Is the Universe eternal?

The Earth and the Universe are about 4.55 billion and 13.7 billion years old, respectively. Compared to these numbers, the brief flicker of our existence seems inconsequential, and yet we cannot resist the temptation to ask the question: What’s the future of the Universe?
Although we have arrived at a certain understanding of the dynamics of the Universe, the question of when and how the Universe will die ranks among today’s deepest mysteries of the cosmos. We don’t have definite answers to these questions. However, physics and cosmology provided us with theories and tools to travel forward in time and speculate about the ultimate fate of the Universe.
We seem to have three cosmic destinies that depend on the shape of the Universe.  A spherical, closed, expanding Universe will pause momentarily at its maximum size, reverse direction, begin to contract and accelerate toward its point of origin for a second rendezvous with singularity – a point which contains a huge amount of mass in an infinitely small space. This is known as the Big Crunch model.
For an open, hyperbolic (saddle-shaped) Universe, one of the possible scenarios is known as the Big Rip. In this scenario, the Universe lacking enough matter will expand at an ever increasing rate, undoing the work of gravity. The expansion, accelerated by a yet to be discovered mysterious force called Dark Energy, will cause the galaxies, stars and planets  to be separated from each other, ultimately ripping them apart. The Universe that was born with a violent explosion will end with an even more violent explosion.
The most widely accepted scenario for the fate of the Universe is known as the Heat Death. The idea comes from the second law of thermodynamics, which states that entropy – a measure of ‘disorder’ – always increases. In this scenario, the Universe will eventually evolve into a state of maximum disorder. When this will happen, there will be no ‘free energy’ available to sustain motion or life and it will be a cold, stationary, dead Universe. The death will be slow and less painful. A vast, empty, frozen Universe is compatible with either a flat or an open Universe.
What if none of the above fates await the Universe and instead, it is eternal?
As the eternal Universe carries on expanding, we will no longer be able to observe galaxies outside our local group 100 million years from now. Over trillions upon trillions of years, a star could exert enough gravitational force on another to kick it out of its galaxy. The remaining star would then fall into a lower-energy orbit closer to the galaxy’s nucleus.
Furthermore, the Universe will run out of hydrogen and helium gases which fuel the thermonuclear reactions of generation after generation of stars. No new stars will form and galaxies will grow dim as the final generation of stars dies off. As a result, all the matter in the universe will consist of either dead stars – white dwarfs, neutron stars and black holes, or cold lumps such as planets, moons, asteroids, meteorites, rocks and so forth.
One hundred quintillion  (10E+20) years from now, all the stars would spiral into the galaxy’s centre coalescing into an enormous black hole. The mass of the resulting black hole could be as much as 100 billion solar mass.
What’s next? Are we going to end up with a Universe filled with monstrous black holes? Although black holes may live longer than any other object known in the Universe, according tol British physicist Stephen Hawking, even they have a finite lifetime. In the early 1970s, he showed that black holes should in fact thermally create and emit sub-atomic particles, known today as Hawking radiation, until they exhaust their energy and evaporate completely. This means further into the future, evaporation will deplete the matter within black holes and even the most massive black holes will at the end evaporate completely if the Universe exists forever.
Calculations show that a solar mass black hole will evaporate after roughly ten unvigintillion years (10E+67). The gigantic black holes in the order of hundreds of thousands to billions of solar masses will evaporate after ten untrigintillion (10E+97) to ten quattuortrigintillion (10E+106) years. That may be far longer than the age of the Universe, but it’s still not forever.
All said and done, is the Universe really eternal? In mathematics eternal is infinity which is a concept and not a number. We cannot do with infinity the normal mathematical operations that we do with numbers without running into irreconcilable logical absurdities. Hence, when we try to figure out whether the Universe is eternal or not, we stand at the frontiers of human knowledge, groping dimly into the remote future for an understanding that might not even exist.

The writer is Professor of Physics at Fordham University, New York.

Is the Universe eternal?

The Earth and the Universe are about 4.55 billion and 13.7 billion years old, respectively. Compared to these numbers, the brief flicker of our existence seems inconsequential, and yet we cannot resist the temptation to ask the question: Whats the future of the Universe? Although we have arrived at a certain understanding of the dynamics of the Universe, the question of when and how the Universe will die ranks among todays deepest mysteries of the cosmos. We dont have definite answers to these questions. However, physics and cosmology provided us with theories and tools to travel forward in time and speculate about the ultimate fate of the Universe. We seem to have three cosmic destinies that depend on the shape of the Universe. A spherical, closed, expanding Universe will pause momentarily at its maximum size, reverse direction, begin to contract and accelerate toward its point of origin for a second rendezvous with singularity a point which contains a huge amount of mass in an infinitely small space. This is known as the Big Crunch model. For an open, hyperbolic (saddle-shaped) Universe, one of the possible scenarios is known as the Big Rip. In this scenario, the Universe lacking enough matter will expand at an ever increasing rate, undoing the work of gravity. The expansion, accelerated by a yet to be discovered mysterious force called Dark Energy, will cause the galaxies, stars and planets to be separated from each other, ultimately ripping them apart. The Universe that was born with a violent explosion will end with an even more violent explosion. The most widely accepted scenario for the fate of the Universe is known as the Heat Death. The idea comes from the second law of thermodynamics, which states that entropy a measure of disorder always increases. In this scenario, the Universe will eventually evolve into a state of maximum disorder. When this will happen, there will be no free energy available to sustain motion or life and it will be a cold, stationary, dead Universe. The death will be slow and less painful. A vast, empty, frozen Universe is compatible with either a flat or an open Universe. What if none of the above fates await the Universe and instead, it is eternal? As the eternal Universe carries on expanding, we will no longer be able to observe galaxies outside our local group 100 million years from now. Over trillions upon trillions of years, a star could exert enough gravitational force on another to kick it out of its galaxy. The remaining star would then fall into a lower-energy orbit closer to the galaxys nucleus. Furthermore, the Universe will run out of hydrogen and helium gases which fuel the thermonuclear reactions of generation after generation of stars. No new stars will form and galaxies will grow dim as the final generation of stars dies off. As a result, all the matter in the universe will consist of either dead stars white dwarfs, neutron stars and black holes, or cold lumps such as planets, moons, asteroids, meteorites, rocks and so forth. One hundred quintillion (10E+20) years from now, all the stars would spiral into the galaxys centre coalescing into an enormous black hole. The mass of the resulting black hole could be as much as 100 billion solar mass. Whats next? Are we going to end up with a Universe filled with monstrous black holes? Although black holes may live longer than any other object known in the Universe, according tol British physicist Stephen Hawking, even they have a finite lifetime. In the early 1970s, he showed that black holes should in fact thermally create and emit sub-atomic particles, known today as Hawking radiation, until they exhaust their energy and evaporate completely. This means further into the future, evaporation will deplete the matter within black holes and even the most massive black holes will at the end evaporate completely if the Universe exists forever. Calculations show that a solar mass black hole will evaporate after roughly ten unvigintillion years (10E+67). The gigantic black holes in the order of hundreds of thousands to billions of solar masses will evaporate after ten untrigintillion (10E+97) to ten quattuortrigintillion (10E+106) years. That may be far longer than the age of the Universe, but its still not forever. All said and done, is the Universe really eternal? In mathematics eternal is infinity which is a concept and not a number. We cannot do with infinity the normal mathematical operations that we do with numbers without running into irreconcilable logical absurdities. Hence, when we try to figure out whether the Universe is eternal or not, we stand at the frontiers of human knowledge, groping dimly into the remote future for an understanding that might not even exist. The writer is Professor of Physics at Fordham University, New York.