Edited by Sanaya Narula.
Humans have always pondered the end of the universe. From abrahamic interpretations of a righteous second coming, to ominous prophecies hailing from indigenous cultures foretelling the universe’s doom, humanity has shown consistent curiosity when it comes to figuring out the end of everything we know, knew and will ever know.
Along with existing theological and mythological interpretations of the ultimate fate of our universe, recent theories posed by physicists based on observational evidence build a clearer picture for what to expect when envisioning the cataclysmic end to the universe.
To understand how the universe will end, we must first understand how the universe began. When Edwin Hubble first proposed the now largely accepted expanding universe theory, it led to the most widely held universe creation theory–The Big Bang Theory (created by Georges Lemaître).¹
It suggests that the universe has been continuously expanding since its beginning; and thus, that every known object in space, be it stars, galaxies, planets or cosmic dust, gets farther and farther away from each other as the expansion continues. This causes the ‘density’ of the universe to decrease over time, as the further away things are in a space, the smaller the density.² (Fun fact: this also means that there wasn’t exactly “nothing” before the birth of our universe. Instead, there was an infinite amount of ultra dense, high energy matter, which started expanding - thus, the “Big Bang”.)
Hubble’s theory pioneered deeper investigations into the theoretical end of everything, and birthed the three most well known theories about how our proverbial “day-of-reckoning” would play out.
One: The Big Freeze (or, Heat Death)
As you may already know, stars begin to form when clouds of space dust, called nebulas, attract each other gravitationally. Eventually, the dust ‘collapses’ in on itself, causing the dust to spiral and spiral until it forms a big ball of hot gas, the centre of which exponentially increases in heat. The energy emitted by this big ball of hot gas is given off as light and heat, becoming what we know as stars.³ They are the main form of light in the universe, with the nuclear reactions at their core creating elements which compose planets, and in turn, life. So what happens when the space dust runs out?
This is the basis of the Heat Death theory, which suggests that eventually, after trillions and trillions (about 10¹² - 10¹⁴) of years,⁴ all the stars in our universe will have burned out, and no space dust will be left to replenish these stars.⁴ Black holes, formed from the matter of collapsing stars, will be the sole entities roaming the universe, eating up any leftover space dust and emitting Hawking’s Radiation, which is theorised to cause their ‘evaporation’.
Thus, eventually, even these black holes would disappear. All that will be left is a dark, cold, energyless universe, befitting the title of the theory: “heat death”.
The laws of thermodynamics summarise this pretty effectively. The first law states that energy loss in the form of heat always occurs, and the second law states that entropy always increases with time (eventually reaching a state where all energy is evenly distributed). Thus, there is always a net loss in energy, causing a “Big Freeze”.
Two: The Big Rip
Remember when we established earlier that the universe is constantly expanding? What if I told you that this expansion isn’t just making things farther away from each other, but could, in due course, rip things apart?
Imagine the universe as a flat piece of fabric, with the “expansion” being the stretching of the fabric. At the rate it is expanding now, defined by a number called the Hubble constant, objects in the fabric of the universe are only getting farther apart from each other. However, the Hubble constant is steadily increasing,⁴ and one day, the rate of acceleration of the universe’s expansion will reach a point where it will override electromagnetic, gravitational and strong nuclear forces keeping things like galaxies and planets together, thereby ripping the fabric of the universe the same way actual fabric rips when you stretch it too fast.⁴ This ripping will begin with galaxies, but will eventually affect all material objects in the universe, regardless of how small they may be. Finally, all forms of universal objects will be disintegrated into simple, unbound particles.
Definitely a messier way for our universe to meet its cataclysmic end!
Three: The Big Crunch & The Big Bounce
This theory poses a question which reverses the original theory of universal expansion: what if all this expansion would eventually cause the universe to contract instead?⁵
The obvious effect of this would be catastrophic; material objects in space would crash and collapse into each other, killing off any sentient life forms inhabiting any planets. Eventually the contraction would become so dense that the entire universe would collapse in on itself, forming a singularity,⁶ which is a fancy word to say universal annihilation by everything crashing into each other all at once.
