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Mass of the Universe

Scientists may 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 today.

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 kilograms).

The full article appears in New Scientist, 16 December 2000.


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