How far away is that oldest object really?

This also means that the Milky Way is 13.4 billion light years away. Or is it really? That column reported that “astronomers speculate”. [GLASS-z13] Now it’s actually about 33 billion li from us.”

How do they know this?

Try this thought experiment. I’m sitting in front of you, blowing a louder and louder note on a whistle every second. You will hear the note every second. No drama there. Now suppose I am actually sitting in a plane. I keep whistling every second, but after my first whistle note in front of you, the plane moves away from you at the speed of sound, which is about 1,250 kmph or 350 meters per second. How often will you hear the whistle? (Assume for this experiment that you can actually hear it.)

OK, the second time I fly it after a second, but until then, I’m 350 meters away from you. It will take a second for that sound to reach you – so you hear my second note two seconds after the first. You can say that the frequency of the notes is halved, from one every second to one every two seconds. Similarly, if the plane was flying towards you, the frequency would double.

This is a simple way to think about the famous Doppler effect. Sound travels in waves. If the sound source is moving away from you, those waves get longer, their frequency decreases, and the sound becomes shorter. Conversely, if it is moving towards you. The classic example familiar to all of us is the horn of a passing train. Its pitch decreases as it moves away from us. In fact, my whistle from a moving plane will also be lower in pitch than the one that sounded before the plane took off.

What does all this have to do with the GLASS-z13 and how far is it from us? As I mentioned in my previous column, there is a clue in that “z13”. This is a measure of what astronomers call “redshift”, or the change in the frequency of light.

Like sound, light is also made of waves. (It’s a little more complicated than that, but let it be.) When light comes from a source that’s moving—like my whistle in a plane—its wavelength changes like that of sound. If the source is moving away, the waves get longer (and their frequency decreases). When this happens on the sound spectrum, you get sound at a lower pitch. With light, you get light that is closer to the red end of the light spectrum: thus the “redshift”. Conversely, if the source is moving towards you: thus “blueshift”.

You must be wondering here: How do we detect this change, whether red or blue? It is not at all that light from a distant source suddenly appears red or blue. Instead, it has to do with the substances the object is made of – such as iron or carbon, or magnesium.

When you heat such a substance, it emits light. A spectroscope (aka spectrometer and spectrograph) uses a prism to break that light up into a spectrum, in the same way that a rainbow is formed from “white” light. Each such substance produces its own unique pattern of lines in that spectrum, each line at a specific frequency.

So if you find telltale lines of iron in a spectrum, you know that whatever your spectroscope is pointing at contains iron. This fingerprint, if you wish, is how we know the chemical composition of distant celestial bodies.

Here’s the fascinating thing. When astronomers first used spectroscopes on light from distant stars and galaxies, they recognized fingerprints, characteristics of different substances, in the spectra. But to his surprise, in every case, these spectral lines shifted along the spectrum. This led to a remarkable conclusion: these distant objects are moving.

Not only this. Since the degree of shift tells how fast the object is moving, we know that these objects are indeed moving very fast. (The Pulsar I wrote about here a few weeks ago is going at 2 million kmph.) Not only that; Hubble’s law tells us that the farther an object is, the faster it is moving away from us, and thus the greater its light. This is because the universe is constantly expanding.

(Note: there are some relatively nearby stars and galaxies whose light is blue, which means they are moving toward us. This is because at those relatively close distances, the gravitational attraction between objects exceeds the extent that they separates.)

Finally, the magnitude of the Doppler effect is measured by comparing the frequency of a shifted spectral line to its frequency at “rest”. Specifically, if you divide the difference between these frequencies by the “rest” frequency, that ratio is the redshift, called “z”. This measurement of redshift tells us how fast the object is moving and how far it is from us.

And that will bring us back to the GLASS-z13. In the name, “z13” stands for a redshift of 13 in the light from the Milky Way. This means it is about 13.4 billion ly away; Or, more correctly, it has taken so long for its light to reach us. But remember that redshift also says that the galaxy is moving away from us. Since we know its speed, we can calculate how far it has traveled in those 13.4 billion years. that number? About 20 billion li. So our best guess is that today, GLASS-z13 is actually about 33 billion years from us.

All that, from a few lines in the light to a tiny speck in the sky. Astronomy always inspires awe.

Dilip D’Souza, once a computer scientist, now lives in Mumbai and writes for his dinner. His Twitter handle is @DeathEndsFun.

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