New study shows universe has less dark energy than previously thought

The make-up and the growth of the universe have never been clearer — or more confusing — as revealed in a massive new study of the markers astronomers use to map the cosmos.

A new analysis called Pantheon+ has narrowed the uncertainty in the extent and composition of the universe. To do this, Pantheon+ builds on two long-standing astronomical projects – one called Pantheon, which combines observations of 1,550 supernovae dating back 10 billion years; and another called SH0ES, measuring relatively nearby pulsating stars known as Cepheids within 10 million light-years.

The analysis of the structure and expansion of the universe recently published in Pantheon+ The Astrophysical Journal notes that 66.2 percent of the universe is made up of dark energy, the mysterious accelerator driving the universe’s accelerated expansion, slightly less than previous estimates of about 68 percent.

Only 33.8 percent of the universe is made up of matter — and the vast majority of that is unobservable dark matter, the existence of which astronomers can only infer from galactic-scale gravitational effects. At the accepted rate of 85 percent dark matter to 15 percent normal (baryonic) matter, that means just under 5 percent of the mass of the universe is what we can see around us.

Pantheon+ was also able to measure the expansion of the universe with an uncertainty of 1.3 percent, close enough that it is now undeniable that the early universe and the current universe are not expanding at the same rate.

Speak with Vice versa, lead author Dillon Brout, a NASA Einstein Fellow at the Center for Astrophysics | Harvard-Smithsonian says that this level of precision means that instead of being limited by the data used to measure the universe’s growth, we are “approaching the limit where we are limited by the uncertainties of our method.”

WHAT’S NEW – Pantheon+ provides as precise a measurement of dark energy, dark matter, and baryonic matter as is currently possible.

And “assembled” is the right word – this work combines analyzes from the original pantheon, which measured dark matter, and the supernova H0 for the equation of state (SH0ES), which measures the Hubble constant at which the universe is expanding.

Pantheon+ consolidates two decades of data from different telescopes and astronomers into a single analysis; it represents “an all-star sample,” Brout explains. And this is the largest group of exploding stars that has been assembled — over 1,500, half the number of an earlier version that focused only on the supernovae.

But Brout notes that’s all that can be achieved with current gear. The limiting factor is time. “We get about one supernova a year, which helps us measure the Hubble constant, and we have 42 of them now. So we’ll have to wait a while before we can double our data set,” he says.

WHY IT MATTERS – Surveys like Pantheon+ allow astronomers to verify their results using different methods and different goals. Some components measure Cepheids, relatively nearby stars that regularly increase and decrease in brightness; others measure supernovae that outshined galaxies up to 10 billion years ago.

For now, this is about as accurate as these types of measurements can get. “A lot of people are going to think, ‘Of course you have to use James Webb,'” Brout says, “and the answer to that is ‘yes’ — but it’s not immediately clear how much it’s going to help us.” The James Webb Space Telescope will Let astronomers take a look at how stardust and observations at different wavelengths affect the observations from the anchors that hold their measurements in place.

The increasing accuracy of this analysis has also magnified one of the biggest problems in cosmology. Pantheon+ has narrowed the speed at which the universe is expanding to 73.4 kilometers per second per megaparsec – plus or minus 1.3 percent. That means space is expanding locally at about 164,000 miles per hour.

But that’s just here and, more importantly, now. Cosmic microwave background measurements show that in its earliest days, the universe was definitely expanding at a slower rate, about 67 kilometers per second per megaparsec. As surveys like Pantheon+ become more accurate, it is becoming clear that this discrepancy – the Hubble voltage – cannot be explained simply by the difficulty of getting clear observations.

The expansion of the universe has undoubtedly accelerated, but it’s not entirely clear why.

WHAT’S NEXT – As an overview of the field, Brout notes that Pantheon+ is a way to capture the state of the art on the verge of tremendous transformation. In the next two years, the Vera Rubin Observatory in Chile will come online, and “the game will change somehow in the future.”

While the work measuring dark matter, dark energy, and the expansion of the cosmos has been built on many different observations with many different tools, “in the future, we have these big, billion-dollar telescopes that are collecting really enormous samples of their own.”

How enormous? The Rubin Observatory expects to find over a million of the right kind of ancient supernovae over the next 12 years – a thousand times more than what Pantheon+ collects.

The size of the teams working on that data will also change: “These are going to be huge collaborations with hundreds of people, and they’re going to nail a lot of these things.” But for now, “before those really big giant telescopes turn on,” Hopes says Brout that Pantheon+ can be the culmination of an era.