Kurzgesagt – In a Nutshell
Sources – Three Ways to Destroy the Universe
We thank following experts for their help
Prof. Matt Caplan
Illinois State University
– Our universe was born 14 billion years ago in the Big Bang and has been expanding ever since.
The latest estimates of the present age of the universe based on the properties of the cosmic microwave background indicate that our Universe is about 13.8 billions years old:
#Planck Collaboration (2020): “Planck 2018 results VI. Cosmological parameters”. Astronomy & Astrophysics, vol. 641
https://www.aanda.org/articles/aa/full_html/2020/09/aa33910-18/aa33910-18.html
This video will deal with huge time-spans, so exact figures will be unimportant. For the sake of simplicity, we’ve rounded off the estimated present age of the universe from 13.8 to 14 billion years.
– For some reason, new empty space is being created out of nothing between galaxies. Space itself is becoming bigger.
The fact that the universe has been expanding since its birth at the Big Bang is one of the cornerstones of modern cosmology. Its importance and various misconceptions are nicely explained in this article:
#Lineweaver, Charles and Davis, Tamara (2005): “Misconceptions about the Big Bang”. Scientific American, vol. 292, no. 3.
https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
Quote: “The expansion of the universe may be the most important fact we have ever discovered about our origins. You would not be reading this article if the universe had not expanded. Human beings would not exist. Cold molecular things such as lifeforms and terrestrial planets could not have come into existence unless the universe, starting from a hot big bang, had expanded and cooled. The formation of all the structures in the universe, from galaxies and stars to planets and Scientific American articles, has depended on the expansion.”
One common misconception about the expansion of the universe is that galaxies are “flying away” from some “center”, but this is not right. Leaving aside some relatively small local velocities, all galaxies are still in their places, like dots painted on the surface of an inflating balloon. The expansion of the universe happens because new space is constantly being created between galaxies:
#Lineweaver, Charles and Davis, Tamara (2005): “Misconceptions about the Big Bang”. Scientific American, vol. 292, no. 3.
https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
Quote: “When some familiar object expands, such as a sprained ankle or the Roman Empire or a bomb, it gets bigger by expanding into the space around it. Ankles, empires and bombs have centers and edges. Outside the edges, there is room to expand into. The universe does not seem to have an edge or a center or an outside, so how can it expand?
A good analogy is to imagine that you are an ant living on the surface of an inflating balloon. Your world is two-dimensional; the only directions you know are left, right, forward and backward. You have no idea what “up” and “down” mean. One day you realize that your walk to milk your aphids is taking longer than it used to: five minutes one day, six minutes the next day, seven minutes the next. The time it takes to walk to other familiar places is also increasing. You are sure that you are not walking more slowly and that the aphids are milling around randomly in groups, not systematically crawling away from you.
This is the important point: the distances to the aphids are increasing even though the aphids are not walking away. They are just standing there, at rest with respect to the rubber of the balloon, yet the distances to them and between them are increasing.”
– Today we think the universe is at the mercy of two cosmic demigods fighting for dominance. The first one is all the stuff in the universe: galaxies, gas, dark matter. While they are very different they just want to do one thing: get together. Matter is attracted by matter, pulled together by gravity. And gravity also pulls on the universe as a whole, slowing the expansion that started at the Big Bang.
Matter is attracted by matter due to the force of gravity. But gravity not only tends to bring galaxies, gas and dark matter closer together. This pull between galaxies also has an effect on the universe as a whole, i.e. it has an effect on the geometry of space itself: It tends to slow down the cosmic expansion.
#Riess, Adam and Turner, Michael (2004): “From Slowdown to Speedup”. Scientific American, vol. 290, issue 2.
https://www.scientificamerican.com/article/expanding-universe-slows-then-speeds/
Quote: “In Einstein’s theory, the notion of gravity as an attractive force still holds for all known forms of matter and energy, even on the cosmic scale. Therefore, general relativity predicts that the expansion of the universe should slow down at a rate determined by the density of matter and energy within it.”
– Our second warrior is empty space. While it seems, well, empty and powerless, empty space has an intrinsic energy. We don’t really have an idea what this energy is or why it is there. It’s really a placeholder that fills a blank in our understanding of reality. But it’s got a cool name: dark energy. Dark energy pushes things apart, a sort of “anti-gravity” that accelerates the cosmic expansion.
In 1998, astronomers discovered that, some 5 billion years ago, the expansion of the universe began to accelerate. This means that the universe is not only getting bigger, but doing so at an increasing speed. This strange acceleration happens because empty space itself seems to be infused with a mysterious substance dubbed “dark energy”:
#ESA (2020): “What is dark energy?”. ESA Science & Technology/Euclid (retrieved 2023)
https://sci.esa.int/web/euclid/-/what-is-dark-energy-
Quote: “Dark energy is an unidentified component of the Universe that is thought to be present in such a large quantity that it overwhelms all other components of matter and energy put together. According to the most recent estimates from ESA's Planck mission, dark energy contributes 68 percent of the matter-energy density of the Universe.
One way to envisage dark energy is that it seems to be linked to the vacuum of space. In other words it is an intrinsic property of the vacuum. So, the larger the volume of space, the more vacuum energy (dark energy) is present and the greater its effects. [...]
By 1998, the two teams had their results and instead of the expected deceleration, both had found that the expansion was accelerating. This was completely unexpected because nothing in known physics was capable of producing this effect. In keeping with the naming of the mysterious dark matter, astronomers began referring to whatever was causing the acceleration as dark energy.”
