Don’t take tension, but the universe expands

And there’s my favourite, the Hubble Constant that says things about this universe we inhabit. But before I explain why I like it, here’s a rundown of how Edwin Hubble, the 20th Century astronomer, came to define the number and give it a value, and what it means for our understanding of the universe.

You’ve heard of the Doppler effect: It’s the change in frequency of a wave you notice, if you and the source of the wave are moving relative to each other. Plainer language, you say?

Certainly you’ve noticed the change in the pitch of a train’s horn as it moves past you. The horn is higher pitched as a train approaches you, lower as it moves away. That’s the Doppler effect: The sound is “shifted" lower or higher depending on how the train is moving.

Well, astronomers noticed distant galaxies producing the same kind of shift. Not in some sound they make, but in the light that they emit that reaches us. That light is Doppler-shifted, too, and (in general) towards the lower-frequency red end of the light spectrum.

What does this “red shift" tell us? That those galaxies are moving, and moving away from us. In turn, that means the universe is expanding.

This was a fundamental, profound discovery about the universe. It raised all sorts of questions. What is it expanding into? Will it stop expanding at some point, and start collapsing? Is the expansion slowing down or accelerating, or is it steady? And how fast is it expanding, anyway?

Edwin Hubble was the first to offer an answer to some of these questions, and it was a surprising one: The further a galaxy is from us, the faster it is receding from us. In other words, the red shift of a given galaxy doesn’t just tell us how fast it is moving; it is also a measure of its distance from us.

The Hubble Constant captures this, and Hubble estimated it at 500km/second per megaparsec (mpc). (The megaparsec is a unit of distance, equivalent to about 3.25 million light years, or mly.) He is now believed to have overestimated—contemporary measurements suggest it is closer to 65km/s/mpc.

Like with most things to do with the universe, this constant involves quantities almost too huge to comprehend. The closest star to us, Proxima Centauri, is about 4.3 light years away—itself about 40 trillion km. Milky Way galaxy is about 100,000 light years across. So, a megaparsec is about 750,000 times the distance to Proxima Centauri, and over 30 times the diameter of the Milky Way.

The Hubble Constant says that with each megaparsec out into space from the Earth, an object’s velocity increases by 65km/s. One way to understand this is to consider a galaxy that’s more or less stationary, one mpc away. With every second, it moves another 65km away. Or imagine that we find a galaxy whose red shift says it is speeding along at 200km/s. The Hubble Constant tells us it is about 3mpc (200/65) away.

If that’s still hard to comprehend, think of friend, Manjula who jumps on her bike, saying: “I’m going to flee from you. With each kilometre I put between us, my speed will increase by 5kmph."

Her Manjula Constant, if you like, is 5kmph/km. With a toss of her hair, she pedals off. Some time later, she zooms past friend Mandeep, who measures her speed and reports to you: 20kmph. You know that Manjula is now 4km (20/5) from you. What’s more, her constant also gives you an estimate of how long Manjula has been pedalling. Invert it to get 1/5, or 0.2 hours, which is 12 minutes. That’s how long it takes to travel 4km at 20kmph. That’s how long it’s been since she left.

This is what makes the Hubble Constant my favourite. Because by simply inverting it, we have an estimate of the age of the universe—we effectively roll the clock back to that gargantuan primeval explosion we know as the Big Bang. Try it: invert 65km/s/mpc and do some relatively straightforward arithmetic. You’ll come up with a figure of a little less than 16 billion years.

The thing is, though, we don’t have the Hubble Constant down precisely. Astronomers have generally used three different methods to calculate it, and the values they get vary: about a 10% difference between figures.

That may not be much, considering there is an inherent uncertainty in measuring red shifts and distances on these cosmic scales. Still, a respected science journalist recently wrote: “[A]s far as anyone can tell, there’s nothing wrong with [these measurements], and there’s no obvious way to get them to agree" (Gravitational lens gives us a third estimate of the Universe’s expansion, John Timmer, Ars Technica, 13 May 2023) So persistent is this difference that it has come to be known as “Hubble Tension".

One method of calculating the Hubble Constant involves a certain class of stars known as the Cepheid variables. A recent paper builds on data from over a thousand Cepheids observed by James Webb Space Telescope (JWST). They were earlier observed with the Hubble Space Telescope (HST).

There was some suspicion that the HST observations were not very accurate; the “superior resolution of JWST negates" such inaccuracies. Even so, the paper “conclude[s] that errors in [HST measurements] of Cepheids...do not significantly contribute to the [Hubble] tension." (JWST Observations Reject Unrecognized Crowding of Cepheid Photometry as an Explanation for the Hubble Tension at 8 Confidence, Adam G. Riess et al., Astrophysical Journal Letters, 6 February 2024)

An entire scientific effort merely to confirm that the Hubble tension still exists! I love astronomy. And mathematics. And science. And reason.

PS: Next week’s column will be my last in this space. It’s been 13 years and something like 400 columns. I’ve loved writing them, I am so grateful that you read them and I hope they gave you things to think about.

 

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