Physics Gets Weird at the End of the Universe

If you think you know how the universe is going to  end, you’re probably wrong. That’s because no one knows for sure what the future has in store for  us. And because — from big crunches to big rips, from cyclic revivals to periods of infinite  darkness — the options appear limitless.

But that’s not to say that forecasting  the end of the universe is beyond the capacities of science. By learning  more about the universe around us today, we can uncover what will help  us predict its ultimate demise. I’m Alex McColgan, and you’re watching  Astrum.

Join me today on a journey to the future of the universe — a future  where there are no longer any planets or stars or galaxies—no longer any atoms  or molecules or life — a future where the universe is lying on its deathbed. How  long do we have until that future is here? And is there an afterlife for the universe? 

These questions might sound impenetrable, but their answers lie in our understanding of  the most familiar force of nature: Gravity. Ordinarily, gravity is an attractive force that  slows down objects trying to move farther apart or speeds up objects moving closer together.  That’s why, before dark energy was discovered, astronomers assumed our expanding universe  was being slowed down by gravity over time.

They even came up with a parameter called q to  measure the rate of this deceleration. But at the close of the 20th century, measurements  of supernovae revealed that q was actually a negative number — the universe wasn’t  decelerating at all. It was accelerating.

This acceleration is fundamentally tied  to the type of matter and energy contained in the universe. If the energy of ordinary  matter gives rise to a gravitational pull, it must take a very different form of energy to  explain the gravitational push accelerating the expansion of our universe. We call this  form of energy “dark energy.

” The tricky thing is that we don’t know what dark energy  is, exactly, or where it comes from. And yet, the very fate of the universe is tied to  its identity. Depending on the nature of dark energy, the accelerated expansion of  our universe can grow stronger, settle down, or even reverse course.

Rather than making a  video on just one of these fates, we’ve decided to collect them all into one big flowchart to  serve as your one-stop guide to the end of the universe. At the top of the flowchart is what dark  energy is, and at the bottom… is how we all die. Most cosmologists would agree that dark  energy is likely to fall into one of two main categories.

First — the most popular theory  — is that dark energy is the energy of the vacuum itself. Second — the leading alternative — is  that dark energy is the energy of new fields in the universe that we simply haven’t discovered  yet. Since fields are just a way to represent collections of particles, this would mean that  dark energy is made of a new form of matter — new types of particles, beyond electrons or  quarks or anything else we know of today.

From these two categories, our flowchart is  going to branch out into theories of dark energy that are wildly different from each other  and predict all sorts of catastrophic futures. On the vacuum energy side, we need to consider  whether the vacuum itself is stable, or whether it can jump into a state of even lower energy  through a process known as quantum tunneling. A stable vacuum is by far the simplest scenario. 

It means that dark energy will continue to exist for an eternity in the same form it takes on  today. Our universe will accelerate forever, growing infinitely larger and colder in what’s  called the Big Freeze. The good news is, you wouldn’t notice anything changing here  on Earth, or even throughout the Milky Way, since there’s enough mass concentrated in this  small region to counteract the gravitational repulsion coming from dark energy.

But if you  zoom out beyond that, you’ll notice that distant galaxies are being accelerated away from our  own in all directions, eventually crossing a threshold known as the event horizon. Just like  with a black hole, once you cross this point, you can no longer communicate or interact with  the world you’ve left behind. That means that, one by one, we’ll lose contact with all galaxies  beyond the local few where gravity is strong enough to keep us close.

All that will remain  of them is a dimming, reddening light from the moments before they crossed the event horizon.  Eventually, this light will become so faint that it too will be unobservable, and future  generations of astronomers — billions of years down the road — might have no idea that anything  ever existed in the universe beyond the Milky Way. If you’re wondering what will become of the stuff  inside the Milky Way, it dies a relatively boring death.

The Second Law of Thermodynamics says that  systems will always tend towards states of higher entropy. When stars fuse hydrogen into helium  and emit light as a byproduct, when planets heat up and winds soar across the atmosphere, and even  when you eat a carrot and wait for it to come out the other side — all of those processes increase  entropy. But the entropy of the Milky Way can only grow so large.

