We knew that we lived on a Goldilocks planet—not too close to the sun, but not too far away, either. Now we know we live in a Goldilocks universe, too.
It’s one of the great, if little known, astrophysical mysteries: Explosions from colliding neutron stars should have killed off all life in the universe by now. Except they haven’t.
When two neutron stars smash together, they release powerful bursts of gamma rays that are capable of dissolving the ozone layers of planets in its own galaxy as well as the next one over. What keeps us, and possibly alien life, safe from these galaxy-sweeping blasts?
Inflation, in a word. The speed at which the universe is expanding ensures that there are neighborhoods in the universe safe from gamma ray storms, according to a new study by an international team of astrophysicists.
The speed with which objects in the universe move apart is controlled by a factor called the cosmological constant. It’s a number that Einstein added to his equations for general relativity to make the mathematics work. He didn’t realize it in 1917, but it turned out to describe the speed with which objects in the universe move away from each other.
The multiverse theory predicts that there are many universes, each with their own physics and cosmological constants. It turns out that our universe might be a bit of a freak. Here’s Ilima Loomis, interviewing cosmologist Alan Heavens for Science Magazine:
In theory, Heavens explains, either the constant should be hundreds of orders of magnitude higher than it appears to be, or it should be zero, in which case the universe wouldn’t accelerate. But this would disagree with what astronomers have observed. “The small—but nonzero—size of the cosmological constant is a real puzzle in cosmology,” he says, adding that the research shows the number is consistent with the conditions required for the existence of intelligent life that is capable of observing it.
When the cosmological constant is low and galaxies are clustered close together, the risk of gamma ray bombs destroying atmospheres is higher. Even if life worked differently on other planets, it would most likely need an ozone layer to shield it from constant bombardment by ultraviolet light. This means that contracting universes would be hard places to live. It also suggests that dense parts of our own universe probably can’t support life.
Dispersed universes have another problem. Stars like our Sun, which burn hydrogen and can warm planets up enough to support life, would be rare in such places. Clouds of space dust might not be thick enough to coalesce and form stars.
For galaxies with just the right density, the rate at which the universe expands needs to be not too fast and not too slow. The researchers say that our universe somehow got a cosmological constant in the sweet spot for supporting life.
This means that our solar system is lucky in two ways. First, it’s in an arm of the Milky Way Galaxy away from the dense and explosive center. Second, we’re in a universe that isn’t contracting into a dense cloud or thinning out into a mere haze. Instead, objects are just spreading out slowly. The researchers suggest this means there are other places in our universe with conditions that could support life, and it could give them a better idea of where to look.