Somewhere Out There
The idea of deep space is both tantalizing and terrifying. To most people, the prospect of ever seeing another planet seems preposterous, let alone living on one, and many major scientists believe that the human race will never exit our solar system.[1] But still, people dream. Anyone who’s watched Doctor Who, or consumed really any sci-fi, is probably entranced by the idea of flying among the stars and onto faraway planets. In a way, it's almost comforting to believe that there’s an endless world out there for our descendants to explore.
But unfortunately, physics informs us that the idea of a boundless universe for human exploration is at best unlikely, and at worst hopelessly naive. To understand why, we need to go back to its very beginning and follow the progress of our world over the millennia.
“all of time and space, everything that ever happened or ever will - where do you want to start?” -Doctor Who Season 5, “The Eleventh Hour”
The Big Bang Theory is a TV show. But, it‘s also the most widely accepted origin for the universe and the basis for almost all of modern physics. It’s pretty well known, but there are still a lot of important details that people are unaware of, so here’s a quick rundown:
In the beginning there was a single point containing all the matter and energy of the universe. It’s impossible to know where it came from, but the unstable singularity began to expand into the universe. Temperatures in the range of trillions of degrees Kelvin[2] caused expansion to be far faster than the speed of light, and the gravity that kept the singularity together gave way.[3] As the newborn universe expanded, overall temperatures lowered resulting in a goop (scientific term) of high-energy basic particles that were constantly being formed and reformed through annihilation and pair production. Eventually, temperatures dropped even more, the production of new particles through pair production effectively ceased, leaving particles and antiparticles (the opposites of normal particles) to destroy one another.[4] In the end, only a minuscule number of the total particles remained (their numbers were greater than antiparticles), and the universe as we know was born.
Eventually, the original extreme expansion slowed, and gravity reasserted itself,[5] compressing newly formed atoms into galaxies, stars, planets, and you. Gravity created a universe of heterogeneity with massive quantities of matter existing in galaxies and stars, and gaps of empty space in between. Even though it had become mostly irrelevant for a time, the fundamental expansion of the universe remained, but just slowed down.[6]
“She’s not fat. She’s just bigger on the inside.” -Doctor Who: Shroud of Sorrow
If the universe as we know it is affected by expansion, how are planets, stars, comets, and people altered?
The simplest way to think about the expansion of the universe is to think about it in a non-3-dimensional way. Imagine a big piece of saran wrap with dots on it. The dots are matter in the universe, and the rest of the saran wrap is just empty space. If you stretch the surface out, the dots get further apart; matter doesn’t expand in size, but the space between it does. Of course, that begs the question of what matter is expanding into. For more on that, see “We Live in a Donut?”
But, here’s the issue: the rate of the universe’s expansion, while relatively slower, is still really fast. So, why isn’t all the matter in the universe falling apart from the sheer force of an expanding universe? Well, the four fundamental forces of the universe are keeping matter together. You probably know something about how forces work, and the four fundamental forces are the interactions at the subatomic level that facilitate higher-level forces. They’re why atoms and planets don’t get pulled apart, but solar systems and galaxies do. The other forces only function at smaller scales, and gravity diminishes over long spaces. So, while we can't see the effects of expansion in everyday life, they are visible outside of our small planet. The universe is just a tug-of-war between gravity and expansion; one causes all matter to accelerate away from each other, and the other pulls everything together.
If you understand what is happening, then the next most important thing is to understand why. In this case, the cause of universal expansion can be discovered by analyzing yet another discrepancy
“Everything you know… is a lie.” -Doctor Who Season 12, “The Timeless Children”
The Law of Conservation of Energy: energy can not be created nor destroyed. Seems familiar enough, right? It’s likely that you were taught this fundamental basis of physics, but you were probably not taught about universal expansion early on, and those two things appear to come into conflict in two key ways:
1.Gravitational Potential Energy
The equation for gravity potential energy specifies that as the distance between objects increases, the GPE (gravitational potential energy) between them increases.[7] But as the universe expands, so does the distance between objects, meaning that GPE seems to increase out of nowhere, violating the conservation of energy.
2. Cosmic Redshifting
In waves of all forms, a greater wavelength means less energy and a lesser one means more. In the process of redshifting the wavelength of a photon or other similar particle in motion increases. This can happen for a variety of reasons, for example, the expansion of the universe. The wave is pulled apart, therefore pulling apart the troughs of each oscillation and increasing the wavelength.[8] The wavelength increases, meaning that some energy from the particle is gone, but it doesn’t obviously go anywhere.
What is happening here? Where does the energy from GPE come from, and where does the energy of the photons go? Essentially, the particles are performing work on the universe and pushing against its boundaries to expand them.
