The make-up and the growth of the Universe has never been clearer, or more confusing, as revealed in a massive new survey of the markers astronomers use to measure the cosmos.
A new analysis called Pantheon+ has reduced the uncertainty in the expansion and composition of the Universe. To do this, Pantheon+ draws 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, which measures relatively nearby pulsating stars known as Cepheids within 10 million light years.
The Pantheon+ analysis of the composition and expansion of the Universe published recently in The Astrophysical Journal finds that 66.2% of the Universe is made up of dark energy, the mysterious accelerator driving the accelerated expansion of the Universe Universe, slightly less than previous estimates of 68%.
Only 33.8 percent of the Universe is 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, this means that 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 to within 1.3 percent uncertainty, close enough that it is now undeniable that the early Universe and the present Universe are not expanding at the same rate.
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 rather than being limited by the data to measure the growth of the Universe, “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 can currently be assembled.
And “assembled” is the right word: this work combines analysis of the original Pantheon, which measured dark matter, and the Supernova H0 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; represents “a sample of stars,” explains Brout. And this is the largest set of exploding stars ever assembled: more than 1,500, half as many as a previous version that focused only on supernovae.
But Brout points out that this is all that can be achieved with the current team. The limiting factor is time. “We have about one supernova per year that helps us measure the Hubble constant, and now we have 42. So we’ll have to wait a while just to double our data set,” he says.
The cosmic microwave background is one of the best ways to understand the early universe. Print Collector/Hulton Archive/Getty Images
WHY IT MATTERS – Surveys like Pantheon+ allow astronomers to compare their results with different methods and different objectives. Some components measure Cepheids, relatively nearby stars whose brightness regularly waxes and wanes; others measure supernovae that eclipsed galaxies up to 10 billion years ago.
For now, this is as accurate as these types 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 allow astronomers to see how stardust and observations at different wavelengths affect the observations of the anchors that hold their measurements in place.
The increasing precision of this analysis has also increased one of the biggest problems in cosmology. Pantheon+ has reduced the rate at which the Universe is expanding to 73.4 kilometers per second per megaparsec, by 1.3 percent. This means that, locally, space is getting faster at about 164,000 miles per hour.
But this is here and, more importantly, now. Measurements of the cosmic microwave background show that in its early days, the Universe was definitely expanding more slowly, about 67 kilometers per second per megaparsec. As surveys like Pantheon+ become more precise, it becomes increasingly clear that this discrepancy (the Hubble strain) cannot be explained solely by the difficulty of obtaining clear observations.
Undeniably, the expansion of the Universe has accelerated, but it is 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 just before a huge transformation. In the next two years, the Vera Rubin Observatory in Chile will come online and “it will be a game changer in the future.”
Although the work measuring dark matter, dark energy, and the expansion of the cosmos has been built from many different observations with many different tools, “going forward we have these big billion-dollar telescopes that they’re collecting really huge samples on their own.”
How huge? The Rubin Observatory hopes to find more than a million of the right kind of ancient supernovae in the next twelve years, a thousand times more than Pantheon+ collects.
The scale of the teams working on that data will also change: “They’re going to be huge collaborations with hundreds of people and nailing a lot of these things.” But for now, “before these really big giant telescopes get fired up,” Brout hopes Pantheon+ could be the pinnacle of an era.
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