New Sound Proof We're Living In A Giant Void

These fluctuations are now imprinted in the CMB, and dubbed“baryon acoustic oscillations” (BAOs). Since these became the seeds for galaxies and other structures, the patterns are also visible in the distribution of galaxies.

By measuring these patterns, we can learn how galaxies are clustered at different redshifts (distances). A particularly striking pattern, with lots of clustering, occurs at an angle called the“angular BAO scale.”

This measurement ultimately helps astronomers and cosmologists learn about the universe's expansion history by providing something physicists call a“standard ruler.” This is essentially an astronomical object or a feature on the sky with a well-known size.

By measuring its angular size on the sky, cosmologists can therefore calculate its distance from Earth using trigonometry. One can also use the redshift to determine how fast the cosmos is expanding. The larger it appears on the sky at a certain redshift, the faster the universe is expanding.

My colleagues and I previously argued that the Hubble tension might be due to our location within a large void. That's because the sparse amount of matter in the void would be gravitationally attracted to the denser matter outside it, continuously flowing out of the void.

In previous research , we showed that this flow would make it look like the local universe is expanding about 10% faster than expected. That would solve the Hubble tension.

But we wanted more evidence. And we know a local void would slightly distort the relation between the BAO angular scale and the redshift due to the faster-moving matter in the void and its gravitational effect on light from outside.

So in our new paper, Vasileios Kalaitzidis and I set out to test the predictions of the void model using BAO measurements collected over the last 20 years. We compared our results to models without a void under the same background expansion history.

In the void model, the BAO ruler should look larger on the sky at any given redshift. And this excess should become even larger at low redshift (close distance), in line with the Hubble tension.

The observations confirm this prediction. Our results suggest that a universe with a local void is about one hundred million times more likely than a cosmos without one, when using BAO measurements and assuming the universe expanded according to the standard model of cosmology informed by the CMB.

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Our research shows that the ΛCDM model without any local void is in“3.8 sigma tension” with the BAO observations. This means the likelihood of a universe without a void fitting these data is equivalent to a fair coin landing heads 13 times in a row.

By contrast, the chance of the BAO data looking the way they do in void models is equivalent to a fair coin landing heads just twice in a row. In short, these models fit the data quite well.

In the future, it will be crucial to obtain more accurate BAO measurements at low redshift, where the BAO standard ruler looks larger on the sky – even more so if we are in a void.

The average expansion rate so far follows directly from the age of the universe, which we can estimate from the ages of old stars in the Milky Way. A local void would not affect the age of the universe, but some proposals do affect it.

These and other probes will shed more light on the Hubble crisis in cosmology.

Indranil Banik is postdoctoral research fellow in astrophysics, University of Portsmouth

This article is republished from The Conversation under a Creative Commons license. Read the original article .

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