Discovery of unexpected ultramassive galaxies won't rewrite cosmology, but still leaves questions

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By observing different parts of the electromagnetic spectrum, the Hubble Space Telescope and the James Webb Space Telescope are able to see different things in the same parts of the universe. Credit: NASA, J. Olmsted (STScI).

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By observing different parts of the electromagnetic spectrum, the Hubble Space Telescope and the James Webb Space Telescope are able to see different things in the same parts of the universe. Credit: NASA, J. Olmsted (STScI).

Ever since the James Webb Space Telescope (JWST) captured the first glimpse of the early universe, astronomers have been surprised to discover that there are more „ultramassive” galaxies than expected. Based on the most widely accepted model of cosmology, they may not have formed until much later in the history of the universe, prompting a change in model.

It will upend decades of established science.

„The growth of objects in the universe is gradual. You start small and get bigger and bigger,” said Julian Muñoz, assistant professor of astronomy at the University of Texas at Austin and a co-author. Latest paper Published in Physical review letters It tests changes to the cosmological model. The study concludes that there is no need to revise the standard cosmological model. However, astronomers may have to rethink their understanding of how the first galaxies formed and formed.

Cosmology studies the origin, evolution and structure of our universe, from the Big Bang to the present day. The most widely accepted model of cosmology is the so-called lambda cold dark matter (ΛCDM) model, or „standard cosmological model”. Although the model is very well known, much about the early universe remains theoretical because astronomers have not been able to fully observe it.

Launched in 1990, the Hubble Space Telescope was instrumental in developing and refining the Standard Cosmological Model. It observes the universe in ultraviolet, visible and some infrared wavelengths of light. However, it is better at seeing some things than others. For example, Hubble is well equipped to observe small galaxies, which often contain young, ultraviolet-emitting stars and less dust that absorbs shorter wavelengths.

Launching in late 2021, JWST provides an important complement to Hubble's capabilities. By observing near- and mid-infrared wavelengths, JWST can detect objects invisible to Hubble.

„We're opening a window for people who don't know,” Munoz said. „We can now test our theories about the universe where we couldn't before.”


An infrared view of the universe captured by the James Webb Space Telescope. Credit: NASA, ESA, CSA and STScI.

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An infrared view of the universe captured by the James Webb Space Telescope. Credit: NASA, ESA, CSA and STScI.

After the Big Bang, things were not the same. Small variations in density had a significant impact on the future structure and evolution of the universe. Areas of higher density attracted more material due to gravity, eventually leading to the formation of larger and larger structures.

To become massive so quickly, the ultramassive galaxies observed by JWST are theoretically only possible if these high-density regions formed after the Big Bang. This requires changing the standard cosmological model.

Munoz and his team tested this hypothesis.

They chose a cosmic time when both JWST and Hubble observations were available. Within this range, they identified the most massive galaxies in the JWST data and calculated the amount of change to the initial density of the Universe required for their formation.

They also calculated how many small galaxies would result from this hypothetical change. These extra small galaxies would have been observed by Hubble.

„But that's not what we're seeing,” Munoz explained. „You can't change cosmology to explain this abundance problem because Hubble's observations are also affected.”

Why is JWST finding so many ultramassive galaxies? One possibility is that they contain supermassive black holes. These black holes heat up nearby gas, making galaxies appear brighter and therefore larger than they actually are. Or the galaxies may not actually exist in the early universe, but look like they do because dust makes their color appear redder than otherwise. This change makes galaxies appear farther away than they are.

In addition to Munoz, study authors include Nashwan Sabti and Mark Kamionkowski of Johns Hopkins University.

More information:
Nashwan Sapti et al., Insights from HST into Ultramassive Galaxies and Early-Universe Cosmology, Physical review letters (2024) DOI: 10.1103/PhysRevLett.132.061002. On that day arXiv: DOI: 10.48550/arxiv.2305.07049

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Physical review letters


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