Lead: New analyses from the Dark Energy Spectroscopic Instrument and a South Korean team suggest dark energy might not be constant, a finding that could reverse the long‑term expansion of the cosmos. The March Desi survey and a November paper led by Prof Young Wook Lee of Yonsei University report evidence that cosmic acceleration has varied over time. If dark energy weakens further, gravity could eventually pull galaxies back together in a Big Crunch rather than an ever‑faster expansion or a Big Rip. These claims remain contested by other senior astronomers and will require further observations to settle.
Key takeaways
- The Universe began with the Big Bang about 13.8 billion years ago and was expected to slow under gravity before dark energy was discovered in 1998.
- The Dark Energy Spectroscopic Instrument, operating from Arizona, produced unexpected March results suggesting the acceleration of galaxies may have changed over time.
- A November paper from a South Korean team led by Prof Young Wook Lee recalibrated supernova brightness using host galaxy ages and found evidence that acceleration is slowing.
- Prof Lee reports analysis of roughly 300 supernova host galaxies and characterizes the statistical significance as about a one‑in‑a‑trillion chance of a fluke.
- Mainstream cosmologists, including Prof George Efstathiou, argue the supernova corrections are uncertain and may reflect messy astrophysical systematics.
- Independent reanalyses have softened but not eliminated the anomalous signals, keeping the debate active.
- Possible cosmic endgames now being discussed include a renewed possibility of a Big Crunch if dark energy weakens further, alongside previously considered scenarios such as a Big Rip or eternal expansion.
Background
Cosmologists inferred the existence of dark energy in 1998 from observations of Type Ia supernovae used as standard candles. Those supernova studies showed the rate at which distant galaxies recede is accelerating rather than decelerating under gravity. The simplest theoretical description is a constant vacuum energy, often denoted by a cosmological constant, that produces steady accelerated expansion.
Alternative proposals allow dark energy to evolve, which changes the universe’s long‑term fate. A stronger accelerating force could spread matter so thin that galaxies and eventually atoms would be torn apart in a hypothetical Big Rip. Conversely, if dark energy weakens or reverses sign, gravity could halt expansion and trigger a collapse called the Big Crunch. Large redshift surveys and precision supernova work are the principal observational routes to constrain these possibilities.
Main event
In March, data releases from the Dark Energy Spectroscopic Instrument, a redshift survey operating from the Kitt Peak site in Arizona, revealed subtle features in the expansion history of the Universe. Project scientists including Prof Ofer Lehav of University College London have noted that Desi tracked millions of galaxies with unprecedented detail and showed hints that acceleration may vary with time.
Building on that thread, a South Korean team led by Prof Young Wook Lee revisited the original supernova datasets that first implied dark energy 27 years ago. They applied a correction for the ages of the host galaxies to refine the intrinsic brightness estimates of Type Ia supernovae and concluded that the acceleration is decreasing.
Prof Lee told BBC News that a weakening dark energy would change modern cosmology and open the door to a Big Crunch. His work was peer reviewed and published in a Royal Astronomical Society journal, and he reports an analysis based on about 300 host galaxies with a quoted statistical improbability of being a chance result at roughly one‑in‑a‑trillion.
Several senior astronomers have pushed back. Prof George Efstathiou of the Institute of Astronomy, Cambridge, said the age‑brightness correlation is loose and that applying such a correction may be risky. Other teams have independently reassessed subsets of the supernova data and while their updates reduced some of the original anomalies, they did not fully remove the hints that dark energy might evolve.
Analysis & implications
If dark energy is not a true constant, the simplest Lambda cold dark matter picture must be extended and new physics invoked. Time variation in dark energy can arise in models where a scalar field evolves across cosmic time or where interactions between dark sectors alter the effective pressure driving expansion. Such changes would force cosmologists to revisit parameter fits that underpin our estimates of cosmic history and content.
