A ‘cosmic stocktake’ announced today at the IAU in Prague estimates that about 20% of the ordinary matter in the Universe has been ‘used up’ by being turned into such things as stars, planets and black holes. The rest resides in the enormous gas clouds that envelop old galaxies and can become the birth places of new galaxies.
Nature doesn’t have a reporter at the IAU quite yet, but we thought this was interesting enough to make a few phone calls. Here’s what reporter Lucy Heady found out…
The numbers aren’t surprising – they’re exactly what would be expected from other surveys of the Universe’s constituents. But it was done in a novel way, and has produced very quotable results.
The project, headed by Simon Driver of St Andrews University, used telescopes in Australia, Spain and Chile to obtain accurate distance information for over 10,000 giant galaxies. This allowed the team to measure the total mass of stars within a representative volume of the local Universe.
To measure the amount of mass contained in supermassive black holes at the centre of each galaxy, Driver and his team calculated how many stars in each galaxy are in the central ‘bulge’ surrounding the black hole. Astronomers know from measurements of nearby galaxies that that there is an extremely tight correlation between various properties of the bulge, such as the concentration of stars, and the mass of the black hole at its centre.
Once these masses had been determined “it was a simple task of summing them up to determine how much of the Universe’s matter is locked away,” explains Driver.
As it turns out, only about 0.01% of the normal matter in the Universe can be accounted for by black holes and only 0.1% is dust (including the large bits of ‘dust’ called planets); star formation, at 20%, has certainly cost the Universe far more ordinary matter than any other activity.
Although these “ultimate big picture calculations” can capture the imagination, “they don’t really tell us anything new about the physics of the Universe” says Mike Merrifield of Nottingham University.
The calculation by the Driver team gives an estimate that all of the ordinary matter in the Universe will be used up in about 70 billion years – after that point there will be no suitable material left to make new stars. But this doesn’t say anything about the lifespan of the Universe itself. “It just tells you when the Universe is going to become a rather dull place,” says Merrifield.
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