By Hayley Covington ’26

The following was written for FYS 101-33.

In one of his most famous poems, Robert Frost questioned whether the world would end in fire or ice. Today, many physicists and cosmologists dedicate themselves to answering his question, only with slightly different terminology. There are two main schools of thought concerning the ultimate fate of the universe: Heat Death and the Big Rip. Heat Death would be slow, steady, and (despite what the name suggests) cold. It is the metaphorical ice that Frost references. Heat Death is the theory that the universe will end via an incredibly slow and steady dissipation of all known matter into waste energy. The Big Rip is a much more dramatic, explosive end to the universe; thus, it is embodied by fire. In this scenario, everything from galaxies to black holes—and even the fabric of space time itself—would be ripped apart. But why would either of these scenarios happen? 

Despite having vastly different endings, these two scenarios stem from the same fact: our universe is not only expanding, but it is doing so at an accelerating rate. Previously, it was thought that the universe expanded at a constant rate (NASA), but observations of a phenomena known as red-shifting have proven that assumption wrong. The universe is expanding, so the space between objects in space is expanding. As the space between two objects expands, light travels from one object to another. Light waves are stretched as they travel through the expanding space, so their wavelengths becomes longer (European Space Agency). The longest wavelength of visible light is red, so stretched light waves appear red and the more a light wave has been stretched the redder it appears (European Space Agency). Cosmologists noticed the red-shifting of distant supernovas was increasing (Nobel Prize, 2011). If the universe were expanding at a constant rate, the red-shifting would stay the same. Therefore, the universe must be expanding at an accelerating rate. Acceleration does not happen without cause, so scientists concluded that there must be an unknown, invisible force causing the accelerating expansion (NASA). They called it dark energy (NASA). We know very little about dark energy. We do not know what it is made of, what it looks like, how much there is, how strong it is, or how dangerous it is, so there is a lot of uncertainty about theories concerning the end of the universe. Despite their uncertainty, cosmologists have produced two models of dark energy, one that would result in Heat Death and the other in the Big Rip. 

One model of dark energy, Heat Death, suggests that it is a cosmological constant (NASA). This model presents dark energy as an intrinsic and fundamental property of space which opposes gravity (Garner), preventing the universe from being crushed back into a tiny dense ball of everything by the force of gravity. The end result of this model is a universe which expands infinitely, eventually reaching rates of expansion faster than the speed of light. Imagine a racetrack with a prize at the finish line. The racers run at the speed of light. When the race starts, the track begins to expand in length, faster and faster, until the track is lengthening faster than the racers can run. Even running as fast as they are, they can never reach the end of the track and win the prize. Essentially, galaxies would be moving apart so fast that light emitted from one galaxy would never be able to reach another galaxy. Additionally, as the universe expands, it would slowly cool overtime due to the increasing entropy of the universe (Mack, 2020). The second law of thermodynamics states that the entropy, or disorder, of an isolated system increases over time (Encyclopædia Britannica, 2022). Essentially, entropy is the decay of organization. For example, when an apple is cut into slices, entropy increases. Entropy is the favored state of the universe, so energy is required to fight it. Ergo, it takes a lot more energy to stitch the slices back together than it did to slice the apple apart. Additionally, in every transformation or transfer of energy, a little bit of energy is lost and released as heat, increasing the entropy of the system (Encyclopædia Britannica, 2022). Continuing with the apple analogy, no matter how carefully the apple is stitched back together, it’s impossible to get all of the juice let out by slicing back into the apple. Theoretically, if the apple was sliced and stitched back together enough times, there would be no more apple to stitch back together. All of the apple would have been lost as entropy. Similarly, physicists expect that our universe will eventually dissipate into waste heat, or entropy. First, as established above, galaxies will become completely isolated. Then, much later, stars will begin to die (Mack, 2020). It will be impossible for new stars to form because the dense clouds of hydrogen and helium responsible for the formation of new stars will be scattered across the ever-expanding galaxy. Then, black holes will degenerate (Mack, 2020). By the time that the supermassive black holes degenerate, the basic particles of the universe such as hydrogen and helium will have dissipated into waste energy (Mack, 2020). There will be nothing left in the galaxy.  

