Decadence: A Journey Into The Far Future Of Our Universe

“Rage, rage against the dying of light” — is more than an apt description of the future of our universe. Not the immediate future, not a hundred years, a thousand years, or even a billion years from now, but a future unimaginably distant from our times. We live, we die. Our parents, families, friends, some random people we meet only once, that stray dog wagging its tail where the alley turns into Broadway, we are all doomed to the same end. Oblivion. What's perfectly normal for living organisms stands true for all things — living or azoic. Everything decays; everyone dies. With the unvarying passage of time, stone monuments turn to dust, rigid iron bars turn to rust, and even the universe ends up in ashes.

A time will come when there won't be any more stars. Stars shine by the fusion of hydrogen into helium. Hydrogen is a non-renewable energy source. All of the universe's hydrogen formed within 380,000 years after the Big Bang — of all visible matter, 75% was hydrogen, and nearly 25% was helium. In these 13.8 billion years, trillions upon trillions of stars across billions and billions of galaxies scattered in every corner of an ever-expanding universe have consumed the hydrogen reserves. Although not even a full percent of the initial mass has been used up so far, eons from now, no free hydrogen will be left to drive star formation. With the last star exploding in a supernova, the universe plunge into eternal darkness. 

 

One day, this will all fade. The universe will plunge into eternal darkness. 
Image Credits: pxhere.com

                        

The following paragraphs give a brief idea of what lies ahead at the largest of scales. In the infinitely remote timescales discussed below, the universe turns to nothing. 

We live in a star system that achieved its first light some 4.6 billion years ago. Our Sun is an average main sequence G-type star. It'll continue to fuse hydrogen into helium for another billion years and get brighter by 1% every 100 million years. At the close of a billion years from now, the Sun will gradually begin to run low on fuel. Hydrogen fusion and the outpouring radiation pressure liberated as a direct byproduct of hydrogen nuclei turning into helium prevents a star from collapsing under its own massive weight. This is virtually common for all types of stars out there. When a star begins to run out of its primary fuel, it looks for other means of energy production. Depending on its mass, being done with hydrogen, the star successively fuses helium into carbon, carbon into oxygen, and so on, all the way up to iron. Our Sun doesn't pack enough mass to fuse helium. So when its hydrogen reserves run out, it'll expand to double its current size, turning into a red giant. The red giant Sun will engulf Mercury and Venus, and probably Earth. 

The Sun's increased brightness will make Earth inhospitable for all life in about a billion years. Even subsurface life will cease to exist as the last molecule of liquid water evaporates under the blazing heat of an expanding red giant sun. If the Sun doesn't swallow Earth, our oasis of a planet will burn to a crisp.

By five billion years, the habitable zone would shift past Jupiter, turning some of the moons of the outer solar system habitable for a fleeting moment. In about 8 billion years, the Sun will ultimately shed its outer layers leaving behind a dense stellar remnant, a white dwarf whose brightness will be comparable to that of a full moon on a clear night sky. With the Sun turned into a white dwarf, the expanding nebula of expelled stellar debris will look like a fallen flower on an unmarked grave. 

But considering the far-flung future of the universe, at 13.8 billion years from now, the cosmos will still be young. Somehow, if humans manage to survive that long, spread across alien star systems, then the show has just begun.  

Following Hubble's Law, the universe continues expanding, with distances between galaxies becoming unimaginably vast. At 150 billion years, the temperature of the Cosmic Microwave Background Radiation(CMBR) falls to a mere 0.3 K, and the cosmic light horizon (the farthest distance from which light reaches us) extends further. At 450 billion years, the local group of galaxies merges into a single galaxy. At 800 billion years, most of the stars in the universe leave the main sequence, turning into white dwarfs, neutron stars, or black holes. At one trillion years, star formation ceases as galaxies run out of molecular hydrogen gas clouds needed to create new ones. The cosmic horizon recedes so far that the evidence of the Big Bang becomes undetectable as per our current technologies. When the universe reaches an age of 100 trillion years, star formation ceases permanently, marking the beginning of the degenerate and the black hole era.

With stars switching off one by one, the cosmos becomes darker and darker. At 120 trillion years from now, black dwarfs (cool white dwarf stars that emit no visible light) will collide to form accidental new stars. Colliding neutron stars and white dwarfs intermittently puncture the cosmos with ultra-bright supernovae, letting out a final beacon into an ever-darkening cosmos. One quadrillion years into the future, the temperature of our Sun (most likely turned into a black dwarf) falls to 5 K. At 100 quadrillion years, the galaxies eject stray matter. Whatever matter remains, planets, asteroids, irregular hunks of rocks, and degenerate stellar cores collide or get sucked into monstrous supermassive black holes at the heart of every galaxy. At 10^30 (one followed by thirty zeros) years, only black dwarf and stray matter remain. With all stars gone, along with their remnant cores, black hole mergers become common.

Physicists speculate that at 2*10^36 years, even subatomic particles, viz., protons, will start to decay. After the era of proton decay at 3*10^43 years, the black hole era will begin. Black holes will eventually evaporate in a sudden flash of radiation from 2×10^66 years and counting. If proton decay is correct, then every speck of matter that might have survived till now will decay into subatomic particles, and the universe will move toward its final energy state. The universe at these timescales will comprise nothing except radiation. Following the second law of thermodynamics, entropy will finally erase everything, making the universe cold and unchanging. Vacuum fluctuations would be everywhere, and quantum tunneling would assume the upper hand. At 10^10^10^56 years, perhaps our universe might give rise to a new one via quantum fluctuations. Physicists are still trying to figure out the nature of dark matter and dark energy, the possibility of a big crunch (sudden collapse of the universe), or a big rip (accelerated expansion), whose knowledge would dramatically change the picture of the future. With entropy increasing the disorder of the universe further, the notion of time itself will come to an end. There will be no change to account for since the universe will become unchanging. And that would mark the end of everything. So, the universe will not end in fire but in ice. 

First Published: 05 April, 2020

Updated On: 04 August, 2023

For a more detailed picture, here's another article - A Trillion Trillion Trillion and a Lot More