The end of the universe is a matter of speculation. Our understanding of cosmology and physics remains limited and the ultimate scenario keeps getting adjusted to accommodate the latest hypotheses. Dr Matt Caplan, an assistant professor of physics at Illinois State University has put forward an interesting new scenario for how it could all end: in a deluge of black dwarf supernovae.
Before getting that far into the future, let’s step back to the current time and talk about stars. The most massive stars eventually explode turning into neutron stars or black holes, but these are a minority. Most stars are small and their fate is to become a white dwarf, a bright, dense ball of degenerate matter instead of a supernova.
White dwarfs do not produce their own energy by fusion like regular stars. They have a certain amount of energy from their formation, but it slowly dissipates into the cosmos. Over time, maybe 10,000 times longer than the age of the universe or maybe a quadrillion times, these white dwarfs will become incredibly cold. They become black dwarfs.
In the Monthly Notices of the Royal Astronomical Society, Caplan provides an end-game scenario for these black dwarfs; an explosive demise he believes will be “last interesting astrophysical transients to occur prior to heat death.” Heat death, aka the Big Freeze, when the universe eventually runs out of free energy and reaches absolute zero and maximum entropy, is one of the possible ways the universe might end. If Caplan's scenario is true, these black dwarfs could go supernova, and it will be the last bang in the universe before it all ends.
There are certain assumptions that have to be made for the black dwarfs going supernova. One of the main ones is that protons, the particles in the nucleus of atoms, should not decay. Turning a white dwarf into a black dwarf takes a huge amount of time, so if protons decay, black dwarfs are unlikely to form as the white dwarfs would just break apart in a shorter timescale. However, the idea that protons don't decay is not too out there when it comes to assumptions, as we are yet to see such decay. So assuming protons decay, white dwarfs will turn into black dwarfs, and these will over time will turn into iron black dwarfs thanks to quantum phenomena that can create heavier atoms within the white dwarf. And this is where things get very interesting.
Both white dwarfs and black dwarfs lose electrons over time. The positrons slowly destroy the electrons in the dwarf's center, and weaken it, causing it to eventually collapse in a huge explosion much like higher mass stars going supernova. White dwarfs can go supernova but only if they have stolen enough mass from a companion to cross the Chandrasekhar mass limit. When they are 1.4 times the mass of the Sun, they collapse on themselves. An iron-rich black dwarf will only need to weigh 1.2 times the mass of the Sun to go kaboom.
This might not seem much, as many many stars won’t be that heavy, but Caplan estimates that 1 billion trillion stars will explode in such a manner. And if you’re wondering when it will happen, he suggests roughly in 101100 years. That's a 1 with 1,100 zeros after it number of years. According to Caplan's calculations, these cosmic fireworks will continue slowly but surely until 1032000 years in the future.
And you thought a few months in lockdown was an eternity.