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Posted 31 October 2007

The End

by Frikkie de Bruyn

In a recent conversation with a fellow amateur astronomer I talked about Stephen Hawking’s statement that humans should leave the Earth if they want to survive as specie. His reply was: “But you don’t have to worry about that”. This is an example of the short term view of things so many of us have. Most of us are concerned about money, what next year will bring, retirement, etc, but in terms of the Universe this means literally nothing. Let us go on a really long journey into the future to see what science can tell us about the end of everything.

The human race will almost certainly not survive long enough to witness the death of the Sun. The majority of all species that ever inhabited Earth have already died. Humans will not escape this fate. Many natural disasters could end our stay on earth, like the event that killed the dinosaurs about 65 million years ago or the event about 251 million years ago when a mass extinction killed 70 percent of land species and 96 percent of marine species. Only creatures living deep underground could survive such disasters. We cannot avoid a similar fate.

In about 5 billion years any form of life on Earth will be wiped out. The Sun will have exhausted all the nuclear fuel at its core and will expand into a red giant, probably reaching the orbit of the Earth. It will be so hot that the oceans will boil. The irony is that the Sun, on which we are dependent for life giving energy, will destroy the Earth in the end. There is uncertainty if the red giant Sun will destroy the Earth. As the Sun expands there will be a change in the Sun’s density. This may cause the Earth to spiral away from the Sun. However, it is possible that the Sun’s outer envelope may envelop the Earth. This will cause friction which will slow down both the Earth’s rotation around its own axis and its movement around the Sun, meaning the Earth will spiral into the Sun, whatever happens, the Earth will be scorched and no life will survive.

But this is not the end of the Sun. When the Sun consumed all its hydrogen it will start burning helium then carbon and finally oxygen. No further nuclear fusion will be possible and its outside layers will form the familiar planetary nebula. The remaining core of the Sun will become a white dwarf. It will contain most of the Sun’s mass but its size will be about half of the Earth’s diameter. It will be very hot but it will slowly cool down until it eventually reaches the temperature of the surrounding space and become a black dwarf. Observation showed that even the oldest white dwarfs still radiate energy at several thousand degrees Kelvin. (Zero degrees Kelvin is the equivalent of -273 C.) It is estimated that it will take about another trillion years for white dwarfs to finally settle down as black dwarfs.

But what will happen to the planets not consumed by the red giant Sun? It is estimated that at least Jupiter, Saturn and the remaining outer planets and the Kuiper belt objects will survive the death of the Sun. However, in computer simulations scientists have put hypothetical planets orbiting a dying star. The result was that the star completely disrupted the stability of the planets. Changes in the mass of the star during its red giant stage cause planets to collide and to disrupt their orbits. Some planets spiralled into the star while others were ejected into interstellar space. If we apply this result to our own Solar System all that remained after the death of the Sun will be the white dwarf and a disc of rapidly rotating planetary wrecks surrounding it. The rest will be ejected into interstellar space.

The second law of thermodynamics provides us with a one directional cosmic time. Since light move at a finite speed and we use light to observe distant objects, we are looking back in time. At the edge of the visible Universe we see light that was emitted about 13.7 billion years ago. The acceleration of the expansion of the Universe poses a problem since the most distant galaxies move away faster and faster. Eventually they will move away from us faster than the speed of light and no light emitted by these galaxies can reach us. They will disappear from view and we will never see them again. In the very distant future, about 3 trillion years from now, all galaxies will disappear from view. But Einstein taught us that nothing can move faster than the speed of light. The answer is that space is being created among the galaxies. They do not move faster than the velocity of light but are merely being carried along by the expanding space. Larger distances mean larger volumes of space and more space is created in a large volume of space than in a small volume. That is why the more distant galaxies appear to be moving away from us faster than the speed of light. If there are humans around at that stage they will not be able to observe any distant galaxies and cosmology as we know it will no longer exist.

There is another reason to believe the expanding Universe eventually means the end of cosmology. We base many of our evidence for the Big Bang on the Cosmic Microwave Background Radiation and it will disappear too. The abundance of elements is further proof of the Big Bang but when galaxies disappear from view this valuable evidence will eventually be hidden by subsequent generations of stars. In about 3 trillion years from now there will be no evidence that the Big Bang happened and the Universe will appear to be static and unchanging.

What will happen to the Milky Way? Galaxies collide and merge. The dwarf galaxy Carina is currently in the halo of the Milky Way and will be totally absorbed. The Large and Small Magellanic Clouds will merge with our galaxy and the Sagittarius Dwarf Elliptical Galaxy is in the process of being absorbed by the Milky Way. Eventually the Andromeda Galaxy will collide with our galaxy in about 2 billion years and when giant galaxies collide they tear each other apart and we end up with a galaxy very different from the original two galaxies. The supermassive black holes at the centre of the two galaxies will merge into one super massive black hole. The shock wave caused by the collision of galaxies will lead to a burst of new stars being formed. This is expected to happen before the death of the Sun and our Solar System will probably end up in the outer reaches of the galaxy’s halo, about 100 000 light years from the centre. Gradually the galaxy will erode and in about 1020 years all the stars will escape into intergalactic space.

Some of the older galaxies have depleted their interstellar gas and dust and no new stars are formed and these galaxies are gradually cooling down. In our galaxy the interstellar gas and dust will also disappear, stars will form red giants and white dwarfs which will turn into cold black dwarfs. In about 100 trillion years every star will be a black dwarf with about the same temperature as the background of the Universe. Neutron stars and black holes will float around in a very dark and cold Universe.

It is interesting to see what will happen to matter as we know it. It is expected that black dwarfs will merge to form black holes which will merge with larger black holes. Eventually only a few very massive black holes will remain. Some theories predict that protons are unstable over long periods of time and any matter that was not consumed by a black hole will decay leaving electrons, positrons, neutrinos and radiation spreading into space. This is expected to happen in about 1030 years. We know that black holes radiate thermal energy like any black body and this radiation increases as the black hole loses mass. In the end every black hole will explode in a burst of X-rays and gamma rays and disappear. All black holes will disappear in about 10100 years. Radiation and elementary particles not absorbed by a black hole is all that will remain. The temperature of the Universe will be just above absolute zero and will never reach zero because the energy of a field cannot be zero in terms of quantum mechanics.

What will happen then? The answer is we don’t know. Dark energy may even push the remaining elementary particles and photons away from each other. There might be another Big Bang or the Universe might be cyclical with the whole process starting again. It is a bleak picture of a cold dead Universe but it is still a very long way away.

Frikkie de Bruyn

Note: The statistics and some of the information used in this article have been obtained from Universe Today and reproduced with the permission of Fraser Cain.

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