The true nature and origin of dark energy are one of the biggest mysteries of current cosmology. The only thing we know is that it is a kind of energy that completely permeates empty space and whose gravity opposes the gravity of matter – it pushes out instead of pulling in, accelerating the cosmic expansion:
#NASA (2018): “Weighing the entire universe: dark matter and dark energy”. Goddard Space Flight Center.
https://imagine.gsfc.nasa.gov/observatories/satellite/wmap/weighing.html
Quote: “Dark energy refers to the force that is accelerating the expansion of the universe. This is an "anti-gravity" force, sometimes referred to as quintessence or a cosmological constant. Gravity pulls things together, and this familiar force should act to slow down the expansion of the universe. Yet the universe continues to fly wide open faster and faster. No one knows why.”
It is important to note that although dark energy is sometimes described as exerting an “anti-gravity”, it doesn’t imply a new force of nature opposed to gravity. Instead, what dark energy exerts is an unusual form of gravity – one which is repulsive rather than attractive:
#Riess, Adam and Turner, Michael (2004): “From Slowdown to Speedup”. Scientific American, vol. 290, issue 2.
https://www.scientificamerican.com/article/expanding-universe-slows-then-speeds/
Quote: ”From the time of Isaac Newton to the late 1990s, the defining feature of gravity was its attractive nature. Gravity keeps us grounded. It slows the ascent of baseballs and holds the moon in orbit around the earth. Gravity prevents our solar system from flying apart and binds together enormous clusters of galaxies. Although Einstein’s general theory of relativity allows for gravity to push as well as pull, most physicists regarded this as a purely theoretical possibility, irrelevant to the universe today. Until recently, astronomers fully expected to see gravity slowing down the expansion of the cosmos. [...]
But general relativity also allows for the possibility of forms of energy with strange properties that produce repulsive gravity. The discovery of accelerating rather than decelerating expansion has apparently revealed the presence of such an energy form, referred to as dark energy.”
– So we have all the matter in the universe, pulling in, and empty space infused with dark energy, pushing out. Whoever wins will kill the universe in fun ways. But who will win? It all depends on the mysterious dark energy: Will its strength stay the same – a common assumption just because it keeps our models simple? Or will it get more powerful, or will it get weaker over time?
The future of the universe will depend on how strong the effect of dark energy will be relative to the gravitational pull between galaxies. The rest of this video explains in detail the three main scenarios.
– If the strength of dark energy stays constant, it will win the war.
In this video, when we talk about the “strength” of dark energy, we refer to its energy density, i.e. the amount of dark energy per unit volume of space. This energy density has been measured and, at least at present moment of cosmic time, it’s equivalent to the energy of about 3-4 protons per cubic meter:
#NASA (2014): “What is the Universe Made Of?”. WMAP mission.
https://wmap.gsfc.nasa.gov/universe/uni_matter.html
Quote: “WMAP determined that the universe is flat, from which it follows that the mean energy density in the universe is equal to the critical density (within a 0.5% margin of error). This is equivalent to a mass density of 9.9 x 10–30 g/cm3, which is equivalent to only 5.9 protons per cubic meter. Of this total density, we now (as of January 2013) know the breakdown to be:
4.6% Atoms. More than 95% of the energy density in the universe is in a form that has never been directly detected in the laboratory! The actual density of atoms is equivalent to roughly 1 proton per 4 cubic meters.
24% Cold Dark Matter. Dark matter is likely to be composed of one or more species of sub-atomic particles that interact very weakly with ordinary matter. Particle physicists have many plausible candidates for the dark matter, and new particle accelerator experiments are likely to bring new insight in the coming years.
71.4% Dark Energy. The first observational hints of dark energy in the universe date back to the 1980's when astronomers were trying to understand how clusters of galaxies were formed. Their attempts to explain the observed distribution of galaxies were improved if dark energy were present, but the evidence was highly uncertain. In the 1990's, observations of supernova were used to trace the expansion history of the universe (over relatively recent times) and the big surprise was that the expansion appeared to be speeding up, rather than slowing down! There was some concern that the supernova data were being misinterpreted, but the result has held up to this day. In 2003, the first WMAP results came out indicating that the universe was flat (see above) and that the dark matter made up only 24% of the density required to produce a flat universe. If 71.4% of the energy density in the universe is in the form of dark energy, which has a gravitationally repulsive effect, it is just the right amount to explain both the flatness of the universe and the observed accelerated expansion. Thus dark energy explains many cosmological observations at once.”
As quoted above, the total energy density of the universe (which includes normal matter, dark matter and dark energy) is equivalent to the mass-energy of 5.9 protons per cubic meter. About 71.4% of this energy corresponds to dark energy, so this means that the density of dark energy is equivalent to 5.9 × 0.714 = 4.2 protons per cubic meter. The exact value will vary depending on the astronomical survey used to derive the data, but all of them indicate a value of a few protons per cubic meter.
This value is extremely low. For comparison, the mass-energy of a cubic meter of air at standard conditions of pressure and temperature is about 1026 times higher. However, since –contrary to ordinary matter– dark energy completely fills the universe, it has a dominant role in its evolution. As explained below, if the measured value of the dark energy density stays consonant in the future, it will overcome the effect of matter and completely dominate the future of the universe.
– Since space is growing, matter is getting more and more diluted, like sugar in a cup being filled with more and more tea.
