The expansion of the Universe is slowing down, contrary to the established theory of acceleration under the influence of dark energy. New research shows that key cosmic "beacons"—Type Ia supernovae—are not as standardized as previously thought: their brightness depends on the age of the stars from which they originated.

Rethinking the Expansion of the Universe

After the Big Bang, which occurred 13.8 billion years ago, the expansion of the Universe slowed due to gravity. However, in 1998, astronomers made a surprising discovery: observations of distant Type Ia supernovae revealed they were dimmer than expected. This meant they were farther away than models predicted, suggesting that about nine billion years after the Universe's birth, its expansion began to accelerate.

This discovery, which earned the Nobel Prize in Physics in 2011, led to the concept of dark energy—a force making up about 70% of the Universe's energy density and acting like anti-gravity, pushing galaxies apart. In the standard cosmological model (ΛCDM), dark energy is considered constant over time.

New Data and Challenges to the Standard Model

In recent years, data have emerged that question the constancy of dark energy. For example, measurements of baryon acoustic oscillations—a kind of "echo" of the Big Bang—hinted at the possibility that dark energy's properties might change. A recent study provided evidence supporting this hypothesis and identified a source of error in previous calculations.

A group of astronomers found that Type Ia supernovae, long considered "standard candles" for measuring cosmic distances, actually depend strongly on the age of their progenitor stars. An analysis of data from 300 galaxies showed with high confidence (99.999%) that even after standard brightness calibration, supernovae from younger stellar populations appear dimmer, while those from older populations are brighter.

Since distant galaxies are generally younger than nearby ones, this creates a systematic bias: distant supernovae seem dimmer and, therefore, farther away than they actually are, creating the illusion of accelerated expansion.

Correction and New Conclusions

Scientists have developed a method to correct for this age-related bias. After applying this correction, the observations no longer matched the standard ΛCDM model, which predicts acceleration. Instead, the corrected data aligned with models based on independent measurements of baryon acoustic oscillations (BAO) and the cosmic microwave background (CMB).

Combining all three data sets—corrected supernovae, BAO, and CMB—allowed researchers to statistically rule out the standard ΛCDM model with high significance. The combined analysis showed that dark energy is weakening over time, and the Universe is no longer accelerating but has entered a phase of slowed expansion. This conclusion may also help resolve the "Hubble tension"—the discrepancy in measurements of the Universe's expansion rate obtained by different methods.

A New Chapter in Cosmology

Thus, the accelerated expansion of the Universe may not be a fundamental property of the cosmos, but rather the result of a systematic measurement error that was previously overlooked. Dark energy is likely not constant, and its strength appears to decrease over time.

If these findings are confirmed, they will lead to a paradigm shift in cosmology and open a new chapter in our understanding of the past and future of the Universe. Further observations with new instruments, such as the Vera Rubin Observatory, will provide even more data to test this hypothesis.