Signals of the universe’s changing expansion rate have been detected by the Dark Energy Spectroscopic Instrument (DESI), located atop a telescope at the Kitt Peak National Observatory in Arizona, USA.
The universe is still expanding at an accelerating rate, but it may have recently slowed compared to a few billion years ago. This is the preliminary finding from the most precise measurement of the universe’s evolution, announced by scientists on April 4.
Although these initial findings are yet to be confirmed, if proven true, they would deepen the mystery of dark energy and highlight significant gaps in our understanding of the universe.
Each of DESI’s 5,000 fiber-optic robots can observe a galaxy in 20 minutes, enabling astronomers to create what they call the largest 3D map of the universe to date.
Arnaud de Mattia, a leading team member, explained DESI’s data: “We’ve measured the positions of galaxies in both space and time, as the farther away they are, the further back in time we look, toward a younger universe.”
In just the first year of its five-year survey, DESI mapped 6 million galaxies and quasars, using light that traveled up to 11 billion years into the universe’s past.
These findings were presented at conferences in the US and Switzerland on April 4, ahead of a series of scientific papers to be published in the Journal of Cosmology and Astroparticle Physics.
DESI’s mission is to unravel the nature of dark energy—a theoretical form of energy thought to make up about 70% of the universe. Another 25% consists of dark matter, equally mysterious, while only 5% is ordinary matter—all that we can see.
For over a century, scientists have known the universe began expanding after the Big Bang, 13.8 billion years ago. But in the late 1990s, astronomers were shocked to discover the universe’s expansion was still accelerating.
This was surprising because gravity from matter—both ordinary and dark—was expected to slow the universe’s expansion.
Clearly, something is causing the universe to expand faster, and scientists have named this mysterious force “dark energy.”
Recently, it was found that the universe’s acceleration surged significantly around 6 billion years after the Big Bang.
The Lambda CDM model of the universe suggests that in the “tug-of-war” between dark matter and dark energy, dark energy appears to dominate.
According to this model, the universe’s rapid expansion, tied to the “cosmological constant,” is closely linked to dark energy.
DESI’s director, Michael Levi, said the instrument’s initial results show “basic consistency with our standard cosmological model… but we also see some intriguing differences that could indicate dark energy evolves over time.”
In other words, the data “seem to suggest the cosmological constant Lambda isn’t truly constant,” as dark energy may exhibit “dynamic” behavior and change, Arnaud de Mattia noted.
DESI researcher Christophe Yeche suggested this could imply that after accelerating 6 billion years post-Big Bang, the universe’s expansion rate has “slowed in recent times.”
Whether dark energy truly changes over time requires further verification from DESI and other instruments, like the Euclid Space Telescope. If confirmed, our understanding of the universe may need to adapt to this strange development.
For example, the cosmological constant might be replaced by a field linked to an unknown particle. Arnaud de Mattia noted we might even need to tweak Einstein’s Theory of Relativity “to work slightly differently on the scale of large structures.”
However, de Mattia emphasized that such a shift is still far off. After all, Einstein’s Theory of Relativity, despite over a century of scrutiny, has consistently proven remarkably accurate and compelling.
