We’re all familiar with the sound of a car passing by. This distinctive drop in pitch is known as the doppler effect and we notice it the most with sounds. But it also affects light in the exact same way. If light moved slow enough, we would also see the car change color as it passed us by. The reality is, light moves far too fast for us to notice anything like this.

But take a look at this image of Galaxy GS-Z13 taken by the James Webb telescope, the most distant galaxy ever imaged. It should look white – but its color has been changed all the way to red. In fact, the light from this galaxy has been changed so much that it isn’t even visible to the human eye – and only James Webb can see it. This is the doppler effect in action, or for light, it’s known as redshift.

In this article, we’re going to look at this newly discovered galaxy and how we are able to see almost right back to the start of our universe.

Most distant galaxy we’ve ever seen

Although it may not look like much, this tiny red blob is the most distant thing anyone has ever seen. With this picture, we are looking back 13.6 billion years into the past, when the universe was only 2% of its current age. But the galaxy itself is over 33 billion light-years away – how does this make sense? 

We know that a light-year is the distance light travels in a year. So if it took the light 13 billion years to reach us, it should be 13 billion light-years away. Well it’s not quite as simple as that.

In the time that light has spent traveling towards us, space itself has been constantly expanding – and the galaxy has been moving further away from us. Because of this, the galaxy in 2023 is now over 33 billion light-years away, but its light from 13 billion light-years away has only just reached us. But how can we possibly measure this distance? The answer lies within the color of the galaxy.

All of the light we see is just a small fraction of the light that actually exists. Every form of light can be thought of as a wave, and the length of that wave can be smaller or longer. This is what determines the colors that we see.

But our eyes are very limited – and so there is much more light bouncing around us that we simply can’t see. So when we discover a galaxy that has changed color, something must have happened to it. 

The Dopppler Effect

Just like the moving car, the light from this galaxy is being changed or redshifted. But there are two kinds of redshift: Doppler redshift and cosmological redshift. In the doppler scenario, the galaxy is moving through space. But with cosmological redshift, the galaxy is both moving and stationary at the same time. We’ll get to that one later.

First, let’s imagine the galaxy as if it was stationary. The light leaves the galaxy in every direction at a constant speed. When the galaxy moves through space, the waves at the front get slightly compressed and the waves at the back get stretched out. Since it’s traveling away from us, we are receiving the stretched light, which has a longer wavelength and therefore a different color. But why is the galaxy moving in the first place? 

The planets in our solar system have movement, our solar system has movement and our galaxy itself is also moving through space. But this isn’t enough to see a dramatic effect in redshift since this relative movement is nowhere near the speed of light. But remember that this galaxy has gone from 13 billion light-years to over 33 billion light-years away – a distance only possible if it was going considerably faster than the speed of light. 

The expanding universe

Since the big bang nearly 14 billion years ago, our universe has been constantly expanding. But understanding what this actually means is pretty difficult. We imagine the big bang as an explosion that radiates out from a central point. But in reality, there is no center of the universe and no single point where space expands from. If you take any point in the universe, everything will appear to be expanding away from it at the same rate.

It’s not that planets, stars or galaxies are getting bigger – but the space in between those galaxies is getting bigger. The rate of this expansion is around 70 km/s for every 3 million light-years of distance. So an object at that distance will be 70 km further away after a second. An object 3 million light-years further down the line will be 140 km further away, and so on… 

If we extrapolate that out to galaxy GS-Z13, it will be moving away from us at around 700,000 km/s. But we’re told that objects can’t move faster than the speed of light. But that is only true for objects moving through space. This is where cosmological redshift comes in.

How our universe expands 

If we look at an unbaked cookie, we have a random scattering of chocolate chips, which we can think of as galaxies. When the cookie gets baked, the dough expands and the chips end up further away from each other. But it’s not that the chips have moved relative to the dough, but the dough in between the chips has expanded, just like our universe.

The galaxies in our universe can’t move through space faster than the speed of light, but there is no limit to how fast the universe can expand. This means that the light currently being emitted from this galaxy will never reach us, since the expansion of the universe at this distance is outracing the speed of light. 

But how do we know all of this? How do we know how far away this galaxy is? Well, although our planets and galaxies don’t grow with the expansion of space, light waves do. As the light from the galaxy has been traveling through space, the expansion of space itself has stretched this wave of light to a much longer wavelength. And so by the time the light reached James Webb, it had been stretched well outside of our normal viewing range.

But the James Webb telescope was designed to view light in the infrared range, so it was just capable of picking this light up. The trick to measuring how old and distant the light is lies within measuring how much that light has been shifted. So James Webb looked for Galaxies whose light had been shifted the most.

The Ultra Deep Field

It first focused on a tiny area of our night sky called the Ultra Deep Field. This area is the equivalent of a coin placed 18 meters away – but it features over 100,000 galaxies, many of which have gone through a large amount of redshift. But the problem is that James Webb’s time is precious and it would take forever to survey every little dot in the sky. So in order to choose which galaxies to study, it used something known as the Lyman technique. 

Light with a wavelength below 90 nanometers gets completely absorbed by hydrogen. This shows up as a dramatic drop-off at around here on the spectrum – so below this, no light should reach us. For light that has been redshifted, that drop off point will appear much further up the spectrum.

And so, by doing a relatively quick spectroscopy measurement, Webb found 4 galaxies that all had a drop off point that had been shifted all the way into the mid infrared range. One of these was Galaxy GS-Z13. This galaxy had the highest amount of redshift that scientists had ever seen – but calculating the exact redshift amount is what allows us to find out how old and distant this galaxy is.

We can use the spectrum of light as a reference point. Light gets absorbed by various elements at different wavelengths, creating a signature on the spectrum, a recognizable pattern that will be the same for all light. As the light from the Galaxy gets redshifted, that signature will also be shifted by the same amount. 

And so with the light from galaxy GS-Z13, we should see this signature appear further up the spectrum. By taking specific points and measuring how much they have been shifted, divided by the original wavelength, we get an exact redshift value. This alone doesn’t tell us much, but after some complex calculations that are too long for this video, we can arrive at the travel time. This is how long the light we see has been traveling through space to get to us. We can calculate this because we already know some important characteristics about the universe.

We know that the universe is expanding and the rate at which it expands. And although this rate is constant, galaxies actually accelerate away from us. This is because the rate at which Galaxies move away from us changes over distance and not time.

When galaxy GS-Z13 was much closer to us, there was less space in between us, therefore less space to expand. Now, the space between us is much larger, and so there is much more expansion going on. And so, by understanding how this expansion changes over time, we can calculate the light from this galaxy to be around 13.6 billion years old.

What is most shocking about all of this is just how much we can’t see and will never see. We talk about if there is life on Mars, but looking at the sheer scale of the universe that we can’t even see, it seems completely inevitable that life exists, somewhere out there.