For almost 100 years, astronomers have known that the universe is expanding. Galaxies are moving away from us, and the further away a galaxy is, the faster it's receding. This fact led cosmologists to devise the Big Bang theory; if you rewind the current expansion of the universe back in time, eventually you get to a stage where everything was condensed into a single point — the singularity — roughly 13.8 billion years ago.
Because it takes time for light to travel through space, the further away we look, the further back in time we are looking, too. Thanks to the help of the now-operational James Webb Space Telescope (JWST), astronomers are now able to look back farther than ever before.
NASA Discovers 6 Galaxies
Using data from the JWST's infrared instruments, astronomers have spotted what appear to be six massive galaxies from the universe's infancy, according to a study published in Nature in February. These colossal cosmic entities, if confirmed, could reshape how we think about the origins of our universe.
Mike Boylan-Kolchin, an astronomer from the University of Texas at Austin, says that scientists now think we're seeing galaxies from as early as 13.48 billion years ago — just 320 million years after the Big Bang.
Read More: Did the Big Bang Happen More Than Once?
“Of course, the stars in these galaxies took some time to form, so these galaxies started to form even earlier in the history of the universe," he says. "This, coupled with the power of JWST, leads us to expect that we can see galaxies from even earlier times, maybe back to 13.55 billion years ago or so [or 250 million years after the big bang].”
Looking at Ancient Galaxies in the Universe
When astronomers look back at ancient galaxies, they aren’t directly measuring their ages. Instead, they're measuring what's called a "redshift." This refers to the wavelength of cosmic light that's being stretched on its journey to Earth; if it hasn’t been stretched at all, it has a redshift of 0, but if it has been stretched to double its original wavelength, it has a redshift of 1.
Redshift Stretch Factor
“The key point that connects this to the expansion of the universe is that the stretch factor — the redshift — is directly proportional to how much the universe has expanded between when the light was emitted and when it was observed," says Boylan-Kolchin. "This is why redshift measurements are so crucial: they give us information about how much the universe has expanded since the light was emitted, and with a cosmological model, we can convert this expansion into a time [or a distance].”
JWST Images Upend Models of the Universe
What's interesting, though, is that some of the observations made by the JWST don’t quite fit in with how cosmologists have modeled the evolution of galaxies in the early universe.
These observations pertain galaxies at the redshift of z~7-10, meaning they’re not the most distant galaxies that we can observe, the ones that we’re seeing as they were around 320 million years after the Big Bang. (Those are at z = 13.) These galaxies, meanwhile, are from a little bit later — about 500 to 700 million years after the big bang.
“The remarkable feature of these galaxy candidates is that even though they’re only a factor of 2 or later in the age of the Universe, they’re seeming to be 100 or 1000 times more massive than the very earliest galaxies," says Boylan-Kolchin. "And that’s really the crux of the issue: We can predict, given our current models of cosmology, the theoretical upper limit to how massive galaxies can be at around 700 million years after the Big Bang. And these galaxy candidates are right at that limit,” he says.
Impact on Formation Theories
Sure, they haven’t exceeded the limit, but according to Boylan-Kolchin, the theoretical upper limit is implausible for real galaxies. Astronomers would expect most galaxies to be 10 times less massive than the upper limit on average, which is what they see for the younger galaxies with higher redshift.
But if astronomers continue to identify redshift z~7-10 galaxies with masses at the upper limit of their models, it will require a new understanding of how these galaxies form — or a revised cosmological model for how they got so big in the first place.
Explanations Behind Massive Galaxies
Boylan-Kolchin explains that there are some possible explanations for what astronomers are seeing.
It could be that some of the light from the massive galaxy candidates isn’t actually coming from stars at all, but rather is coming from accretion — the accumulation of particles into a massive object thanks to gravitational forces — into supermassive black holes. This would mean astronomers are overestimating how massive these galaxies are.
Conversion of Light
Alternatively, it could be that the conversion of light to stellar mass is incorrect: meaning, the light of these galaxies is dominated by the most massive stars, but their total mass is dominated by lower-mass stars, because massive stars are so rare. If astronomers are not making this conversion correctly, it could mean that we are once again overestimating how massive these galaxies are.
Efficient Galaxy Formation
It could also be a possibility that very early in our cosmic history, galaxy formation was extremely efficient, which would have made it possible to convert much of the available mass into stars in short time periods. This would require a revision to our current models of galaxy formation, and cosmologists are already working on this.
“If these galaxies are confirmed and/or more similar systems start showing up in forthcoming JWST observations, it would make a strong case that we need to evaluate ways to modify the cosmological model,” says Boylan-Kolchin.
JWST Changes Views of the Universe
What should we expect to learn from the James Webb Space Telescope? As more data rolls in from the JWST over the next year, Boylan-Kolchin thinks astronomers will get a better idea of whether there actually is a crack in their model, or if they can explain their observations with some of the ideas mentioned earlier.
It's certainly an exciting time to be interested in our cosmic origins.