How fast is the universe expanding? It depends who you ask. If we look to the relatively nearby stars and galaxies that surround us in space, we will arrive at a certain number of this value, known as the Hubble constant. But if you look further into space, you’ll get slightly different numbers. This discrepancy, known as the Hubble tension, is small but has a significant effect. This tension may simply be caused by imperfections in our measurement results. Alternatively, it could indicate a fundamental gap in our understanding of the structure of the universe. Indeed, even in the absence of any tension, there are deep mysteries involved in the rate of expansion of the universe. In other words, the fact that the universe is accelerated by dark energy. Dark energy is still unexplained and largely unknown. Now, new measurements of the Hubble constant, made by observing mirror images of distant exploding stars, or supernovae, further complicate matters.
In a study published today in the journal chemistry, Patrick Kelly and colleagues at the University of Minnesota measured the Hubble constant using a time delay from a distant supernova known as Refsdal. They reached an expansion rate of 66.6 kilometers per second per million parsecs (km/s/Mpc), or 66.6 kilometers per second per 3.26 million light years, with an uncertainty he gave of 1.5%. (A previous study of supernovae done in 2017 yielded similar results, but with much greater statistical uncertainty.)
This number (66.6 km/s/Mpc) is strangely inconsistent with other supernova-based measurements in the so-called local universe. These tend to produce higher values of the Hubble constant, around 73 km/s/Mpc. However, 66.6 km/s/Mpc is strikingly similar to Hubble constant measurements from far distant origins in the ‘early’ universe, yielding values of about 67 km/s/Mpc. “We should agree on supernova measurements, but we don’t,” says Kelly. “And neither can be right.”
The Hubble constant can be measured in various ways. For the local universe, most of it is a variety of standard candles, i.e., certain types of supernovae and other celestial bodies that have known and little-varying intrinsic luminosities that allow us to more easily ascertain their distance and motion relative to us. It depends on physical celestial bodies. Measurements from multiple types of standard candles can be stitched together to allow astronomers to measure the Hubble constant over longer distances. Each standard candle becomes one “step” of the so-called “cosmic distance ladder”. But the space-like ladder of distance begins to wobble and roll over truly vast distances. To measure the Hubble constant, which was prevalent in the early universe, researchers primarily use the cosmic microwave background (CMB). This is essentially the remnant heat of the Big Bang when the universe was barely more than her 400,000-year-old fireball. Sound waves rippling through cosmic fire, he carved into the CMB a testamentary pattern that astronomers can use as a standard ruler for drawing the subsequent expansion of the universe.
In 1964, Norwegian astrophysicist Sjur Refsdal first proposed another method that could measure the Hubble constant using supernovae. If the light of a distant supernova happens to pass around the gravity of a large object, such as a cluster of galaxies, on its way to Earth, the light undergoes “gravitational lensing” or follows multiple divergent paths to Earth. It may be distorted or bent due to , there is a long one and a short one. The net result is that a single supernova will appear multiple times at slightly offset locations in the sky, with the delay between each appearance corresponding to the total distance traveled by its light. Combining this delay with information about how fast the supernova is moving away from us, obtained by measuring a property called redshift, and the mass of the lens group, yields the value of the Hubble constant.
In November 2014, Kelly and his colleagues, then at the University of California, Berkeley, discovered the first known example of such a phenomenon, the supernova Lefsdal, about 14 billion light-years away from Earth. They accurately predicted that the lens image from the supernova would reach Earth at the end of 2015, about 360 days later. This time, the research team finally succeeded in measuring the expansion rate of the universe using Refsdal. “This is unlike anything that has ever been done,” Kelly says. To derive a value, the team divided into groups and independently evaluated the blinded data and arrived at a figure of approximately 66.6 km/s/Mpc, which fits the unexpectedly turbulent early universe. .
The result is a “huge addition” to our knowledge of the Hubble constant, said Wendy Friedman, an astronomer at the University of Chicago who studies the expansion rate of the universe. “Completely independent of other kinds of methods.”
Astronomers have used lenses to measure the expansion of the universe before, but with quasars (the extremely bright nuclei of certain galaxies) rather than supernovae. In 2017, a team called H0LiCOW used this method to reach values of around 72 km/s/Mpc. Sherry Suyu, his H0LiCOW leader at the Max Planck Institute for Astrophysics in Garching, Germany, says lensed quasars are “more abundant” in the sky and the method has some advantages. says. However, supernovae show more pronounced changes in brightness. This means that exact time delays in the image can be measured more accurately, possibly with a higher level of accuracy. “I can really see this dramatic change,” he says Suyu.
But while quasars may shine for millions of years—for us, forever—supernovae are short-lived, shining brightly for only a few weeks or months. “It has to be found early,” says Suyu. “If you miss it, it’s gone.” To date, only a handful of time-delayed supernovae are known. The latest, named H0pe, was discovered earlier this year by the James Webb Space Telescope (JWST). Refsdal is therefore the first such event to be used to measure the expansion of the universe, but certainly not the last.
If Kelly and his team are correct, it probably suggests that our best guesses about the nature of dark matter need to be fine-tuned. Dark matter is a mysterious, invisible substance that gives galaxies and clusters of galaxies most of their mass and seems to regulate gravitational lensing. . If true, the results “suggest that the model of dark matter in galaxy clusters must be flawed,” Kelly says. Updates to these models are cascaded and may require changes to the so-called standard model of cosmology. In this model, it is assumed that certain rather inert, ‘cold’ forms of dark matter and certain types of dark energy act together to guide growth and evolution. Relationships between galaxies and star clusters over cosmic time.
“We still don’t understand what dark matter and dark energy are,” says Friedman. “Measuring the Hubble constant locally is a way to test that model directly. will be.”
But not everyone is convinced that such a cosmological shift is yet to come. Duke University’s Daniel Skolnick says the uncertainty in this result is seemingly small at 1.5%, but it is still marginal and large enough to fall within the results of other regions. said. “If the uncertainty were much smaller, everyone would be looking hard at themselves in the mirror right now,” says Skolnick, who was not involved in the study. “This would be very confusing, as all local measurements would appear to be in agreement with the higher values.”
To know for sure, we need to study more time-delayed supernovae and check their Hubble constant values. Such results may appear sooner or later. H0pe measurements are due in the next few months at JWST, and Chile’s Vera Rubin Observatory, which is due to open next year, should significantly increase the number of known time-delayed supernovae. “You’ll find a lot more,” Kelly says. “If they all support a lower value for the Hubble constant, it will further strengthen the disagreement. I hope we can figure out where the problem lies.”