However, Heat Death is not the only possible end of the universe. The other model of dark energy predicts a much more dramatic ending. The Big Rip model represents a universe in which the strength of dark energy increases as the universe expands (Mack, 2022).  In this scenario dark energy is referred to as phantom dark energy (Rutgers University). The core of this theory is that phantom dark energy eventually overwhelms all other forces in the universe. The phantom dark energy would begin by overwhelming the force of gravity between galaxies and the expansion of space would force galaxies further and further apart until they are completely isolated (Mack, 2022). Then, the phantom dark energy would overwhelm the gravity holding individual galaxies together, causing them to unwind and the objects within the galaxies, such as planets and stars, would be ripped apart as well (Mack, 2022). The theory also states that the electrical and nuclear forces holding together individual atoms would be unable to compensate for the increasing strength of the phantom energy, meaning atoms would be torn apart into their subatomic particles (Mack, 2020). Nuclei would split into singular protons and neutrons and then separated into their fundamental particles, quarks. Eventually, the universe would be expanding so quickly that the fabric of spacetime would rip (Mack, 2020). This would, of course, destroy the universe.  

In summary, Heat Death is the slow dissipation of all matter into entropy and the Big Rip is the tearing apart of all matter and the fabric of spacetime. Heat death occurs when the strength of dark energy is constant, whereas the Big Rip occurs when the strength of dark energy is increasing. Mathematically, the strength of dark energy is represented by the variable w (Rutgers University). If w is greater than negative one, but less than zero, Heat Death will occur (Rutgers University). However, if w is less than negative one the dark energy, then the strength of it is increasing and the Big Rip will occur (Rutgers University). The closer to negative one w is, the longer it will take for the Big Rip to occur and the further from negative one w is, the sooner the Big Rip will occur (Rutgers University). To understand the difference between the two scenarios, imagine the track analogy from before. In a Heat Death scenario, the track is expanding faster and faster, eventually lengthening faster than the racers can run. In a Big Rip scenario, the track is still expanding faster and faster, but the rate at which the track is lengthening is also getting faster and faster. This means that the track lengthens faster than the racers can run much earlier than in a Heat Death scenario. Essentially, in Heat Death the expansion of the universe is accelerating; however, in the Big Rip, the expansion of the universe is accelerating at an accelerating rate due to the increasing strength of phantom dark energy. That is what causes the stark difference between the two scenarios.  

It is difficult to determine which is more likely to occur. The Chandra Satellite calculated the value of w as approximately -0.991 (Vikhlinin, A., et al., 2008), meaning that dark energy is not strong enough to cause a Big Rip. However, a more recent study from 2014 calculated a w value of -1.124 (Rest, A., et al., 2014), indicating the Big Rip will occur, albeit not soon. While the 2014 study is more recent, it is not necessarily more reliable. Calculating the strength of dark energy is extremely complicated and there is room for lots of error in both studies. However, most physicists regard Heat Death as the most likely end to the universe. Heat Death is, at its core, just a continuation of the second law of thermodynamics, so it is less of a leap of logic than the Big Rip. Of course, both of these theories assume that the acceleration of the universe will continue. Some cosmologists theorize that the expansion will reverse and the universe will begin contracting or that we will hit a maximum expansion and stop at that size. However, these theories don’t hold much merit as there is no evidence the accelerating expansion will stop.  

