Most of you might know that our Universe is expanding and it has also been observed to do so. Galaxies are moving away from each other. Hubble discovered this. But, I question that what would be the future of an expanding universe like ours, what will be its ultimate fate? There are several theories that answers this question, and still people all around the world are searching for answers.
Actually, the future of an expanding universe is bleak. If a cosmological constant accelerates the expansion of the universe, the space between clusters of galaxies will grow at an increasing rate. Stars are expected to form normally for 1×10^12 to 1×10^14 years, but eventually the supply of gas needed for star formation will be exhausted. Once the last star has exhausted its fuel, stars will cease to shine. According to theories that predict proton decay, the stellar remnants left behind would disappear, leaving behind only black holes which themselves eventually disappear as they emit Hawking radiation. Ultimately, if the universe reaches a state in which the temperature approaches a uniform value, no further work will be possible, resulting in a final heat death of the universe. But, this might not be completely true. There are many probable ways for our universe to end. Here I discuss the most widely accepted facts.
The universe is currently 1.37×10^10 (13.7 billion) years old. This time is in the Stelliferous Era. About 155 million years after the Big Bang, the first star formed. Since then, stars have formed by the collapse of small, dense core regions in large, cold molecular clouds of hydrogen gas. At first, this produces a protostar, which is hot and bright because of energy generated by gravitational contraction. After the protostar contracts for a while, its center will become hot enough to fuse hydrogen and its lifetime as a star will properly begin. Stars whose mass is very low will eventually exhaust all their fusible hydrogen and then become helium white dwarfs. Stars of low to medium mass will expel some of their mass as a planetary nebula and eventually become a white dwarf; more massive stars will explode in a core-collapse supernova, leaving behind neutron stars or black holes.
The Andromeda Galaxy is currently approximately 2.5 million light years away from our galaxy, the Milky Way Galaxy, and the galaxies are moving towards each other at approximately 120 kilometers per second. Approximately three billion years from now, or 17 billion years after the Big Bang, the Milky Way and the Andromeda Galaxy may collide with one another and merge into one large galaxy. Because it is not known precisely how fast the Andromeda Galaxy is moving transverse to us, it is not certain that the collision will happen.
Assuming that dark energy continues to make the universe expand at an accelerating rate, 2×10^12 (2 trillion) years from now, all galaxies outside the Local Supercluster will be red-shifted to such an extent that even gamma rays they emit will have wavelengths longer than the size of the observable universe of the time. Therefore, these galaxies will no longer be detectable in any way. By 10^14 (100 trillion) years from now, star formation will end, leaving all stellar objects in the form of degenerate remnants. This period, known as the Degenerate Era, will last until the degenerate remnants finally decay.
It is estimated that in 10^14 (100 trillion) years or less, star formation will end. The least massive stars take the longest to exhaust their hydrogen fuel (see stellar evolution). Thus, the longest living stars in the universe are low-mass red dwarfs, with a mass of about 0.08 solar masses, which have a lifetime of order 10^13 (10 trillion) years. Coincidentally, this is comparable to the length of time over which star formation takes place.
After 10^40 years, black holes will dominate the universe. They will slowly evaporate via Hawking radiation. A black hole with a mass of around 1 solar mass will vanish in around 2×10^66 years. However, many of these are likely to merge with supermassive black holes at the center of their galaxies through processes described above long before this happens. As the lifetime of a black hole is proportional to the cube of its mass, more massive black holes take longer to decay. A supermassive black hole with a mass of 10^11 (100 billion) solar masses will evaporate in around 2×10^99 years.
After all the black holes have evaporated (and after all the ordinary matter made of protons has disintegrated, if protons are unstable), the universe will be nearly empty. Photons, neutrinos, electrons, and positrons will fly from place to place, hardly ever encountering each other. Gravitationally, the universe will be dominated by dark matter, electrons, and positrons (not photons). By this era, with only very diffuse matter remaining, activity in the universe will have tailed off dramatically (compared with previous eras), with very low energy levels and very large time scales. Electrons and positrons drifting through space will encounter one another and occasionally form positronium atoms. These structures are unstable, however, and their constituent particles must eventually annihilate. The universe now reaches an extremely low-energy state.
What happens after this, is speculative. It is possible that a Big Rip event may occur far off into the future. Also, the universe may enter a second inflationary epoch, or, assuming that the current vacuum state is a false vacuum, the vacuum may decay into a lower-energy state. Finally, the universe may settle into this state forever, achieving true heat death.