The mass-energy of matter (which means normal matter + dark matter) stays constant, since no new galaxies nor dark matter are being created. But since the universe is expanding, all kinds of matter are being constantly diluted as time goes on. This means that, on cosmic timescales, the effect of matter on the universe as a whole is decreasing.
– But as the universe expands, new empty space is created, which brings more dark energy, which pushes everything apart even more, which creates more empty space, which makes the universe grow even faster. A feedback loop that will make the universe expand at an exponential rate. Every 12 billion years or so it will double in diameter – forever.
As quoted above, dark energy is linked to empty space itself. Therefore, if its density stays constant, its total amount –and its effects on the universe as a whole– will increase as the universe expands:
#ESA (2020): “What is dark energy?”. ESA Science & Technology/Euclid (retrieved 2023)
https://sci.esa.int/web/euclid/-/what-is-dark-energy-
Quote: “One way to envisage dark energy is that it seems to be linked to the vacuum of space. In other words it is an intrinsic property of the vacuum. So, the larger the volume of space, the more vacuum energy (dark energy) is present and the greater its effects.”
In the scientific jargon, this possibility is known as dark energy being given by a “cosmological constant”.
As matter is constantly being diluted as the universe becomes bigger, its density will effectively become zero in the far future. However, since dark energy is attached to empty space, an expanding universe will only bring more and more dark energy. This in turn accelerates the expansion of the universe, creating more space and more dark energy. The result is an exponential expansion in which the universe doubles in size every 12 billion years or so:
#Baez, John (2016): “The End of the Universe”. Department of Mathematics, University of California Riverside.
https://math.ucr.edu/home/baez/end.html
Quote: “As the universe expands these things eventually spread out to the point where each one is completely alone in the vastness of space. If dark energy works as we expect, the distance between things that aren't gravitationally bound to each other will double every 12 billion years.”
– But while dark energy is winning the war, matter is winning at least one battle: At short distances, it can keep things together. Local galaxy bubbles can overcome the push of dark energy.
Although dark energy tends to push things apart, if it stays constant, it will push things apart with a constant “strength” at all distances. However, the gravitational pull between galaxies increases as the galaxies become closer to each other. This means that the fight between the gravitational pull of matter and the push of dark energy will depend on the distance between objects. For two galaxies that are very far away, the pull of gravity will be very weak and the push exerted by dark energy will prevail. But for galaxies that are close enough, the pull of gravity will overcome the push of dark energy.
For the measured value of the dark energy density quoted above (equivalent to just a few protons per cubic meter), this means that, although dark energy will dominate the universe at the largest scales, galaxy clusters will remain bounded by the pull of gravity:
#Mack, Katie (2020): “Tearing Apart the Universe”. American Scientist, vol. 108, number 6.
https://www.americanscientist.org/article/tearing-apart-the-universe
Quote: “Dark energy is often assumed to be a cosmological constant that stretches space out, accelerating cosmic expansion by imbuing the universe with some inherent inclination for swelling. On large scales, this is a pretty good description. But within galaxies, solar systems, or in the close vicinity of organized matter generally, a cosmological constant has no effect. It can be more properly thought of as a force for isolation—if two galaxies are already distant from one another, they get more distant, and individual galaxies, clusters, or groups of galaxies find themselves more and more alone as time goes on. They also form a bit more slowly in the presence of a cosmological constant than they otherwise would. What the cosmological constant cannot do is break apart anything that is already, in any sense, a coherent structure: What therefore gravity hath joined together, let not a cosmological constant put asunder.
The reason for this small mercy of the cosmological constant (which, to be fair, does still destroy the whole universe eventually) lies in the “constant” part of the story. If dark energy is a cosmological constant, its defining feature is that the density of dark energy in any given part of space is constant over time, even as space expands. The expansion rate isn’t constant, just the density of the stuff itself, in any given volume of space. This makes sense in a way, if every bit of space is automatically assigned a set amount of dark energy within it, but it’s still super weird, because it means that as space gets bigger, the amount of dark energy increases to keep the density constant. It also means that if you draw a sphere of a given size anywhere in the universe and measure the amount of dark energy inside the sphere, and then do the same at some future time, you’ll always get the same number, regardless of how much the outside universe has expanded in the meantime. If your original sphere contains a cluster of galaxies and some quantity of dark energy, in a billion years the amount of dark energy in that region will still be the same, so if it wasn’t enough to mess up the galaxy cluster before, it won’t be in the future. The balance between matter and dark energy in that sphere does not significantly change even as the rest of the cosmos seems to inexorably empty out.”
– In a few billion years, our local group of galaxies will merge into a gigantic ball with trillions of stars.
For nearby galaxies, the pull of gravity will be large enough to overcome the outward push exerted by dark energy. Our nearby galaxies include Andromeda, Triangulum and others. Andromeda and the Milky Way (and later the other galaxies) are expected to merge in a few billion years from now:
#NASA (2018): “Crash of the Titans: Milky Way & Andromeda Collision”. Scientific Visualization Studio
https://svs.gsfc.nasa.gov/30955
Quote: “The three largest galaxies in our Local Group of Galaxies are our Milky Way along with the Andromeda (also known as Messier 31) and Triangulum (also known as Messier 33) galaxies. This scientific visualization of a computer simulation depicts their joint evolution over the next several billion years and features the inevitable massive collision between the Milky Way and Andromeda.”
