The universe is misbehaving.
The cosmos expanded after its birth, in the big bang nearly 14 billions years ago. And for most of the 20th century, scientists assumed gravity would gradually slow down that expansion, with all the universe’s matter acting as drag.
But observations in the late 1990s showed that’s far from the case. Careful, high-precision estimates of cosmic distances using a special class of exploding stars called type Ia supernovae revealed that, against all expectations, the universe’s expansion is actually speeding up. On cosmic scales, this is like throwing a ball up in the air and watching it rise at an ever-increasing speed. The matter of “why” this can happen remains One of the most pressing questions in physics.
“The theorists are having a field day,” says Tamara Davis, an astrophysicist at the University of Queensland in Australia. “There are hundreds upon hundreds of theories about what’s driving cosmic acceleration.”
Scientists call it dark energy but no one really knows what it is. Its behavior, however, may offer a potent clue to its identity: if dark energy’s expansion-accelerating effect holds steady over time, this would fit quite comfortably within what’s known as the standard model of cosmology, the best overarching explanation of the universe’s evolution that scientists have yet devised. The trouble is that no one has been able to say with certainty whether dark energy is actually so fixed—and if dark energy’s strength Can you imagine?Gravity is a constant that changes over time. To reconcile this with the rest physics, we may need to rethink our understanding of gravity.
A group of over 400 scientists, including Davis began collaborating on an observational programme called the Dark Energy Survey about a decade back. The researchers developed a camera that was designed to be used with the Victor M. Blanco Telescope located at the Cerro Tololo Inter-American Observatory, Chile. They also reported The latest, most authoritative search resultsThe group’s efforts were rewarded earlier this month, at the annual meeting held by the American Astronomical Society. The group has also The findings of the study were posted on this websitePreprint servers arXiv.org
Night after night for five years, the team hunted for the same kind of supernovae that first led to dark energy’s discovery. Type Ia supernovae all release about the same amount of light when they occur, which makes them handy “rulers” for reckoning cosmic distances. Astronomers can measure the brightness of a type Ia to determine its distance. Combining that distance with another measurement of the supernova’s speed away from Earth gives us the speed at the cosmos expanded when the stellar cataclysm occurred. This second measurement involves examining a supernova’s spectrum—splitting its light into constituent colors and seeing how reddened they are from the light’s wavelengths being stretched by cosmic expansion. The higher an object’s “redshift” is, the faster that object is receding from us.
Dark energy was discovered in 1998 by a group of about 50 supernovae. Most had low redshifts, and were from a relatively nearby universe. To understand how dark energy has changed over time, it is necessary to measure the distances of supernovae and their speeds from a larger area of spacetime. With DES, Davis says, “we wanted to see whether dark energy has been the same throughout the universe’s history, and the only way to do that is by seeing what it was in the past.” Ultimately, the project found and studied more than 1,500 of the telltale stellar explosions, many of them with high redshifts from the far depths of the cosmos.
New Analysis, same perplexing results
To spot fresh type Ia supernovae, “we had to play the game of ‘find the differences in the photos,’” says Maria Vincenzi, a cosmologist at Duke University and a DES collaborator, who co-led the group’s supernova cosmology effort. “You look at images from one night to another to see if you see something pop.”
DES researchers had to weed through about 19,000 “pops” that they identified in search of the right type. To sort through such a huge dataset, the team used new machine-learning methods that made the process 100 times faster.
But even that was not enough: although clever automation saves time in the initial selection of candidates, determining each one’s redshift typically requires getting more data. This can be an urgent, fraught task because gathering a supernova’s diagnostic spectrum must occur before its light fades away and requires securing oodles of fiercely competed-for observing time on a large telescope. “It only takes about five minutes to take a picture of a supernova versus several hours to get its spectrum,” Davis says.
The DES team’s solution was to measure redshifts for each computer-flagged candidate’s presumed host galaxy, rather than the candidate itself, using dedicated time on the Anglo-Australian Telescope in Coonabarabran, Australia. “So that’s the thing that allows you to sit back, relax and take the spectrum at your leisure whenever you like because you’re not in a rush to catch the supernova’s light,” Davis says.
And from this––the biggest, best survey of distant supernovae ever––what did they find?
“The DES paper explores whether there is a compelling need for additional complexity in the [standard] cosmological model and finds that the answer is no,” says Charles Bennett, a cosmologist at Johns Hopkins University, who was not involved in DES. “This does not mean that nature is not more complex but rather that we don’t have sufficiently compelling evidence for more complicated models.”
Yet, as Vincenzi points out, “the results are literally borderline between supporting the standard model and suggesting instead that the universe’s acceleration hasn’t been constant over time.”
The team essentially used more and improved data to arrive at the same vexing conclusions that motivated its search in the first instance. Despite being the very best supernova-based estimate of dark energy yet performed, DES’s result is almost uncannily placed in the shrunken liminal space where certainty remains elusive. Even now, no-one can say if gravity laws need to be revised.
The Path to Illumination
In the future, the DES team’s machine-learning methods could be applied to larger datasets. This will put dark energy under even greater scrutiny. Vera C. Rubin Observatory, in Chile, will likely find fresh type Ia-supernovae rather than hundreds or thousands. You can save up to tens of millions by using this websiteIt will be necessary to find more efficient methods of analyzing all these data.
“It wouldn’t be possible to follow up every single supernova live; there just are not enough telescope resources in the world,” Davis says.
Astronomers will also use observatories such as NASA’s upcoming Nancy Grace Roman Space Telescope to probe the universe’s expansion history even further back in time by studying supernovae and taking a host of Other approaches.
“We have to measure the history of the universe’s expansion and structure growth in great detail to understand the nature of dark energy and potential modifications to Einstein’s theory of gravity,” says Yun Wang, a senior research scientist and dark energy expert at the Infrared Processing and Analysis Center at the California Institute of Technology, who was not involved in DES. Early results from various far-seeing studies have already revealed a possible hint that dark energies are more complex than the simple constant cosmologists assumed. It’s “a problem called the Hubble tension,” Wang says. “Different ways of measuring the present-day expansion of the universe give very different answers. The modification of our laws of gravity is a possible solution to this tension, which may also be the origin of dark energy.” Sleuthing out the cause behind the strangely Inconsistent measurements on the current rate of cosmic expansionThe DES analysis could provide clues to the dark-energy puzzle.
“I find it very strange that the standard model works so well and explains a great diversity of precise data with only a few parameters but also has one and only one area of failure,” Bennett says. He notes that many attempts to adjust the model in order to resolve the Hubble tension result in the violation of other physical laws which are well supported by observation. “It’s hard to change the model of the universe without affecting multiple measurable properties.”
For now, “the mystery of dark energy persists,” Wang says, “and the ultimate fate of the universe hangs in the balance.” If cosmic acceleration has a constant strength, the universe will expand forever; its fate is sealed. But if dark energies can change with time, then it opens up an entire universe of new possibilities. “To me, it’s the most exciting problem in physics and astronomy today.”