Everyone knows that the Universe began with the Big Bang. But what is seldom spoken about is the end of the Universe. In this episode, we’re going to answer one of the most profound questions we can ask: What is the ultimate fate of the Universe and of all life?

Have you ever wondered if the Universe will exist forever? And if it does have an end… what will it be like? And what will happen to Earth? And to life? Today we’re going to talk about the end of all endings.

Spoiler alert! I will reveal how the history of the Universe ends.

A little bit of cosmology

Cosmology is the branch of science that studies the origin, evolution and fate of the Universe. In ancient Mesopotamia it was thought that the Earth was all that existed, and that it was flat and surrounded by ocean. Since those early cosmological models until the present day, our understanding of the Universe has changed a lot, especially in recent years.

We now know that the Earth is an insignificant dot in the immensity of the Universe. And the most accepted cosmological models describe a Universe that began as an extremely dense soup of particles, and has not stopped expanding ever since.

In addition, everything indicates that the Universe expands in the same way in all directions. I mean, everywhere we look, we see galaxies moving away from us. And the further away they are, the faster they move. So, in a situation like this, we can ask ourselves, will it continue to expand forever?

There are different theories about the ultimate fate of the Universe. If you want to go deeper into the subject, you can read “The End of Everything (Astrophysically Speaking)” by my former colleague Katie Mack. But for simplicity’s sake, today we’ll consider three scenarios:

  1. The Universe exists forever.
  2. The Universe ends abruptly.
  3. The Universe ends slowly.

Which one would you bet on?

Scenario 1: The Universe exists forever

In this scenario, the expansion of the Universe begins to slow down until it stops. The Universe achieves the perfect balance between the forces of expansion and attraction.

In this state of spatial tranquillity, the light from other galaxies is not affected anymore by the expansion of the Universe. So, over time, light begins to reach us from many more galaxies than we currently see. The night sky is getting brighter and brighter, and we are privileged to see more and more galaxies than we could ever see in an expanding universe.

It is a constant astronomical spectacle, which could not even be compared to the clearest night you have ever seen from the heights of Tenerife.

Photo by Xavier Mestdag from Pexels.

You may find the idea of a static Universe appealing. Einstein and most other physicists of his day also thought so for a while, and Einstein even went as far as to “force” his Theory of Relativity to ensure that the Universe would be static… only to admit later that it was one of the biggest blunders of his life.

Because it turns out that it’s actually very difficult for the Universe to become static, as it would be an unstable equilibrium. A bit like your phone falling onto the floor in an upward position. Rather, if the Universe stops expanding, the most likely outcome is that it will start shrinking again, until it eventually collapses due to gravity, which brings us to the next scenario.

Scenario 2: The Universe ends abruptly

In this scenario, like in the previous, the Universe stops expanding. But now it doesn’t stay there forever. Instead, gravity does what gravity does best, and starts pulling all matter back, so that the Universe contracts more and more until it reaches the Big Crunch, which is kind of a reversed Big Bang.

In the Big Crunch, we also reach that moment when the expansion stops and the sky becomes simply spectacular… but it would merely be the omen of a catastrophic death. Soon the Universe starts contracting, galaxies engulf each other, and all the civilizations in the Universe are destroyed, ending up again in a dense soup of particles like the one from which we emerged.

But, if the Universe is capable of expanding and contracting once, you may be thinking: Couldn’t it repeat the process again? Yes, the Universe could be constantly bouncing between Big Bang, Big Crunch, Big Bang, Big Crunch… indefinitely. And this is indeed another cosmological possibility, called the Big Bounce.

Luckily, the Big Crunch, and hence the Big Bounce, are also fairly unlikely. For the past few billion years, the Universe on a large scale has been dominated by something very powerful: Dark energy.

Dark energy is what causes the Universe to expand. And, despite its sinister name, it may be our salvation, as it will cause the expansion to counteract the gravitational pull, thus avoiding the Big Crunch.

Hypothetically, dark energy could also mess with us and start growing uncontrollably. This would cause the expansion to become too fast, to the point of forcing the stars in our own galaxy to start moving away from each other, the Earth would begin separating from the Moon, and eventually, even the atoms in our body would end up moving away from each other. In this possible future, called the Big Rip, all civilizations would end up blitzed into oblivion once again.