The number of stars in the Milky Way is of the order of 100 billion and that of Andromeda is of the order of one trillion:
#NASA (2011): “Andromeda is So Hot 'n' Cold”. Herschel Mission (retrieved 2023)
https://www.nasa.gov/mission_pages/herschel/pia13771.html
Quote: “Andromeda is our Milky Way galaxy's nearest large neighbor. It is located about 2.5 million light-years away and holds up to an estimated trillion stars. Our Milky Way is thought to contain about 200 billion to 400 billion stars.”
Therefore, after merging with the other (smaller) galaxies of the Local Group, the number of stars in the resulting supergalaxy will be of the order of trillions of stars.
– It will soon become our last view of the cosmos. All other galaxies will be pushed away by the expansion. For us, it will look like the rest of the universe is rushing away, until in a few hundred billion years we won’t see other galaxies at all. We will be alone, surrounded by a seemingly infinite, dark void.
All other galaxies beyond our local cluster will be carried away by the exponential expansion of the universe. Technically speaking they would still be visible, but they won’t be in practice: As they are pushed further and further away by the cosmic expansion, their light will be more and more redshifted (i.e. the wavelengths of the light they emit will become larger and larger). In a few hundred billion years from now, this redshift will be so extreme that they will be undetectable:
#Krauss, Lawrence and Starkman, Glenn (2000): “Life, the Universe, and Nothing: Life and Death in an Ever-expanding Universe”. The Astrophysical Journal, Volume 531, Number 1
https://iopscience.iop.org/article/10.1086/308434/meta
Quote: “Thus, in roughly 150 billion years, light from all objects outside our Local Supercluster will have redshifted by more than a factor of 5000, with each successive 150 billion years bringing an equal redshift factor. In a little less than 2 trillion years, all extrasupercluster objects will have redshifted by a factor of more than 1053. Even for the highest energy gamma rays, a redshift of 1053 stretches their wavelength to greater than the physical diameter of the horizon. [...] The resolution time for such radiation will exceed the physical age of the universe.”
– In about 100 trillion years, all stars of our supergalaxy will have died out. All gas that could create new stars has been consumed, and no new gas can come in. The galaxy will be dark and filled with stellar corpses. Over quadrillions of years, white dwarfs and neutron stars will slowly cool until becoming truly dark – turning off the last lights of the universe.
It is thought that after some 100 trillion years, the “stelliferous era” (“star-forming age”) will come to an end:
#Busha, Michael T. et al. (2003): “Future Evolution of Cosmic Structure in an Accelerating Universe”, The Astrophysical Journal, Volume 596
And since the longest lived stars have a much lower expected lifespan (about 17 trillion years, see table above), this means that the last star will also die in about 100 trillion years from now.
Interstellar space is filled with gigantic clouds of gas and dust. Stars form when the densest chunks of these cosmic clouds collapse under their own weight. The image below shows a big cosmic cloud of about 1,500 light-years in diameter giving rise to new stars (the bright blue dots scattered inside the cloud):
#NASA (2012): “Giant Stellar Nursery”. Hubble Space Telescope (retrieved 2023)
https://www.nasa.gov/multimedia/imagegallery/image_feature_2409.html
Quote: “Stars are sometimes born in the midst of chaos. About 3 million years ago in the nearby galaxy M33, a large cloud of gas spawned dense internal knots which gravitationally collapsed to form stars. NGC 604 was so large, however, it could form enough stars to make a globular cluster. Many young stars from this cloud are visible in this image from the Hubble Space Telescope, along with what is left of the initial gas cloud. Some stars were so massive they have already evolved and exploded in a supernova. The brightest stars that are left emit light so energetic that they create one of the largest clouds of ionized hydrogen gas known, comparable to the Tarantula Nebula in our Milky Way's close neighbor, the Large Magellanic Cloud.”
After stars are born, they start to synthesize new chemical elements and partially re-inject new gas into the interstellar medium. After trillions of years, however, this process will eventually come to a halt, since all the available interstellar gas will have been used in creating new stars and all that will remain will be stellar corpses like white dwarfs, neutron stars and black holes, which are no longer able to re-inject new gas into space. If no new gas can enter the galaxy from the outside (because all other galaxies will have been taken so far away by the cosmic expansion), all reserves of gas will have been exhausted and no new stars will be able to form. As mentioned above, it is expected that star formation will end in about 100 trillion years.
– All structures, big and small, will slowly dissolve. One by one, all dead stars and planets will leave the supergalaxy, which slowly dissolves over sextillions of years. Every object will end up on its own, which means that dark energy takes over again, creating more and more empty space between everything. Objects will be so far apart that it will be as if each had a universe for itself. Not much happens anymore, until in a googol years, all black holes will have evaporated.
The dead stars will eventually leave the galaxy even if the galaxy exerts a gravitational pull on them because, over eons, stars will randomly collide against each other and these collisions will end up pushing most of the stars out the galaxy. This process is known as “stellar evaporation”. A detailed timeline of the events that will unfold in the long term future of an expanding universe is given in table 10.2 of the following classical book by astronomer John Barrow and physicist Frank Tipler (for reference, a sextillion years is 1021 years a and a googol years is 10100 years; the above figures are only intended to give approximate orders of magnitude):
#Barrow, John and Tipler, Frank (1986): The Anthropic Cosmological Principle, Oxford University Press.
– In the end, entropy and dark energy won’t stop until their job is finished. Over time spans you might as well call forever, all remaining structures might even dissolve into single particles that will be pushed away from each other by an ever growing empty space. Imagine a whole universe – with just a single, lonely particle traveling through nothingness.
