Mass of the
finally have a reliable estimate of the
mass of the universe based on a faint
afterglow from microwave radiation created
when the universe was formed. This glow,
and the microwave radiation responsible
for it, can be seen in every part of the
sky from all points on the earth.
A hundred thousand years after the "big
bang", conditions were similar to those
inside the sun today. An almost uniform
plasma of electrons, hydrogen and helium
ions filled the entire universe. The free
electrons scattered and gave off energy in
the form of photons that rendered the
universe opaque. As matter formed, these
photons gathered around areas of higher
density that eventually became the
galaxies of today's Universe.
After some 300,000 years, the temperature
of the universe cooled. Electrons no
longer had enough energy to resist being
captured by nuclei thus forming atoms.
Photons were no longer scattered and the
Universe became transparent.
But the early photons did not disappear,
they simply continued in whatever
direction their last scattering sent them.
Some of these photons scatter in our
direction and we can still detect them
As the microwave background photons travel
towards us, their paths are bent by the
matter in the universe. The more matter
there is in the universe, the more the
light paths are bent. Thus by studying
this residual microwave background,
scientists can measure how much matter is
needed to create the distortion.
A balloon-borne telescope, Boomerang, was
launched earlier this year from the
McMurdoch Station on Ross Island,
Antarctica. The telescope spent 10 days
riding the polar stratospheric vortex in a
long arc above theSouth Pole. It mapped
400 square degrees or one per cent of the
sky. The data was used to make an accurate
determination of the density of the
universe. Convert this to a mass by
considering the volume of the visible
universe, and the universe weighs in at
100 trillion, trillion, trillion, trillion
tonnes (one metric tonne = one thousand
The full article appears in New Scientist,
16 December 2000.