The end of the world: Five to seven billion years from now, the Sun will loom in our sky as a blazing 'red giant'.

Credit: Photolibrary

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One thing we do know about dark energy: the extra 'push' it delivers would seem to guarantee an open, ever-expanding cosmos. Today astronomers look out across the universe and see galaxies lumped together in clusters, with those clusters grouped together in superclusters. The superclusters, in turn, appear to be strung out in vast string-like filaments that stretch for hundreds of millions of light years across the cosmos. Gravity has crafted these structures – but dark energy will tear them apart.

The discovery of dark energy came at an awkward time for Adams, who had just published his book on the long-term fate of the cosmos, The Five Ages of the Universe (with co-author Greg Laughlin). How does the presence of dark energy affect his forecast? "Perhaps the most important update is that we now 'know' that the universe is accelerating," he told me by e-mail (using quote marks to emphasise the fact that, in science, no result is ever 100 per cent certain). "Since the expansion of the universe is speeding up, essentially no more cosmic structure will form."

In other words, those clusters and superclusters and stringy filaments are the end of the line in terms of cosmic evolution. "The things that we have now in the universe will be all that you get – ever," Adams said.

Thanks to dark energy, those large-scale structures will gradually disintegrate, and the universe will eventually look very different from what we see today. Things will appear fairly normal for the first few trillion years; stars will continue to shine and any planets they may harbour could be reasonably hospitable places. Adams calls this the 'stelliferous era' (meaning "filled with stars"); it is the era we now inhabit. (The first of Adams's eras is the 'primordial era', which covers roughly the first million years of cosmic history, from the Big Bang to the creation of the first stars.)

Eventually, the stars will exhaust their nuclear fuel, and – perhaps 100 trillion years from now – no new stars will be able to form. The stelliferous era will have come to an end, and we will enter what Adams calls the 'degenerate era'. The most prominent objects in the universe at this stage will become 'degenerate stellar objects' – essentially, the wasted cores of stars that no longer shine. Ordinary stars will have evolved into white dwarfs, while heavier stars will become ultra-dense neutron stars or black holes.

Ultimately, we shouldn't get too attached to these stellar remnants either, Adams cautions. After a mind-numbing period of time, white dwarfs and neutron stars will disintegrate through a process called proton decay, in which solid matter gives way to radiation. The lifetime of a proton is not yet known, but theory suggests that they last for 1030 to 1040 years. These are very large numbers indeed; the age of the universe at the moment is only about 1010 years.

After that point, the only sizable objects left in the universe will be black holes, and we enter the aptly-named 'black hole era'. Black holes are the most enduring objects that our universe and the laws of physics are able to craft. And yet they, too, must succumb to the endless time of an expanding universe. Black holes will ultimately disappear, evaporating by a process known as Hawking radiation – a quantum-mechanical process first described by British physicist Stephen Hawking in the 1970s. A black hole with the mass of the Sun may last for 1065 years; a supermassive black hole may endure for 10100 years.

After the last black hole has disappeared in a puff of Hawking radiation, the universe will be nearly empty. All that will remain will be a sparse flotilla of fundamental particles, drifting endlessly across a frozen, featureless void. Adams calls this final epoch the 'dark era'.

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