As indicated above, over extremely long timescales all matter will turn to iron (the most stable atomic nucleus) due to quantum processes and later collapse to black holes, which will eventually evaporate. In the end, all structures will disintegrate into its constituent and most stable particles:
#Baez, John (2016): “The End of the Universe”. Department of Mathematics, University of California Riverside.
https://math.ucr.edu/home/baez/end.html
Quote: “But the overall picture seems to lean heavily towards a far future where everything consists of isolated stable particles: electrons, neutrinos, and protons (unless protons decay). If the scenario I'm describing is correct, the density of these particles will go to zero, and eventually each one will be cut off from all the rest by a cosmological horizon, making them unable to interact. Of course there will be photons as well, but these will eventually come into thermal equilibrium forming blackbody radiation at the temperature of the cosmological horizon — perhaps about 10–30 Kelvin or so.”
– What if dark energy gets stronger? In this case empty space won’t just win over matter – it will literally rip it to pieces. In the Big Freeze scenario, matter lost the war but won some battles. But here matter wins nothing. Dark energy is growing stronger over time, overcoming the pull of gravity and creating new empty space on smaller and smaller distances.
Although a constant dark energy density (or a cosmological constant) is the simplest option to describe dark energy, it is not the only one. Another possibility is that dark energy is given by a field (like the magnetic field, say) that fills the universe. In such a case, the properties and future evolution of dark energy will depend on the dynamics of that field. Such a possibility has been studied over the last years in various contexts.
One of the most studied models is given by the so-called “phantom energy”. In this scenario, dark energy is explained by a field (a “phantom field”) that fills the universe and whose internal dynamics is such that it makes its energy density grow with time. This model was put forward in the following article and has been much studied ever since:
#Caldwell, R. R. (2002): “A phantom menace? Cosmological consequences of a dark energy component with super-negative equation of state”. Physics Letters B, volume 545, issues 1–2.
https://www.sciencedirect.com/science/article/abs/pii/S0370269302025893
https://arxiv.org/abs/astro-ph/9908168 (open-access version)
Quote: “The phantom energy has positive energy density, ρp > 0, but negative pressure, such that ρp + pp < 0. (It is no fatal flaw for a component to violate the dominant energy condition for a finite time, as can arise from a bulk viscous stress due to particle production [2].) It immediately follows from the equation of state w < −1 that the phantom energy density grows with time.”
The main consequence of this models is that the dark energy density grows in such a way that it becomes infinite in a finite amount of time, completely destroying all structures and destroying the universe in a dramatic comic event dubbed “Big Rip”:
#Caldwell, R. R. et al. (2003): “Phantom Energy: Dark Energy with w < −1 Causes a Cosmic Doomsday”. Physical Review Letters 91, 071301.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.91.071301
https://arxiv.org/abs/astro-ph/0302506 (open-access version)
Quote: “The positive phantom-energy density becomes infinite in finite time, overcoming all other forms of matter, such that the gravitational repulsion rapidly brings our brief epoch of cosmic structure to a close. The phantom energy rips apart the Milky Way, solar system, Earth, and ultimately the molecules, atoms, nuclei, and nucleons of which we are composed, before the death of the Universe in a “big rip.” “
The implications of this scenario are explained below.
– In this scenario things will escalate quickly – it could start as early as in 20 billion years from now.
The timeline of the phantom energy scenario depends on the particular details of the phantom field. These are unknown, so scientists have calculated the possible physical outcomes under various assumptions for the parameters defining the mathematical model. For concreteness, in this video we’ve taken the example analyzed in the following well-known article, in which the Big Rip happens in about 20 billion years from now:
#Caldwell, R. R. et al. (2003): “Phantom Energy: Dark Energy with w < −1 Causes a Cosmic Doomsday”. Physical Review Letters 91, 071301.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.91.071301
https://arxiv.org/abs/astro-ph/0302506 (open-access version)
Quote: “For example, for w = −3/2 and H0 = 70 km sec−1 Mpc−1, the time remaining before the Universe ends in this “Big Rip” [31] is 22 Gyr.”
– First dark energy will create empty space between individual galaxies. Our galaxy will leave its local cluster and begin to drift alone in a rapidly inflating and ever darker cosmos. Some billion years later empty space starts to push between individual stars, dissolving the galaxy. If you lived on a planet in a star system, the night sky will start looking sad and gloomy, as other stars are pushed away too far to be seen.
A few million years after the sky turns dark, dark energy starts to create empty space inside star systems. Your planet is pushed away from its star and all life in the universe freezes to death. There is not much time left, as a few months later dark energy is creating empty space inside solid objects.
Stars, neutron stars, planets, asteroids, everything solid is being ripped into pieces. If you are on a spaceship, you only have a short time before you are ripped apart. Half an hour later, even atoms are destroyed as new space is being created so furiously that electrons and nuclei are separated. Now the universe just has a fraction of a second left.
The timing of all the events mentioned in the preceding paragraphs can be read off from the following table summarizing the consequences of the specific model mentioned above:
#Caldwell, R. R. et al. (2003): “Phantom Energy: Dark Energy with w < −1 Causes a Cosmic Doomsday”. Physical Review Letters 91, 071301.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.91.071301
https://arxiv.org/abs/astro-ph/0302506 (open-access version)
– In this final moment, only dying black holes are left, drained and defeated by dark energy. They are tiny, septillions of times smaller than an atom – and they explode with the power of a trillion supernovae in a trillionth of an octillionth of a second.
