There is a crater on the moon named after him. There is a Norwegian band named after him. There is an asteroid named after him. There was once a spacecraft named for him. And on 17 July, Google celebrated his 124th birthday with a doodle.

Yet it is a good bet plenty of us have not heard of Georges Lemaître, the Belgian astronomer and Catholic priest.

Should we have heard of him? I’ll say just this much: you’ve heard of the theory that you and I and your dog named Boo and Lemaître himself and in fact this entire planet and in fact our entire solar system and universe … that we all owe our existence to a cataclysmic explosion several billion years ago? The theory that’s called the Big Bang? Well, Georges Lemaître postulated it. That’s a measure of the man.

After earning a doctorate in Physics in 1920 at the Catholic University of Leuven in Belgium, and being ordained a priest three years later, Lemaître went to study at the University of Cambridge. A year later, he moved to MIT. At both institutions, his subject was astronomy; in particular, the findings that were shaking the field in those early decades of the 20th century.

Consider: Albert Einstein’s general theory of relativity and Vesto Slipher’s discovery of a Doppler “redshift" in the light received from distant galaxies (see my column “Of galaxies’ Doppler shift") offered researchers in cosmology plenty of new material to chew on. Harlow Shapley came up with a good estimate of the size of our Milky Way galaxy—much larger than it had been presumed to be. Edwin Hubble showed that there were plenty of other galaxies out there, beyond the Milky Way. He was also able to estimate how far away they are. Then, by correlating redshifts to distance, he stumbled on a simple but compelling relationship: the further the galaxy is, the faster it is receding. For every million light years (mly) further away a galaxy is from us, Hubble calculated, its speed increases by 150km/second. This is called the “Hubble constant".

And even though this figure for the Hubble constant was soon found to be an overestimate, what the relationship implied was stunning indeed. Galaxies were not just static objects sitting around somewhere in the cosmos. They were receding from us and from each other. There’s a now-famous analogy: think of baking a loaf of raisin-bread, and picture how those raisins move through the dough as the bread rises. That’s how galaxies are moving away from each other—though, let’s be honest, at somewhat higher speeds than a raisin achieves. At the distant reaches of the universe, those speeds approach the speed of light.

Lemaître was the first to suggest what now seems like the obvious implication: the universe is expanding.

Lemaître must have also asked himself the natural question: what could possibly have set off such expansion? I mean, this is not like the thunderbolt serve of a Roger Federer, nor even the blast of a rocket that leaves Earth en route to Mars. To hurl entire galaxies out into space in every possible direction at these remarkable speeds, you need an explosion of energy of a magnitude it is truly impossible to comprehend—and yet the weight of observation and science pointed inexorably to just such an explosion. Not only that. If you wind time backwards and reel all those galaxies in, they come together into … well, into what? Whatever it was, it held inside it the seeds of everything we see around us today—the entire universe that we observe and so love.

In 1927, it was Lemaître who used Hubble’s data to estimate the Hubble constant—even before Hubble himself did-

Lemaître’s conclusion in a 1931 paper: back at the beginning of time —whatever that may mean, whenever that was—there was a single inconceivably dense primeval particle that exploded. What it had looked like an instant earlier, how and why the gargantuan explosion happened—these are hard questions to answer. But Lemaître believed it happened. We now call that the Big Bang.

And in fact, in 1927 it was Lemaître who first used Hubble’s data to estimate the Hubble constant—even before Hubble himself did—at about 200km/second per mly. Over the next half-century or so, a series of astronomers made observations and calculations and fought over the Hubble constant. Today, the generally accepted value, confirmed by many studies, is about 21.3km/s/mly.

The Big Bang flung matter out into space—if we can call whatever was out there at the time “space"—with such force that several billion years later, galaxies are still speeding outward in every direction. But I want to return to that brief mention above of reeling galaxies in and winding time backwards—all the way back to Lemaître’s primeval particle. Just how long back will we go in time before we get to the Bang?

In other words, as you may have noticed, what I’m asking is, what is the age of the universe?

There is a sense, using Hubble’s constant, in which we can indeed wind time backwards. Bear with me while I explain. Let’s assume galaxies are zooming outward at a steady, unchanging speed. Hubble’s constant tells us that if there’s a galaxy a million light years away, it’s moving at 21.3km/s. So at that steady speed, how long did it take to reach that point a million light years away?

Simple: divide the distance (1 mly) by the speed (21.3km/s) — in other words, invert the Hubble constant—and there you have it: about 14 billion years. That’s how long the galaxy has been chugging along. And when did it start its journey through space? At the beginning of time: the Big Bang. In other words, as you may have concluded, we have the age of the universe: 14 billion years.

Caveat: there are some sweeping assumptions here. One is that the galaxies rush along at a never-changing speed. Also, remember that the distance we are talking about is from us here on Earth. Can we really assume that’s the same as the distance a galaxy has travelled from the moment of the Big Bang, especially because we on Earth, part of a galaxy called the Milky Way, are also moving through space at a frenetic pace?

Correcting for these and more changes the number to about 13.7 billion years, and scientists today generally think that’s reasonable. Though as I alluded to above, it’s taken astronomers a long time and much argument to settle on a value for the Hubble constant. Remember that Lemaître’s original estimate was 200km/s/mly. That number gives us an age for the universe of only about 1.5 billion years. But by the 1930s, geologists had found rocks on the Earth that were 3 billion years old. Obviously the Earth cannot be older than the universe it exists in. That contradiction helped spark the years of wrangling over the Hubble constant.

But having wound time backwards, let’s try winding it forward. Lemaître told us that there was a Big Bang and the universe has been expanding ever since. But will it expand forever? Or is the expansion slowing down and will it stop at some point? What happens then? Will the galaxies, boomerang-like, hurtle back toward each other? And how will that end, in some kind of equally unimaginable Big Crunch? What happens after that?

Beguiling questions, all. While this is not the place to consider them in any detail, the evidence we have so far suggests to most scientists that the universe will keep expanding for a long time. Eventually, it will run out of material to produce new stars, existing stars will fade away, and we’ll have a dying, steadily darkening universe. Consider yourself warned then, and set your alarm clocks for several trillion years from now.

So much cosmological exploration, from Lemaître’s early deduction about an expanding universe! But in 1951, possibly because Lemaître was also a priest, someone claimed even more for his theory. It scientifically validated Catholicism, said none other than Pope Pius XII.

Lemaître made it clear to the Pope how unhappy he was with this peculiar pronouncement. He practised both science and religion with distinction, but believed they were separate endeavours. I think he would have agreed with what his great German contemporary, Hermann Weyl, once said: “Mathematical inquiry in itself... lifts the human mind into closer proximity with the divine than is attainable through any other medium."

In fact, for those who believe in the divine, I think many million light years does qualify as “closer proximity".

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

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