- Three independent experiments now agree dark energy is weakening over time.
- Dark energy behaving as a field implies it is linked to particles.
- The signal sits at 3 sigma, short of a confirmed discovery.
Sabine Hossenfelder wasn't expecting good news. The German theoretical physicist turned science communicator has spent years watching tentative cosmological results dissolve under closer inspection. So when new dark energy data arrived, she put her personal "bullshit meter" at a 2 out of 10.
The data didn't cooperate with her skepticism.
Three independent experiments have now converged on the same finding: dark energy, the force behind the universe's accelerating expansion, appears to have been stronger in the past and may be fading. The cosmology crisis is no longer just a rumour. The question is whether cosmology's foundational model needs replacing.
The Model That Held for Decades
For most of modern cosmology, dark energy has been treated as a cosmological constant: a fixed, unchanging background pressure built into the fabric of space. Einstein introduced the idea to balance gravity and keep the universe static. He abandoned it after Edwin Hubble confirmed that the universe was expanding. Astronomers revived it in the late 1990s when supernova observations revealed that expansion was accelerating.
The simplest explanation, and for two decades the dominant one, was that empty space carries a small but invariable energy density.
What is the cosmological constant?
The cosmological constant is a term in Einstein's field equations representing the energy density of empty space. If dark energy is a true constant, its strength never changes and the universe expands forever at an accelerating rate. If it behaves instead like a dynamic field, its strength can vary over time, and the universe's long-term fate becomes far less certain.
Three Experiments Point the Same Way
The first signal came from the Dark Energy Spectroscopic Instrument (DESI), which spent three years mapping more than 14 million galaxies and quasars. DESI measures dark energy through baryon acoustic oscillations. These are subtle ripple patterns in the distribution of matter, left over from sound waves in the early universe's plasma.
These patterns act as a cosmic ruler. By measuring their apparent size at different distances, DESI can reconstruct how the universe's expansion rate has varied over 11 billion years. The data, published in peer-reviewed form in March 2025, pointed toward dark energy that was stronger in the past.
A second signal came from the Dark Energy Survey, which tracked supernova brightness across cosmic distances.
Key figure
6.2σ
The South Pole Telescope's measure of the Hubble tension – how far early- and late-universe expansion rate measurements have diverged from each other.
The third comes from the South Pole Telescope, a 10-meter instrument with 16,000 detectors at Antarctica's Amundsen-Scott Station. The team released two years of cosmic microwave background data covering one twenty-fifth of the sky. Combined with DESI, their analysis confirmed the dark energy signal and measured the Hubble tension at 6.2 sigma from local measurements.
Each experiment is independent. Each covers a different era of cosmic history. All three lean the same way.
What the Evidence Doesn't Yet Settle
Hossenfelder characteristically declines to declare a crisis resolved. Her 2 out of 10 rating reflects a genuine concern: all three experiments fit their data against the same Lambda-CDM model of the universe. If the model's assumptions are subtly wrong, the anomaly could be an artifact rather than a real change in dark energy.
If dark energy is not a constant, this is a clue that it's sort of a field that permeates the universe.
Sabine Hossenfelder, Science with Sabine
The 3 sigma significance, roughly a 3-in-1,000 chance of being random noise, falls short of the 5 sigma threshold physicists typically require before claiming a discovery. The Hubble tension has persisted at similar significance for years without a clean resolution emerging.
If the Signal Holds, the Implications Are Large
A non-constant dark energy would mean it behaves as a field, and in physics, fields couple to particles. That opens two paths: an undiscovered particle is responsible, or dark energy connects to an existing one. The Higgs boson is among the candidates that theorists have proposed, though no version of this idea has been convincing so far.
More On Hubble Tension
The Silence Between Black Holes
Black holes are colliding in silence across the cosmos. That silence could help explain cosmic expansion.
→The implications for the universe's long-term future are also real. A constant dark energy guarantees endless accelerating expansion. A weakening dark energy raises the possibility that expansion eventually slows and perhaps reverses. In effect this could lead to cyclic cosmologies, in which the universe collapses and rebounds through recurring big bangs.
This is now more plausible than it has seemed in decades. Our earlier coverage of DESI's baryon acoustic oscillations evidence explains the measurement technique in more detail.
DESI continues collecting data, and the Euclid satellite and the Vera Rubin Observatory will both measure dark energy with considerably greater precision over the coming years. By the late 2020s, the question of whether the cosmological constant truly deserves its name should be considerably closer to answered.
Go Deeper
- DESI DR2 Results Guide - The DESI collaboration's overview of their three-year dataset and what it implies for dark energy models
- Growing evidence for evolving dark energy (Phys.org) - The South Pole Telescope team explains their CMB measurements and what combining them with DESI data reveals
- New DESI Results Strengthen Hints Dark Energy May Evolve (Berkeley Lab) - Accessible summary with commentary from DESI team members
Fact Check: Claim-by-Claim Verification Verified
The article accurately reports hints of evolving dark energy from DESI DR2, DES supernova analyses, and South Pole Telescope combinations, with appropriate hedging on significance and uncertainties. [1][5][6]
Commentary
- Dark Energy Survey's supernova role is supportive ("hints," ~2-3σ) rather than explicit independent BAO-like signal; article simplifies but doesn't misstate.
- 6.2σ Hubble tension is approximate/local vs. early universe; exact value varies slightly by combination but directionally correct.
- Results fit within ΛCDM tensions but prefer evolving DE; future data (Euclid, Rubin) needed for confirmation. [1]
Sources used for verification
Academic/Peer-reviewed:
- Extended Dark Energy analysis using DESI DR2 BAO measurements - arXiv [2]
- DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations - arXiv
- Evidence for evolving dark energy from DESI DR2 - in2p3.fr [1]
- Dynamical Dark Energy Beyond Planck? Constraints from SPT, ACT, DESI - arXiv
Other reliable sources:
- DESI DR2 Results Guide - desi.lbl.gov [5][1]
- Is Dark Energy Getting Weaker? New Evidence Strengthens the Case - quantamagazine.org [6]
- New DESI Results Strengthen Hints Dark Energy May Evolve - lbl.gov [9]
- Growing evidence for evolving dark energy - phys.org
Fact-checked by Perplexity Sonar Pro on 2026-03-06