Eventually, stars will run out of  hydrogen to fuse, planets will be left to freeze, winds will abate, and there will be no more  carrots left to digest. Matter will have nothing to do but collapse into black holes, and black  holes will evaporate into a pool of radiation. What we once called the Milky Way will be in a  state of total thermal equilibrium.

Nothing will be happening. There will be nothing to happen.  This fate that awaits the Milky Way is sometimes called Heat Death — not because it’s hot (in  fact, it will be quite cold), but because there will be no more heat flow, no more temperature  gradients, no more transfers of energy from one place to another.

Admittedly, it might take a  while to reach Heat Death — some estimates place it 10106 years away, or 1096 times the current  age of the universe. But if dark energy really comes from a perfectly stable vacuum, then there  will be no avoiding it: Heat Death is coming. There is one possible silver lining at the end of  this story, however.

It has been conjectured by Sir Roger Penrose, a Nobel laureate and brilliant  physicist, that Heat Death might not be the end of our story, but merely a new beginning. In  his model known as Conformal Cyclic Cosmology, Penrose proposes that the radiation-filled  universe in the infinite future will be indistinguishable from the state of the universe  at the Big Bang, and indeed they may be one and the same. Each aeon of infinite expansion  in this model merely leads to a new Big Bang and a new cycle of cosmology.

There isn’t much  evidence in support of this hypothesis as of yet, but it may be a source of hope to cling onto  if our universe ends up heading toward a Big Freeze. You and I would not survive, but maybe the  universe could — and at least that’s something. If the idea of a Big Freeze scares you, I’ve got  some good news and some bad news.

The good news is that there are reasons to believe that  even if dark energy comes from the vacuum, that vacuum state might not last forever,  meaning we might not be headed toward a Big Freeze after all. One reason to doubt the  stability of the vacuum comes from observations of the Higgs field. Current measurements from CERN  indicate that the Higgs might have a lower-energy vacuum state than the one we live in, though it  could take something like 10100 years to tunnel into this preferred vacuum.

To paint a  mental picture of how this all works, we can imagine an apple falling off a tree,  hitting our friend Isaac on the head, and rolling off into a small dip in the grass. A short  distance away, there’s an even bigger dip, which the apple would absolutely be drawn toward, if not  for the barrier blocking its path. Classically, the apple is stuck, but quantum-mechanically,  the apple can and will tunnel through the barrier into this lower-energy state, if you  just wait long enough.

The Higgs field works the same way, with its different vacuum states  corresponding to the different dips in the grass. A second reason to doubt the stability of the  vacuum actually comes from string theory. While string theory hasn’t made any quantitative  predictions, it’s given us some pretty strong hints that stable vacuum states with positive  energies are really hard to come by.

String theory also predicts the existence of new fields  that could conceivably tunnel to different vacuum states much faster than the Higgs — say, within  the next trillion or even hundred billion years. Here comes the bad news, though: If we live  in a meta-stable vacuum that can decay at the whim of a quantum-mechanical fluctuation,  we’ve got bigger things to worry about than the Big Freeze. Quantum tunneling into a more  stable vacuum won’t just affect the expansion of space, but it can also fundamentally alter the  properties of particles and the laws of physics governing our universe.

If the Higgs field falls  into a new vacuum state, ordinary particles like protons and electrons will have completely  different masses, and atoms and molecules as we know them will cease to exist. Or if we tunnel  into a new vacuum in the string theory landscape, our universe might suddenly contain new particles  we’ve never seen before! In either case, life on Earth would all but certainly be toast.

As for the  universe as a whole — in this scenario, we can’t predict its fate more precisely without more  information about the new vacuum we fall into. Let’s step back and take a look at the other side  of our flowchart, where dark energy comes from the presence of new fields rather than the vacuum.  There are different ways in which fields can evolve over time, which would in turn dictate  the future trajectory of our universe.

Unlike vacuum states, which would have perfectly constant  energies between tunneling transitions, fields can continuously gain or lose energy over time. Just  like a ball rolling across a rough patch of grass, fields experience friction as they evolve, so it’s  most natural to expect their energy to decrease. But physicists have shown that it’s possible to  have exotic types of fields called “phantom dark energy” that overcome this “friction” and actually  gain energy over time.

