Final data from groundbreaking cosmology telescope reveals 30 models of the universe were wrong

After a decades-long mission to understand the nature of the universe, the telescope on a mountain plateau in northern Chile will say goodbye in 2022. Now, the release of final data reveals a field in tension that is the telescope’s legacy.

In October 2007, the Atacama Space Telescope (ACT) observed its first light. But it wasn’t light from a star or a distant galaxy. Instead, ACT was designed to look for microwaves, specifically the types of microwaves left over from the earliest days of the universe. This “fossil” light, known as cosmic microwave background radiation (CMB), was emitted when the universe was only 380,000 years old.

The CMB provides cosmologists with a raw picture of the infant Universe. ACT was designed to complement other surveys, such as the European Space Agency’s Planck satellite. Project Planck launched an orbiting spacecraft to provide an all-sky survey of the CMB. However, its resolution was limited, especially for studying the CMB’s polarization (the direction in which the oscillations of the CMB’s magnetic and electric fields point as the light travels). In contrast, although ACT is ground-based, it is capable of deeper exploration of smaller pockets of the CMB sky at much higher resolution.

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ACT is particularly good at observing the polarization of the CMB, which can tell us a lot about the conditions of the early Universe. Changing the amount of dark matter in the universe, how it’s distributed, the number of neutrinos, or a dozen other properties of the universe changes how the CMB sees light.

Final ACT

In November, the ACT team released its sixth and final public dataset in three papers published in the Journal of Cosmology and Astroparticle Physics. While cosmologists will continue to mine for data for years to come, the core team also provided a final set of analyzes and studies before saying goodbye for good.

Their findings were consistent with what studies like Planck’s had already identified: Something strange is happening as the universe expands. Current measurements of the expansion rate, known as the Hubble rate or Hubble constant, measured by early space probes such as Planck and ACT, reveal values ​​that are much slower than estimates based on nearby measurements such as supernova dimming.

This contradiction has become known as the Hubble tension, and is perhaps the greatest unsolved mystery in modern cosmology. But ACT not only confirmed the existence of tension; It also ruined some very good ideas.

ACT axis 30 space models

Cosmologists have been busy concocting many theoretical explanations for the Hubble tension. Many of these are called “extended” cosmological models because they take the standard cosmological picture and add some additional components or forces to the universe.

However, these ingredients and powers do not only exist today. These would have also been present when CMB was first released. Therefore, ACT’s superior view of the CMB allowed the team to test many of these models (approximately 30 in fact).

They all failed.

But in science, you only lose if you learn nothing. Negative results of ACT are useful for cosmologists’ research. In other words, you can only know the correct answer by erasing all the wrong answers.