An international research team has raised the possibility that dark energy, known as the force accelerating the expansion of the universe, may have weakened. This finding contradicts existing physical theories, leading some scientists to evaluate that this discovery could represent a significant turning point in modern astronomy.
Approximately 900 researchers from over 70 institutions worldwide participated in the Dark Energy Spectroscopic Instrument (DESI) international research team, which announced their research results indicating that dark energy has weakened over the past 4.5 billion years on the 19th (local time). DESI is a facility installed at the Kitt Peak National Observatory in Arizona, where it uses 5,000 optical fiber devices to rapidly scan numerous galaxies in the universe to study changes in dark energy.
Dark energy was first discovered in 1998. Until then, scientists had predicted that the universe, which originated from the big bang 13.8 billion years ago, would expand but gradually slow down due to gravity. However, researchers from the United States and Australia observed that the rate of expansion of the universe is actually accelerating. They coined the term dark energy, reporting an unknown force that pushes the universe outward.
Modern physics explains that all the self-luminous stars combined account for only 5% of the entire universe. Scientists believe that there exists matter that has gravity but does not emit light, known as dark matter, which accounts for 27% of the universe. The remaining 68% is estimated to be dark energy, which has a repulsive force instead of gravity.
Scientists have believed that dark energy maintains a constant force. However, last year the DESI researchers captured data suggesting that the strength of dark energy may vary over time for the first time. Current findings indicate that the energy density of dark energy is approximately 10% lower than it was 4.5 billion years ago. While many scientists initially thought it was likely a simple data error, signals of that change have grown clearer even after a year.
Seshadri Natarajan, a professor at the University of Portsmouth in the United Kingdom, noted, "The evidence has become stronger now," and added, "We conducted additional tests to rule out the possibility of data errors, and our confidence in the results is increasing." Catherine Heymans, a professor at the University of Edinburgh in the United Kingdom, emphasized, "Until last year, many scientists believed that further verification was necessary, but now, more data has accumulated and the academic community is reviewing it closely."
Scientists are searching for the cause of the changes in dark energy, but no one has yet provided a clear answer. Ofer Lahav, a professor at University College London, said, "No one knows (the cause) yet," adding, "If this research result is correct, we need to find a new theory that can explain the changes in dark energy. That's why this research is interesting."
Jung Dong-hwi, a professor at Pennsylvania State University in the United States, told National Public Radio (NPR) on the same day, "It's still early to conclude that this research result is decisive, but our understanding of dark energy is gradually advancing." Although Professor Jung did not participate in this study, he is researching the dark energy of the early universe with the Hubble-Avery Telescope experiment, which is earlier than DESI. He stated, "If dark energy actually changes over time, it will open up a path for rethinking the origins of dark energy completely," and added, "We need to consider how to analyze the characteristics of dark energy more precisely through current and future data."
The DESI research team plans to verify whether this research result is conclusive by analyzing 50 million galaxies and celestial bodies over the next two years. They will also study dark energy by observing farther parts of the universe with the Euclid telescope launched by the European Space Agency in 2023.
References
DESI(2025), https://data.desi.lbl.gov/doc/papers/