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…