The most interesting part of this theory is the suggestion that this “crunch” will cause the birth of a new universe, a poetic circumstance where the end of everything that ever has been, is the beginning of everything that ever will be. Funnily enough, this epochal event is named “The Big Bounce.” ⁷
(A tangent, but these physicists must’ve been snickering and giggling when they decided to name all the ways the ultimate apocalypse will play out “The Big _____”. Either that or they just appreciate making end theories known in layman’s terms.)
As romantic as this theory is, most physicists believe it to be the most unlikely one, thanks to overwhelming observational evidence which suggests that our universe is not a closed one. A “closed” universe essentially means a universe with walls, with a defined limit and finite shape.⁸ Think of it like the original 2010 Minecraft worlds where you could actually find the edge of the world; the limit of its expansion. Physicists believe that expansion will happen infinitely, and won’t be limited by a figurative wall which will cause the universe to contract. And so, as romantic as a phoenix-like cyclical universe sounds, where the existence of a past universe before ours with a whole different set of sentient beings could be a possibility, it's more likely that Armageddon is just our universe running out of fuel or being ripped apart by its own infinite expansion.
Us human beings are an inconsequentially existential species. Despite how small and insignificant our time in this universe has been, from the very dawn of humanity to the present day being eclipsed by the monumental, unimaginable scale of the age of the universe, there are still those among us who try to figure out how all of this will end. Some may see these efforts as foolish, wasteful and in vain, saying that there is no utilitary use to this knowledge; “it is not like we would ever be able to stop the end of the universe”. And yet, this is the most human part of this pursuit, in that these theories aren’t made to prepare humanity for the end, or even to try and stop the end. For all we know, humanity won’t even be around to witness the universe’s ultimate fate. But humans don’t try to figure things out just for us to be able to apply the knowledge in a practical setting, humans figure things out simply because there are things to be figured out. It is the basic principle of what it means to be human; the one thing differentiating us from all other species being our seemingly needless, pure curiosity to know more and more and more.
Katie Mack, an astrophysicist at North Carolina State University, says that “We are a species poised between an awareness of our ultimate insignificance and an ability to reach far beyond our mundane lives, into the void, to solve the most fundamental mysteries of the cosmos.” ⁹
And I, for one, am inclined to agree with her.
“Sit in and watch the sunlight fade
Honey, enjoy, it's gettin' late
There's no plan, there's no hand on the rein
As Mack explained, there will be darkness again”
Andrew Hozier Byrne, ‘No Plan’, 2019
References:
Lematre, A. G. “A Homogeneous Universe of Constant Mass and Increasing Radius Accounting for the Radial Velocity of Extra-Galactic Nebulae.” Monthly Notices of the Royal Astronomical Society, vol. 91, no. 5, 13 Mar. 1931, pp. 483–490, academic.oup.com/mnras/article/91/5/483/985165, 10.1093/mnras/91.5.483. Accessed 3 Apr. 2022.
Wollack, E. J. (2010, December 10). universe 101: Big Bang Theory. Cosmology: The Study of the universe. Retrieved April 3, 2022, from https://web.archive.org/web/20110514230003/http://map.gsfc.nasa.gov/universe/
“Stars | Science Mission Directorate.” Nasa.gov, 2022, science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve. Accessed 4 Apr. 2022.
Baez, J. (2016, February 7). end. UCR Math. Retrieved April 3, 2022, from https://math.ucr.edu/home/baez/end.html
WMAP- Fate of the universe. (2015, June 29). Wilkinson Microwave Anisotropy Probe. Retrieved April 3, 2022, from https://map.gsfc.nasa.gov/universe/uni_fate.html
The Big Crunch. (2018). Archive.org. https://web.archive.org/web/20100316211915/http://www.windows.ucar.edu/tour/link=/the_universe/Crunch.html
Brandenberger, R., & Peter, P. (2017). Bouncing Cosmologies: Progress and Problems. Foundations of Physics, 47(6), 797–850. https://doi.org/10.1007/s10701-016-0057-0
Ryden, B. (2017). Introduction to Cosmology (B. Ryden, Ed.). Cambridge University Press.
Mack, K. (2020). The End of Everything: (Astrophysically Speaking). Scribner.
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