Why the universe is so dang empty
From the folks behind PHD Comics
The following is an excerpt from We Have No Idea by Jorge Cham and Daniel Whiteson.
Another big mystery about the structure of the universe is: Why is the universe as empty as it is? Why are the stars and galaxies not closer together—or farther apart?
To give you some perspective, our solar system is about 9 billion kilometers wide, but the nearest star is about 40,000 billion kilometers away.
And our galaxy is about 100,000 light-years wide, but the nearest galaxy, the Andromeda Galaxy, is about 2,500,000 light-years away.
However big space is and whatever shape it has, there seems to be plenty of room to have things closer together. It’s not as if some cosmic parent had to separate all of the stars and galaxies because they were squabbling in the backseat.
Luckily, emptiness is a matter of perspective, and we can divide this question into two different questions:
Why can’t we move faster than the speed of light?
Why did space expand during the Big Bang, and why is it still expanding today?
The speed of light is the cosmic yardstick that defines what we mean by “close” and “far.” If the speed of light was much, much faster, then we would be able to see farther and travel more quickly, and things would not seem so far away. If the speed of light was much slower, our distant neighboring stars would seem even more impossible to visit or send texts to. 1
On the other hand, we can’t blame it all on the speed of light. If space hadn’t been stretched so much during the first fractions of a second after the Big Bang, everything would be much closer together today. And if dark energy wasn’t currently pushing everything even farther away, then the prospects for interstellar travel wouldn’t be getting worse by the minute.2 We can imagine a universe where inflation had limited itself to blowing up the universe by a more reasonable amount than the absurd factor of 1032.
So the emptiness of our universe comes from the interplay between these two quantities: the speed of light that defines the distance scales and the expansion of space, which is pulling everything apart. We don’t know why either of these quantities are what they are, but if you changed them, you would get a universe that looks very different from ours. As with many of the big mysteries, we have only our single universe to study, so we don’t know if this is the only way the universe could be organized, or if in other universes there was very little expansion and everyone feels much closer together than we do.
- The roaming charges would be killer.
- Uncool, dark energy. Very uncool.
Excerpted from We Have No Idea by Jorge Cham and Daniel Whiteson. Published May 2017 by Penguin Random. Published with permission.
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