What about black holes? A curious consequence of this scenario is that, as the universe approaches the Big Rip, black holes will absorb the phantom energy in a way that works as if the black hole was absorbing negative energy. This drains the mass of all black hole, making them smaller and smaller as the universe nears the Big Rip:
#Babichev, E. et al (2004): “Black Hole Mass Decreasing due to Phantom Energy Accretion”. Physical Review Letters 93, 021102.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.93.021102
https://arxiv.org/abs/gr-qc/0507119 (open-access version)
Quote: “Solution for a stationary spherically symmetric accretion of the relativistic perfect fluid with an equation of state p(ρ) onto the Schwarzschild black hole is presented. This solution is a generalization of Michel solution and applicable to the problem of dark energy accretion. It is shown that accretion of phantom energy is accompanied by the gradual decrease of the black hole mass. Masses of all black holes tend to zero in the phantom energy Universe approaching the Big Rip.”
In the final moment before the Big Rip, all black holes have reached the mass known as “Planck mass” (about 20 micrograms) and “evaporate” (explode) via the phenomenon known as Hawking radiation (a well understood black hole process unrelated to phantom energy):
#Babichev, E. et al (2004): “Black Hole Mass Decreasing due to Phantom Energy Accretion”. Physical Review Letters 93, 021102.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.93.021102
https://arxiv.org/abs/gr-qc/0507119 (open-access version)
Quote: “This means that the phantom energy accretion prevails over the Hawking radiation until the mass of black hole is the Planck mass. However, formally all black holes in the universe evaporate completely at Planck time before the Big Rip due to Hawking radiation.”
A black hole of the Planck mass will fully evaporate in a time close to the Planck time (about 10–44 sec), which means that all its mass (20 micrograms) will converted into energy via E = mc2 in that time, implying a power (energy liberated per unit time) of about 1047 watts (joules per second):
#Toth, Viktor T: “Hawking Radiation Calculator” (used 2023)
The total energy liberated by such a black hole explosion is not that big (about 2·109 joules), but it is emitted in such a short time that the energy liberated per unit time is way higher than that of a supernova:
#Branch, D. (2003): Supernovae. In: Encyclopedia of Physical Science and Technology, Academic Press
https://www.sciencedirect.com/topics/physics-and-astronomy/type-ia-supernovae
Quote: “At its brightest, a normal Type Ia supernova (SN Ia) reaches an absolute visual magnitude of −19.5 and has a luminosity exceeding 1043 erg/sec, billions of times that of the Sun.”
1 erg is 10–7 joules, which means that the power of a typical supernova is of the order of 1036 watts. Given that the power of our Planck-mass black hole explosion is almost of the order of 1048 watts, this is about 1012 (one trillion) times more powerful than a supernova.
The time in which this energy is liberated is of the order of 10–40 seconds, which is basically a trillionth (10–12) of an octillionth (10–27) of a second.
Finally, the size of these Planck-mass black holes is of the order of 10–35 m (see table above), which is about a septillion (1024) times smaller than a hydrogen atom (about 10–11 m).
– Finally, the very fabric of reality is torn to its core, obliterating spacetime itself. The Big Rip. Space and time have lost their meanings, making it impossible to predict what will happen next.
Once the Big Rip comes, spacetime itself is torn apart:
#Mack, Katie (2020): “Tearing Apart the Universe”. American Scientist, vol. 108, number 6.
https://www.americanscientist.org/article/tearing-apart-the-universe
Quote: “In the last tiny fraction of a second, molecules crack open, and any thinking beings still holding on are destroyed, torn atom-from-atom from within. Beyond that point, there is no possibility of watching the destruction, but it carries on nonetheless. Nuclei themselves, the ultradense matter in the centers of atoms, are the next to go. The impossibly dense cores of black holes are eviscerated. And at the final instant, the fabric of space itself is ripped apart.”
– Poor matter. But there is one scenario where it wins: If the strength of dark energy decreases with time, and if this decrease is strong enough, the pull of gravity will win and all the stuff in existence will move towards each other – unfortunately making the universe collapse onto itself.
The last possibility corresponds to the case in which the energy density of the dark energy field decreases with time, and it does so in a drastic enough way to firs halt and then reverse the cosmic expansion. This is a feature of many models in which dark energy is given by a so-called “quintessence” field:
#Andrei, Cosmin et al. (2022): “Rapidly descending dark energy and the end of cosmic expansion”. Proceedings of the National Academy of Sciences, vol. 119, no. 15.
https://www.pnas.org/doi/10.1073/pnas.2200539119
Quote: “If dark energy is a form of quintessence driven by a scalar field ϕ evolving down a monotonically decreasing potential V(ϕ) that passes sufficiently below zero, the universe is destined to undergo a series of smooth transitions. The currently observed accelerated expansion will cease; soon thereafter, expansion will come to end altogether; and the universe will pass into a phase of slow contraction.”
– No one knows when this may begin, but it could be as soon as in a few hundred million years. How will it look?
