The Biggest Crisis in Cosmology May Finally Have a Solution

The persistent, indeterminate problem of the expansion rate of the Universe may not require a rewriting of everything we know about physics.

New measurements taken using the James Webb Space Telescope suggest that the local Universe is moving away from us at a rate of about 70 kilometers (about 43 miles) per second per megaparsec. .

This is great if true. It could finally resolve the difference between the measurements of the rapid expansion of the Universe that has plagued scientists for the better part of a century: the Hubble tension, which is sometimes called the greatest disaster in cosmology.

We’ll need a lot more work before we can say that the problem has been definitively solved, but the new measurements – made on three different types of stars – could be an important step forward. Surveys are sent to The Astrophysical Journal and are available on arXiv.

“On the basis of these new JWST data and using three independent methods, we find strong evidence of the Hubble tension,” says astronomer Wendy Freedman of the University of Chicago. “On the contrary, it appears that our standard cosmological model for explaining the evolution of the Universe continues.”

Here is the price. The universe is expanding at a rapid rate known as the Hubble constant. To calculate this constant, we can use different types of visual objects, all of which provide their own measurements.

Observations from the early universe include the microwave background – the leftover microwave radiation from the early light that flooded the universe – and baryon acoustic oscillations, which are the scattering patterns of distant galaxies. corresponding to the waves that once floated through the early Universe. .

These two signs are known as common rulers, because we know how big they are. They allow us to obtain precise measurements of space directly, and suggest that the Universe is expanding at a rapid rate of 67.4 kilometers per second per megaparsec.

Signals from the nearby Universe are known as ordinary candles. These are known intrinsically luminous objects, such as Cepheid variable stars and Type Ia supernovae. Because we know how bright they are, we can also accurately calculate their location. And they suggest that Hubble is always 74 kilometers per second per megaparsec.

Now, both types of measurements have overlapping error bars, so this difference is not the end of cosmology as we know it. But it would be very good to settle with confidence on a more accurate number for the same expansion rate. Or,​​​​if there are multiple expansion rates, an explanation for why different parts of the Universe expand differently.

Freedman has worked for many years to measure Hubble’s regularity using methods different from the usual standard candles. In particular, he is focusing on stars at the end of the red giant branch, or TRGB stars.

These stars come in roughly the same size and brightness, making them an ideal tool for measuring the distances of nearby galaxies. Using observations from various instruments such as Hubble and Gaia, Freedman and his colleagues made several TRGB measurements that returned Hubble a frequency of 69 to 70 kilometers per second. megaparsec.

Enter the James Webb Space Telescope, the most powerful telescope ever used. Now, Freedman and his team have used it to measure TRGB stars, as well as Cepheid variable stars, and a type of carbon-rich giant star that, they say, is a new type of candle. common based on their consistent brightness.

By measuring the distance of three stars independently, the researchers obtained a lot of information that they can use to check the systematic errors to find an independent measurement of Hubble’s constant.

For TRGB stars, the researchers found a value of 69.85 kilometers per second per megaparsec. For carbon stars, they got 67.96. The Cepheid variance was much smaller, at 72.05, but the error bars for all three converge.

“Getting good agreement from three different types of stars, to us, is a strong indication that we’re on the right track,” Freedman says.

We are not out of the woods yet. Although the measurement falls within the error bars of standard bushings and standard candles, we have been getting different values ​​for too long for the problem to be solved suddenly. In fact, earlier this year JWST measurements of Cepheid variable stars and type Ia supernovae were used to confirm Hubble’s estimate of 73 kilometers per second per megaparsec.

So, we’re going to need to measure a lot, and measure again, and measure again. Just be sure. However the new calculations suggest that differences between different observations can still account for the discrepancy, without the need to develop major new theories.

However, who knows? Perhaps we will encounter new physics in our search for an answer.

Inquiries sent to The Astrophysical Journaland is available on arXiv.