Think about it like a trampoline (which uses elastic potential energy). It naturally wants to snap back to a position at rest, but when people jump, its boundaries are stretched. In the case of the universe, gravity naturally pulls everything together but that force is resisted by the energy pushing the universe apart. This same energy from redshifting photons and other particles is what gives the universe the energy to increase GPE between objects during its expansion. Whether energy conservation is being observed is unclear, as General Relativity doesn’t have a strict definition of energy (it isn’t obvious whether the universe growing could be counted as a use of energy), but this relationship is still interesting and useful to discuss.[8]
Remember the universe isn’t a 2-dimensional space and it doesn’t actually have edges that are being pushed against; everything just moves away from everything else.[9] If you want to know more about the shape of the universe, see “We Live in a Donut?”
“My entire understanding of physical space has been transformed! Three-dimensional Euclidean geometry has been torn up, thrown in the air, and snogged to death! My grasp of the universal constants of physical reality has been changed forever!” -Doctor Who Season 9, “The Husbands of River Song”
This theft of energy from particles is the most widely accepted reason for the universe’s expansion. We know that everything moved apart rapidly after the Big Bang but later decreased in speed a great deal. We also know that the universe had extremely high energy early on. Remember the temperatures in the range of trillions of degrees at the beginning of the universe? That heat didn’t just decrease because it was distributed evenly across a larger space (remember the cosmic goop), it also literally “disappeared” because it was used to expand the universe.[8] This is why our universe is swelling at slower rates now; there is literally less particle energy to be put into its expansion.
When the Big Bang occurred, no energy was “invested” into the expansion of the universe, but as time goes on, more and more particle energy is lost. Remember the tug-of-war analogy; if energy from the big bang keeps being redshifted, eventually the fuel that expands the universe will decrease and run out. In this case, it’s only natural to think that gravitational forces would slowly take control as expansion slows down and bring the distant matter back together. But against the expectations of past physicists, current predictions show that rather than falling off, the expansion of the universe is getting faster.[10]
“The universe is big.” -Doctor Who Season 5, “The Pandorica Opens”
Astronomer Edwin Hubble in the 1920s proved that galaxies are moving away from each other, and around seventy years later three physicists published findings that proved that the rate at which the universe was expanding was increasing (acceleration).[11] Both of these discoveries ignited heated debate and research among physicists, about how fast the universe is expanding (see “Tension in Physics”), and why the growth is accelerating. These discussions continue today, and there are several possible explanations that physicists have proposed.
The most widely accepted current explanation for the universe’s expansion is the standard model of cosmology (the Lambda-CDM model), a physical model that aims to explain the movement of objects at the astronomical level based on Einstein's Theory of General Relativity.[12] It states that the accelerating expansion is caused by a mysterious force known as dark energy.
Dark energy, in the most general sense, is the energy that inhabits nothing; empty space. The idea is that all empty space has energy, a compound that just like all other energy can fuel the expansion of the universe.[12] The form dark energy takes is generally unknown and much debated, but in practice it acts like a cosmological constant; a consistent repellant force throughout the universe. But, as the universe expands, more empty space comes into existence, so there is more energy, which makes the universe swell, which creates even more empty space. Current physics says that dark energy makes up around 68% of all mass and energy in the universe, while normal matter only makes up 5%.[13] This positive feedback loop explains the acceleration of universal expansion. Unlike normal energy or matter, dark energy doesn’t dilute over time, so its quantity is always increasing while other energy decreases because of cosmic redshifting. This creates a different view of the future of the universe, rather than contracting because of the decrease in energy from the big bang, it will expand increasingly fast until… we’re not really sure (see “The End of Everything” for information on the end of the universe).
Unfortunately for physicists, the current form of the Lambda-CDM is most likely incorrect in how it talks about the expansion of the universe. This is because of the Hubble Tension, a discrepancy between the predicted and observed expansion of our universe that contradicts currently accepted scientific theories (once again, see “Tension in Physics”).[14] Recently, physicists have been trying to come up with modifications to the standard model that would explain this difference.
