Dark energy's sign reversal raises questions about persistent Hubble tension

On July 10, 2026, researchers including Sehjal Khandelwal and Abraão J. S. Capistrano revealed new insights about dark energy and the ongoing Hubble tension. Their study, published in Physical Review D, explores the implications of dark energy potentially flipping its sign over cosmic history.

Understanding Dark Energy and Cosmic Expansion

For nearly a century, astronomers have recognized that the universe is expanding. In the late 1990s, the Supernova Cosmology Project and the High-Z Supernova Search Team discovered that this expansion is accelerating, a finding that earned the Nobel Prize in Physics in 2011. The leading explanation for this acceleration is dark energy, often modeled as a constant known as Lambda, which pushes space apart.

This theory, combined with cold dark matter, forms the LCDM model, a framework that has successfully explained numerous cosmic phenomena. However, a significant issue persists: the Hubble tension, which highlights discrepancies in measuring the universe's expansion rate.

The Hubble Tension Explained

The Hubble constant (H₀) quantifies the current expansion rate of the universe. There are two main methods for measuring this constant:

• The cosmic microwave background (CMB), which predicts today's expansion rate based on early universe physics.
• Direct measurements using nearby supernovae, calibrated against pulsating stars known as Cepheids.

However, these methods yield conflicting results, disagreeing by five to seven standard deviations. This significant gap suggests that our understanding of the universe might be incomplete, prompting various proposed solutions, including new particles and modified gravity theories.

Introducing LsCDM: A New Perspective

One intriguing proposal is the Luminosity Sign Change in Dark Energy Model (LsCDM). This model retains most aspects of LCDM but introduces a critical change: dark energy may have once had a negative value, pulling matter together before flipping to a positive value as the universe aged. This transition is believed to have occurred when the universe was less than one-third of its current age.

Previous studies suggested that this sign change could alleviate both the Hubble tension and a related issue known as the S₈ tension without compromising the model's ability to explain early universe observations. However, the new study focuses on how cosmologists measure the tension between datasets.

Using advanced statistical methods, the researchers analyzed the latest CMB data from the Planck satellite, the Atacama Cosmology Telescope, and the South Pole Telescope, along with galaxy clustering data from the Dark Energy Spectroscopic Instrument (DESI) and the Pantheon Plus supernova catalog.

Key Findings and Implications

The research yielded mixed results. While the CMB and galaxy clustering data showed remarkable agreement when employing non-Gaussian statistical methods, the inclusion of nearby supernova measurements revealed persistent tensions in both the LCDM and LsCDM models.

The sign-flipping model did navigate predictions closer to the locally measured expansion rate, yet the observed H₀ remained in an unexpected region of the model's predictions. This highlights the importance of rigorous statistical analysis in cosmology, as oversimplified methods may obscure genuine insights.

Ultimately, this study emphasizes that while progress is being made, the foundational questions regarding dark energy and the Hubble tension remain unresolved.