It has been over two decades since the discovery of dark energy.
Scientists have therefore had more than 20 years to decode the secrets of this invisible substance that appears to be pulling the universe apart. Yet, they still know close to nothing about it. Dark energy, in fact, may not even be a substance. It could be a force or even an intrinsic property of space itself.
For instance, the standard model of cosmology — our leading theory of cosmic evolution — does suggest dark energy is unwavering across the universe and throughout time, making it a fundamental property of space. If constant, the mysterious dark energy that makes up a whopping 70 percent of the universe would push away all stars and galaxies. However, the biggest survey of the universe’s cosmic history could indicate that dark energy, also known as a hypothetical "anti-gravity" force, may evolve with time rather than remain constant, hinting at a less lonely future for residents of the universe.
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If this early result holds with future observations, cosmologists may have to, at the very least, explore systematic uncertainties in the prevailing Lambda CDM (LCDM) model, a mathematical model of the universe in which lambda represents dark energy. They may also need to start sifting through dozens of other models of our universe to find the true best fit. Still, the evidence is tentative — it does not reach what's known as the "5-sigma threshold," which determines whether a signal can be celebrated as an official discovery. So, continuously emerging interpretations about dark energy's evolution could change with more data scheduled to come within the next few years.
"If this is true, this just turns cosmology upside down," said Dillon Brout of Boston University, who measures the acceleration of the universe with supernovas. Such a discovery would be a "paradigm shift in our thinking of what our best understanding of our universe is."
Streetlights of the universe
Perched atop the Nicholas U. Mayall 4-meter telescope at Arizona's Kitt Peak National Observatory, the Dark Energy Spectroscopic Instrument, or DESI, pinpoints positions of a million galaxies each month. Through these observations, cosmologists can measure the universe's expansion rate as it increased over the past 11 billion years. These faraway galaxies, which can be likened to the "streetlights of the universe," are thus helping cosmologists study the universe-permeating enigma of dark energy.
And, on Thursday (April 4), the DESI collaboration shared the largest-ever 3D map of the universe. It includes high-precision measurements of the universe's expansion rate over the past 11 billion years as well. In its first year of operations alone, DESI has proven to be twice as powerful at measuring the expansion history of the early universe as its predecessor, the Sloan Digital Sky Survey, which took more than a decade to build a similar 3D map.
This "is the next generation of data we've been waiting a long time for, so it's really nice to see it having arrived," said Brout, who is not involved with the DESI collaboration.
In addition to countless galaxies clustered together like knotted threads, DESI's new 3D map spotlights a faint pattern in the early universe known as Baryon Acoustic Oscillations, or BAO. These subtle, 3D wrinkles had flown through matter that existed during the first 380,000 years of our universe's history, freezing with time and turning into relics of an infant cosmos. By mapping the sizes of those frozen BAOs, researchers managed to estimate the distances to galaxies and infer how fast the universe was expanding at various points in time.
Because light from typical galaxies is too faint to see, as those galaxies sit very far away from us and the light they emit is relatively low-intensity, the DESI collaboration also studied over 400,000 intensely bright objects called quasars. As light from these objects glides through interstellar space, it gets absorbed by clouds of gas and dust, helping cosmologists map pockets of dense matter in a similar way to mapping galaxies.
"It lets us look out further to when the universe was very young," Andreu Font-Ribera, a scientist at the Institute for High Energy Physics in Spain and a member of the DESI collaboration, said in a statement. "It's a really hard measurement to do, and very cool to see it succeed."
'If this is real, we're in uncharted territory'
The preliminary conclusion that dark energy could be evolving with time comes from an early analysis of DESI data combined with data from other cosmological data. The researchers found a varying dark energy model agreed better with the data compared to the standard model. To be clear, no single dataset by itself convincingly reveals the time-evolving nature of dark energy, but the signal becomes slightly stronger when all datasets are combined.
"It is not a strong enough preference that I would say Lambda CDM is wrong," Kyle Dawson, the co-spokesperson for DESI at the University of Utah, told Space.com. "We've actually never found deviations from that model before with any real meaning."
From the early analysis, however, it appears dark energy is transitioning from being a strong driver of acceleration of our universe to tapering off to some degree, said Dawson.
"If this is real, we're in uncharted territory," said Brout. The DESI collaboration used the second simplest model of our universe after Lambda CDM, which is unremarkable except for its ability to help cosmologists check for deviations from the standard model. If future observations in the pipeline indeed find dark energy is evolving with time, dozens of other models too would become viable, and cosmologists would have to start testing all of them individually, said Brout.
"If it's not Lambda CDM, who knows?"
