There Is Something "Very Wrong" With Our Understanding Of The Universe, Telescope Final Data Confirms

From October 2007 until mid-2022, the Atacama Cosmology Telescope (ACT) looked at the millimeter and microwave universe. It discovered the most extreme galaxy cluster and many other peculiar things, though its true focus has always been studying the cosmic microwave background (CMB). 

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This is the light echo of the Big Bang, the first light that freely moved through the universe. This relic is crucial to our understanding of the universe, and now the ACT team has published the final data release, confirming that there is something very wrong with our understanding of the universe.

The crux of it all is the Hubble Constant, a very important parameter that expresses the expansion rate of the Universe today. It is possible to measure this in multiple ways. An important one is by collecting the distance and recession velocity of many galaxies in the nearby universe. You can also obtain that from the CMB. There are other methods, but these two are the ones for which we have the highest precision, and there is a big problem. They produce two different numbers!

This is known as the Hubble Tension. Any approach that aims to measure it should ideally find roughly the same value (at least within measurement uncertainty). But the stellar method and the CMB method find two values with no overlap. The CMB data comes from the European Space Agency’s satellite Planck, and ACT just confirmed those results.

“Our new results demonstrate that the Hubble constant inferred from the ACT CMB data agrees with that from Planck - not only from the temperature data, but also from the polarization, making the Hubble discrepancy even more robust," Colin Hill, cosmologist at Columbia University and co-lead of one of the papers, said in a statement.

One way that astronomers have tried to fix the tension is by proposing theoretical alternatives to the standard model of cosmology. These are known as extended models. Well, they were known as extended models, because the ACT data release shows that 30 of the main extended models have no support.

“They’re gone,” Erminia Calabrese, cosmologist at Cardiff University who led that particular study, added. “We assessed them completely independently. We weren’t trying to knock them down, only to study them. And the result is clear: the new observations, at new scales and in polarization, have virtually removed the scope for this kind of exercise.”

Polarization is a property of light. Light is polarized if its electromagnetic field is oscillating in a particular direction. The polarization of the CMB is very important as it provides new insights into the early universe, which mapping the CMB can’t deliver. It can also allow better tests of cosmological models and with greater precision. ACT delivered a significant improvement in polarization maps.

“This is mainly because ACT  has a larger diameter — six meters compared to Planck’s one and a half meters — and sharpness increases with mirror size,” explained Sigurd Naess of the University of Oslo, one of the paper's leads. “But it's also because ACT’s images of the polarized light are much more sensitive than Planck's.”

This doesn’t mean that Planck data is obsolete, quite the contrary. The two build and rely on each other. The findings of this work expand what we know of the earliest time of the universe, as the CMB was emitted just 380,000 years after the Big Bang. The questions remain and loom large, but the findings are exciting.

The three papers were published in the Journal of Cosmology and Astroparticle Physics: here, here, and here.