A shift from steady acceleration toward deceleration would have profound observational and theoretical consequences. Observationally, measurements of distances and growth of structure at multiple redshifts would need to be reconciled with the new expansion history. Theoretically, models would have to explain why dark energy begins to weaken only after billions of years, and particle physicists would be tasked with embedding that behavior into a consistent framework.
Practically, resolving this requires more data and better control of astrophysical systematics. Larger, independent supernova samples, improved host‑galaxy characterisation, and complementary probes such as baryon acoustic oscillations and weak gravitational lensing will be essential. The debate highlights how small methodological choices, like host age corrections, can shift cosmological conclusions when measurements approach percent‑level precision.
Comparison & data
| Model | Observational signature | Long‑term fate |
|---|---|---|
| Constant dark energy (Lambda) | Steady acceleration across redshift, consistent supernova distances | Continued expansion, cold, dilute Universe |
| Weakening dark energy | Declining acceleration at low redshift, altered distance modulus for Type Ia supernovae | Possible halt and reversal of expansion, Big Crunch |
| Strengthening dark energy | Increasing acceleration, divergence in large‑scale structure growth | Possible Big Rip if divergence accelerates without bound |
The table summarizes three broad classes of behaviour. Current datasets including Desi redshifts and calibrated supernova samples probe the low to intermediate redshift window where differences emerge. The South Korean recalibration of roughly 300 supernova hosts is one such test; larger samples and cross‑checks are required to shift consensus.
Reactions & quotes
Below are representative responses from project participants and critics, each given in context.
Now with this changing dark energy going up and then down, again, we need a new mechanism. And this could be a shake up for the whole of physics.
Prof Ofer Lehav, University College London, Desi collaborator
This comment accompanied Desi team discussions about the March survey anomalies and highlights how variable dark energy would demand new theoretical ideas.
If dark energy is not constant and it’s getting weakened, this will change the whole paradigm of modern cosmology.
Prof Young Wook Lee, Yonsei University, lead author
Prof Lee made this statement after publishing his RAS paper that applied host galaxy age corrections to historic supernova samples and reported slowing acceleration.
I think that this is just reflecting the messy details of supernovas. The correlation with age is not very tight, so I think it is dangerous to apply a correction.
Prof George Efstathiou, Institute of Astronomy, Cambridge
Prof Efstathiou emphasized caution and attributed the signal to possible astrophysical systematics rather than cosmological change.
Unconfirmed
- The precise magnitude and sign of the reported change in dark energy remain under debate because systematics in supernova standardization may mimic evolution.
- The Desi March anomalies and the South Korean recalibration have been partially softened by independent reanalyses but not fully ruled out, so the claimed one‑in‑a‑trillion significance should be interpreted with caution.
- It is not yet established whether a single new physical mechanism can account for both the Desi hints and the adjusted supernova distances across all redshifts.
Bottom line
The possibility that dark energy evolves is among the most consequential questions in modern cosmology because it determines the Universe’s ultimate fate. Current hints from Desi and a South Korean reanalysis of supernovae are intriguing but not decisive, and they expose how sensitive conclusions are to data treatment and systematic corrections.
Resolving whether the cosmos will expand forever, experience a Big Rip, or eventually collapse in a Big Crunch will require larger, independent datasets and tighter control of astrophysical biases. Over the next decade, coordinated surveys and cross‑method consistency checks should clarify whether we are witnessing a genuine rewrite of cosmology or the subtleties of complex astronomical measurements.
Sources
- BBC News — Media report summarizing recent papers and reactions.
- Dark Energy Spectroscopic Instrument (Desi) — Official project site, KPNO/NOIRLab collaboration, observational data effort.
- Royal Astronomical Society (RAS) — Publisher/academic society where the South Korean paper appeared.
- NOIRLab / Kitt Peak — Host site and facility information for the Arizona spectrograph.
- Institute of Astronomy, Cambridge — Institutional comment and critique from senior astronomers.