Dark energy may have once been ‘springier’ than it is today − DESI cosmologists explain what their collaboration’s new measurement says about the universe’s history

The Mayall 4-meter Telescope at the Kitt Peak National Observatory houses the DESI instrument. KPNO/NOIRLab/NSF/AURA/P. Marenfeld

Gravity pulls us to earth, a lesson you learn viscerally the first time you fall. Isaac Newton described gravity as a universal attractive force, one that holds the Moon in orbit around the Earth, the planets in orbit around the Sun, and the Sun in orbit around the center of our galaxy.

In the 1990s, astronomers made the astonishing discovery that the expansion of the universe has sped up over the past 5 billion years, which implies that gravity can push as well as pull.

Einstein’s theory of general relativity explains gravity as a consequence of curved space-time, where it allows for both attraction and repulsion. However, producing gravitational repulsion requires a new form of energy with exotic physical properties, often referred to as “dark energy.”

New results from a large survey of the universe, announced in March 2025, are challenging the conventional picture of dark energy.

Dark energy and cosmic expansion

The simplest explanation for cosmic acceleration assumes a form of energy that fills apparently empty space and stays constant over time, instead of diluting as the universe expands.

In fact, quantum mechanics predicts that “empty” space is filled with particles that flare briefly into and out of existence. At first glance, it seems like this effect could explain a constant dark energy, but no simple estimates of the effect’s magnitude line up with actual observations. Nonetheless, constant dark energy is a simple assumption that has proven successful in explaining many cosmological measurements.

Today’s standard cosmological model incorporates this kind of constant dark energy. It also incorporates atoms and dark matter, which exert the attractive gravity that resists dark energy’s repulsion.

New dark energy measurements

The new measurements from the Dark Energy Spectroscopic Instrument, or DESI, collaboration, which we are affiliated with, pose the sharpest challenge yet to this standard model.

Relative to the constant dark energy predictions, the new DESI measurements suggest that the universe was expanding slightly faster a few billion years ago – by 1% to 3% – before relaxing to the expansion rate predicted today. One explanation for this temporary speed up is that the “springiness” of dark energy – a combination of energy and pressure that determines its repulsive effect – was higher in the past. The springiness then declined as the universe expanded further.

Astronomers can measure the history of the universe from our vantage point in the present because light travels at a finite speed. So, we see distant objects as they were in the past. Cosmic expansion stretches the wavelength of light – a phenomenon known as redshift. A precise measurement of an object’s light can reveal the size of the universe at the time the light was emitted.

The new DESI results are based on measuring the redshifts of more than 14 million galaxies, creating a three-dimensional map that spans 12 billion years of cosmic history. To determine the distances light traveled across this map, DESI measured a subtle feature imprinted on the clustering of these galaxies by acoustic waves that traveled through the early universe.

An exciting result

DESI’s evidence for evolving dark energy comes from combining its own distance and redshift measurements with other measurements of the average density of matter in the universe. The higher the density of matter, the more strongly it can pull against dark energy’s expansive push. The matter density measurements come from the European-led Planck space mission, which mapped structure in the cosmic microwave background.

The combination of DESI and Planck data favors evolving dark energy, instead of constant dark energy, with a statistical significance of 3.1 standard deviations. This result has only a 1 in 500 chance of occurring randomly.

Despite the long odds, physicists consider such a finding to be solid but not overwhelming evidence, in part because even the most careful experimenters may underestimate uncertainties in their measurements.

To strengthen the statistical case, DESI scientists added measurements of cosmic distances made by the Dark Energy Survey collaboration, which applied a different measurement technique based on the brightness of light from supernova explosions.

The combination of DESI, Planck and Dark Energy Survey supernovae favors the evolving dark energy model by odds of 40,000 to 1. However, other supernova surveys give results that agree more with constant dark energy, so most cosmologists aren’t yet ready to abandon the standard cosmological model.

Even if DESI’s findings hold up, they still can’t say what dark energy is. But they can provide much stronger clues than cosmologists had before.

The DESI-based model implies that dark energy changed its properties surprisingly quickly. Dark energy began to lose its repulsive strength at about the same time it became the dominant form of energy in the cosmos.

Extrapolating to the past, this model also implies that dark energy once had an extraordinary springiness, at a level that no simple theory of a dark energy field can explain. As future data sharpens these measurements, the findings could point us in a weird new direction – perhaps even challenging Einstein’s theory of gravity itself.

In the model that fits the DESI data, the density of dark energy goes up and then declines, shown as a blue curve, instead of staying constant as assumed in the standard cosmological model, indicated by the horizontal dotted line. In either case, the density of atoms and dark matter dilutes as the universe expands, shown as a red curve, and today it is only about half that of dark energy. The repulsive effect of dark energy began to exceed the attractive effect of matter when the universe was about 8 billion years old, marked as ‘acceleration begins.’ David Weinberg

An ambitious experiment

DESI is an extremely ambitious undertaking and an example of “big science” at its best. The instrument itself is mounted on the 4-meter Mayall Telescope at the Kitt Peak National Observatory. It uses 5,000 optical fibers mounted on tiny robotic positioners that guide the light from individual galaxies to scientific instruments that dissect that light and record the data for measuring redshifts.

Every 15 minutes, the telescope shifts to a new area of the sky, and the robots move the fibers to point to 5,000 new galaxy locations. After five years of design and construction, DESI has operated continuously since 2021.

A close-up of the DESI focal plane showing a few of the 5,000 fiber positioners. The white spots inside the bluish circles are the optical fibers that guide the light collected from distant galaxies to the spectrographs about 40 meters away. Dr. Claire Poppett, DESI Collaboration

Led by the Department of Energy’s Lawrence Berkeley National Laboratory, DESI is a collaboration of over 900 scientists at 70 institutions around the world. At our university alone, more than 20 faculty, students, postdocs and research staff have worked on DESI over the past decade.

This work includes contributions to building and installing spectrographs, which measure the properties of light, as well as writing software to record data, leading instrument operations, observing and troubleshooting at the telescope, designing galaxy and quasar surveys, creating catalogs for statistical analysis, testing measurement techniques with computer simulations, interpreting results and writing papers – all in tight communication with our collaborators.

If the evidence for evolving dark energy holds up — and despite our instinctive caution, we think it has a good chance of doing so — it will join a list of remarkable 21st-century discoveries achieved with large U.S. national investments.

These discoveries include the first detection of gravitational waves by the National Science Foundation-funded Laser Interferometer Gravitational-Wave Observatory, LIGO, and the spectacular measurements of galaxies and exoplanet atmospheres by NASA’s James Webb Space Telescope.

These achievements show what the support of science by U.S. taxpayers and dedicated, creative researchers across the globe can accomplish.

David Weinberg receives funding from the National Science Foundation and NASA that supports his dark energy research.

Ashley Ross receives funding from Lawrence Berkeley National Lab to support his work on DESI and NASA to support work on related experiments.

Klaus Honscheid receives funding from Department of Energy.

Paul Martini receives funding from the Department of Energy.