These exotic fields wreak absolute havoc on the universe. Because their  energy is growing everywhere and all the time, they eventually overpower the energy from  all other forms of matter, even within the Milky Way. This increase in energy heats up  the entire universe, fully reversing course from all the cooling the universe has done over  the past 14 billion years.

In this scenario, the expansion of space doesn’t just continue to  accelerate, but it accelerates so fast that any region of space will grow to infinite volume in a  finite time — maybe even just a few billion years from today. And if you want to call for help,  that gets increasingly harder, as the cosmic event horizon we spoke about earlier will shrink  down to the size of our galaxy, then our planet, then our home, and then us. As this is happening,  the repulsive gravitational effects of phantom dark energy grow ever stronger, ultimately  tearing apart all of the molecules, atoms, and nuclei in the universe into their constituent  parts.

Then, we hit the singularity. This new kind of singularity—where the universe is infinitely  big and hot rather than infinitesimally small and hot — has been called the Big Rip. But I’m  still hopeful that this grim ending from phantom dark energy exists only in the minds of theorists,  while the real dark energy in our universe behaves more peacefully and loses energy over time. 

So what would fate have in store for us then? Here’s where our flowchart starts to get a little  messy, because there are so many different ways a field can evolve over time. There are two main  questions that have to be answered to predict the fate of the universe in this scenario: First,  can the dark energy field have negative potential energy; and second, does the field’s energy change  slowly or quickly?

If the field’s energy is always positive and it loses energy very slowly, it would  hardly be distinguishable from vacuum energy. In this case, we’d just be headed toward another Big  Freeze. On the other hand, if the field’s energy is drained away very quickly, the universe would  soon have no dark energy left at all, it would stop accelerating, and its future would depend  entirely on its spatial curvature - or the shape of the universe itself.

A flat or open universe  would eventually evolve toward Heat Death, but there would be no cosmic event horizon to  prevent us from communicating with galaxies as far as the eye can see. This may be the most peaceful  of all possible futures for the universe. On the other hand, a closed universe would eventually  stop expanding, reverse course, and collapse back in on itself, which sounds … less peaceful. 

This kind of recollapse is known as a Big Crunch. Having a closed universe is just one of  two paths that can lead to a Big Crunch. The other way to see our Milky Way crushed  to smithereens is for the dark energy field to reach a negative potential energy.

At that  point, it will only be a matter of time before the universe stops expanding and begins to  contract. During this contraction phase, gravity turns cosmic friction into cosmic  anti-friction, speeding up the field’s evolution and giving it more energy rather than  draining it. If the field gains energy slowly, there will be plenty of time for the universe to  recollapse and experience a full-on Big Crunch.

On the other hand, if the field gains energy fast  enough, the universe could reach a temperature mimicking conditions at the Big Bang at a rate so  fast that this would occur all before the Milky Way is even predicted to collide with Andromeda.  Even though space does contract in this scenario, it doesn’t “crunch. ” In fact, the amount of  contraction is hardly noticeable by the time Earth turns to toast.

So what happens next?  Models suggest that if the universe reaches extremely high temperatures while contracting,  it could be forced to transition into a new phase of expansion, where it has time to cool off  once more. This transition is called the Big Bounce.

In fact, the universe could go through  a Big Bounce following a Big Crunch, too. These scenarios open the door to a new type of cyclic  universe, entirely different from Penrose’s idea, which we’ve discussed in previous videos, in  which the universe goes through an infinite sequence of expansion, cooling, contraction, and  reheating over and over again. In this picture, even our own Big Bang 14 billion years  ago could have actually been a Big Bounce, connecting our own cycle of the universe  to all those that came before it.

This wraps up our master flowchart for all  the different ways the universe could end. It could freeze to death, be ripped to shreds,  be crushed to oblivion, or simply heat up so quickly that it bounces into a brand new life  cycle. We might not have any certainty yet, but even just narrowing it down to four  possible options is already an enormous feat of science.

So there’s a great  new topic for small talk with strangers and dinner conversations. You’re welcome. If you had control over the fate of the universe, which of these four destinies would you pick  for it?

Let us know in the comments below. Thanks for watching. And thanks to our crew of Astrumnauts over a Patreon who help us make us make science knowledge freely available to everyone.

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