Timing the Race of the Galaxies

 Runners at the "City to Surf Fun Run" in Sydney, Australia

Consider this: The universe of expanding galaxies is very much like a marathon race. Just as all the runners begin in a tightly packed area and move outward after the starting gun, all the galaxies in the universe started in a tiny, infinitely dense point and expanded outward after the Big Bang. In a marathon, as fast runners speed ahead and stragglers fall behind, the field gradually spreads apart. Likewise, galaxies expand over time at different rates, making space more diffuse.

Now imagine you’re a roadside observer standing alone at the ten-mile point and you want to know when the race started. All you’ve got are a watch and a radar gun. What do you do?

Being the clever problem-solver you are, you know that any object moving at a constant rate follows this basic mathematical rule: the distance the object travels divided by its velocity gives you the amount of time it’s been moving. That means if you know how fast a particular marathon runner is moving (her velocity) and the distance she’s covered, you can figure out how long she’s been running.

Fortunately, you have those numbers at your fingertips: the radar gun can tell you the runner’s velocity, and you already know you’re at the ten-mile mark. Assuming your runner is moving at a constant rate, you can calculate, for example, that a six-mile-per-hour runner took 60 minutes to run ten miles. Subtracting 60 minutes from the current time on your watch gives you an estimate of when the starting gun fired. Because most runners don’t move at a constant rate, though, you decide to repeat this calculation for other runners as they pass by, and average your results to refine your answer. Problem solved.

So what does timing marathon runners have to do with the age of the universe? Let’s recall our analogy of the universe as a "race" of expanding galaxies, all of which started in the same place and at the same time (just before the Big Bang). Why can’t astronomers apply your marathon-timing method to determine when the galactic marathon started?

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