With Webb’s help, astronomers verified previous Hubble Telescope measurements of the expansion of the universe. Remarkably, both space telescopes come to the same conclusion. And so the mystery of the Hubble constant is still unsolved.
Astronomers have been trying for years to find out how fast our universe is expanding. They use the Hubble constant for this; the cosmological parameter that determines the absolute scale, size and age of the universe. It’s one of the most direct ways to find out how the universe is evolving. However, there is a striking discrepancy between the value of the constant measured with a wide range of independent distance indicators and the value predicted from the afterglow of the Big Bang. And so hopes were placed on the powerful James Webb telescope to unravel this mystery.
Hubbleconstante
Back to the beginning. The Hubble constant is therefore an important parameter in cosmology that describes the rate at which the universe is expanding. But researchers are still unable to put their finger on it. This is because it has still not been possible to determine the exact value of the Hubble constant. And while several ways have already been tried to measure this. A common way is to observe Cepheids (pulsating stars) and supernovae. This shows that the universe is expanding at about 74 kilometers per second per megaparsec. Another way is to make measurements of the cosmic microwave background radiation from the early universe. And data from the Planck satellite on the cosmic background radiation reveals that our universe is expanding at a rate of 67.4 kilometers per second per megaparsec. A difference of six or seven kilometers per second!
Cepheïden
It is not without reason that astronomers look at Cepheids. The brightness of certain stars in distant galaxies tells us how far away they are and therefore how long this light has traveled to reach us. The redshift of the galaxies tells us how much the universe has expanded during that time, thus revealing the rate of expansion. Cepheids give us an accurate measurement of distance because these stars are extremely bright: they are supergiants, a hundred thousand times brighter than our Sun. In addition, they pulsate for several weeks. The longer this period, the brighter they are. They are therefore the golden instrument for measuring the distances of galaxies a hundred million light-years away or more, a crucial step in determining the Hubble constant. Unfortunately, stars in distant galaxies are often closely packed from our vantage point and it is not yet possible to distinguish them well from their ‘nearer’ neighbors.
Hubble
This was one of the reasons why the Hubble Space Telescope was built. Before Hubble’s launch in 1990 and subsequent Cepheid measurements, the universe’s expansion rate was so uncertain that astronomers weren’t even sure whether the universe had been expanding for 10 billion or 20 billion years. Thanks to Hubble, which operates from space and is therefore not affected by the clouding effects of the Earth’s atmosphere, we got a much better picture of this. Hubble can distinguish individual Cepheid stars in galaxies more than a hundred million light-years away and can measure the period over which these stars change brightness.
Dust
Yet Hubble’s measurements also left much to be desired. That’s because Hubble can’t properly peer through the dust that stands between us and the Cepheids. Dust absorbs and scatters blue optical light, making distant objects appear faint. This makes them appear further away than they actually are. Unfortunately, Hubble sees less clearly in red light than in blue light, causing the light from Cepheids to mix with other stars in its field of view. But this shortcoming was remedied with the arrival of James Webb.
James Webb
Space telescope James Webb is very strong at detecting infrared light. This telescope is specifically designed to operate in the infrared part of the electromagnetic spectrum and it has several instruments on board that can detect and analyze infrared radiation. With its large mirror and sensitive optics, the telescope can therefore easily separate the light from Cepheids from neighboring stars.
Same conclusion
Webb’s observations in infrared have led to measurements of Cepheids with much less noise than was the case with Hubble. “This is an improvement that astronomers dream of,” the researchers wrote in a statement. In total they observed more than 320 Cepheids. And that has now led to a startling conclusion. “We found that Hubble’s previous measurements were generally correct,” the researchers said. “We also observed four more galaxies using the Webb telescope, which also produced similar results.”
This image shows how Hubble and Webb measure the distance to Cepheid stars (stars that are far away from us). These stars help us understand how quickly the universe is expanding. Because Cepheids often sit in busy star regions, light from other stars can distort the measurements. The Webb telescope can distinguish Cepheids well with the help of its infrared camera and now confirms that Hubble has been making accurate measurements over the past 30 years. Image: NASA. ESA, J. Kang (STScI). Science: A. Riess (STScI)
Accurate
The results mean that the measurements of Cepheids – and therefore the expansion rate of the universe – are at least accurate, as two independent telescopes have come to similar findings. But this does not solve the mystery about the exact value of the Hubble constant. “What the results still don’t explain is why the universe appears to be expanding at such a fast rate,” the statement said. “We can predict the expansion rate of the universe by observing its baby picture, the cosmic background radiation. We then use our best model of how the universe has grown over time to figure out how fast it should be expanding today. The fact that the current measurement of the expansion rate is significantly higher than predicted is a problem that has now lasted a decade.” This means that there is something wrong with the universe, or that we are missing something in our understanding of the universe.
The last word about the Hubble constant has certainly not yet been said. Astronomers are now investigating a number of possibilities that could explain the persistent discrepancy. “It could indicate the presence of exotic dark energy, exotic dark matter, a revision in our understanding of gravity or the presence of a unique particle,” the researchers summed up. “A more mundane explanation is that measurement errors are the cause. But the new measurements with Webb now show that there were probably no measurement errors when observing Cepheids. The more interesting options therefore remain on the table for the time being.”
2023-09-13 16:58:46
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