Dark Energy

Dark energy is the biggest mystery in modern physics. It makes up about 68% of the energy in the entire universe, but we have no idea what it actually is. We only know it exists because it is somehow pushing galaxies away from each other faster and faster. Dark energy is the reason the universe is not just expanding, but expanding at an accelerating rate. It is the leading idea about how our universe will eventually end.

  • % of universeapprox. 68%The biggest single ingredient
  • EffectPushes universe apartExpansion is accelerating
  • Discovered1998From observations of supernovae
  • Nobel Prize2011Perlmutter, Schmidt, Riess
  • Best candidateCosmological constantEnergy that fills empty space
  • Mystery levelTotalAlmost everything about it is unknown

What is dark energy?

Dark energy is the name astronomers give to the unknown thing causing the expansion of the universe to speed up. Ordinary matter, dark matter and even light all have gravity that pulls things together. Dark energy seems to do the opposite: it pushes things apart, against gravity. The further apart two galaxies are, the harder dark energy pushes.

The most common idea is that dark energy is a cosmological constant: a property of empty space itself. Every cubic centimetre of empty space appears to contain a tiny, fixed amount of energy that pushes outwards. There is so little of it per cubic centimetre that we can never detect it in a lab. But across the unimaginable volume of the universe, it adds up to the biggest force in the cosmos.

How dark energy was discovered

In the late 1990s, two independent teams of astronomers (the Supernova Cosmology Project led by Saul Perlmutter, and the High-Z Supernova Search Team led by Brian Schmidt and Adam Riess) were carefully measuring the brightness of Type Ia supernovae in distant galaxies. Type Ia supernovae all reach almost the same true peak brightness, so they can be used as "standard candles" to measure distances across the universe.

Both teams expected to find that the universe's expansion was slowly decelerating, because the combined gravity of all the matter in the universe should be slowing it down. To their amazement, both teams found the opposite. The distant supernovae were dimmer than they should be, meaning they were further away than the expected slowdown would put them. The expansion was actually speeding up. The result was so surprising that each team double-checked everything before publishing. They announced it in 1998 and shared the 2011 Nobel Prize in Physics.

Why dark energy is so strange

The strangest thing about dark energy is how much there is of it. Astronomers now think the entire universe contains:

  • approx. 68% dark energy
  • approx. 27% dark matter
  • approx. 5% ordinary matter (atoms, you, stars, planets, gas)

In other words, the stars, planets and people that fill almost every astronomy book are only about 5% of the universe. The other 95% is invisible stuff we do not yet understand. And the biggest single ingredient is something we did not even know existed before 1998.

Fact Dark energy and dark matter are completely different things despite the similar names. Dark matter is an invisible substance whose gravity pulls things together: it holds galaxies and clusters of galaxies in shape. Dark energy is an invisible property of empty space that pushes things apart: it stretches the universe faster and faster. Confusingly, the two also fight each other on the largest scales.

What could dark energy be?

Physicists have a few main ideas for what dark energy might actually be, but none of them is proven.

  • Cosmological constant: a fixed amount of energy locked into the fabric of space itself, the same everywhere and at all times. Albert Einstein included exactly such a term in his original 1917 equations of general relativity (although he later called it his "biggest blunder"). It might be a real property of the universe after all.
  • Quintessence: an energy field whose strength can change over time, rather than being fixed. If quintessence is real, the rate of cosmic acceleration may speed up or slow down as the universe evolves.
  • Modified gravity: maybe dark energy is not really a thing at all. Maybe our theory of gravity needs adjusting on the largest scales, just as some people argue for dark matter on the scale of galaxies.
  • Vacuum energy: quantum mechanics predicts that empty space is full of "vacuum energy" from particles popping in and out of existence. The trouble is, simple calculations give a vacuum energy of around 10120 times too much. Working out why the answer is so wrong is one of the biggest open puzzles in physics.

How will the universe end?

If dark energy is a cosmological constant (the most popular idea), then the universe will keep expanding faster and faster forever. Distant galaxies will be pushed beyond our cosmic horizon and eventually disappear from view. In tens of billions of years, an astronomer in our merged Milky Way + Andromeda galaxy will see nothing but their own galaxy in the sky: a single, lonely island of stars in an otherwise empty universe. Eventually all stars will burn out and the universe will enter a long, cold heat death.

If dark energy is something more exotic that gets stronger over time, the future could be even more dramatic: a Big Rip in which everything (galaxies, stars, planets, atoms) is eventually torn apart by the accelerating expansion. Current measurements lean against this scenario, but it cannot yet be ruled out.

Did you know? Einstein originally added a cosmological constant to his equations in 1917 because he wanted a static universe, where the constant exactly balanced gravity to keep the universe from expanding or shrinking. When Hubble discovered the universe was expanding anyway in 1929, Einstein dropped the constant and called it his "biggest blunder". The 1998 discovery of cosmic acceleration suggests Einstein's "blunder" was actually correct after all, just for a different reason than he originally thought.
Deeper dive: the vacuum energy catastrophe

One of the biggest open puzzles in modern physics is sometimes called the cosmological constant problem or the vacuum energy catastrophe. It is a clash between our two best theories: general relativity (which describes gravity and the universe at the largest scales) and quantum field theory (which describes the particles and forces of the very smallest scales).

Quantum field theory predicts that empty space is not really empty at all. Particle-antiparticle pairs constantly pop in and out of existence in what is called the quantum vacuum. Each of those fluctuations carries a tiny bit of energy, and if you add up all of them across all possible energies, the total energy of even one cubic centimetre of empty space ought to be astronomically large.

Astronomy lets us measure the actual energy of empty space, because that is essentially what dark energy is. The measured value is incredibly tiny: positive, but only about 10-9 joules per cubic metre. The naive prediction from quantum theory is around 10120 times bigger. That is the worst disagreement between theory and experiment in the history of physics, by a long way. Either quantum theory is missing something huge, or some unknown new physics cancels out almost all of the predicted energy and leaves only the tiny amount we measure. Solving the cosmological constant problem may be the key to a true "theory of everything".

For the other mysterious ingredient, see dark matter. For the growing universe in general, see the expanding universe and the observable universe.