What expansion means
In the 1920s, astronomers found that distant galaxies are moving away from us, and that the farther away a galaxy is, the faster it tends to recede. The usual interpretation is that space itself is stretching, carrying galaxies apart, rather than galaxies flying outward through a fixed space. Edwin Hubble’s name is attached to the early measurements, though others contributed to the idea.
A common picture is a rubber sheet with dots on it. Stretch the sheet and every dot moves away from every other dot, with no special center. The analogy has limits, but it captures why no galaxy is at the “middle” of the expansion.
Why people expected it to slow down
For decades, the natural expectation was that expansion should be slowing. Matter attracts other matter through the gravitational force, so the combined pull of everything in the universe ought to act like a brake. The open question was whether the braking was strong enough to eventually halt expansion, or whether the universe would expand forever at an ever-slower rate. Measuring how much the expansion had slowed was a major goal of observational cosmology.
The surprise of the late 1990s
In 1998, two independent research teams reported results from studying distant Type Ia supernovae, a kind of stellar explosion that astronomers use as a standard reference for brightness. If these explosions all have roughly the same true brightness, then how dim they look tells you how far away they are.
The teams found that the distant explosions were fainter than expected for a universe whose expansion was slowing down. The most natural reading was that they were farther away than predicted, which means expansion had been speeding up over the last several billion years. Leaders of both teams shared the 2011 Nobel Prize in Physics for this discovery.
Since then, other lines of evidence have pointed in a similar direction, including measurements of the faint afterglow of the Big Bang and of how galaxies are distributed in space. When independent methods agree, scientists become more confident in a result.
What “dark energy” refers to
Dark energy is the name given to whatever is causing, or mimicking, the acceleration. It is important to be clear that the name is a placeholder. It describes an effect, not a fully understood substance.
Several ideas are discussed:
- A cosmological constant. Einstein once added a constant term to his equations of gravity. In modern language, it corresponds to a fixed energy density built into empty space itself. It is the simplest explanation and fits many observations well.
- An evolving field. Some models propose a field whose strength changes over time, so that the push of dark energy would not be constant.
- A change to gravity. Others suggest that the laws of gravity may differ on very large scales, with no new energy needed.
No option has been established beyond doubt. Calculating the energy of empty space from quantum physics gives a number that is wildly different from what is observed, and that mismatch is generally regarded as an unsolved problem.
How much is there?
In the standard model of cosmology, usually called the Lambda-CDM model, dark energy is estimated to make up roughly two thirds of the universe’s total energy content today. Dark matter accounts for roughly a quarter, and ordinary matter, the atoms that make up stars, planets and people, for only about five percent. These percentages are approximate and come from fitting the model to many observations, so they carry some uncertainty and can be refined as measurements improve.
Dark energy and dark matter are different problems. Dark matter is extra mass that holds galaxies together, while dark energy is linked to the accelerating expansion on the largest scales.
Why acceleration happens only now
Early in cosmic history, matter was densely packed, and its gravity dominated. As space expanded, matter thinned out. If dark energy behaves like a cosmological constant, its density does not thin out, so eventually it began to outweigh matter’s pull. Roughly speaking, the change from slowing to speeding up is thought to have occurred several billion years ago. The exact timing depends on the model.
Possible futures
If dark energy stays constant, distant galaxies will keep receding faster, and over extremely long times they would pass beyond our view. Our own galaxy and its neighbors are held together by gravity and are not expected to be pulled apart. Some more speculative models allow dark energy to strengthen or weaken, which could lead to different outcomes. Because the nature of dark energy is unknown, these scenarios should be treated as possibilities rather than predictions.
Open questions
One current puzzle is that different ways of measuring the present expansion rate give slightly different answers, a disagreement often called the Hubble tension. Whether this reflects measurement errors, or hints at new physics, is not settled. New survey telescopes and space missions are collecting more data to test whether dark energy is truly constant.
For readers who want the wider context, the field of cosmology treats the expanding universe as one pillar of its picture of cosmic history, and the fundamental forces known to physics do not yet include an obvious explanation for dark energy.