272 The Poetry of Physics and The Physics of Poetry
claim its is linked to the cosmological constant Λ, a parameter in
Einstein’s General Theory of Relativity that we spoke of earlier in this
chapter. Others suggest that it is due to a scalar field that cannot be
directly observed. Dark energy is believed to make up 72% of the
universe with remainder being dark matter which we will describe in the
next section making 23% of the universe and the observable matter made
up of protons, neutrons, electrons and the other elementary particles that
we see in our labs everyday making up a mere 5% of the Universe. This
is the challenge facing cosmologists and elementary particle physicists.
They only can see 5% of the total universe and from these observations
they have to figure out how the other 95% of the universe that they are
unable to observe directly behaves and what is the nature of these dark
quantities.
Dark Matter and the Lumpiness of the Universe
The other mysterious substance representing an estimated 23% of the
universe is dark matter, which is an electrically neutral form of matter
that does, however, exert a gravitational pull or push. Dark matter is a
form of matter that is undetectable by electromagnetic radiation but its
presence can be inferred from the gravitational effects it has on visible
matter. The existence of dark matter was proposed to explain the strength
of the gravitational forces within galaxies and between galaxies. The
stars in a galaxy rotate about the center of a galaxy where the greatest
concentration of stars exists. From the rate of rotation of stars on the
periphery of the galaxy and the distance of those stars from the centre of
the galaxy one is able to estimate the amount of mass that is required to
generate the forces that are observed. In many galaxies there is not
enough observable matter in the stars, nebulae and intergalactic gas and
dust in the galaxy to account for the strength of the gravitational pull. It
is assumed that this deficit is due to dark matter, i.e. matter that cannot
be seen because it does not have an electric or nuclear charge.
The existence of dark matter is also required to explain why as a
result of the Big Bang the matter we can observe is not uniformly
distributed about the universe. Dark matter is responsible for the fact that
observable matter gathered together in clumps to form clusters, galaxies
and stars. It is suggested that the dark matter, which dominated the early
universe amplified tiny inhomogeneities in the distribution of matter
causing matter to clump into dense regions and leaving other regions