Wonders of the Universe

THE BEGINNING OF THE END

On the northern coast of Namibia, where the cold waters of the South Atlantic meet the Namib Desert, lies one of the most inhospitable places on Earth. The Skeleton Coast has been feared for as long as sailors have travelled near its shores; seventeenth-century Portuguese mariners used to call this place ‘the gates to hell’, and the native Namib Bushmen named it ‘the land God made anger in’. Today, you can just about make it to the coast in a sturdy 4x4, or effortlessly cruise in from the port city of Walvis Bay in a helicopter. But even so, when you stand on the sands beside the South Atlantic, the gods still have anger left. Each morning a dense ocean fog rolls along the coastline, fed by the upwelling of the cold Benguela current. Coupled with the constantly shifting shape of the sandbanks in the intense Atlantic winds, this toxic navigational conspiracy has meant that over the years thousands of ships have been wrecked along the Skeleton Coast. The decaying carcasses of the rusting ships and the bleached bones of marine life swept ashore by the currents all add to the coast’s gothic feel. The name Skeleton Coast also reflects the large number of human lives lost here over the centuries; even if you made it ashore after a shipwreck, the onshore currents are so strong that there is no way of rowing back out to sea, and the only route to safety is through hundreds of miles of inhospitable desert. This genuinely was a place of no return: if you were shipwrecked here, this was the end of your universe.

Just as the ship’s iron will eventually rust and be carried away by the desert winds, so we think the last matter in the Universe will eventually be carried off into the void.

One of the ships to end her days here was the Eduard Bohlen, a 91-metre (300-foot), 2,272-tonne steamship that ran aground here on the 5 September 1909 on a journey from Germany to West Africa. A century’s shifting sands have carried her hundreds of metres inland and the Atlantic winds have attacked her carcass, leaving her rusting and skeletal. When we arrive she is guarded by a phalanx of jackals who are less wary of us than I expected. She forms an abstract backdrop to our story; the symbolism is immediate, brutal even, and for me surprisingly powerful. These wrecks, complex structures dismantled by the passage of time, are like our last stars.

The Skeleton Coast: one of the most inhospitable places on Earth, where humans have perished for centuries, and where only jackals and the strongest life forms remain.

In the far future of the cosmos, the last remaining beacons of light will no more be permitted to evade the second law of thermodynamics than the Eduard Bohlen. Even the white dwarfs must fade as the laws of physics methodically dismantle the Universe. Slowly, as the glowing embers of the last stars lose their warmth to space, they will cease to emit visible light. After trillions of years, the final beacons burning in the cosmic sky will turn cold and dark – their remnants are known as black dwarfs.

Black dwarfs are dark, dense, decaying balls of degenerate matter. Nothing more than the ashes of stars, they take so long to form that after almost 14 billion years, the Universe is currently too young to contain any at all. Yet despite never seeing one, our understanding of fundamental physics allows us to make concrete predictions about how they will end their days. Just as the iron that makes up the ships of the Skeleton Coast will eventually be carried away by the desert winds, so it is thought that the matter inside black dwarfs, the last matter in the Universe, will eventually evaporate away and be carried off into the void as radiation, leaving nothing behind. The processes by which matter might, given enough time, decay, are not understood. Physicists need a more advanced theory of the forces of nature, known as a Grand Unified Theory, to speak with certainty about the behaviour of protons, neutrons and electrons over trillion-year timescales. There are reasons to expect that such a theory may exist, and that a mechanism for even the most stable sub-atomic particles to decay into radiation might be present in nature. For this reason, experiments to measure the lifetime of protons are ongoing in laboratories around the world, but as yet nobody has observed proton decay, and we are therefore now in the realm of speculation. But here is one possible, and given our understanding of physics today, probable, story of how our universe will end.

NASA

This composite X-ray image from the Chandra X-ray Observatory shows gas blowing away from a central supermassive black hole in the active galaxy NGC 1068.

NASA

Once the last remnants of the last stars have decayed away to nothing…the story of our universe will finally come to an end.

In trillions of years, our universe will be littered with black dwarfs. From the ashes of stars, dark, dense and decaying balls of degenerate matter will form.

With the black dwarfs gone, there will not be a single atom of matter left in the Universe. All that will remain of our once-rich cosmos will be particles of light and black holes. After an unimaginable expanse of time, it is thought that even the black holes will evaporate away, and the Universe will consist of a sea of light; photons all tending to the same temperature as the expansion of the Universe cools them towards absolute zero. When I say unimaginable period of time, I really mean it: ten thousand trillion trillion trillion trillion trillion trillion trillion trillion years. In scientific notation, that’s 10¹⁰⁰ years. That is a very big number indeed; if I were to start counting with a single atom representing one year, there wouldn’t be enough atoms in all the stars and planets in all the galaxies in the entire observable universe to get anywhere near that number.

Once the last remnants of the last stars have decayed away to nothing and everything reaches the same temperature, the story of our universe will finally come to an end. For the first time in its life the Universe will be permanent and unchanging. Entropy finally stops increasing because the cosmos cannot get any more disorganised. Nothing happens, and it keeps not happening forever.

This is known as the heat death of the Universe, an era when the cosmos will remain vast, cold, desolate and unchanging for the rest of time. There’s no way of measuring the passing of time, because nothing in the cosmos changes. Nothing changes because there are no temperature differences, and therefore no way of moving energy around to make anything happen. The arrow of time has simply ceased to exist. This is an inescapable fact, written into the fundamental laws of physics. The cosmos will die; every single one of the hundreds of billions of stars in the hundreds of billions of galaxies in the Universe will expire, and with them any possibility of life in the Universe will be extinguished