Well, that's one probable way that our universe is gonna die. It might be true but we still don't know.
Actually, the future of an expanding universe is bleak. If a cosmological constant accelerates the expansion of the universe, the space between clusters of galaxies will grow at an increasing rate. Stars are expected to form normally for 1×10^12 to 1×10^14 years, but eventually the supply of gas needed for star formation will be exhausted. Once the last star has exhausted its fuel, stars will cease to shine. According to theories that predict proton decay, the stellar remnants left behind would disappear, leaving behind only black holes which themselves eventually disappear as they emit Hawking radiation. Ultimately, if the universe reaches a state in which the temperature approaches a uniform value, no further work will be possible, resulting in a final heat death of the universe. But, this might not be completely true. There are many probable ways for our universe to end. Here I discuss the most widely accepted facts.
The universe is currently 1.37×10^10 (13.7 billion) years old. This time is in the Stelliferous Era. About 155 million years after the Big Bang, the first star formed. Since then, stars have formed by the collapse of small, dense core regions in large, cold molecular clouds of hydrogen gas. At first, this produces a protostar, which is hot and bright because of energy generated by gravitational contraction. After the protostar contracts for a while, its center will become hot enough to fuse hydrogen and its lifetime as a star will properly begin. Stars whose mass is very low will eventually exhaust all their fusible hydrogen and then become helium white dwarfs. Stars of low to medium mass will expel some of their mass as a planetary nebula and eventually become a white dwarf; more massive stars will explode in a core-collapse supernova, leaving behind neutron stars or black holes.
The Andromeda Galaxy is currently approximately 2.5 million light years away from our galaxy, the Milky Way Galaxy, and the galaxies are moving towards each other at approximately 120 kilometers per second. Approximately three billion years from now, or 17 billion years after the Big Bang, the Milky Way and the Andromeda Galaxy may collide with one another and merge into one large galaxy. Because it is not known precisely how fast the Andromeda Galaxy is moving transverse to us, it is not certain that the collision will happen.
Assuming that dark energy continues to make the universe expand at an accelerating rate, 2×10^12 (2 trillion) years from now, all galaxies outside the Local Supercluster will be red-shifted to such an extent that even gamma rays they emit will have wavelengths longer than the size of the observable universe of the time. Therefore, these galaxies will no longer be detectable in any way. By 10^14 (100 trillion) years from now, star formation will end, leaving all stellar objects in the form of degenerate remnants. This period, known as the Degenerate Era, will last until the degenerate remnants finally decay.
It is estimated that in 10^14 (100 trillion) years or less, star formation will end. The least massive stars take the longest to exhaust their hydrogen fuel (see stellar evolution). Thus, the longest living stars in the universe are low-mass red dwarfs, with a mass of about 0.08 solar masses, which have a lifetime of order 10^13 (10 trillion) years. Coincidentally, this is comparable to the length of time over which star formation takes place.
After 10^40 years, black holes will dominate the universe. They will slowly evaporate via Hawking radiation. A black hole with a mass of around 1 solar mass will vanish in around 2×10^66 years. However, many of these are likely to merge with supermassive black holes at the center of their galaxies through processes described above long before this happens. As the lifetime of a black hole is proportional to the cube of its mass, more massive black holes take longer to decay. A supermassive black hole with a mass of 10^11 (100 billion) solar masses will evaporate in around 2×10^99 years.
After all the black holes have evaporated (and after all the ordinary matter made of protons has disintegrated, if protons are unstable), the universe will be nearly empty. Photons, neutrinos, electrons, and positrons will fly from place to place, hardly ever encountering each other. Gravitationally, the universe will be dominated by dark matter, electrons, and positrons (not photons). By this era, with only very diffuse matter remaining, activity in the universe will have tailed off dramatically (compared with previous eras), with very low energy levels and very large time scales. Electrons and positrons drifting through space will encounter one another and occasionally form positronium atoms. These structures are unstable, however, and their constituent particles must eventually annihilate. The universe now reaches an extremely low-energy state.
What happens after this, is speculative. It is possible that a Big Rip event may occur far off into the future. Also, the universe may enter a second inflationary epoch, or, assuming that the current vacuum state is a false vacuum, the vacuum may decay into a lower-energy state. Finally, the universe may settle into this state forever, achieving true heat death.
Well, that's one probable way that our universe is gonna die. It might be true but we still don't know.
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