Over the last decade, various measurements have disagreed over the rate that the Universe is expanding. Astronomers use Cepheid variables as standard candles to measure relatively nearby galaxies and then use them to calibrate more distant candles like Type 1a supernovae. The most accurate measurements have been done with the Hubble Space Telescope, but a new survey of 330 Cepheid variables has been done with JWST, narrowing down the error bars even more and building a perfect distance ladder to Type 1a supernovae.
Very broadly, the cosmic distance ladder uses three rungs of measurement. The first rung is, where we use simple geometry to measure stellar distances, which is extremely accurate. The second rung looks at a type of variable star known as Cepheid variables. Their rate of oscillation correlates with their overall brightness. The third rung measures the apparent brightness of Type Ia supernovae, which always explode with consistent brightness.
One proposed solution to the Hubble tension suggests that perhaps our scale for Cepheid variables is wrong. While we can measure parallax distances for lots of Cepheids, measuring brightness can be a challenge. When a Cepheid variable is near several other stars, it can be difficult to separate its brightness from the background brightness of nearby stars. It’s an effect known as crowding and could be skewing our data.
To determine whether this bias is significant, the team compared observations of Cepheids made by the Hubble Space Telescope with observations from the James Webb Space Telescope. The Hubble observations have long been the basis of the Cepheid rung of the cosmic distance ladder, but since JWST observes Cepheids at infrared wavelengths, clustering is less of a problem. The team used 560 Cepheid measures from Hubble and 325 Cepheid measures from JWST.
They found that JWST observations increased the precision of the Cepheid scale, but the overall scale was not changed. In other words, the clustering issue seen in Hubble data does not significantly bias the cosmic distance ladder. So this doesn’t resolve the Hubble tension. The new results actually make the tension slightly worse, because the Cepheid scale is now more precise.
So much for that idea. But there’s still a bit of hope. As JWST gathers observations of distant supernova, the third rung of the cosmic distance ladder can be tested to see if there’s bias there.
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