This is an analogy to the Big Bang. The blue, green, and red "cars" start at left at the same time. The green car moves
twice as fast as the blue one, and the red car moves 3 times as fast as the blue one. At any time after the start
of the race, therefore, the green car has moved 2 times further from the start than the blue one, and the red car
has moved 3 times further from the start than the blue one.
If you arrived at the starting line after the race had begun, you would be able to deduce from the cars'
locations & speeds that they had all started together at the beginning. (Why?) (How would their
positions later on be different if one of the cars had had a head start at the beginning?)
When we look at distant galaxies, we observe that the speed they are moving away from us is proportional to
their current distance away from us. This is exactly analogous to the car race, and it allows us to deduce that
all those galaxies were once all together at the "starting line." This start is the Big Bang --- all the material
that is currently in all those different galaxies was once compressed into one small very small region.
In fact, we are better able to deduce the Big Bang from the current galactic positions and speeds than we
would be able to deduce the start of the car race from the cars' current positions and speeds. With the cars,
we wouldn't be able to rule out the possibility that their earlier motions had been quite different from
what we see now. (Maybe the blue car started really fast, then had engine trouble and stopped, and now
is limping along.) But with galaxies, the laws of physics allow as unambigously to deduce their earlier
velocities from their current velocities. So we have stronger grounds to deduce what happened in the
past. (Of course, we are assuming that the laws of physics didn't change between then & now. There is good
evidence for the laws not changing --- many predictions from the Big Bang theory are confirmed by other
experiments --- but there's always a possibility that there was a change.)
(Read the pictures from left to right, then down)
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