New results from the DESI (Dark Energy Spectroscopic Instrument) Collaboration have revealed signs of a time-varying dark energy.
Two ‘fans’ corresponding to the two main areas DESI has observed, above and below the plane of our Milky Way Galaxy. Image credit: DESI Collaboration / DOE / KPNO / NOIRLab / NSF / AURA / R. Proctor.
“The Universe never ceases to amaze and surprise us,” said Dr. Arjun Dey, NOIRLab’s DESI Project Scientist and Mid-Scale Observatories’ associate director for strategic initiatives.
“By revealing the evolving textures of the fabric of our Universe as never before, DESI and the Mayall telescope are changing our very understanding of the future of our Universe and nature itself.”
Taken alone, the DESI data are consistent with our standard model of the Universe: Lambda CDM, where CDM is cold dark matter and lambda represents the simplest case of dark energy, where it acts as a cosmological constant.
However, when paired with other measurements, there are mounting indications that the impact of dark energy may be weakening over time and that other models may be a better fit.
Those other measurements include the light leftover from the dawn of the Universe (Cosmic Microwave Background, or CMB), distance measurements of supernovae, and observations of how light from distant galaxies is warped by the gravitational influence of dark matter (weak lensing).
So far, the preference for an evolving dark energy has not risen to 5 sigma — the gold standard in physics that represents the commonly accepted threshold for a discovery.
However, different combinations of DESI data with the CMB, weak lensing, and supernovae sets range from 2.8 to 4.2 sigma.
The analysis used a technique to hide the results from the scientists until the end, mitigating any unconscious bias about the data.
This approach sets a new standard in how data from large spectroscopic surveys are analyzed.
DESI is a state-of-the-art instrument mounted on NSF’s Nicholas U. Mayall 4-m telescope at Kitt Peak National Observatory, a Program of NSF NOIRLab.
It can capture light from 5000 galaxies simultaneously, enabling it to conduct one of the most extensive surveys of the cosmos ever.
The experiment is now in its fourth of five years surveying the sky, with plans to measure roughly 50 million galaxies and quasars (extremely distant yet bright objects with black holes at their cores) and more than 10 million stars by the time the project ends.
The new analysis uses data from the first three years of observations and includes nearly 15 million of the best measured galaxies and quasars.
It’s a major leap forward, improving the experiment’s precision with a dataset that is more than double what was used in DESI’s first analysis, which also hinted at an evolving dark energy.
DESI tracks dark energy’s influence by studying how matter is spread across the Universe.
Events in the very early Universe left subtle patterns in how matter is distributed, a feature called Baryon Acoustic Oscillations (BAO).
That BAO pattern acts as a standard ruler, with its size at different times directly affected by how the Universe was expanding.
Measuring the ruler at different distances shows researchers the strength of dark energy throughout history.
The DESI Collaboration will soon begin work on additional analyses to extract even more information from the current dataset, and DESI will continue collecting data.
Other experiments coming online over the next several years will also provide complementary datasets for future analyses.
“Our results are fertile ground for our theory colleagues as they look at new and existing models, and we’re excited to see what they come up with,” said Dr. Michael Levi, DESI director and a scientist at Berkeley Lab.
“Whatever the nature of dark energy is, it will shape the future of our Universe. It’s pretty remarkable that we can look up at the sky with our telescopes and try to answer one of the biggest questions that humanity has ever asked.”
“These are remarkable results from an incredibly successful project,” said Dr. Chris Davis, NSF program director for NSF NOIRLab.
“The potent combination of the NSF Mayall Telescope and DOE’s Dark Energy Spectroscopic Instrument shows the benefits of federal agencies working together on fundamental science that improves our understanding of the Universe.”
The physicists shared their findings today in a series of papers that will be posted on arXiv.org.