As in the previous scenario, the exact timing of such a cosmic evolution will depend on the specific mathematical parameters chosen to describe the quintessence field, which are unknown. However, scientists have proposed models in which the quintessence field could start reversing the cosmic expansion in just 100 million years from now – a surprisingly short time in an astronomical context:
#Andrei, Cosmin et al. (2022): “Rapidly descending dark energy and the end of cosmic expansion”. Proceedings of the National Academy of Sciences, vol. 119, no. 15.
https://www.pnas.org/doi/10.1073/pnas.2200539119
Quote: “Although the universe is expanding at an accelerating rate today, this paper presents a simple mechanism by which a dynamical form of dark energy (known as quintessence) could cause the acceleration to come to end and smoothly transition from expansion to a phase of slow contraction. That raises questions, How soon could this transition occur? And at what point would it be detectable? The conclusions are that the transition could be surprisingly soon, maybe less than 100 million y from now, and yet, for reasons described in the main text, it is not yet detectable today.”
– As the universe begins to contract, over billions of years, galaxies and galaxy clusters approach each other until they eventually collide. They are mostly made of empty space, so a collision is like the gentle merger of two clouds. At any rate, first galaxies and later individual stars get closer and closer.
Galaxies or galaxy clusters are not “solid” objects, so a collision between them is more similar to en encounter between clouds that a crash between, say, two rocks:
#Royal Astronomical Society (2018): “Supercomputer simulation of a collision between two galaxy clusters”. RAS YouTube Channel (retrieved 2023)
https://www.youtube.com/watch?v=aDyohDWYPF8
Quote: “A supercomputer simulation of a collision between two galaxy clusters, similar to the real object known as the 'Bullet Cluster’, and showing the same effects tested for in Abell 3827. All galaxy clusters contain stars (orange), hydrogen gas (shown as red) and invisible dark matter (shown as blue). Individual stars, and individual galaxies are so far apart from each other that they whizz straight past each other. The diffuse gas slows down and becomes separated from the galaxies, due to the forces between ordinary particles that act as friction. If dark matter feels only the force of gravity, it should stay in the same place as the stars, but if it feels other forces, its trajectory through this giant particle collider would be changed.”
– As the universe goes on collapsing, you might worry about stars and planets eventually crashing against each other. This will happen, but it’s not your worst problem. If space itself shrinks, this also concentrates all the radiation emitted in the past by all the stars, supernovae and quasars that ever existed. Now ‘empty’ space is filled with radiation, the dark nothing between stars is heating up, making life unpleasant and then impossible as planets just burn. Slowly at first, then rapidly, space gets as hot as it was after the big bang. Stars are pretty hot, but now the space around them is hotter –they are literally boiled from the outside.
This process has been vividly described in the following book by astrophysicist Katie Mack:
#Mack, Katie (2020): The End of Everything: (Astrophysically Speaking). Simon & Schuster
https://www.simonandschuster.com/books/The-End-of-Everything/Katie-Mack/9781982103552
https://www.sciencefriday.com/articles/end-of-everything-excerpt/ (excerpt)
Quote: “The expansion of the universe as it is occurring today does more than just stretch out the light of distant galaxies. It also stretches out and dilutes the afterglow of the Big Bang itself. One of the strongest pieces of evidence for the Big Bang is the fact that we can actually see it, simply by looking far enough away. What we see, specifically, is a dim glow, coming from all directions, of light produced in the universe’s infancy. That dim glow is actually a direct view of parts of the universe that are so far away that, from our perspective, they are still on fire—they’re still experiencing the hot early stage of the universe’s existence, when every part of the cosmos was hot and dense and opaque with roiling plasma, like the inside of a star. The light from that long-burned-out fire has been traveling to us all this time, and, from sufficiently distant points, has just now arrived.
The reason we experience this as a low-energy, diffuse background (the cosmic microwave background) is that the expansion of the universe has stretched out and separated the individual photons to the point that they’re now merely a bit of faint static. And the fact that they show up as microwaves is due to extreme redshifting. The expansion of the universe can do a lot, including taking the heat of an unimaginable inferno and diluting and stretching it out until it’s just a faint microwave hum we might experience only as a tiny bit of static on an old-fashioned analog TV.
If the expansion of the universe reverses, this diffusion of radiation does too. Suddenly the cosmic microwave background, that innocuous low-energy buzz, is blueshifting, rapidly increasing in energy and intensity everywhere, and heading toward very uncomfortable levels.
But that’s still not what kills the stars.
It turns out that there is something that can create more high-energy radiation than concentrating the afterglow from space itself being on fire. As the universe has evolved over time, it has taken what was, at the very beginning of the cosmos, a fairly uniform collection of gas and plasma and used gravity to collect that gas into stars and black holes (and other minor things like planets and people, but for the purpose of this discussion we can ignore those). Those stars have been shining for billions of years, sending their radiation out into the void to be dispersed, but not to disappear. Even the black holes have had their chance to shine, producing X-rays as the matter falling into them heats up and creates high-energy particle jets. The radiation produced by stars and black holes is even hotter than the final stages of the Big Bang, and when the universe recollapses, all that energy gets condensed too. So rather than being a nicely symmetric process of expansion and cooling followed by coalescence and heating, the collapse is actually much worse. If you’re ever asked to choose between being at a random point in space just after the Big Bang, or just before the Big Crunch, choose the former. (To quote the legendary D:Ream, “things can only get better.”) The collected radiation from stars and high-energy particle jets, when suddenly condensed and blueshifted to even higher energies by the collapse, will be so intense it will begin to ignite the surfaces of stars long before the stars themselves collide. Nuclear explosions tear through stellar atmospheres, ripping apart the stars and filling space with hot plasma.”
– As the universe collapses into itself, all galaxies and all stars merge into a single ball of hot plasma – the Big Crunch is complete. From here on there are two possibilities. Either the universe will collapse completely into a singularity, a point of zero size and infinite density, without space and time. Something it might have been before the big bang. Or the universe could “bounce back”, restarting the cosmic expansion, creating a new baby universe. Somewhat poetic, everything died, but everything is reborn. But to be clear, we have zero evidence that this happened before or will in the future.
