Dark Energy

Field note 01 / Cosmology

The universe is speeding up.

“Dark energy” is our name for whatever is driving that acceleration. We can measure its influence across billions of light-years. We still do not know what it is.

Follow the evidence
Estimated composition of the universeDark energy accounts for about 68 percent, dark matter 27 percent, and ordinary matter 5 percent.

≈68% dark energy

≈27% dark matter

≈5% ordinary matter

It is not a substance we have captured. It is a gap in the model—named for the effect we observe.

What tipped us off

Three views of one accelerating cosmos

Exploding stars

In 1998, two teams found distant Type Ia supernovae dimmer than expected. The expansion of space had accelerated while their light was traveling to us.

Ancient light

The cosmic microwave background records the early universe. Its pattern points to a geometrically flat cosmos with far more energy than matter can supply.

A cosmic ruler

Baryon acoustic oscillations leave a standard scale in the distribution of galaxies. Measuring it at different eras traces how expansion has changed.

Reading the curve

Gravity slowed expansion. Then the balance shifted.

Matter pulls inward and thins as space grows. If dark energy keeps roughly constant density, it eventually dominates—and expansion accelerates.

Schematic history of cosmic expansionA curve shows expansion slowing under gravity in the early universe, then accelerating as dark energy becomes dominant.gravity dominatesacceleration beginstime →
Schematic only—not to scale.

Leading ideas

One effect, competing explanations

Vacuum energy

Einstein’s cosmological constant treats empty space as having a fixed energy density. It fits observations well—but theory predicts a wildly different value.

A changing field

“Quintessence” imagines a field whose strength evolves over cosmic time. Detecting change would separate it from a true constant.

Different gravity

General relativity may need revision at the largest scales. In that case, acceleration could be gravity behaving differently—not a new ingredient.

The open case

The decisive clue is w.

w≈ −1

The equation-of-state parameter w compares dark energy’s pressure with its density. A cosmological constant gives exactly −1.

  • Is w precisely −1?
  • Has it changed over time?
  • Does gravity follow relativity everywhere?

New galaxy maps, supernova surveys, and gravitational-lensing measurements are narrowing the answer.