How the Universe Ends lessons
Will the universe last forever?
The size of space over time, solved from the Friedmann equation with today's measured values: one measured past, then a Big Freeze, a Big Rip and a Big Crunch, with a w slider to bend the future.
The universe began with the Big Bang 13.8 billion years ago, and space has been stretching ever since. The big question is what happens next.
The answer depends on dark energy, the unknown cause of the expansion speeding up. Change how dark energy behaves, and the future bends, as the chart shows.
This course compares the main endings: the Big Freeze, the Big Rip and the Big Crunch. Along the way it says plainly what is measured and what is still a guess.
Why the expansion speeds up
Gravity from matter pulls everything together, so you would expect the expansion to slow down. In 1998, astronomers found it is speeding up instead.
A box of space grows over time: its matter thins out while dark energy stays the same in every cubic metre, so dark energy's share rises past matter's and the expansion starts speeding up.
As space grows, the same matter is spread through more volume, so it thins out. Dark energy seems to stay the same in every cubic metre, so its share keeps growing.
Its push began to beat gravity about 6 billion years ago, and it has outweighed matter for about the last 4 billion. Today dark energy is about 70% of the universe, and matter about 30%.
Practice: Thin out the matter
Work out how many times thinner matter gets when every distance in space grows by a given factor, or the reverse.
Three possible endings
Each ending follows from what dark energy does next. The cards below use the same model as the opening chart.
Three cards, one per ending, each with its computed size of space over time, what dark energy does, the time left and the final state.
Today's measurements fit the Big Freeze best, which is also called the heat death. The other two need dark energy to change, and that has not been confirmed.
A fourth idea, the Big Bounce, says a crunch could rebound into a new Big Bang. It is speculation, because nobody can yet describe a universe crushed that small.
Practice: Which ending?
Sort what a far-future observer would see into the Big Freeze, the Big Rip or the Big Crunch.
The equation of state
Cosmologists sum up dark energy with one number, its equation of state w. It sets how the density of dark energy changes as space grows.
Dark energy density: \rho_{\text{DE}}\propto a^{-3(1+w)}
Drag w and watch dark energy's density as space grows: flat at w = -1, rising below it, falling above it, with matter thinning out for comparison.
Here a is the size of space, with today as 1. At w = -1 the power is zero, so the density never changes: this is Einstein's cosmological constant.
Below -1, dark energy gets denser as space grows, which is called phantom energy. Above -1 it fades, and only dark energy that keeps falling below zero leads to a crunch.
Practice: Tune dark energy
Set w so that dark energy's density changes by a target factor each time space doubles, then see which ending it leads to.
The Big Freeze
If dark energy stays constant, space grows faster and faster forever. Galaxies beyond our own small group are carried out of sight, one by one.
Gas runs out, stars burn out, and everything cools towards absolute zero. It gets ever closer but never quite arrives.
This end state is called the heat death. All the energy is still there, but it is spread so evenly that nothing can happen any more.
Entropy and the arrow of time
Entropy counts how many ways the pieces of something can be arranged. Spread-out states have far more arrangements than tidy ones, so things spread out on their own.
Particles start in one half of a box and hop at random. They spread out on their own, and bars count the arrangements behind every split.
The second law of thermodynamics says the entropy of an isolated system never goes down. This one-way rule gives time its direction, the arrow of time.
The second law: \Delta S\geq 0
The universe began with very low entropy, and it has been rising ever since. The heat death is where it stops rising, because nothing is left to spread out.
Practice: Count the ways
Tap every arrangement of four particles that puts a given number in the left half, to count the ways each split can happen.
The last stars
Galaxies now make new stars about ten times more slowly than they did 10 billion years ago. Their gas is running low.
A clock runs on a scale of powers of ten while stars of different masses burn out one by one, from a 10 Sun giant to the smallest red dwarf.
Small, dim red dwarfs burn their fuel so slowly that they last for trillions of years. The universe is so young that not one has died of old age yet.
When the last red dwarfs fade, around 10^{14} years from now, the stellar era ends. After that, almost nothing shines.
