the Race of the Galaxies
at the "City to Surf Fun Run" in Sydney, Australia
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.
imagine youre a roadside observer standing alone at the
ten-mile point and you want to know when the race started. All
youve got are a watch and a radar gun. What do you do?
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 its been moving. That means if
you know how fast a particular marathon runner is moving (her
velocity) and the distance shes covered, you can figure
out how long shes been running.
you have those numbers at your fingertips: the radar gun can
tell you the runners velocity, and you already know youre
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 dont
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.
what does timing marathon runners have to do with the age of
the universe? Lets 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 cant astronomers apply your marathon-timing
method to determine when the galactic marathon started?
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