Makeup and the growth of the Universe have never been clearer – or more confusing – than they have been revealed in a massive new survey of the markers astronomers use to measure the cosmos.
A new analysis called Pantheon+ has reduced uncertainty in the expansion and composition of the Universe. To do this, Pantheon+ relies on two long-standing astronomical projects – one called Pantheon, combining 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 Panthéon+ analysis of the composition and expansion of the Universe recently published in The Astrophysical Journal finds that 66.2% of the Universe is made up of dark energy, the mysterious accelerator behind the accelerating expansion of the Universe, slightly less than past estimates of around 68%.
Only 33.8% 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 gravitational effects at galactic scale. At the accepted ratio of 85% dark matter to 15% normal (baryonic) matter, this means that just under 5% 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 less than 1.3% uncertainty, close enough that it is now undeniable that the early Universe and the current Universe are not expanding. at the same pace.
Talk with Reverse, lead author Dillon Brout, NASA Einstein Fellow at the Center for Astrophysics | Harvard-Smithsonian says this degree of precision means that instead of being limited by data from measuring the growth of the Universe, “we are approaching the limit where we are limited by the uncertainties of our method.”
WHAT’S UP – Pantheon+ offers as accurate a measurement of dark energy, dark matter, and baryonic matter as is currently possible to assemble.
And “assembled” is the right word – this work combines analysis of the original Pantheon, which measured dark matter, and 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,” says Brout. And it’s the largest set of exploding stars that have been put together – over 1,500, half as many as a previous version that focused only on supernovae.
But Brout notes that’s about all that can be won with current equipment. The limiting factor is time. “We get about one supernova a year which helps us measure the Hubble constant, and we have 42 now. So we’re going to have to wait a bit to double our dataset,” he says.
WHY IS IT IMPORTANT – Surveys like Pantheon+ allow astronomers to cross-check their results across different methods and different targets. Some components measure the Cepheids, relatively nearby stars that steadily wax and wane in brightness; others measure supernovae that overtook galaxies up to 10 billion years ago.
At the moment, that’s 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 question is ‘yes’ – but it’s not immediately clear how much that’s going to help us.” The James Webb Space Telescope will be let astronomers examine how stardust and observations in different wavelengths impact 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+ reduced the rate at which the Universe expands to 73.4 kilometers per second per megaparsec, or 1.3%. This means that locally space is expanding at about 164,000 miles per hour.
But it’s right here and, more importantly, now. Measurements of the cosmic microwave background show that in its early days the Universe was expanding much more slowly, at around 67 kilometers per second per megaparsec. As surveys like Pantheon+ become more precise, it becomes increasingly clear that this discrepancy – the Hubble strain – cannot simply be explained by the difficulty of obtaining clear observations.
The expansion of the Universe has undeniably accelerated, but it is unclear why.
AND AFTER – 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. Within the next two years, the Vera Rubin Observatory in Chile will be commissioned, and “the game is kind of going to change in the future.”
While the work of 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 large telescopes of a billion dollars collecting really huge samples themselves.”
How huge? The Rubin Observatory expects to find more than a million ancient supernovae of the right type over the next twelve years, a thousand times more than Pantheon+ collects.
The scale of the teams working on this data will also change: “It will be huge collaborations with hundreds of people and they will implement a lot of these things.” But for now, “before those really big giant telescopes turn on,” Brout hopes Pantheon+ can be the pinnacle of an era.
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