Dead stars and proton decay
Next comes the degenerate era. Galaxies hold only dead stars: cooling white dwarfs, neutron stars, brown dwarfs and black holes.
Some theories predict that protons decay, very slowly, into lighter particles. If so, every dead star and planet would slowly turn into radiation.
A water tank like Super-Kamiokande: pick a proton lifetime and see how many decays a year it should flash, against the none that have been seen.
Nobody has ever seen a proton decay. Super-Kamiokande's 50,000 tonne tank of water has watched since 1996, which shows the proton lasts more than 2.4\times 10^{34} years.
Practice: Waiting for a decay
Use decays per year = protons divided by lifetime to find a decay rate, or a lifetime, with powers of ten.
The black hole era
If protons decay, ordinary matter is gone by about 10^{40} years. Only black holes are left, so this stretch is called the black hole era.
In 1974, Stephen Hawking showed that black holes should glow very faintly. This Hawking radiation carries off mass, so a black hole slowly shrinks and finally vanishes.
Pick a black hole from one Sun to a hundred billion Suns and read its Hawking temperature and how long it takes to evaporate, on a ruler of powers of ten.
Bigger black holes are colder and last far longer. The largest take about 10^{100} years, and after them come only faint light and stray particles, the dark era.
Practice: Timeline of the far future
Match each era and event of the far future to when it ends, in powers of ten of years.
The Big Rip
If w is below -1, dark energy gets denser as space grows. The expansion then runs away so fast that space tears apart at a definite moment, the Big Rip.
A countdown to the Big Rip for a chosen w: the Milky Way, Earth's orbit, the Moon's orbit and Earth itself come apart in turn, each at a computed time before the end.
Things held together weakly go first. For w = -1.5, the Milky Way breaks up tens of millions of years before the end, Earth leaves the Sun a few months before, and atoms tear apart in the final split second.
For w = -1.5 the Rip would come about 22 billion years from now. Measurements so far put w much closer to -1, so a Big Rip is not favoured.
Practice: Torn apart in order
Put the things a Big Rip tears apart in order, first to last.
The Big Crunch and the Big Bounce
If dark energy fades and then turns negative, it pulls instead of pushing. The expansion slows, stops and reverses, and galaxies start rushing together.
A countdown to a Big Crunch: as space shrinks, the leftover glow of the Big Bang is squeezed and the sky heats up past room temperature, red heat and the Sun's surface.
As space shrinks, the faint glow left over from the Big Bang is squeezed to shorter waves. The sky warms, glows red hot, and everything ends crushed together, like the Big Bang in reverse.
Some physicists suggest a crunch could bounce into a new Big Bang. Nobody knows, because describing it needs a theory of quantum gravity that does not exist yet.
Practice: Read the future
Read computed curves of the size of space over time and name the ending each one shows, and what dark energy must be doing.
What DESI found
The Dark Energy Spectroscopic Instrument (DESI) maps millions of galaxies to measure how fast space grew over the last 11 billion years. If w is held constant, its results put it within about 3% of -1.
The best-fit w over cosmic time from DESI's 2025 results with each supernova set, against a constant w = -1, with how sure each result is in sigma.
Let w change over time, though, and DESI's 2024 and 2025 results hint that dark energy is weakening. Combined with other data the hint is 2.8 to 4.2 sigma, and 5 sigma is the usual bar for a discovery.
In 2026, a recalibration of one supernova survey brought its figure down to about 3 sigma, so the question is still open. If dark energy is weakening, the Big Freeze may not be our fate after all.
Practice: Measured or not?
Decide whether each claim about the future is measured, predicted by well-tested physics, or unconfirmed.
Why the far future is uncertain
Every forecast here stretches today's physics far beyond anything we have tested. A tiny change in dark energy now can grow into a completely different ending.
Nobody knows what dark energy actually is. Whether protons decay, and what happens at a crunch, are open questions too.
New surveys are already at work, including DESI, the Vera C. Rubin Observatory and the Euclid space telescope. Over the next few years they will show far more clearly how dark energy changes over time.