“Rule 1: The Doctor lies.” - Doctor Who Season 6, “Let’s Kill Hitler”
The problem with trying to modify the standard model, even if there is an issue, is that any small tweak or addition could have drastic consequences on the rest of the theory. Being the foremost cosmological theory, it’s well-refined and so, difficult to modify.[15] These justifications for the Hubble Tension are new and theoretical, most of them have to do with redefining either dark matter or dark energy, and so also provide more rigid explanations for vague concepts. Here they are in (a very general) order of most to least proven (or accepted):
Dark Matter
Outside of the normal particles that physicists are aware of, the standard cosmology also includes a mysterious phenomenon known as dark matter (not to be confused with dark energy) that makes up around 27% of all mass and energy of the universe.[13] Essentially, dark matter is a substance that interacts with “normal” energy and matter only gravitationally; so it can’t be seen or touched, but its high quantity means that its gravitational force is crucial in holding our universe together. With our current understanding, it’s more beneficial to think of the universe as tug of war between dark energy and dark matter, as they both have the most significant effect on the universe’s expansion at the current. The reason physicists believe that it exists is because of unusual patterns of astronomical objects that suggest gravitational interactions outside of visible matter. However, little is known and much is up for debate about this type of matter. Some physicists have created physically consistent models that align with our knowledge of the universe’s expansion in which dark matter decays over time into particles with less mass just like normal particles do (remember e = mc^2, mass and energy are interchangeable).[15] This would mean that as time goes on there is less dark matter in the universe, and so less gravitational force pulling everything together, relieving the Hubble Tension. However, considering that dark matter is so unknown, this isn’t the only explanation that uses its properties to tweak the standard model.
Dark Energy over Time
Just like dark matter, dark energy is a newer discovery even though it makes up most of the universe. Seeing as scientists proposed it as the key to the acceleration of the expanding universe, it would be natural to adjust it to fix the Hubble Tension. The current understanding of the substance is that it's like the cosmological constant Einstein described in his first equation for general relativity (see “Einstein’s Blunder or a Stroke of Genius?”); it is equal in all empty space at all times in the universe, but what if that isn’t true? What if the amount of dark energy in the universe oscillates over time (creating a universe that slows and speeds up in expansion at different times), or what if more dark energy was added to the universe at some point for a reason we don’t understand.[16] This would violate the original principle that dark energy never dilutes or increases in density, but if it did it could theoretically speed up expansion disproportionately based on when the observations occurred. Additionally, some have even proposed that dark matter decays into dark energy, which would both decrease the quantity of gravitationally bound dark matter, and increase the quantity of dark energy.[16] All of these ideas, while possible, have flaws and are still relatively unproven. From here on out though, the theories get a lot wilder.
Quintessence
Quintessence is another explanation for the unnaturally fast expansion of our universe.
It suggests that most of our universe is inhabited by a massive wave, but because of its large scale, one with an extremely large wavelength and low energy density. Unlike other forms of matter and energy, quintessence would have a repulsive gravitational force that would help it to push the universe apart.[17] The substance takes different forms in different theories, but it generally has a few key flaws. First of all, such a wave, while not interacting with other particles much, would still interact with them a small amount gravitationally or even through kinetic energy.[17] Second, the wave wouldn’t spread equally, so it would cause inconsistent expansion (faster in some places than others). Despite some evidence that expansion may be disproportionate in some areas, both of these ideas can be disproved by looking at data from the cosmic microwave background, the same data that confirmed the Hubble tension.[17]
Virtual Particles
Virtual particles are a fundamental part of Quantum Physics. They’re massless particles/waves that are small fluctuations in quantum fields and transfer energy to other particles.[18] Think of photons (light particles), waves that transfer energy in an electromagnetic field from one atom to another, and imagine them like an unnatural wave on the beach, where the surface of the water is an electromagnetic field; they’re energetic disturbances in the natural rhythm of the sea that only exist until they impart their energy to another object. In quantum field theory, a version of quantum physics, these disturbances can happen in empty space without any energy being used, through the use of zero-point energy. This concept is complicated, but basically, a virtual “particle” and “antiparticle” are created at the same time and disappear almost instantly. This phenomenon creates quantum vacuum energy that would perform a similar function to dark energy, pushing the universe apart.[19] This is some pretty weird stuff, but what is most important to remember is that none of it makes any sense. The reason for that is that when physicists tried to calculate the force of expansion that would be generated by this, it was over ONE HUNDRED ORDERS OF MAGNITUDE too high.[20] That’s an insane difference that presents a an issue far larger than the Hubble tension. Whether it’s quantum field theory or the Lambda model, something is causing our equations to be vastly different from the observable universe. Even if this explanation falls short, it can’t be ignored completely.
Einstein Was Wrong
Most modern astronomical theories are based on Einstein’s theory of general relativity, and if all else fails there's always that possibility that his theory was incorrect. There are several alternative theories of gravity such as modified Newtonian dynamics and string theory, but they are either unprovable or flawed by our current understanding.[21] That doesn’t mean a better alternative theory is impossible though, and there may be new discoveries in the future that put Einstein’s theory to rest.