The good news is that we will probably be dead by then, regardless of what occurs. The average lifespan of a mammalian species is one million years (Ord, 2020). Of course, humans are very different from the average mammalian species, but even if we managed to double or quadruple our expected lifespan it would still be trillions of years before Heat Death poses a threat (Dunbar). However, the accelerating expansion of the universe could complicate our quest for space travel. Humans have always looked to the stars and today it seems more likely than ever that we could one day live amongst the stars. Technocrats in Silicon Valley are drawing up plans for terraforming Mars and advancing space travel while idealist philosophers at Oxford dream of a utopian future in which our descendants traverse galaxies free of suffering, disease, and war (Ord, 2020). Intragalactic travel seems possible, even inevitable, assuming humans manage to stay alive long enough to develop light speed technology, but intergalactic travel is more complicated. As established previously, the universe will eventually be expanding so quickly that it will be impossible for light to travel from one galaxy to another. As such, even if we develop space crafts capable of light speed travel, it might still be impossible to reach other galaxies. It all depends on how long it takes to develop the technology and how quickly the universe expands. In a Big Rip scenario, we would have less time to develop the technology and be able to use it for intergalactic travel because the phantom dark energy would be accelerating the accelerating expansion of the universe. We would have more time to develop the technology for intergalactic travel if the universe truly is heading towards Heat Death because the universe would not be expanding as quickly.  

In conclusion, Heat Death is the slow dissipation of all matter into entropy until thermodynamic equilibrium is reached, whereas the Big Rip is more dramatic, causing the fabric of spacetime to tear. We have no way to definitively know which will occur, as it is very difficult to calculate the strength of dark energy to determine whether it is increasing, but physicists generally seem to support the theory of Heat Death over the Big Rip, if only because it is less of leap of logic. Regardless of which occurs, the point at which the two scenarios pose a legitimate threat to life is so far into the future that we need not worry about it. The only threat to us is us.  

Works Cited 

Mack, K. “How ‘Heat Death’ Will Destroy the Universe.” BigThink, YouTube, 25 Dec. 2020, https://www.youtube.com/watch?v=jRPkNMsYnLU. 

Mack, K. “The End of Everything (Astrophysically Speaking): Katie Mack.” Science and Nonduality, YouTube, July, 2022, https://www.youtube.com/watch?v=Z1AcdfMkyQE 

Mack, K. “How the Universe will End: The Big Rip | Katie Mack.” Penguin Books UK, Youtube, 2020, https://www.youtube.com/watch?v=Z1AcdfMkyQE 

“Dark Energy, Dark Matter.” NASA, NASA, https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy. 

Dunbar, Brian. “Dark Energy Changes the Universe.” NASA, NASA, https://www.nasa.gov/missions/deepspace/f_dark-energy.html. 

Garner, Rob. “WFIRST Will Help Uncover Universe’s Fate.” NASA, NASA, 12 Sept. 2019, https://www.nasa.gov/feature/goddard/2019/nasa-s-wfirst-will-help-uncover-universe-s-fate. 

“The Nobel Prize in Physics 2011.” NobelPrize.org, https://www.nobelprize.org/prizes/physics/2011/summary/. 

Ord, Toby. The Precipice. Bloomsbury Publishing, 2020.  

Phantom Energy: Dark Energy with W <-1 Causes a Cosmic Doomsday. Rutgers University. https://www.physics.rutgers.edu/~saurabh/690/Mar27-Zhang-phantom.pdf.  

Rest, A., et al. “Cosmological Constraints from Measurements of Type Ia Supernovae Discovered during the First 1.5 Years of the Pan-STARRS1 Survey.” ArXiv.org, 31 Aug. 2014, https://arxiv.org/abs/1310.3828. 

“Second Law of Thermodynamics.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., https://www.britannica.com/science/second-law-of-thermodynamics.  

Vikhlinin, A., et al. “Chandra Cluster Cosmology Project III: Cosmological Parameter Constraints.” NASA/ADS, 2014 https://ui.adsabs.harvard.edu/abs/2009ApJ…692.1060V/abstract.  

“What Is ‘Red Shift’?” ESA, https://www.esa.int/Science_Exploration/Space_Science/What_is_red_shift.