The endpoint of a Big Crunch has been discussed by cosmologists over the years. According to general relativity, the endpoint should be a singularity similar to the one that was supposed to give rise to the Big Bang: a “point of infinite density” at which the theory loses its predictive power. However, physicists think that before reaching an actual singularity, the (as yet unknown) quantum effects of gravity will come into play, which may restart a new phase of expansion:
#Mack, Katie (2020): The End of Everything: (Astrophysically Speaking). Simon & Schuster
https://www.simonandschuster.com/books/The-End-of-Everything/Katie-Mack/9781982103552
https://www.sciencefriday.com/articles/end-of-everything-excerpt/ (excerpt)
Quote: “A collapsing universe will, in the final stages, reach densities and temperatures beyond what we can produce in a laboratory or describe with known particle theories. The interesting question becomes not “Will anything survive?” (because by this point it is very clear that the answer to that is a straightforward No), but “Can a collapsing universe bounce back and start again?”
Cyclic universes that go from Bang to Crunch and back again forever have a certain appeal in their tidiness. Rather than a beginning from nothing and catastrophic, final end, a cycling universe can in principle bounce along in time arbitrarily far in each direction, with endless recycling and no waste.
Of course, like everything in the universe, it turns out to be significantly more complicated. Based purely on Einstein’s theory of gravity, general relativity, any universe with a sufficient amount of matter has a set trajectory. It starts with a singularity (an infinitely dense state of spacetime) and ends with a singularity. There isn’t really a mechanism in general relativity to transition from an end-singularity to a beginning one, however. And there is reason to believe that none of our physical theories, general relativity included, can describe the conditions of anything close to that kind of density. We have a pretty good understanding of how gravity works on large scales, and for relatively (ha!) weak gravitational fields, but we have no idea how it works on extremely small scales. And the kinds of field strengths you’d encounter when the entire observable universe is collapsing into a subatomic dot are all kinds of incalculable. We can be fairly confident that for that particular situation, quantum mechanics should become important and do something to make a mess of things, but we honestly don’t know what.”
– Most scientists think that dark energy will stay constant, so the likely fate of the universe is the Heat Death – eternal cold and utter boredom.
Today, the most accepted cosmological model is the so-called “Lambda-CDM” model of the universe, in which dark energy is described by a cosmological constant (usually denoted by the greek letter Lambda, hence the name of the model):
#ESA (2023): “Mission Science”. Euclid Mission (retrieved 2023)
Quote: “Astronomers have called these components dark matter and dark energy to reflect their mysterious nature. Dark matter adds gravity to the Universe on the scale of galaxies and clusters of galaxies, whereas dark energy acts on the larger scale and causes the accelerated expansion of the Universe.
By combining this dark sector with ordinary matter cosmologists have constructed an extraordinarily accurate description of the Universe's behaviour. It is called the lambda-CDM model, where lambda indicates the dark energy and CDM stands for cold dark matter.
Lambda is also known as the Cosmological Constant. It was first introduced by Albert Einstein in his model of the Universe. At the time everyone believed in the assumption that the Universe was a static, unchanging place. So, Einstein used lambda to help balance the gravity in the Universe. [...]
ESA's Planck mission mapped the CMB to unprecedented levels of precision between 2009 and 2013. Simulations of the Universe, based on lambda-CDM gave such a good fit to the observations that when Planck's first all-sky map was released in March 2013, the cosmologists involved called it an almost perfect Universe.”
As explained above, the long-term evolution of a universe in which dark energy is given by a cosmological constant will end in the Heat Death, or Big Freeze.
A cosmological constant is not only the simplest possibility to describe dark energy from the theoretical point of view, but is also in agreement with the observations of the early universe, which until now have shown no evidence that the density of dark energy could have varied with time. In this sense, phantom energy models (leading to a Big Rip) or quintessence models (leading to a Big Crunch) are more constrained and, although widely studied, aren’t considered the most likely explanation of dark energy by most cosmologists.
– Which seems sad but has a huge upside: In this scenario we get to have the universe for the longest. It gives us trillions of years to expand, jump from star to star, maybe even from galaxy to galaxy. We might even find a way to keep consciousness around forever. We don’t know. So we just have to wait and see, and make the most of the amazing universe we have right now.
The possibility of keeping an artificial consciousness “alive” for an infinite amount of time in an eternally expanding universe was considered in 1979 by physicist Freeman Dyson in this classical paper:
#Dyson, Freeman (1979): “Time without end: Physics and biology in an open universe”. Reviews of Modern Physics, vol. 51, 447.
https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.51.447
Quote: “Quantitative estimates are derived for three classes of phenomena that may occur in an open cosmological model of Friedmann type. (1) Normal physical processes taking place with very long time-scales. (2) Biological processes that will result if life adapts itself to low ambient temperatures according to a postulated scaling law. (3) Communication by radio between life forms existing in different parts of the universe. The general conclusion of the analysis is that an open universe need not evolve into a state of permanent quiescence. Life and communication can continue for ever, utilizing a finite store of energy, if the assumed scaling laws are valid.”
However, Dyson’s model was conceived for an ever-expanding universe lacking dark energy (dark energy was discovered two decades later) and it is not clear that it could work in a future expanding universe infused with dark energy.