What happens at the end of time?

“Some say the world will end in fire, some say in ice” - Robert Frost, Poet. 1920

To explain what happens at the end of time, we have to start at the beginning of time.

The universe is all of space and time, including planets, stars, galaxies, and all other forms of matter and energy.

The Big Bang theory is the most popular theory for how it all started. It suggests that the universe developed 13.799±0.021 billion years ago an extremely small hot dense ball that expanded and cooled into the universe that we know today, forming stars, galaxies, galaxy clusters - and has been expanding ever since.

Scientists have several ideas for how the universe will end and what will be there afterwards, also known as ‘future evolution’. Here are some of the most popular: the Big Crunch, the Big Freeze, or the Big Rip.

The Big CrunchHere gravity acts like a cosmic boomerang. Instead of continuing to expand out, everything gets pulled back in, reversing the Big Bang. Imagine the universe like a giant balloon, slowly sucking all the air out until everything collapses into a super-dense singularity (think the opposite of a black hole!). What happens then? Honestly, we don't know. Maybe another Big Bang, maybe oblivion – the physics get a little wonky at that point.

The Big FreezeAnother common theory is that the universe will just continue to expand forever. As the temperature descends towards absolute zero, all phenomenon and all processes will cease. This time imagine the universe as a giant ice cube slowly melting. Over trillions of years, all stars will burn out, shutting off in the night sky one by one. Black holes will engulf any leftover gas and stars and eventually even the black holes will evaporate, the universe will be cold and dark except for faint whisper of heat, due to what’s known as Hawking radiation.

The Big RipStick with idea that the expansion of the universe is rapid and accelerating, another theory is that we will go from the Big Bang to a Big Rip, where the universe gets ‘too big for its boots’ and everything inside it is ‘stretched’ to the point of destruction i.e. until distances between particles becomes infinite. Imagine the universe stretching like taffy, ripping apart atoms and everything else into nothingness. In this theory, the sun will run out of fuel in its core and expand until it eventually collides with and engulfs planets in orbit near to it. From Earth, we’d initially be far enough away that we could imagine looking at a sunrise on the horizon that fills up half the sky!

So, what happens after the universe ends?The twist in these tales is dark energy. We still don't know enough about these forces that could end it all. But by definition, the laws of physics that we understand might cease to exist. Some theories propose a multiverse, where our universe is just one bubble in a vast collection, constantly popping in and out of existence.

The big news? We're talking timescales so vast – easily 20 billion years away – for even the soonest estimates. So, plenty of time to worry (or not worry) about our fate. In the meantime, there's a whole universe out there to explore!

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Sources:

Burman, A. and Roy, S., (2022) Will the current rate of expansion of the universe result in a Big Freeze, Big Crunch or Big Rip?.

Fèvre, R. (2021) From the Big Bang to the Big Rip: One Cycle of a Closed Granular Friedmann-Planck Universe. Journal of High Energy Physics, Gravitation and Cosmology, 7, 377-390.

Mack, K., 2021. The end of everything:(astrophysically speaking). Simon and Schuster.

Sasikumar, C., (2021) A theory of universe is oscillation of begin of big bang and end with ultimate fate of big rip