How fast is the universe expanding? It depends who you ask. Cast your gaze at the relatively nearby stars and galaxies around us in space, and you’ll come up with a number for this value, known as the Hubble constant. But look into the much more distant universe and you’ll get a slightly different number. This gap, known as Hubble Voltage, is small but has heavy ramifications. The voltage could simply be caused by flaws in our measurements or it could indicate fundamental shortcomings in our understanding of cosmic structure. Certainly, even without any tension, there are deep mysteries related to the rate of expansion of the universe, namely the fact that it is accelerated by dark energy, a still unexplained force of which we know almost nothing. Now, a new measurement of the Hubble constant, made by observing a mirror image of a distant exploding star, or supernova, further complicates matters.
in research published today in the journal Science, Patrick Kelly of the University of Minnesota and his colleagues used the delay of a distant supernova known as Refsdal to measure the Hubble constant. They arrived at an expansion rate of 66.6 kilometers per second per million parsecs (km/s/Mpc), or 66.6 km per second per 3.26 million light-years, with an uncertainty of 1.5%. (A previous study of the supernova, from 2017, reached a similar result but with significantly larger statistical uncertainty.)
This number – 66.6 km/s/Mpc – is oddly at odds with other supernova-based measurements in the so-called local universe. These tend to give a higher value for the Hubble constant: around 73 km/s/Mpc. Yet 66.6 km/s/Mpc is strikingly similar to measurements of the Hubble constant from much more distant sources in the “early” universe, which provide values of around 67 km/s/Mpc. “We should be okay with the supernova measurement, but we’re not,” Kelly says. “And they can’t both be right.”
The Hubble constant can be measured in several ways. For the local universe, most rely on various standard candles – certain types of supernovae and other astrophysical objects that possess known intrinsic luminosity, barely varying, making it easier to determine their distances and motions relative to We. Measurements of several kinds of standard candles can be linked to allow astronomers to estimate the Hubble constant at ever greater distances, each standard candle being a “step” on what is known as the “distance scale”. cosmic”. But the scale of cosmic distances begins to shift and shift over truly vast distances. To measure the Hubble constant that prevailed in the early universe, researchers primarily use the cosmic microwave background (CMB) – essentially the waste heat from the big bang when the universe was little more than a ball. fire 400,000 years old. The sound waves rippling through this cosmic fire imprinted telltale patterns on the CMB that astronomers can use as standard rulers for plotting the further expansion of the universe.
In 1964, Norwegian astrophysicist Sjur Refsdal first suggested another way to use supernovae to measure the Hubble constant. If, on its way to Earth, light from a distant supernova passed around the gravitational grip of a massive object, such as a cluster of galaxies, the light could be “gravitational lensed”, or distorted and bent to follow several divergent paths to Earth. , some longer and some shorter. The end result would be a single supernova appearing multiple times in slightly offset positions in the sky, with the delay between each appearance corresponding to the total distance its light traveled. Combining these delays with knowledge of how fast the supernova was moving away from us – obtained by measuring a property called redshift – and the mass of the lens cluster would provide a value for the Hubble constant.
In November 2014, Kelly, then at the University of California, Berkeley, and his colleagues discovered the first known example of such an event: the supernova Refsdal, which occurred some 14 billion light-years from Earth. They correctly predicted the arrival of a lens image of the supernova, which reached our planet some 360 days later in late 2015. The team was finally able to use Refsdal to measure the expansion rate of the supernova. ‘universe. “It’s unlike anything that’s been done before,” Kelly says. To arrive at a value, the team worked in groups that independently evaluated the blinded data to arrive at its unexpected figure of around 66.6 km/s/Mpc.
The result is “a great addition” to our knowledge of the Hubble constant, says Wendy Freedman, a University of Chicago astronomer who specializes in study of the expansion rate of the universe and was not involved with the new newspaper. “It’s completely independent of any other type of method.”
Astronomers have used the lens before to measure the expansion of the universe, but with quasars – the extremely bright nuclei of some galaxies – rather than with supernovae. In 2017, a team called H0LiCOW used this method to arrive at a value of about 72 km/s/Mpc. Lens quasars are “more abundant” in the sky, which gives this method some advantages, says H0LiCOW leader Sherry Suyu of the Max Planck Institute for Astrophysics in Garching, Germany. But supernovae show more obvious changes in brightness, meaning the exact delay in the images can be measured more precisely, perhaps giving a higher level of precision. “You really see this drastic variation,” Suyu says.
But whereas quasars can shine for millions of years – essentially forever for us – supernovae are short-lived and only shine for weeks or months. “You have to be able to find them early on,” says Suyu. “If you miss it, they’re gone.” To date, only a few delayed supernovae are known. The most recent, named H0pe, was discovered by the James Webb Space Telescope (JWST) earlier this year. So, while Refsdal is the first such event to be used to measure the expansion of the universe, it may not be the last.
If the value of Kelly and his team is confirmed, it would suggest that we may need to adjust our best guesses about the nature of dark matter – the enigmatic and invisible substance that seems to give galaxies and galaxy clusters most of their mass and thus modulates the gravitational lens. If true, says Kelly, their result “implies that there must be a flaw in our patterns of dark matter in galaxy clusters.” Updating these models may in turn require changes to the so-called standard model of cosmology, which assumes that some rather inert “cold” form of dark matter and a specific type of dark energy act together to guide growth and evolution. of galaxies and clusters through cosmic time.
“We don’t yet understand what dark matter and dark energy are,” Freedman says. “Measuring the Hubble constant locally is a way to directly test this model. If it shows that there is a fundamental piece of physics missing from the Standard Model, that will be very exciting.
However, not everyone is yet convinced that such cosmological changes are coming. Daniel Scolnic of Duke University argues that the seemingly small 1.5% uncertainty in the result is still large enough at the margins to place it within the bounds of other local results. “If they have much smaller uncertainties, then everyone should be looking in the mirror right now,” says Scolnic, who was not involved in the study. “It would be really confusing because all the local measurements seem to agree on higher values.”
To know for sure, we will have to study more time-delayed supernovae and determine their values of the Hubble constant. Such results could come sooner rather than later: a measurement of H0pe is expected from JWST in the coming months, and the upcoming Vera Rubin Observatory in Chile, due to start next year, is expected to significantly increase the population of known supernovae at delay. “We’ll find many more,” Kelly said. “If they are all in favor of a lower value of the Hubble constant, it would reinforce the disagreement. Hopefully we can understand where the problem is.