But the unknown leaped out at them.
When the scientists combined their map with other cosmological data, they were surprised to find that it did not quite agree with the otherwise reliable standard model of the universe, which assumes that dark energy is constant and unchanging. A varying dark energy fit the data points better.
As artist’s impression of an asteroid orbiting close to the sun. The asteroid was found by the A Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile.Credit: DOE/The New York Times
“It’s certainly more than a curiosity,” Palanque-Delabrouille said. “I would call it a hint. Yeah, it’s not yet evidence, but it’s interesting.”
But cosmologists are taking this hint very seriously.
Wendy Freedman, an astrophysicist at the University of Chicago who has led efforts to measure the expansion of the universe, praised the new survey as “superb data”. The results, she said, “open the potential for a new window into understanding dark energy, the dominant component of the universe, which remains the biggest mystery in cosmology. Pretty exciting.”
Michael Turner, an emeritus professor at the University of Chicago who coined the term “dark energy,” said in an email: “While combining data sets is tricky, and these are early results from DESI, the possible evidence that dark energy is not constant is the best news I have heard since cosmic acceleration was firmly established 20-plus years ago.”
The Victor M. Blanco four-metre telescope dome in Chile, where a Dark Energy Camera detects asteroids that orbit between Earth and the sun.Credit: Cerro Tololo Observatory/ The New York Times
Dark energy entered the conversation in 1998, when two competing groups of astronomers, including Riess, discovered that the expansion of the universe was speeding up rather than slowing, as most astronomers had expected. The initial observations seemed to suggest that this dark energy was acting just like a famous fudge factor – denoted by the Greek letter Lambda – that Albert Einstein had inserted into his equations to explain why the universe didn’t collapse from its own gravity. He later called it his worst blunder.
But perhaps he spoke too soon. As formulated by Einstein, Lambda was a property of space itself: The more space there was as the universe expanded, the more dark energy there was, pushing ever harder and eventually leading to a runaway, lightless future.
Dark energy took its place in the standard model of the universe known as LCDM, composed of 70 per cent dark energy (Lambda), 25 per cent cold dark matter (an assortment of slow-moving exotic particles) and 5 per cent atomic matter. So far that model has been bruised but not broken by the new James Webb Space Telescope. But what if dark energy were not constant as the cosmological model assumed?
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At issue is a parameter called w, which is a measure of the density, or vehemence, of the dark energy. In Einstein’s version of dark energy, this number remains constant, with a value of -1, throughout the life of the universe. Cosmologists have been using this value in their models for the past 25 years.
But this version of dark energy is merely the simplest one. “With DESI we now have achieved a precision that allows us to go beyond that simple model,” Palanque-Delabrouille said, “to see if the density of dark energy is constant over time, or if it has some fluctuations and evolution with time”.
The DESI project, 14 years in the making, was designed to test the constancy of dark energy by measuring how fast the universe was expanding at various times in the past. To do that, scientists outfitted a telescope at Kitt Peak National Observatory with 5000 fibre-optic detectors that could conduct spectroscopy on that many galaxies simultaneously and find out how fast they were moving away from Earth.
As a measure of distance, the researchers used bumps in the cosmic distribution of galaxies, known as baryon acoustic oscillations. These bumps were imprinted on the cosmos by sound waves in the hot plasma that filled the universe when it was just 380,000 years old. Back then, the bumps were 500,000-light-years across. Now, 13.5 billion years later, the universe has expanded a thousandfold, and the bumps – which are now 500-million-light-years across – serve as convenient cosmic measuring sticks.
The DESI scientists divided the past 11 billion years of cosmic history into seven spans of time. (The universe is 13.8 billion years old.) For each, they measured the size of these bumps and how fast the galaxies in them were speeding away from us and from each other.
When the researchers put it all together, they found that the usual assumption – a constant dark energy – didn’t work to describe the expansion of the universe. Galaxies in the three most recent epochs appeared closer than they should have been, suggesting that dark energy could be evolving with time.
“And we do see, indeed, a hint that the properties of dark energy would not correspond to a simple cosmological constant” but instead may “have some deviations,” Palanque-Delabrouille said. “And this is the first time we have that.” But, she emphasised again, “I wouldn’t call it evidence yet. It’s too, too weak.”
Time and more data will tell the fate of dark energy, and of cosmologists’ battle-tested model of the universe.
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“LCDM is being put through its paces by precision tests coming at it from every direction,” Turner said. “And it is doing well. But, when everything is taken together, it is beginning to appear that something isn’t right or something is missing. Things don’t fit together perfectly. And DESI is the latest indication.”
Riess of Johns Hopkins, who had an early look at the DESI results, noted that the “hint,” if validated, could pull the rug out from other cosmological measurements, such as the age or size of the universe. “This result is very interesting and we should take it seriously,” he wrote in his email. “Otherwise why else do we do these experiments?”
This article originally appeared in The New York Times.