Asked by: Jacob Williams, by email
This is as much about philosophy and semantics as science.
Let’s start with everything on Earth. In about five billion years’ time, our planet will be engulfed by the Sun as it expands into a red giant star. Our planet’s crust will melt, the atmosphere will be driven off into space and the heavier atoms will become part of the Sun. After many more billions of years, the Sun will stop nuclear fusion altogether and gently cool down as an inert, but hot, white dwarf. After about a quadrillion years (roughly 100,000 times the current age of the Universe), the white dwarf will have cooled enough to stop glowing.
But will the cold, dead heart of the Sun continue to drift through space forever? Not necessarily.
Some theoretical models predict that protons might eventually decay into photons and positrons (anti-electrons). If they do, the atoms in our Sun and every other star would disintegrate, but this would take (roughly) another 10,200 years. The fundamental particles in this thin soup may last forever. Or, the expansion of the Universe might reverse into a ‘Big Crunch’ that restarts the whole cycle.