What was the universe like at the cosmic dawn?
That poetic phrase is what astronomers call the time just a few hundred million years after the big bang when the very first stars switched on, flooding the cosmos with light.
To see this era, we’d need a time machine, and, astoundingly, we have one. We have many, actually: telescopes. Light travels very quickly—at the speed of a billion kilometers per hour!—but still, galaxies are so far away that it takes their light millions or even billions of years to reach us. The farther they are from Earth, the longer it takes for their light to reach us. So, in a sense, when we glimpse their far-flung photons, we see backward through time to observe these ancient galaxies as they were long ago.
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The light from galaxies at the cosmic dawn has been traveling for more than 13 billion years. Attenuated across the vast distance—which has grown all the while as the universe expands—that light arrives to us not only exceedingly faint but also very redshifted, meaning that its once-optical wavelengths have stretched out into the infrared portion of the electromagnetic spectrum. Gathering enough of it to do cutting-edge science—let alone make pretty pictures—requires a huge telescope that is keenly sensitive to any infrared glow.
This is precisely why astronomers built the James Webb Space Telescope (JWST). With its huge 6.5-meter segmented mirror and multiple infrared-attuned instruments, it can see galaxies as they were not too long after the first stars were born and possibly maybe even all the way back to the cosmic dawn.
To push the limits of JWST’s capabilities, astronomers designed a special observing program called the JWST Advanced Deep Extragalactic Survey, or JADES. It looks at a very small area in the sky for long periods of time to get “deep-field” images of the vastly remote objects across the universe from Earth. And now researchers are leveraging the success of that project in JWST’s first year in space. A carefully selected subsection of that area is now the target of the JADES Origins Field, which in the telescope’s current sophomore year will push the observations even deeper into the universe, hopefully taking the measure of galaxies closer to the cosmic dawn.
The program has already returned riches from those almost unfathomable depths. And you can see and explore them for yourself, all from the cozy comfort of your desk (or anywhere else you might use your smartphone)! A mosaic of the observations so far is available online. When first displayed, it looks like a patchy, irregular black field dotted with smudges. But if you zoom in and in and in, you’ll find that the brightest objects are glorious galaxies: many of them are recognizable as spirals, but most, by far, are more irregular in shape. A similar galactic bestiary populates the magnificent Hubble Deep Fields, earlier images from the Hubble Space Telescope that revealed smaller “fragments” of galaxies apparently in the act of merging to form the larger galaxies we see today.
A scattering of stars in our own galaxy can be distinguished here and there in the JADES field. Each of them stands out from objects far more distant via its very sharp, pointlike appearance, as well as the presence of diffraction spikes—three pairs of lines that radiate away from a star’s center and are caused by the bending of light around the hexagonal one-meter mirror segments that make up JWST’s primary mirror. Close inspection also shows a faint horizontal line bisecting each Milky Way star, which is caused by light bending around a support strut of the telescope’s secondary mirror.
If you look very carefully, however, you’ll even find some far-distant fuzzy objects with very bright cores that also display diffraction spikes, such as object 169604. These objects are likely active galaxies, each harboring a central supermassive black hole that is eagerly—and sloppily—eating huge amounts of gas and dust. This material heats up viciously and blasts out radiation as it spirals to its doom at a hefty fraction of the speed of light before finally vanishing across the black hole’s event horizon—the ultimate point of no return.
In fact, one hope for the JADES Origins Field is to find distant galaxies where these gargantuan black holes are just getting their start; one of the biggest mysteries in cosmology is how these black holes grow to be behemoths with a billion times the sun’s mass in less than a billion years.
Many of the astronomical objects in this mosaic can be identified by their color, that is, how bright they appear at different wavelengths. The Near-Infrared Camera, JWST’s workhorse instrument, has filters that select for different wavelengths, several of which are used by JADES. You can switch the display between filters by clicking the top “layers” icon in the upper right corner of the mosaic’s webpage to see how appearances change when you change what flavor of light is observed. The distribution of light across colors is called the spectral energy distribution, or SED, and the bottom “layers” icon has an option to show this. Astronomers use the SED to determine what kind of object they’re seeing and, for galaxies, the approximate distance.
Poking around the image I found plenty of odd objects to ogle. For example, object 196582 is clearly some sort of galaxy that is likely more than seven billion light-years away. But it has a fuzzy, oval-shaped arc stretching above it. Sometimes the gravity of a massive galaxy can bend the light from more distant background galaxies, warping them into unusual shapes in a process called gravitational lensing. But those arcs are usually sharply defined and thin, and this is clearly not. Another possibility is a galactic disruption: galaxies sometimes pass close to each other as they fly through space, distorting and twisting from their gravitational interplay and pulling out streamers of stars like cosmic strands of stretched taffy. If one galaxy is much smaller than the other, it can be totally torn apart. That could be what we’re seeing here.
There are also less extreme encounters. Objects 171522 and 171523 are a pair of interacting galaxies that seem to be roughly equal in size, and each is distorting the other. After enough time, it’s possible this duo will physically merge, becoming a single, larger galaxy. Many such examples of galaxy collisions can be found in the mosaic, adding to our understanding of how galaxies interacted and grew in the universe’s early days.
The most amazing thing about this image probably isn’t any specific object but rather the sheer number of objects that JWST captures in the entire JADES field. Astronomers have counted on the order of 100,000 galaxies there, yet it covers only a tiny fraction of the sky, appearing about the same size as would a piece of very fine gravel on your fingertip held out at arm’s length. Extrapolated across the entire sky, this means that the heavens above hold hundreds of billions of galaxies, which is comparable to previous estimates; the depths of the JADES Origins Field will reveal greater numbers of them still, giving us more to study and more precise measurements of them as well.
It’s a powerful reinforcement of a staggering fact: the universe is positively brimming with galaxies. Our Milky Way is just one of them, and were we to look at it with JWST from 10 billion light-years away, it would be just another among the teeming throng, like those in the JADES field. It’s humbling, certainly, but for me, it’s also a source of joy and even pride: we can grasp this fact. Through science, driven by our ever-hungry curiosity and ever more powerful technology, we can know our true place in the universe—and even bring its most distant depths to our fingertips.
But don’t take my word for it. Find out for yourself by diving into the JADES treasure trove, teeming with galaxies at the cusp of cosmic dawn.