If you’ve ever looked over at your shockingly productive colleague and said, “how do you find the time?”, you know how cosmologists are currently feeling about the early Universe.
Since it first started sending back science data in mid-2022, the internationally funded, state-of-the-art James Webb Space Telescope (JWST) has been giving us images of distant galaxies that appear to have formed and matured far earlier than our models predicted. It’s enough of a problem that some are calling it a challenge to our entire cosmic timeline.
Researchers have likened the situation to flipping through someone’s old photo album, expecting to find baby pictures, and seeing a full-grown adult instead. With a person, you might just conclude that they’re older than you thought, but with early galaxies, you very quickly run into a problem with the age of the Universe itself.
JWST is looking at galaxies that are so distant that their light has taken more than 13 billion years to reach us. Based on what we think we know about these galaxies, and what we think we know about the age of the Universe, it looks like there hasn’t been enough time since the Big Bang for massive galaxies to have formed.
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Headlines have circulated calling this a crisis for cosmology, a threat to the Big Bang theory, or support for speculative hypotheses about other cosmic mysteries, like dark matter. But before we throw out all our cosmology textbooks, let’s dig a little deeper into the data.
As amazing as JWST is, when it comes to the very earliest galaxies, the information it provides is not completely straightforward. While it has shown us spectacularly breathtaking views of nearby nebulae, star clusters, and galaxies, its images of the most distant galaxies look, in general, like fuzzy little dots.
They do have some discernable shape, but we’re not exactly seeing spiral arms and dust lanes here. If we’re lucky, we might see enough extended emission to label a bit of it ‘haze’ in the publication figure.
For the most part, the useful information we get from these images is actually from the spectrum of the light – how much light is arriving at different colors (or wavelengths).
There are two ways that JWST can examine a source’s light. It can take a spectrum by spreading out the light with a spectrograph (which works a bit like a prism) and examining the brightness at each color, or it can use filters that block all but a select range of colors.
The latter is a photometric measurement, and is essentially a very low-resolution spectrum. In both cases, to determine properties like the total mass of a galaxy’s stars, their ages, or their chemical abundances, we compare the data to simulations of the spectrum we expect for a galaxy with those properties.
These measurements are also how we determine the galaxy’s redshift, which tells us what moment in the Universe’s history we’re looking at by telling us how much the light has been stretched by cosmic expansion. The galaxies we’ve seen with the highest redshift values, greater than 10, are sending us their light from within the first 400 million years after the Big Bang.
It's here that we run into a problem. Based on model spectrum comparisons, many of these galaxies seem to have too many stars, or stars that are too old, for the time in which they lived. But there are several ways we could be mistaken – some observational, and some theoretical.
On the observational side, photometric measurements can sometimes be inaccurate; a few apparently-high-redshift galaxies turned out to be much closer when we took spectra. There have also been telescope calibration issues (likely all settled now).
And then there’s the fact that we’re only seeing very small patches of the sky: we could have stumbled onto a clump of galaxies not representative of the norm.
On the theory side, there’s even more uncertainty. Our models of galaxy spectra are based on much closer galaxies. What if early galaxies had different populations of stars (more massive stars and fewer small ones, for instance)?
What if star formation happened more rapidly in the past due to different physical conditions, or varied substantially over time? We’re already seeing hints that our models need adjusting based on weird balances of chemicals in the spectra.
The most exciting conclusion, of course, would be that those galaxies really are super massive and couldn’t have formed in the time allotted, meaning we have to completely rethink cosmic evolution.
But the more conservative position is that both the theory and observations are too uncertain for solid conclusions just yet. Perhaps when we figure out which knobs to turn in our galaxy formation models, we’ll find new insights into the formation of structure in the Universe, if not new fundamental physics.
Based on what we know now, it seems plausible that the Universe formed its galaxies as soon as it could, while still in keeping with physical feasibility. Just like your amazingly productive officemate, what looks utterly impossible to us might be less about altering physical laws and more about using the time we have incredibly efficiently.
Read more about the James Webb Space Telescope: