Not long after the James Webb Space Telescope (JWST) began its scientific operations, astronomers announced that they had discovered galaxies in the early universe that were too large, bright and full of stars for their age. While headlines around the world claimed that these galaxies were “shattering” our understanding of the Big Bang, the truth is much more nuanced – and much more interesting.
The Big Bang theory is our overall picture of the history of the universe, beginning in its deep past, when the cosmos was much smaller, hotter, and denser than it is today. This model, originally developed in the early 20th century, has survived a series of observational tests and is remarkably good at explaining a variety of cosmological observations, including the drift of light from distant galaxies, the appearance of residual radiation in the form of the cosmos. the microwave background, the abundance of light elements, and the evolution of galaxies and larger structures.
While the Big Bang theory cannot say with certainty which galaxies will appear, it can speak to probabilities. For example, cosmologists can say approximately how many small galaxies, how many medium-sized galaxies, and how many large galaxies should appear in a given volume at a given age of the universe. But until JWST, we didn’t have direct observational access to the earliest stages of galactic evolution—something the telescope was explicitly designed to study.
In 2022, astronomers announced that they had found extremely distant galaxies that were surprisingly, surprisingly large. They had measured red shift of galaxies to be over 16, implying that these galaxies existed only 200 million to 250 million years after the Big Bang. However, they were giants and appeared to be fully formed, spiral arms and all.
Connected: No, the Big Bang theory is not ‘broken’. Here’s how we know.
These galaxies seemed far outside the expectations of the Big Bang theory; they were like finding teenagers in a kindergarten class. So what was going on?
Bend cosmology
Follow the brazen headlines proclaiming the death of the Big Bang theory. But those stories left out a crucial detail: astronomers estimated the redshift of those galaxies through a technique known as photometry, which is incredibly uncertain. A full assessment of the ability of these galaxies to “break” the cosmology would have to wait for a more precise measurement of their redshift, and hence their age.
When these more accurate measurements finally came in a few months later, those galaxies went from record-shattering to just… normal galaxies. For example, the redshift of one galaxy was revised from over 16 to just 4.9, moving its age from 240 million years after the Big Bang to over a billion years. This is more than enough time for the normal Big Bang theory to explain their sizes and shapes.
But along with those less exciting revisions came several new confirmed redshifts of other galaxies, including JADES-GS-z14-0, the current most distant known galaxy, with a redshift of 14.32. This galaxy was alive and well when the cosmos was only 290 million years old.
Astronomers fully expected galaxies to exist 290 million years after the Big Bang; that’s why they built JWST. And as galaxies go, JADES-GS-z14-0 is definitely a young one — it’s only 1,600 light-years across, compared to the Milky Way’s 100,000 light-years. But interestingly, the galaxy is quite bright and full of stars – not enough to break cosmology, but enough to open up some questions about the origin and development of the first galaxies that appeared in the universe.
Construction of cosmology
It is quite possible that the Big Bang theory is wrong; scientists must maintain the mental discipline to accept the possibility. But with such a wealth of evidence behind it, the Big Bang is unlikely to be undone by a single observation. And it’s worth repeating that JWST is doing exactly what we designed and built it to do: answer some major ongoing questions about how the first stars and galaxies formed.
It is entirely possible that cosmologists will be able to explain the appearance of galaxies like JADES-GS-z14-0 within the framework of the Big Bang without having to make any major revisions. For example, large black holes may have appeared before these galaxies did, and their superpowered gravitational pull may have triggered bright bursts of star formation. Or maybe supernova feedback and other mechanisms caused the first galaxies to be richer in stars than today’s galaxies, making those early galaxies appear powerful despite their small size.
Or perhaps our initial observations are biased toward these small but bright outliers, and further surveys will reveal larger populations of more common galaxies, thereby reducing the tension with models of galaxy formation.
And finally, maybe we need to add some new ingredients to the universe, like allowing dark energy to evolve over time, to produce these types of galaxies at such early times.
This is exciting enough on its own, without the need to overturn the Big Bang as we know it. There are more than enough mysteries and hidden corners within the universe to keep astronomers awake at night wondering about the possibilities—and in the morning continuing to work on how to solve them.