The accelerating expansion of our universe, attributed to dark energy, has been a cornerstone of modern cosmology since its discovery in 1999. Recent findings from the Dark Energy Spectroscopic Instrument (DESI) may significantly transform our understanding of cosmic acceleration [1, 2]. In this article, we explore the implications of these new measurements and their potential to reshape fundamental physics.

1.1 The journey from a static to an accelerating universe

Our understanding of the universe has undergone a profound evolution over the past century. Between 1920 and 1930, Alexander Friedmann, Georges Lemaître, and Edwin Hubble revolutionized cosmology by establishing that the universe is expanding rather than static [3,4,5], an idea consistent with Einstein’s general relativity framework.

For decades afterward, scientists generally assumed that this expansion must be decelerating due to gravity’s attractive nature on cosmic scales. The first major paradigm shift occurred in 1999 when two independent research teams—the High-Z Supernova Search Team and the Supernova Cosmology Project—discovered that the universe’s expansion rate was actually accelerating rather than slowing down [6, 7]. This unexpected finding, which earned Saul Perlmutter, Brian Schmidt, and Adam Riess the 2011 Nobel Prize in Physics, necessitated the introduction of “dark energy” to explain this acceleration.

1.2 Equation of state: the key to dark energy’s nature

A fundamental question emerged: Is dark energy constant throughout cosmic history, or does it evolve with time? This property is typically characterized by the equation of state parameter w, which relates dark energy’s pressure to its density. For a cosmological constant—the simplest dark energy model—w equals exactly − 1, indicating unchanging energy density throughout cosmic time.

For 25 years, the scientific community has developed increasingly sophisticated methods to measure potential deviations from w = − 1. Recently, DESI’s second analysis of its initial data (approximately one-third of its projected total) revealed a tantalizing hint that may differ from − 1, with significance between 2.8 and 4.2 sigma. While not yet meeting the conventional 5-sigma threshold for definitive discovery, this finding—if confirmed with more data—could be as transformative as the original discovery of cosmic acceleration.

1.3 DESI’s revolutionary findings

The evolution of the universe’s scale factor (denoted by a) describes how space expands over time. DESI’s measurements suggest a potentially revolutionary scenario: the universe appears to be transitioning from an accelerating phase toward a decelerating one in our cosmic future. This differs markedly from the standard cosmological constant model, which predicts continued acceleration.

Even more intriguing is DESI’s indication that the equation of state parameter w may cross the − 1 boundary. When w > − 1, dark energy density decreases over time, potentially explained by scalar field models. Conversely, when w < − 1, dark energy density increases, leading to theoretical challenges known as “phantom” scenarios. A transition across w = − 1 is particularly significant because it cannot be explained by single-field theories of dark energy. If confirmed, this would necessitate multi-field theoretical frameworks, representing a major shift in our understanding of fundamental physics [8].

1.4 How DESI measures cosmic expansion

DESI is a groundbreaking spectroscopic instrument capable of simultaneously measuring optical spectra for 5000 galaxies. By analyzing these spectral “fingerprints,” scientists can determine precise distances to millions of galaxies using their redshifts—the stretching of light wavelengths due to cosmic expansion.

This remarkable technical achievement enables the creation of a three-dimensional map of the universe spanning vast cosmic distances and lookback times. By examining large-scale structures within this map, cosmologists can measure the universe’s expansion history with unprecedented precision. The current results come from the 3 years of DESI’s operations, with additional data already collected and more observations planned.

1.5 Future prospects

DESI has now completed 4 years of observations, with the collaboration actively analyzing this expanded dataset to refine their measurements. The full 5-year survey, expected to be completed and analyzed by 2027, will provide significantly improved constraints on dark energy’s properties.

Complementary experiments, including the space telescope Euclid and the Vera C. Rubin Observatory, will further enhance our understanding by providing independent measurements using different techniques. Together, these efforts may lead us toward a more complete theory of gravity and cosmic evolution.

1.6 Implications for fundamental physics

If DESI’s initial findings are confirmed at higher significance, the implications for theoretical physics would be profound. A deviation from w = − 1 would rule out the cosmological constant as the explanation for dark energy. Furthermore, a crossing of the phantom divide (w = − 1) would necessitate more complex multi-field theories, potentially involving quantum gravity or modifications to Einstein’s general relativity.

These developments represent an exciting frontier in physics, where astronomical observations provide crucial insights into the fundamental nature of space, time, and energy. As DESI continues to collect and analyze data, we may be on the cusp of another revolutionary transformation in our understanding of the cosmos—one that began with the shift from a static to an expanding universe, evolved through the discovery of acceleration, and now points toward an even more dynamic and complex cosmic evolution.