The makeup and the growth of the Universe have never been clearer — or as confusing — as they’ve been revealed to be in a massive new survey of the markers astronomers use to measure the cosmos.
A new analysis called Pantheon+ has narrowed down the uncertainty in the expansion and makeup of the Universe. To do this, Pantheon+ builds on two long-standing astronomical projects — one called Pantheon, combining observations of 1,550 supernovae reaching back 10 billion years; and another called SH0ES, which measures relatively close pulsing stars known as Cepheids within 10 million light years.
The Pantheon+ analysis of the makeup and expansion of the Universe published recently in The Astrophysical Journal finds that 66.2 percent of the Universe is made up of dark energy, the mysterious accelerator driving the Universe’s speeding expansion, slightly less than past estimates of about 68 percent.
Only 33.8 percent of the Universe is matter — and the vast majority of that is impossible-to-observe dark matter, whose existence 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 slightly less than 5 percent of the mass of the Universe is the stuff we can see around us.
Pantheon+ was also able to measure the Universe’s expansion to within 1.3 percent uncertainty, close enough that it is now undeniable that the early Universe and the current Universe don’t expand at the same pace.
Speaking with Inverse, lead author Dillon Brout, a NASA Einstein fellow at the Center for Astrophysics | Harvard-Smithsonian, says that this degree of precision means that instead of being limited by the data for measuring the growth of the Universe, “we’re approaching the limit where we’re limited by the uncertainties of our method.”
WHAT’S NEW — Pantheon+ offers as precise a measurement of dark energy, dark matter, and baryonic matter as can currently be assembled.
And “assembled” is the right word — this work combines analysis 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 expands.
Pantheon+ synthesizes 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 biggest set of exploding stars that have been put together — over 1500, half again as many as an earlier version that focused just on the supernovae.
But Brout notes that’s about all that can be gained with current equipment. The limiting factor is time. “We get about one supernova per year that helps us measure the Hubble constant, and we’ve got 42 of them now. So we’re going to have to wait a while just to double our data set,” he says.
WHY IT MATTERS — Surveys like Pantheon+ allow astronomers to cross-check their results across different methods and different targets. Some components measure Cepheids, relatively nearby stars whose brightness waxes and wanes regularly; others measure supernovae who outshone galaxies up to 10 billion years ago.
For the time being, this is about as accurate as these kinds of measurements can get. “A lot of people will think ‘of course, you have to use James Webb,’” says Brout, “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 look at the ways stellar dust and observations in different wavelengths impact observations of the anchors that hold their measurements in place.
The increasing accuracy of this analysis has also increased one of the biggest problems in cosmology. Pantheon+ has narrowed down the speed at which the Universe is expanding to 73.4 kilometers per second per megaparsec — give or take 1.3 percent. This means that, locally, space is getting bigger at about 164,000 miles per hour.
But that’s just here and, more importantly, now. Measurements of the cosmic microwave background show that in its earliest days, the Universe was definitely expanding slower, about 67 kilometers per second per megaparsec. As surveys like Pantheon+ get more accurate, it gets clearer and clearer that this discrepancy — the Hubble tension — can’t just be explained away by the difficulty of getting clear observations.
The Universe’s expansion has undeniably sped up, but it’s not quite clear why.
WHAT’S NEXT — As an overview of the field, Brout notes Pantheon+ is a way of capturing the state of the art right before an enormous transformation. Over the next two years, the Vera Rubin Observatory in Chile will come on line, and “the game is going to kind of change in the future.”
While work measuring dark matter, dark energy, and the expansion of the cosmos have been built on many different observations with many different tools, “going forward we have these big, billion-dollar telescopes that are collecting really enormous samples on their own.”
How enormous? The Rubin Observatory expects to find over a million of the right kind of ancient supernovae in the next dozen years – a thousand times more than what Pantheon+ collates.
The scale of the teams working on this data will change, too: “these are going to be huge collaborations with hundreds of people and they’re going to nail a lot of these things down.” But for now, “before these really big giant telescopes turn on,” Brout hopes Pantheon+ can be the apex of an era.