Another hypothetical way to drastically end our existence is the theory in which our Universe exists in an unstable state, called a false vacuum. And anywhere, at any time, a more stable vacuum bubble could arise, rapidly expanding and destroying everything in its path. And by ‘rapidly’ I meant travelling at the speed of light, which implies that we would not be able to prevent it: By the time we can see it, it has already happened. But this option is very controversial, so we can simply discard it.

In a nutshell, in the worst case, the premature death of the Universe could occur in a few billion years. But dark energy doesn’t appear to have the characteristics needed for a Big Crunch, a Big Bounce, or a Big Rip, which brings us to the third and final scenario – the most likely fate of the Universe.

Scenario 3: The Universe ends slowly

The Universe continues expanding forever, but not fast enough to tear us apart. Instead, it happens slowly but surely.

The groups of galaxies continue to move faster and faster away from each other indefinitely, to the point that each galaxy ends up alone in its own observable universe.

The civilizations that develop then think that they live in a static universe, composed only of their own galaxy. They are not as lucky as we are today to be able to distant galaxies, and to know that the Universe is actually enormous and expanding.

After billions of years, the Universe, or rather, all those independent universes, are running out of gas to form new stars, and they start switching off. The pieces of solitary matter that float through the immensity are disintegrating. And the supermassive black holes that lurk at the centre of each galaxy are slowly cleaning up what is left around them, including any celestial bodies capable of supporting life. Finally, after an absurdly huge number of years, these lonely black holes evaporate through the so-called Hawking radiation.

So, at the end of time, the Universe is a dark, cold, empty place. And any possible remnant of civilization ends up alone and frozen in the vastness of space. For this reason (and perhaps also due to a lack of imagination) this scenario is called the Big Freeze.

Like it or not, this turns out to be the most likely scenario. In fact, one could say that the Universe has already begun to switch off, because if we count the total number of stars that will ever exist over all time, around 90% of them have already formed.

But if this cold, dark ending scares you, fear not! The Earth will have already disappeared long before all that, in an even more dramatic (and far hotter) way.

The end of the Earth

In about five billion years, the Sun will become a red giant, expanding its radius all the way to the orbit of Mars. Long before that happens, humanity (or whichever civilisation occupies the Earth by this point) will have had to migrate to other planets in order to survive. Earth will become the ball of fire that it once was when it first formed.

We might linger a little longer somewhere in the Solar System, but eventually we will have to go out and explore exoplanets, that is, planets orbiting other stars.

Luckily by then we’ll have quite a few more stars to choose from, as the Milky Way will be on its way to merging with Andromeda and other galaxies in our cosmic neighbourhood, also known as the Local Group.

NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger.

Watching the merger of two galaxies from within will be a one-of-a-kind stellar performance. And it will also be our last meeting of cosmic neighbours, since it is highly probable that we will never be able to interact with any other galaxy beyond the Local Group.

By now you may be thinking that you don’t care about stars and galaxies, and fair enough! At the end of the day, what interests us is: What will happen to life?

The end of life

Like the Universe itself, life faces three different scenarios:

  1. Life exists forever.
  2. Life ends abruptly.
  3. Life ends slowly.

Do you dare to bet on the most likely one?

Well, scenario 1 can be ruled out: At some point during the cooling of the Universe, the stars will stop shining. And then there will be no energy left to supply any kind of civilization. Even in a static universe, it would be difficult to live forever. Hence life, sooner or later, will come to an end.

So will it be scenario 2 or 3? It’s possible that a natural catastrophe, such as a meteorite, wipes out most forms of life on Earth. But, even so, it’s likely that some would survive, and it’s also very likely that some other extraterrestrial forms of life would continue to exist somewhere else in the Universe.

In other words, the most likely scenario is number 3: There will be life in the Universe for a long time to come. What is not so clear is whether there will still be human beings around to appreciate it.

The end of humanity?

Since life cannot exist forever, humanity will logically disappear as well. What we don’t know is when and how. There are many dangers that lie in wait for us in the future: Climate change, pandemics, nuclear wars… Does a long future await us, or will humanity end sooner than expected? I will talk about these and other existential risks in the next episode of Altruphysics.