Are galaxies at the start of the universe too bright?

Does this change what we know about the universe?
17 October 2023

Interview with 

Guochao Sun, Northwestern University


Big Bang


The James Webb Space Telescope was launched with three main directives in mind: To see light from the early universe, to study the formation of new galaxies and cosmic bodies, and to look for signs of life on other planets. So, in the past year or so, what has the James Webb Space Telescope been seeing in regards to these three things?

Let’s begin with a mystery surrounding the very start of our universe.

Will - The James Webb telescope is powerful, so powerful. It can look across the cosmos and see the light produced from the very earliest stars. You can age galaxies by looking at the photons particles of light that these galaxies are producing when stars. And therefore the galaxies that they form in are young. They produce photons with shorter wavelengths, which appear blue. So by measuring the ratio of short to long wavelength, photons being emitted by a galaxy, you can measure its age. Young galaxies aren't as bright, nor are they as massive as older galaxies as these things increase over time. But when Guochao Sun and his team at Northwestern University took a look at some of these earliest galaxies, they spotted a problem.

Guoshao Son - One of the biggest puzzles was that we found too many bright galaxies in those very early epochs. If you are thinking about a normal galaxy or typical galaxy, there's like a pretty general mapping or correspondence between how bright they are and how massive they are. So if we take that general mapping that we build, then for those very bright galaxies, we would infer a pretty high mass. So that ends up being in trouble with some of the very fundamental knowledge we have about the early universe.

Will - Some early galaxies are too bright, seemingly too massive to have formed in the short period of time that the universe had existed. Now, people studying the universe adhere to the standard cosmological model. The notion that the universe started with a big bang, went through a period of exponential inflation and has been expanding ever since. But if galaxies can become this bright this quick, perhaps our understanding of the universe is wrong. Well, before we rip apart the fabric of reality, let's look for alternative explanations. And unfortunately for this program, early blame was pointed at the James Webb Space Telescope's equipment

Guoshao Son - In terms of whether the exposure is saturated or something like that. I think astronomers, they of course did some very careful job making sure this is not something that will contaminate or bias their measurement. But, on a related note, there was indeed some calibration issue in the early analysis of the data. So that of course had led to some systematic offset in terms of the brightness of any other things that were determined from the brightness. But that has been fixed. And the current data that we analyse and compare with models, they have been corrected for this issue. So we think we are immune to those contamination or systematic uncertainties.

Will - Thank goodness, then. Our technical marvel is not responsible. So is it then a fundamental shift in our knowledge of the universe? Well, Guochao and his team run some simulations and have a different theory.

Guoshao Son - So I think the key factor here that is like central to our study is that in order for those like early galaxies to be bright, they can either be really massive or there's actually an alternative way of them to be become very bright, which is that they can form stars in a highly like time variable manner, which means that those early galaxies, they don't need to form their stars in a pretty like regular or steady rate. The rate of star formation can actually fluctuate very crazily over time. So you can end up with a very strong recent burst of star formation, meaning that you form a lot of stars than you would normally expect in a fairly short amount of time. And that will temporarily boost the brightness of the galaxies without making the galaxy itself being too massive or forming too many stars.

Will - So these periods of unusually high star production dubbed bursty star formations are instead what might be responsible for such unusual brightness at the beginning of the universe. Rest easy. Everyone. The universe is still very much as we believe it to be.



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