# Of Galaxies&#8217; Doppler shift

Of Galaxies’ Doppler shift

You’ve heard it, surely. You know when a train rushes past sounding its horn, the horn’s tone changes, going high to low? Maybe you never stopped to think about it, but I’m going to try to make you do so.

It’s called the Doppler shift, that change in tone. If you measure the shift, you can calculate how fast the train is moving. Not that I’m recommending this as the method of choice, next time you want to know a train’s speed. There are less stupid ways. But there are some among us, called astronomers, who do use this method to estimate the speed of some other objects. For them, it’s the only sensible way.

What objects? Galaxies.

Not that I’m suggesting galaxies are zipping around your neighbourhood. Though in celestial terms, they actually are.

The light from a galaxy has certain characteristics. One is that it is Doppler shifted, exactly like the horn’s tone. The fact that it is shifted says that the galaxy is moving, but how fast? Well, if we measure its Doppler shift, we will know its speed. In 1912, astronomer Vesto Slipher did just that with Andromeda, the closest galaxy to us (apart from our own, the Milky Way). “Closest", but it is still two million light years (mly) away, meaning its light takes, yes, two million years to reach us.

Anyway, Slipher estimated that Andromeda is speeding towards us at 170km per second. That’s a few clicks faster than your average Renault Logan, but it will still be four billion years before Andromeda barrels into us. No need to interrupt your lunch, but your grandchildren might want to search for other planets to inhabit.

Andromeda is actually a rarity. According to their Doppler shifts, most galaxies are speeding away from us. The fascinating implication: the universe is expanding. But I’ll return to that.

Apart from their speeds, astronomers are always trying to figure out how far away celestial objects are. Like Edwin Hubble. In the 1920s, Hubble estimated distances to several galaxies with a delectable combination of assumption, observation and deduction, though that’s a subject for another essay. But he noticed something intriguing: the further the galaxy, the faster it recedes. For every one mly further, Hubble found that a galaxy’s speed increases by about 150km per second. More accurate modern measurements place that figure—the Hubble constant—at only about 23km per second per mly, but the relationship between distance and speed holds nevertheless.

Which, if you consider that the universe is expanding, makes sense. Imagine you’re a queen on a chessboard, with someone stationed on each square. Imagine the board expanding evenly in all directions. You’d see everyone receding from you, exactly as the others would as well. People two squares away recede twice as fast as your neighbours, since there are two squares, both expanding, between you and them. The further away someone is, the faster they move away.

Exactly what Hubble found about galaxies.

All right, so the universe is expanding. But when did that start? Astronomers have a name for that moment: the Big Bang, when the universe was born in an incomprehensibly vast cataclysm, when matter exploded out in a myriad directions. And from the Hubble constant, we can calculate when the Bang happened: about 14 billion years ago.

That’s how old your universe is.

What’s compelling about this whole chain of reasoning is how the Doppler shift tells us so much more than just the speed of a galaxy. Think of it thus: if a galaxy has a very large Doppler shift, Hubble’s constant says that it is very far away. If it is far away, its light has taken a very long time to reach us. If the light has taken a long time, the galaxy is very old. So from a galaxy’s Doppler shift, we get its age, too.

And for me, the profoundly humbling thing is that when we look out at these objects, we look back in time. To understand this, remember that Andromeda is two mly away. The light you see it by tonight actually left Andromeda that many years ago, about when certain apes started walking upright, starting on the evolutionary path that would lead inexorably to you, me and, yes, Lady Gaga, too. In that sense, most celestial objects represent a time many millennia ago. In fact, earlier this year a team of astronomers reported what’s thought to be the oldest galaxy ever seen (fittingly, they used the Hubble Space Telescope). Its Doppler shift marks its age as 13 billion years. Out at the edge of the universe, here’s something nearly as old as the universe itself.

Think of that. In looking at this singular object, we peer at the very beginning of the universe. The very beginning of time. The Big Bang.

Think of that, the next time the Frontier Mail whizzes past you, horn blaring.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter of Numbers will explore the joy of mathematics, with occasional forays into other sciences. Comments are welcome at dilip@livemint.com

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