A theoretical physicist at Illinois State University, in the US, has calculated that the last phenomena that will happen in our universe is what he calls ‘black dwarf supernovas’. Assistant physics professor Matt Caplan has had his theoretical paper accepted for publication by the Monthly Notices of the Royal Astronomical Society. Currently, the expectation is that, in the far distant future, our universe will die not with a bang, but with a whimper. All the stars will finally (if massive) explode or (the rest) burn out. “It will be a bit of a sad, lonely, cold place,” explained Caplan. “It’s known as ‘heat death’, where the universe will be mostly black holes and burned-out stars.” Even black holes will eventually ‘evaporate’.
But, at the moment, massive stars die in super powerful explosions known as supernovas. All stars are powered by nuclear fusion reactions, as a consequence of which stars convert hydrogen into ever-heavier elements, which they are able to do because of their density and gravity. Massive stars are so dense and have so much gravity that they end up producing iron in their cores. But they cannot burn iron, so the metal accumulates in their cores, killing the star from the inside, finally resulting in a supernova, which blasts all the elements created by that star, including the iron, across space. What is left often collapses into a black hole.
However, smaller, less massive stars suffer a different fate. At the end of their lives, they collapse into super dense bodies called white dwarfs, which continue to radiate for a very long time afterwards. White dwarfs are still composed of lighter elements such as carbon and oxygen; a white dwarf is about the size of our Earth but has about the same mass as our Sun, meaning that the interior of a white dwarf has a density millions of times greater than the density of the Earth.
“Stars less than about ten times the mass of the Sun do not have the gravity or density to produce iron in their cores the way massive stars do, so they can’t explode in a supernova right now,” he observed. “As white dwarfs cool down over the next few trillion years, they’ll grow dimmer, eventually freeze solid, and become ‘black dwarf’ stars that no longer shine.”
But they will still not be quite dead. “White dwarfs are ash, they’re burnt out, but fusion reactions can still happen because of quantum tunnelling, only much slower,” he elucidated. “Fusion happens, even at zero temperature, it just takes a really long time.”
This process would result in the creation of iron within black dwarfs, ultimately triggering supernovas. Caplan has calculated how long it would take for the first such black dwarf supernova to occur. It is about 10 1100 years. “In years, it’s like saying the word ‘trillion’ almost a hundred times. If you wrote it out, it would take up most of a page. It’s mind-bogglingly far into the future.”
“Only the most massive black dwarfs, about 1.2 to 1.4 times the mass of the Sun, will blow,” he pointed out. “Even with very slow nuclear reactions, our Sun still doesn’t have enough mass to ever explode in a supernova, even in the far future. You could turn the whole Sun to iron and it still wouldn’t pop.”
By the time the first black dwarf supernova happens, the universe would be unrecognisable to us. “Galaxies will have dispersed, black holes will have evaporated, and the expansion of the universe will have pulled all remaining objects so far apart that none will ever see any of the others explode,” he noted. “It won’t even be physically possible for light to travel that far.”
About 1% of the stars existing today – numbering around a billion trillion – are likely to become black dwarf supernovas. The other black dwarfs will remain black dwarfs. Caplan has further calculated that the most massive black dwarfs will explode first, progressively followed by the less massive ones. The last such black dwarf supernova should happen in about 1032000 years. “It’s hard to imagine anything coming after that, [the] black dwarf supernova might be the last interesting thing to happen to the universe,” averred Caplan. “They may be the last supernova[s] ever.”