JWST helped scientists solve a cosmic mystery!

Scientists solved a HUGE mystery about the early universe that has been puzzling scientists since JWST’s first observations. Basically, scientists observed that early galaxies were much larger and more luminous than expected, and they questioned everything, from what they knew about star formation to theories about the early universe

But now, scientists think they may have an answer that doesn’t break the standard model of cosmology. Let’s dive into why JWST is so good at observing the early universe, what the early universe looked like, the crisis in cosmology surrounding these large early galaxies, and what these scientists uncovered to solve the mystery — as well as the questions we still have.

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Why JWST is so good at the early universe

JWST is incredibly good at observing the early universe because that’s what it was designed for. It’s an infrared optimized telescope, while Hubble is optimized for visible and UV light (though it does have the capability to “see” in some near-infrared wavelengths). Infrared is important because as light has traveled through the universe as the universe is expanding, the wavelength has stretched. It’s shifted to the longer, or red, end of the spectrum — what you may know as redshift.

That means to see the earliest light of the universe, the light of the first stars and galaxies, we can’t just be observing the universe in visible light because the wavelength has stretched into the infrared. JWST is optimized for near- and mid-infrared observation with NIRCam and MIRI, the mid-infrared instrument. And it’s VERY good at observing these wavelengths of light

Remember that looking through a telescope is like looking back in time because of the time this light takes to reach us. So when we look really far into the universe, we’re observing how the universe was in the distant past.

With previous telescopes, scientists could observe early galaxies. Hubble allowed observation of UV dominant early galaxies, but wasn’t sensitive enough in the infrared wavelengths to pick up IR ones. Spitzer, a now-retired infrared-optimized telescope, was able to detect some of this light but because it wasn’t nearly as sensitive as JWST, scientists had trouble distinguishing light sources and things kind of blurred together.

But JWST has blown all expectations out of the water. It has shown unprecedented ability to focus light from distant targets (what’s called a point-spread function) — it’s actually twice as sharp as design requirements, so it’s the best telescope we’ve ever had for this purpose and it’s why JWST has been revolutionizing our understanding of the early universe since its deployment.

What we think we know about the early universe

So, then, what’s this mystery? Well, when scientists first started observing the universe with JWST, they immediately started uncovering the most distant galaxies we’ve ever seen. And many of them were big. Much bigger than was theorized by our understanding of the early universe an Lambda CDM, or the Standard Model of Cosmology.

The issue was that these galaxies were bright. Too bright. And if all of that luminous matter came from stars, then it was clear we seriously had misunderstood star formation in the early universe.

Here’s what we think we know about the early universe. After the Big Bang, the universe was incredibly hot — a thick, hot soup of subatomic particles. As the universe began to cool down, the particles began to combine into the first ionized atoms. This happened during the era of recombination, around 240,000 to 300,000 years after the Big Bang, and this is really the earliest point of light we can observe, because before that the universe was opaque and light couldn’t freely travel through it.

This is when CMB, or cosmic microwave background radiation, formed, and this is the first light of the universe, but then came the dark ages. This was the universe after CMB and the era of recombination but before the first stars and galaxies formed. We think these began forming at around 400 million years after the Big Bang.

The first stars in the universe formed out of clumps of hydrogen gas. The clumps basically grew and grew until they were dense enough to collapse and form the first stars. These were likely large, massive stars, so they were gravitationally attracted to one another to form the first star clusters, then to form the first galaxies, then galaxy clusters.

The problem: Bright and massive early galaxies

Scientists use galaxy brightness as a proxy to estimate their mass and number of stars, and these galaxies were way too massive and had too many stars. So then, the theory goes that the first galaxies would be filled with large, massive stars, but they’d be relatively small and dim because it would take time for galaxies to accumulate enough matter to grow larger and brighter. But that’s not what scientists were finding.

In February of 2023, scientists pinpointed six different galaxies that formed in the universe’s first 700 million years. All six were up to 100 times more massive, according to the amount of stars they must have contained, than our current theories predict. They were so massive, in fact, that the six galaxies had more mass among them than should have been available in the universe at that time. Something was seriously wrong, either with the observations, or our theories, or our assumptions.

The answer: That’s no star

We’ve been chipping away at this mystery, finding different answers for this problem. One is starburst — basically stars aren’t forming at an even rate but all at once, which makes a galaxy brighter for a period of time. Another was refining our simulations to have higher resolution in order to predict the pattern and rate of star formation in the early galaxies. Now this new study helped us unlock another piece of the puzzle.

The answer came courtesy of a study in the Astronomical Journal led by UT Austin graduate student Katherine Chworowski. Basically, the answer to these galaxies having too many stars, and therefore being too bright and too massive is — it’s not because of stars. It’s because of black holes.

JWST has already shown us that massive black holes existed in the early universe far earlier than we thought. It was basically the same problem — it takes time for black holes to gather enough matter to form. And it takes time for them to consume enough matter to become big. The question was could they form large with enough collapsing matter to just create a massive black hole at the beginning, or were these the product of stellar-sized black holes merging and growing more rapidly than we thought? Evidence has pointed to the former — basically giant clouds collapsing into large black holes in the early universe.

And this appears to be the answer to the massive early galaxy problem as well. The research team looked at galaxies from the CEERS, or Cosmic Evolution Early Release Science, survey, which targets a sliver of the sky to provide parallel observations from multiple instruments, supported by data from a similar CANDELS survey with Hubble. This helps scientists “see” distant redshifted galaxies and pinpoint distinct sources of light from distant objects.

The team went through the CEERS data looking for redshifted galaxies, focusing on those that were at z > 4 (z corresponds to the fractional change in wavelength and is number value for redshift). For reference, a z value of about 5 corresponds to light emitted about a million years after the Big Bang. A z value of 9 is light emitted when the universe was about .55 billion years old.

What they found, basically, is that many galaxies that appears way too bright and were puzzling scientists actually had accreting massive black holes within them. We can’t directly detect black holes, but when they start consuming matter at a high rate, they begin to gather gas and dust around them — called an accretion disk outside the event horizon of a black hole. As black holes consume matter, the friction in this gas and dust is incredibly luminous, which is what we’re detecting. When the scientists removed these galaxies with black holes from their data, the rest conformed to the expectations of the Standard Model.

This doesn’t completely solve the mystery of the early universe, though. We still need to figure out why there are so many more massive black holes than we expect. And this study doesn’t explain why there are more galaxies than expected either. Those are still mysteries we need to solve, and hopefully JWST will operate for a long time and continue to give us more insight into the early universe.

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