Invisible matter

Part of this missing mass of the Universe is believed to be made
up of something known enigmatically as dark matter. It is believed
that there is probably between ten and forty times as much matter,
or ‘stuff’, out there in space than we can actually see. This is
not because it is so far away or hidden behind other objects, but
because it is literally invisible. Here we go again, you’re thinking,
how can scientists even be sure about the things out in deep space
that they can see, let alone stuff that is invisible! Well, yet again, the
answer is surprisingly simple: galaxies appear to weigh a lot more
than the sum of stars and other visible objects they contain and
must also be made up of a cloud of invisible material that extends
beyond the visible stars. This strange conclusion is reached from
two quite independent routes.
The first is by studying the way stars on the outer rims of
galaxies orbit the centre. If most of the mass of a galaxy is
concentrated in its core, which is what one would expect since that
is the region most densely populated with stars, then the outer stars
should be revolving much more slowly than they actually are. The
only way to explain the way these stars are observed to behave is
if there is some kind of invisible form of matter, dubbed the dark
matter halo, that surrounds, and extends further out from, the
visible matter (the stars). This halo would have to contain many
times more matter in it than all the visible forms of matter put
together.
Another indication that galaxies are more massive than they
appear is found by directly measuring their weight! This is done
using Einstein’s idea that the gravity of a massive object warps space around it. Remember from the last chapter that the general
theory of relativity was first tested experimentally by observing the
deflection of the light from distant stars as it passed close enough
to the Sun’s gravitational field. In the same way, a galaxy will
deflect the light from a more distant galaxy when it is in the line of
sight between the more distant one and us. The amount by which
the light is bent tells us how much mass the nearer galaxy has.
Again, we find that galaxies contain much more matter than just
the visible stuff.
Until recently, it was not clear what this dark matter could
comprise of. It was initially thought that it could be made up
entirely of cold dead stars, black holes, planets, lumps of rock plus
any other non-luminous (and therefore not visible) material that
might be floating around out there that you thought was still down
the back of your sofa. Such objects have been dubbed MACHOs,
which stands for Massive Astronomical Compact Halo Objects.
However, it turns out that there is a limit to how much of this
kind of matter there could be, which is set by the proportion of the
elements synthesized just after the Big Bang.
So a problem remains. We are now certain not only that dark
matter exists but that most of it must be made up of a new kind of
substance we have yet to discover.
Experiments carried out in Japan and announced in mid-
1998 have suggested that part of it is probably made up of
elementary particles called neutrinos. The trouble with these tiny
entities, which were predicted theoretically in the early 1930s and
discovered in laboratory experiments in 1956, was that no one
had been sure they actually weighed anything at all. They are
extremely elusive since they travel through solid matter, including
our measuring devices, as though it was not there. In fact, billions
of neutrinos, mainly produced in the Sun, are at this moment
streaming through your body without you knowing it. Japanese
scientists havenowdiscovered that they do indeed have a very tiny
mass which is enough to account for part of the invisible matter of
galaxies, due to their sheer number. Even out in deep space it is
estimated that there are, on average, several hundred of the little
fellas in a volume the size of a thimble. Even taking into account all the normal material in galaxies
(whether visible or not) plus all the neutrinos, we still cannot
account for all the mass that galaxies appear to have. Neutrinos
would make up what is called ‘hot dark matter’ because they zip
around at high speeds. We are now confident that there must
be more, probably in the form of slower moving heavy particles
that would make up ‘cold dark matter’. The search is currently
on in a number of laboratories around the world to find such
new particles. My favourites are the WIMPs (weakly interacting
massive particles) which might contribute many times more to
the mass of the Universe than all the visible matter put together.
Such particles have never been seen—well you wouldn’t see them
if they were invisible would you—but scientists can figure out
what properties they must exhibit and are designing experiments
to detect them.
So, the best current estimates for all the matter in the Universe
(both visible and dark matter) only account for about a third of the
density required to make omega equal to one and the Universe flat.
It is looking increasingly likely that there is nothing more out there;
that omega is in fact much less than one. Howdoes this square with
the theory of inflation which requires a flat universe? Arewe going
to have to modify it, or even abandon it altogether?