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).
Since the James Webb Space Telescope (JWST) captured its first glimpse of the early universe, astronomers have been surprised by the presence of what appear to be more “ultramassive” galaxies than expected. Based on the most widely accepted cosmological model, they should not have been able to evolve until much later in the history of the universe, giving rise to claims that the model needed to be changed.
This would overturn decades of established science.
“The development of objects in the universe is hierarchical. You start small and get bigger and bigger,” said Julian Muñoz, assistant professor of astronomy at the University of Texas at Austin and co-author of a paper recently published in Physical Examination Letters which tests changes to the cosmological model. The study concludes that there is no need to revise the standard cosmological model. However, astronomers may need to revisit what they understand about the formation and evolution of early galaxies.
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 called the Lambda Cold Dark Matter Model (ΛCDM) or the “standard cosmological model”. Although the model is very well informed, much of the early universe has remained theoretical because astronomers have not been able to observe it completely, if at all.
Launched in 1990, the Hubble Space Telescope played a central role in the development and refinement of the Standard Cosmological Model. It observes the universe in ultraviolet, visible, and some near-infrared wavelengths of light. However, it allows you to see some things better than others. For example, Hubble is well equipped to observe smaller galaxies that often contain larger populations of young ultraviolet-emitting stars and less dust that tends to absorb shorter wavelengths.
Launched at the end of 2021, JWST provides an important complement to Hubble’s capabilities. By observing in near- and mid-infrared wavelengths, JWST can detect objects invisible to Hubble.
“We open a window into the unknown,” Muñoz said. “We are now able to test our theories about the universe where we could not before.”
Infrared view of the universe captured by the James Webb Space Telescope. Credit: NASA, ESA, CSA and STScI.
Shortly after the Big Bang, things were not perfectly uniform. Tiny variations in density have had a huge impact on the future structure and evolution of the universe. Denser regions attracted more material due to gravity, ultimately leading to the formation of larger and larger structures.
Growing so large so quickly, the ultramassive galaxies observed by JWST would in theory only be possible if more of these higher-density regions had developed just after the Big Bang. This would require changing the standard cosmological model.
Muñoz and his team tested this hypothesis.
They chose a cosmic time range for which JWST and Hubble observations are available. Within this range, they identified the most massive galaxies available in the JWST data and calculated the magnitude of change in the initial density of the universe that would be necessary for their formation.
They also calculated how many smaller galaxies would result from this hypothetical change. These additional smaller galaxies would have been observed by Hubble.
“But that’s not what we’re seeing,” Muñoz said. “One cannot change the cosmology enough to explain this abundance problem, since Hubble observations would also be affected.”
So why does JWST find so many ultramassive galaxies? One possibility is that they contain supermassive black holes. These black holes would heat nearby gas, making galaxies brighter and therefore more massive than they really are. Or, galaxies may not be found in the early universe at all, but they appear to be because dust makes their color redder than it would otherwise be. This shift would make the galaxies appear farther away than they are.
In addition to Muñoz, the study’s authors are Nashwan Sabti and Marc Kamionkowski of Johns Hopkins University.
More information:
Nashwan Sabti et al, HST Insights into Ultramassive Galaxies and the Cosmology of the Early Universe, Physical Examination Letters (2024). DOI: 10.1103/PhysRevLett.132.061002. On arXiv: DOI: 10.48550/arxiv.2305.07049
Provided by the University of Texas at Austin
Quote: Discovery of unexpected ultramassive galaxies may not rewrite cosmology, but still leaves questions (February 13, 2024) retrieved February 14, 2024 from
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