Astronomers peering out into the deepest corners of the Universe have found its expiration date, stamped into the light from distant galaxies.
By analysing the brightness of that light, the astronomers discovered the Universe today is only half as energetic as it was two billion years ago. But according to their calculations, we don’t need to worry just yet; our 13.8-billion-year-old Universe is still in its first flush of youth. It will take another 100 billion years for the Universe to become cold, dark and then “nod off for an eternal doze”, according to University of Western Australia astronomer Simon Driver and colleagues who presented their finding at the General Assembly of the International Astronomical Union in Hawaii in August.
We have known that the Universe is dying – though not how quickly – since the 1990s. During that decade, astrophysicists Saul Perlmutter, Brian Schmidt and Adam Riess showed that after the Big Bang, the expansion of the Universe was not slowing down – it was accelerating.
The energy ledger of our Universe shows a gain when gas clouds collapse into stars and galaxies merge, or a loss when stars explode into supernovae. But as the Universe expands, the debits outweigh the gains. And the faster the expansion, the faster the energy balance runs down. When gas clouds and galaxies become too dispersed, stars will eventually stop forming altogether. A further threat comes from supermassive black holes sucking matter in and taking it out of the equation.
To calculate the Universe’s likely expiration date, Driver and his team used 10 telescopes to measure 200,000 galaxies at 21 light wavelengths, from super-high-energy ultraviolet light to the lazy long wavelengths of the far infrared – making this survey the most through examination of the light reaching Earth ever conducted.
Comparing the light reaching us from nearby galaxies and distant galaxies (which we see as they were millions or billions of years ago, because of the time their light takes to reach us), the team found that light at all wavelengths had faded over the past couple of billion years. Wherever they looked, the Universe was losing energy. “It’s moving inexorably towards a heat death,” says University of Melbourne theoretical astrophysicist Katie Mack.
Aside from the Universe’s end date, the new measurement will allow “us to measure the evolution of galactic components” ranging from stars to dust clouds, says Swinburne University astrophysicist Alister Graham.
So is the Universe safe for the next 100 billion years? Not necessarily.
“There’s also another scenario – the Big Rip,” Mack says, which could bring about the death of the Universe.
In contrast to normal matter, the density of dark energy – the mysterious energy source that is causing the Universe to expand ever faster – does not appear to be lessening as the Universe expands. If it increases – even by a little – galaxies would be torn asunder, then (progressively) stars, planets, molecules, atoms and subatomic particles – until every subatomic particle was infinitely far from every other.
Because we know so little about dark energy, it’s hard to say whether we are doomed to rip apart or merely drift into that dark cold eternal night. In our immediate neighbourhood there’s likely to be at least one major spectacle before then. Roughly seven billion years from now, our own ageing Sun will expand into a red giant, engulfing and vaporising the inner Solar System. So even if humans still inhabit Earth, they won’t see the end of the Universe anyway. Probably for the best, really.