New York Times: ‘More Than a Hint’ That Dark Energy Isn’t What Astronomers Thought
New data further challenge the best scientific theory of the history and the structure of the universe. But a separate recent result reinforces it.
Using the Dark Energy Spectroscopic Instrument, or DESI, at Kitt Peak Observatory, scientists have assembled the largest three-dimensional map of the universe to date.
Read the article in the New York Times
Astronomers often compare galaxies in an expanding universe to raisins in a baking cake. As the dough rises, the raisins are carried farther apart. The farther they are from each other, the faster they separate.
In 1998, two groups of astronomers measured the expansion of the universe by studying the brightness of a certain type of supernova, or exploding star. Such supernovas generate the same amount of light, so they appear predictably fainter at farther distances. If the expansion of the universe were slowing, as scientists believed at the time, light from faraway explosions should have appeared slightly brighter than foreseen.
To their surprise, the two groups found that the supernovas were fainter than expected. Instead of slowing down, the expansion of the universe was actually speeding up.
No energy known to physicists can drive an accelerating expansion; its strength should abate as it spreads ever more thinly across a ballooning universe. Unless that energy comes from space itself.
Bob Stupak, left, an electronics maintenance supervisor, and Matthew Evatt, a mechanical engineering manager, working inside DESI’s spectrograph room. Credit: Marilyn Sargent/Berkeley Lab
This dark energy bore all the earmarks of a fudge factor that Albert Einstein inserted into his theory of gravity back in 1917 to explain why the universe was not collapsing under its own weight. The fudge factor, known as the cosmological constant, represented a kind of cosmic repulsion that would balance gravity and stabilize the universe — or so he thought. In 1929, when it became clear that the universe was expanding, Einstein abandoned the cosmological constant, reportedly calling it his biggest blunder.
But it was too late. One feature of quantum theory devised in 1955 predicts that empty space is foaming with energy that would produce a repulsive force just like Einstein’s fudge factor. For the last quarter-century, this constant has been part of the standard model of cosmology. The model describes a universe born 13.8 billion years ago, in a colossal spark known as the Big Bang, and composed of 5 percent atomic matter, 25 percent dark matter and 70 percent dark energy. But the model fails to say what dark matter or dark energy actually are.
If dark energy really is Einstein’s constant, the standard model portends a bleak future: The universe will keep speeding up, forever, becoming darker and lonelier. Distant galaxies will eventually be too far away to see. All energy, life and thought will be sucked from the cosmos.
‘Something to go after’
DESI also captures the light from stars in our Milky Way, as shown in this video created from the instrument’s first data release. Generally, the redder the star, the younger its age. Credit: DESI collaboration and Sergey Koposov/University of Edinburgh
Astronomers on the DESI team are trying to characterize dark energy by surveying galaxies in different eras of cosmic time. Tiny irregularities in the spread of matter across the primordial universe have influenced the distances between galaxies today — distances that have expanded, in a measurable way, along with the universe.
Data used for the latest DESI measurement consisted of a catalog of nearly 15 million galaxies and other celestial objects. Alone, the data set does not suggest that anything is awry with the theoretical understanding of dark energy. But combined with other strategies for measuring the expansion of the universe — for instance, studying exploding stars and the oldest light in the universe, emitted some hundred thousand years after the Big Bang — the data no longer lines up with what the standard model predicts.
Enrique Paillas, a postdoctoral researcher at the University of Arizona who announced the DESI measurement publicly on Wednesday, noted that the data imply that the cosmic acceleration driven by dark energy began earlier in time, and is currently weaker, than what the standard model predicts.
The discrepancy between data and theory is at most 4.2 sigma (in the units of uncertainty preferred by physicists), representing one in 50,000 chances that the results are a fluke. But the mismatch is not yet at five sigma (equal to one in 3.5 million chances), the stringent standard set by physicists to claim a discovery.
Still, the disconnect is enticingly suggestive that something in the cosmological model is not well understood. Scientists might need to revise how they interpret gravity or make sense of the ancient light from the Big Bang. DESI astronomers think the problem could be the nature of dark energy.
“If we introduce a dynamical dark energy, then the pieces of the puzzle fit together better,” said Mustapha Ishak-Boushaki, a cosmologist at the University of Texas at Dallas who helped lead the latest DESI analysis.
Will Percival, a cosmologist at the University of Waterloo in Ontario and a spokesperson for the DESI collaboration, expressed excitement about what lies on the horizon. “This is actually a little bit of a shot in the arm for the field,” he said. “Now we’ve got something to go after.”
Maps showing different views of the cosmic microwave background from the Atacama Cosmology Telescope’s sixth data release, based on data collected between 2017 and 2022. Credit...Naess et al., Atacama Cosmology Telescope
In the 1950s, astronomers claimed that only two numbers were needed to explain cosmology: one related to how fast the universe was expanding and another describing its deceleration, or how much that expansion was slowing down. Things changed in the 1960s, with the discovery that the universe was bathed in light from the Big Bang, known as the cosmic microwave background. Measuring this background radiation allowed scientists to investigate the physics of the early universe and the way that galaxies subsequently formed and evolved. As a result, the standard model of cosmology now requires six parameters, including the density of both ordinary and dark matter in the universe.
As cosmology has become more precise, additional tensions have arisen between predicted and measured values of these parameters, leading to a profusion of theoretical extensions to the standard model. But the latest results from the Atacama Cosmology Telescope — the clearest maps to date of the cosmic microwave background — seem to slam the door on many of these extensions.
DESI will continue collecting data for at least another year. Other telescopes, on the ground and in space, are charting their own views of the cosmos; among them are the Zwicky Transient Facility in San Diego, the European Euclid space telescope and NASA’s recently launched SPHEREx mission. In the future, the Vera C. Rubin Observatory will begin recording a motion picture of the night sky from Chile this summer, and NASA’s Roman Space Telescope is set to launch in 2027.
Each will soak up the light from the sky, measuring pieces of the cosmos from different perspectives and contributing to a broader understanding of the universe as a whole. All serve as ongoing reminders of just what a tough egg the universe is to crack.
“Each of these data sets comes with its own strengths,” said Alexie Leauthaud, a cosmologist at the University of California, Santa Cruz, and a spokesperson for the DESI collaboration. “The universe is complicated. And we’re trying to disentangle a lot of different things.”