SPACE — The James Webb Space Telescope (JWST) has reexamined the work of its predecessor, the Hubble Space Telescope. Hubble’s measurements of the universe’s expansion rate further increased the so-called Hubble strain.
Simply put, measurements of the universe’s expansion rate, determined by a property called the Hubble constant, cannot add up. What’s the story? here’s the problem…
On the one hand, observations of the cosmic microwave background (CMB) radiation, which resembles a picture of the infant cosmos 379,000 years after the Big Bang, say the universe is currently expanding at a speed of about 67.8 kilometers per second per megaparsec. This means that every volume of space measuring one million parsecs (3.26 million light years) will expand at a speed of 67.8 kilometers every second.
On the other hand, an alternative way of measuring the expansion of the universe is by climbing the cosmic distance ladder, where each rung is formed by a different astrophysical milestone, such as Cepheid variable stars and Type Ia supernovae (neutron star explosions). How bright the objects are can tell you their distance, which is then compared with their redshift value to determine the extent of the expansion of the universe as the light reaches us.
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But the problem is, the second method gives a very different value for the Hubble constant, namely around 73.2 kilometers per second per megaparsec. This apparent paradox between the two measurements is what cosmologists call the Hubble tension. No one knows what causes it, but some hypotheses require new physics to explain the apparent contradiction.
One possible explanation is the existence of measurement errors at the bottom rung of the cosmic distance ladder, which is where the Cepheid variables reside. These are stars whose luminosity fluctuates as they move in and out.
The longer the pulse period between moments of maximum luminosity, the greater the maximum luminosity. This relationship between period and luminosity allows scientists to accurately calculate the distance to Earth.
The pulse period can be measured to calculate the maximum luminosity. Meanwhile, how bright a Cepheid variable is in the sky can determine how far away it is. However, it is not an easy method to carry out.
The Hubble Space Telescope is able to observe Cepheid variables in distant galaxies. However, the farther the distance, the more difficult it is for the variable to be distinguished from the other stars clustered around it.
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Because of this, there is concern that stars close to Cepheid variables in distant galaxies will increase the Cepheid brightness values, thereby creating invisible and systematic errors in the measurements. Interstellar dust can also affect the brightness of Cepheid variables, dimming them from our perspective on Earth.
This is where the new measurements by the James Webb Telescope are important. Webb’s measurements of five galaxies hosting more than a thousand Cepheid variables have ruled out the possibility of such errors.
James Webb’s infrared vision was able to penetrate interstellar dust, while its greater resolution was able to resolve Cepheid variables clearly so they stood out from the crowd. From the Webb telescope’s measurements, astronomers led by Adam Riess from Johns Hopkins University concluded that Hubble’s original measurements were correct.
“We have examined the full range of Hubble observations and we can rule out measurement error as a cause of Hubble tension with very high confidence,” Riess said in a statement. The research team’s results have been published in The Astrophysical Journal Letters on February 6, 2024.
The farthest of the five galaxies observed by the Webb telescope is NGC 5468 which is 130 million light years from Earth. The galaxy has been home to a total of eight Type Ia supernovae over the past few decades.
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The supernova had a luminosity curve that could be standardized, and formed the next rung on the cosmic distance ladder above the Cepheids. Because the previous rung is needed to calibrate the next rung, Webb’s observations of Cepheid variables make distance measurements using Type Ia supernovae more accurate. Because the supernova is quite bright even though it is seen in a galaxy more distant than Cepheid.
The observations also told scientists that there were contradictions in various measurements of the Hubble constant. “With measurement errors removed, what remains is the real and intriguing possibility that we have misunderstood the universe,” Riess said. Yes, the Hubble suspense remains unanswered. Source: Space.com
2024-03-12 22:49:00
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