James Webb Space Telescope complicates the paradox of the expanding universe by checking out Hubble’s work

By | March 13, 2024

The James Webb Space Telescope (JWST) has double-checked the work of its older brother, the Hubble Space Telescope. Hubble’s measurements of the expansion rate of the universe are exemplary, the groundbreaking observatory has discovered, creating the so-called “Hubble tension.”

Simply put, measurements of the expansion rate of the universe, defined by a property called the Hubble constantjust don’t add up.

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On the one hand, observations of the cosmic microwave background (CMB) radiation, which resembles a baby photo of the cosmos from just 379,000 years after the Big bang, say that today the universe should be expanding at a rate of about 67.8 kilometers per second per megaparsec. This means that each volume contains one million spaces parsec (3.26 million light years) should expand at a rate of 67.8 kilometers (42.1 miles) per second.

An alternative way to measure this expansion is to climb the cosmic distance ladder, with each rung formed by a different astrophysical landmark, such as Cepheid variable stars and Type Ia. supernovas. How bright these objects are can tell us their distances, which we can then compare to their distances redshift values ​​to determine how much the universe has expanded as their light has traveled to us. The problem, however, is that this method gives us a completely different value of the Hubble constant: somewhere around 73.2 kilometers (45.5 miles) per second per megaparsec.

The apparent paradox between the two measurements is what cosmologists have come to call the Hubble tension. No one knows what causes this, but some hypotheses require new physics to explain the apparent contradiction.

One possible explanation is that there is a measurement error on the bottom rung of the cosmic distance ladder, where the Cepheid variables are located. These are stars with brightness that fluctuates predictably as the stars pulse in and out. The longer the pulsation period between moments of maximum brightness, the greater the maximum brightness. This relationship between period and brightness allows us to accurately calculate their distances from Earth; It is possible to measure the pulsation period to calculate the maximum brightness. Based on how bright a Cepheid variable appears to us in the sky, we can then calculate how far away it must be to appear that bright.

However, it is not a completely foolproof method.

The Hubble Space Telescope is able to observe Cepheid variables in distant galaxies, but the further away they are, the harder it becomes to distinguish between all the other stars crowding around them. As such, there was concern that unresolved stars adjacent to Cepheid variables in these distant galaxies would increase the apparent brightness values ​​of the Cepheids, creating an invisible and systematic error in the measurements. Interstellar dust can also affect the brightness of Cepheid variables, dimming them from our vantage point on Earth.

Side-by-side black-and-white images show a variable star Cepheid as a white, blurry dot.

Side-by-side black-and-white images show a variable star Cepheid as a white, blurry dot.

But new measurements with the James Webb Space Telescope of five galaxies housing a total of more than a thousand Cepheid variables have ruled out this possible error. The JWST’s infrared vision can cut through the interstellar dust, while the greater resolution allows the Cepheid variables to be clearly distinguished, making them stand out from the bulk. From these JWST measurements, astronomers led by Adam Riess of Johns Hopkins University determined that Hubble’s original measurements were exactly right.

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“We have now covered the entire range of what Hubble observed and we can rule out with very high confidence a measurement error as a cause of the Hubble strain,” Riess said in a press release. rack.

The five galaxies observed by the JWST, of which NGC 5468 is the farthest at 130 million light-years from us, have also hosted a total of eight Type Ia supernovae in recent decades. These supernovae that are causing the destruction of… white dwarfs, have a standardizable brightness curve and are the next rung on the cosmic distance ladder above Cepheids. Because the previous step is needed to calibrate the next step, the JWST’s observations of the Cepheid variables therefore make the distance measurements using Type Ia supernovae – which are bright enough to be seen in much more distant galaxies than Cepheids – more precise. And they also tell us that there is a contradiction in the different measurements of the Hubble constant.

“Now that the measurement errors have been ruled out, the real and exciting possibility remains that we have misunderstood the universe,” Riess said.

The team’s results take a long time to come, while that was already the case before available on the pre-print server arxiv en earn chatter at the end of last year. But now that they have been published in full, perhaps we can finally close the chapter of blaming Hubble’s strain on none other than Hubble itself.

The Riess team’s results were published on February 6 in The astrophysical diary letters.

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