They say that all good things must come to an end and our universe is of no exception. But just how it’ll be snuffed out is a subject continually up for debate amongst astronomers and depends on the battle between dark energy and gravity as well as its density. There’s three possible fates for the universe, one is called the Big Crunch, where gravity takes over and begins to pull the cosmos back, compressing to one point. Another extreme is the Big Rip, where the expansion of the universe just gets faster until galaxies, stars, planets, atoms and space itself is ripped apart. Somewhere between these two extremes is what the now-defunct Wilkinson Microwave Anisotropy Probe (WMAP) mission’s data showed to be the likely scenario where the expansion of the universe is not great enough for a big rip and gravity is not strong enough for a big crunch. Instead it will continue to expand, carrying everything we see today over the cosmic horizon where they will be so far away, that their light cannot reach us. The sky will be dark and as all of the stars begin to go out, the universe will grow, cold and lifeless in a so-called big freeze.
Scenario 1: What will happen in a closed universe?
In a closed universe – where the density of the universe is more than five atoms of hydrogen per cubic metre (a critical density) -there’s no repulsive effect of dark energy and gravity eventually halts the expansion of the universe. Beginning to then contract, all of the matter in the Universe collapses to a point – the Big Crunch.
Scenario 2: What will happen in an open universe?
If the geometry of space is open (and curved like a horse saddle), the universe will continue to expand forever, whether there is dark energy present or not. If it is, then dark energy will drive the expansion. The result? Heat death, the Big Freeze or the Big Rip is imminent. Here the universe’s density is less than the critical density.
Scenario 3: What will happen in a flat universe?
With no dark energy, a flat universe will expand forever but at a continually decelerating rate. If dark energy is around, then at first the expansion of the universe initially slows down – thanks to gravity – but eventually speeds up. The ultimate fate of the universe is the same as if it were open. The universe’s density and critical density are equal.
You can read more about the Birth of the Universe in issue 21 of All About Space
Image courtesy of NASA (top) and ESA (bottom)