Dark Energy: Why the Universe Speeds Up

Two dark mysteries, and why they are not one#

The cosmic inventory is humbling. Add up everything the universe is made of and ordinary matter — every atom in every star, planet, and person — comes to only about 5%. Another 27% is dark matter, unseen mass that gravitates. And the remaining 68%, the largest slice by far, is dark energy: the component driving the universe's accelerating expansion.

The two dark components share an adjective and almost nothing else, and the single most common confusion in cosmology is to treat them as the same thing — two names for one mystery. They are not. They are close to opposites.

Dark matter is mass. It has gravity, it pulls, and its whole job in the story is to hold things together: it is the extra gravitating stuff that keeps galaxies from flying apart and lets cosmic structure form. Dark energy does the reverse. It behaves like an energy — a pressure — of empty space itself, and its effect is to push: to stretch the expansion of the universe and make it speed up. One clumps and attracts; the other is smooth and repels. Keeping that contrast straight is the whole point of this article.

The expansion should have been slowing down#

Space is expanding — every distant galaxy recedes, and the farther ones recede faster, as the expanding universe makes plain. For most of the twentieth century the natural expectation was that this expansion must be slowing down. The reasoning was simple gravity: the universe is full of matter, all of it pulling on everything else, so the mutual gravitational tug of all that mass should act like a brake, decelerating the expansion over time. The only open question, people thought, was whether gravity was strong enough to halt and reverse it one day, or merely to slow it forever.

Before going further, retire a second misconception that often rides along. The galaxies are not flying outward through space away from some central point, hurtling into an empty void. It is space itself that stretches, carrying the galaxies with it; every galaxy sees all the others recede, so there is no centre and no edge that the universe expands "into." When we ask whether the expansion speeds up or slows down, we are asking about the growth rate of space, described by a single number called the scale factor $a(t)$ — how large the universe is at time $t$, set to $a_0 = 1$ today. Deceleration means $a(t)$ grows ever more slowly; acceleration means it grows ever faster.

To settle which was happening, astronomers needed to measure how the expansion rate has changed across cosmic history. That means looking far out — and because light takes time to arrive, looking far out is looking back in time. Compare the expansion long ago with the expansion now and you can read off whether space has been braking or flooring the accelerator.

1998: the supernovae that broke the brakes#

The measurement needed a standard candle — an object of known intrinsic brightness, so that its apparent faintness reveals its distance, a rung near the top of the cosmic distance ladder. Type Ia supernovae are close to ideal: a white dwarf detonating at a well-calibrated peak luminosity, bright enough to be seen billions of light-years away.

In 1998 two independent teams measured dozens of these distant supernovae, expecting to quantify how much the expansion had slowed. Instead the supernovae came in fainter than expected — meaning farther away than a decelerating universe would place them. The expansion had not been braking. Over roughly the last 5 billion years it has been accelerating: space is stretching faster and faster. The result was so unexpected that both teams checked it exhaustively before believing it, and it earned the 2011 Nobel Prize in Physics.

Something was overpowering the collective gravity of all the matter in the universe and pushing the expansion to speed up. That something is what we now call dark energy.

Notice, in the animation, the two eras. Early on, when matter was packed into a smaller volume and its density was high, gravity won and the expansion decelerated — the galaxies still drew apart, but ever more slowly. Then, as space grew and matter thinned out, a different influence took over and the expansion began to accelerate, the galaxy separations opening up faster and faster. And note what is expanding: not galaxies swimming through space, but the grid of space itself stretching between them, seen the same from every galaxy.

A constant energy of empty space#

What could push? The oldest and simplest candidate is a number Einstein first wrote into his equations in 1917 and then abandoned: the cosmological constant, $\Lambda$. In modern language it represents an energy that belongs to space itself — a vacuum energy. Empty space, on this idea, is not truly empty; every cubic metre carries a small, fixed energy density, and that energy exerts a negative pressure that acts to push space apart.

The decisive feature of a cosmological constant is that its density stays constant as space expands. This is what sets it apart from matter and explains the whole history above. Matter dilutes: double the size of the universe and the same atoms are spread through eight times the volume, so matter's density falls. But if dark energy is a property of space, then more space simply means more dark energy, at the same density everywhere. Schematically, the matter density falls as

$$\rho_\text{matter} \propto \frac{1}{a(t)^3}, \qquad \rho_\Lambda = \text{constant.}$$

Early on, when $a$ was small, matter was dense and its gravity dominated — hence deceleration. As $a$ grew, matter thinned toward irrelevance while dark energy held steady, until it became the dominant component and its push took over — hence acceleration. Dark energy did not switch on; it simply outlasted everything else. This also means its share of the cosmic budget grows with time: the 68% of today was much smaller in the early universe and will approach 100% in the far future.

What we know, and what we don't#

It is worth being honest about the size of the remaining puzzle. That the expansion accelerates is now solid, confirmed by supernovae, the cosmic microwave background, and the large-scale distribution of galaxies. What dark energy actually is remains genuinely unknown.

The cosmological constant fits the data well, but when physicists try to compute the vacuum energy from quantum theory, the natural estimate overshoots the observed value by many tens of orders of magnitude — one of the worst mismatches between theory and observation in all of physics. Perhaps dark energy is a true constant; perhaps it is a slowly evolving field (sometimes called "quintessence") whose strength changes over time; perhaps it signals that our theory of gravity itself needs revision on the largest scales. Current and coming surveys are measuring whether the push has held perfectly steady or drifted, because the answer decides the fate of the universe: eternal, ever-faster expansion into cold darkness, or something stranger.

For now the essential picture is clear and worth holding onto. Dark matter pulls; dark energy pushes. One is clumpy mass; the other is smooth energy of space. And it is the pusher, invisible and unexplained, that has taken the wheel of the cosmos.