Is dark energy changing, or is it just more of the same? Last month, astronomers announced the startling finding that dark energy 鈥� which is thought to cause the accelerating expansion of the universe 鈥� might weaken over time. This has forced physicists to consider upending the standard cosmological model of the universe but now, some researchers are saying this may be premature.
Since it started scanning the sky in 2021, the Dark Energy Spectroscopic Instrument (DESI) in Arizona has been carefully measuring the distances between millions of galaxies to better understand how our universe is changing over time.
In April, the DESI collaboration announced that, when they combined their latest measurements with other datasets measuring how matter is distributed in the cosmos, like the cosmic microwave background (CMB) and supernovae, they found weak hints that the acceleration of the universe might be slowing, implying an evolving dark energy and a sharp departure from the standard model of cosmology, which is called Lambda cold dark matter (Lambda-CDM).
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But at the University of Cambridge, who helped lead the Planck mission that gathered our best view of the CMB, disagrees with this conclusion and is now trying to convince the rest of the physics community to take his side. 鈥淭o claim that there鈥檚 any evidence for evolving dark energy, you鈥檙e on really thin ground.鈥�
To assess whether dark energy is changing, cosmologists need to measure how matter is distributed throughout the universe at different times. To do this, they look at what are called baryon acoustic oscillations (BAOs), which are echoes from the universe鈥檚 first moments, when matter and energy could move more easily through space due to hotter temperatures. Like water bubbles solidified in ice, BAOs are patterns preserved in the distances between objects in space and can be used to work out how space has expanded over time.
The DESI researchers used patterns in the CMB, as seen by Planck, to map BAOs in the early universe, and they used DESI鈥檚 measurements of the distances between galaxies and stars to assess BAOs as they appear today. They also used data on supernovae to provide another measure of how space has expanded.
By combining these measurements, the researchers could assess whether the data was best explained by predictions from Lambda-CDM or from a model containing evolving dark energy. They concluded that the evolving dark energy model worked slightly better.
But Efstathiou argues that this difference only becomes apparent when the supernova data is included in the analysis 鈥� and based on his experience with this dataset, he thinks it isn鈥檛 accurate enough to be used in this way. 鈥淭he strongest evidence for evolving dark energy is coming from a catalogue where I think you can actually see that there are systematic errors,鈥� he says.
He also argues that the DESI team鈥檚 Bayesian statistical analysis, which involves assigning 鈥減rior鈥� probabilities to which model is more accurate before adjusting these to reflect the data, gives too much weight to the evolving dark energy model. Since we have other lines of evidence that Lambda-CDM is correct, our prior probability should be higher than for the evolving model, he says.
鈥淚 agree with his conclusions,鈥� says at the Institute for Advanced Study in Princeton, New Jersey. 鈥淚 don鈥檛 think the DESI results necessarily point to an evolving dark energy with any significance.鈥�
Zaldarriaga also thinks that comparing models isn鈥檛 as simple as giving them both equal weighting, as DESI has done, because a more complicated model might be able to better fit the data without necessarily being more physically plausible. 鈥淵ou don鈥檛 necessarily have to believe it,鈥� he says.
In a private meeting this month, Efstathiou presented his analysis to DESI researchers, who he says disagree with his arguments. 鈥淲e鈥檝e looked at what Efstathiou did, and he hasn鈥檛 indicated to us that we need to change our analysis in any way, so that鈥檚 good,鈥� says at the University of Waterloo in Canada,聽speaking on behalf of the DESI collaboration. 鈥淗e obviously has a strong interpretation of the data, but that seems more subjective than quantitative and based on statistical reasoning.鈥�
鈥淗e is advocating for the scientific prior that balances these two models to be weighted incredibly strongly for Lambda-CDM, to the extent of why even mention dynamical dark energy?鈥� says Percival. 鈥淏ut it鈥檚 a potential physical explanation, if there is no other [explanation] for this discrepancy that we see.鈥�
鈥淕eorge Efstathiou is absolutely right to make this case. While not decisive, I think it is compelling. This just means that, at this stage, we just need to wait for the data to get better,鈥� says at the University of Oxford. That could happen soon, with the next DESI data due potentially as early as next year.
鈥淚f I鈥檓 right, the evidence will not strengthen,鈥� says Efstathiou. 鈥淭hat鈥檚 a very definite prediction. If they were really onto something, then you鈥檇 expect something to show up.鈥�
arXiv
Prepare to have your mind blown by CERN, Europe's particle physics centre, where researchers operate the famous Large Hadron Collider, nestled near the charming Swiss lakeside city of Geneva.
CERN and Mont Blanc, dark and frozen matter: Switzerland and France
