles with more energy than ever
before, nature may begin to reveal
more secrets of its inner workings.Q
Could dark matter be
something else?A
So far we’ve been assuming
t hat da rk matter is ta ngible,
something that truly exists. But what
if it doesn’t? What if it’s a pha ntom – a
symptom of the fact that we don’t
understand gravity properly? That’s
exactly what proponents of a theory
called Modified Newtonian Dynamics
(MOND) advocate.
Remember, one of the original
reasons dark matter was introduced
was to account for the fact that stars in
the Milky Way don’t slow down the
further they are from the galactic
centre, unlike the planets of our Solar
System. But what if there is one rule
for gravity on small scales (such as a
solar system) and another for large
scales (like a galaxy)? While Newton’s
laws of gravity allow us to send people
to t he Moon or spacecraf t to t he
planets, stretching those rules to
regions to where they don’t apply
might explain why we’re puzzled by
the strange motions of stars.
The idea was first put forward by
Israeli physicist Mordehai Milgrom in- He suggested that the strength of
gravity could become stronger where
acceleration levels are small. These
ideas can help to explain some details
about how gala xies work in ways t hat
the dark matter theory cannot. But
there is currently no reason to suspect
that gravity behaves differently on
different scales.Q
Has dark matter got anything
to do with dark energy?A
No. Dark energy is the name
given to the mysterious entity
thought to be accelerating the overall
expansion of the Universe – a sort of
anti-gravity. In contrast, dark matter
can be thought of as gravitational glue
that helps bind galaxies and clusters of
galaxies together. We’re literally in the
da rk as to what t hey a re.Q
How much dark matter is
there?A
Dark matter completely
dominates the ordinary matter
of which people, planets and stars are
made. Our Milky Way is thought to be
about 90 per cent dark matter and only
10 per cent ‘normal’ matter (baryonic
matter). Of all of the matter in the
Universe, 85 per cent is dark matter
and only 15 per cent is baryonic.
But, according to Einstein’s famous
equation E=mc^2 , mass and energy are
two sides of the same coin. This leads
cosmologists to often talk about the
mass-energy of the Universe – all the
mass and all the energy put together.
In t hese ter ms, t he Universe is 68 per
cent dark energy, 27 per cent dark
matter and just 5 per cent atoms. If we
discount the energy part, the numbers
revert to above – 85 per cent dark
matter, 15 per cent baryonic matter.by COLIN STUART (@skyponderer)
Colin is a science writer and author, and a Fellow
of the Royal Astronomical Society.WHAT IS THIS?
NEED TO KNOW