How the Universe Ends: an interactive course

How will the universe end? Nobody knows for certain, because it depends on dark energy, the unknown cause of the universe's speeding-up expansion. If dark energy stays constant, the universe expands forever and slowly goes dark in a Big Freeze. If it grows stronger, it could tear everything apart in a Big Rip. If it fades and reverses, the universe could collapse in a Big Crunch. The How the Universe Ends course works through all three, in 22 short sections.

It opens with a chart of the size of space over time, solved from the Friedmann equation with today's measured values: one measured past, then three possible futures.

What the course covers

Why dark energy decides it

Gravity from matter should slow the expansion, but in 1998 astronomers found it is speeding up. Matter thins out as space grows, while dark energy, as far as we can tell, does not, so dark energy wins in the long run. Cosmologists sum it up with one number, w: at exactly −1 it stays constant; below −1 it grows; above −1 it fades.

The Big Freeze

With constant dark energy, distant galaxies are carried out of sight, star formation runs down and the last stars burn out. The course follows the long eras after that, from dead stars to possible proton decay and the black hole era, and explains why entropy, the number of ways things can be arranged, gives time its direction.

The Big Rip and the Big Crunch

If w is below −1, dark energy gets denser as space grows and eventually tears apart galaxies, stars, planets and atoms. If dark energy fades and turns negative, the expansion could reverse into a Big Crunch, or perhaps a bounce. The course looks at what the DESI survey's measurements suggest and is careful about what is measured, what is well-tested prediction and what is speculation.

Try a question from the course

As space expands, the same matter spreads through more volume. If every distance in space doubles, every volume grows by 2 × 2 × 2 = 8 times, so matter becomes 8 times thinner. If distances grow 3 times, matter becomes 3³ = 27 times thinner.

Now work out a new one, then press Mark to check your answer in the course.

Who this course is for

• A-level and other advanced physics students who want to go beyond the Big Bang into cosmology's open questions.
• Curious learners who have heard of heat death or the Big Rip and want the real reasoning.
• Anyone who enjoys thinking about very large numbers and very long times.

What you need to know first

The Big Bang Theory course is the natural starting point, for expansion, redshift and dark energy. You need powers and powers of ten: the far future is measured in numbers like 10⁴⁰ years.

How long it takes

There are 22 sections and most take a few minutes. Learners without a plan can open two sections of a course a day, in order, so the whole course takes 11 days. A plan removes the daily limit and lets you go straight to any ending.

Read next

Physics · CoursesCosmology course: the Big Bang, redshift and dark energyAn interactive cosmology course in 24 short sections: expansion, redshift, Hubble's law, the age of the universe, inflation, the CMB and dark energy.