So with that long detour, you know everything you’ll ever need to know about the way the universe expands, now let's get back to the driving question.
“a straight line may be the shortest distance between two points, but it is by no means the most interesting.” -Doctor Who (1963) Season 11, “The Time Warrior”
Now that we’ve finished our very long, and very interesting (I hope) detour, let’s summarize. The expansion of space was rapid at the beginning of the universe because of the energy from the big bang, but eventually slowed down as it decayed. However, as more space was created the amount of dark energy, the energy of empty space, increased and is now accelerating expansion. Where does that leave us now? Well basically, stuck. Here’s why.
Imagine if you set off right at this moment in a magical spaceship that could fly at the speed of light. If you moved away from the Earth in a straight line, how far could you make it before there was nothing else left to see, i.e. before the universe ends. Of course the end of the universe probably won’t be a single event[22] (see “The End of Everything”), but the question remains the same, and the answer is deceivingly simple. All you have to do is multiply the number of years before the death of the universe (up for debate) and take that measurement in light years (the distance light travels in a year), and you would get the correct value. But this isn’t right for any practical purposes, and that’s because if someone was to travel to a star 50 light years away, after 50 years they wouldn’t see it. In fact, they wouldn’t be anywhere near it. That person would have traveled 50 light years in the most useless sense, but everything else would have traveled “away” because of the expansion of the universe. In the same way, all the things that humans could theoretically travel to if we set off now, are slowly moving past the distance that we can reach.
And in the same way that we couldn’t reach things that should be at a possible distance, there are things that we can see in the distant night sky that are beyond our grasp forever. Think about the observable universe; it’s just the photons that are reaching Earth right now from distant galaxies.[23] These photons could have been sent during the big bang (the cosmic microwave background), or just a few minutes ago by the sun, and just now be finally be making it to Earth. Additionally, during that time it’s possible that the universe expanded so much that the objects are now past the line of no return. In this way, the universe taunts us. But not forever, because when something passes that line it releases the final photons (move at the speed of light, just like the theoretical ship going in the other direction) that will ever be seen on Earth. And current theories of the universe say that objects will keep passing that line, that radius around Earth, even do so faster, so in the far, far future, what is left for us and our dying planet?
“Night will fall and the dark will rise.” -Doctor Who Season 7, “A Good Man Goes to War”
The answer is, something actually. The way we think about dark matter is that until the universe decays into nothingness (very cheery), it won’t have the power to actually tear most galaxies apart. What’s most likely is that it will leave the Milky Way, along with possibly a few nearby galaxies to keep themselves together gravitationally.[24] This would leave more than enough to keep our ancestors occupied, but it’s not the sparkling jewel of hope that many think of the far future as. In fact, it’s pretty depressing. What’s even more depressing is that as individual humans we’ll never see our posterity form intergalactic civilizations. But, the moral of this story is in fact the opposite; it shows just how lucky we are.
By the time our local galaxies have linked together into one supergroup, the rest of the universe will be dark. For all essential purposes, it will be dark forever, because most cosmic objects will have passed the “point of no return” and their last photons will have graced our view. But right now, it isn’t. If you look up into the night sky you’ll see stars, that while no human may ever reach, will only be visible from our rocky planet for possibly a small amount of time. We don’t know what is happening there, and we never will, but the fact that we get to see those stars, see those planets, and see those magnificent supernovas from lightyears away is a privilege. And one future humans might not share.
Sources:
TRTWorld. 'An Ecocide': How the Conflict in Ukraine Is Bombarding the Environment, TRT World, 28 Apr. 2022, https://www.trtworld.com/magazine/an-ecocide-how-the-conflict-in-ukraine-is-bombarding-the-environment-56730.
“Ukraine and the Others: The Environmental Impacts of War.” European Youth Portal, https://youth.europa.eu/year-of-youth/young-journalists/ukraine-and-others-environmental-impacts-of-war_en.
Anonymous, Authored by. “The War in Ukraine Is an Environmental Catastrophe.” The Union, 8 Dec. 2022, https://theunion.org/news/the-war-in-ukraine-is-an-environmental-catastrophe#:~:text=In%20conclusion%2C%20the%20environmental%20devastation,ability%20to%20anticipate%20or%20prevent.
Rannard, Georgina. “COP27: War Causing Huge Release of Climate Warming Gas, Claims Ukraine.” BBC News, BBC, 14 Nov. 2022, https://www.bbc.com/news/science-environment-63625693.
Roscini, Flavia. “The Environmental Cost of the War in Ukraine.” International Relations Review, International Relations Review, 14 Apr. 2022, https://www.irreview.org/articles/the-environmental-cost-of-the-war-in-ukraine.