We’ve arrived late for the cosmic show – too late for the best seat, says Harvard theorist Avi Loeb. He says the ideal time to study the cosmos was more than 13 billion years ago, just half a billion years after the Big Bang.
Two contradictory factors affect the best time to observe the cosmos. In the young universe, the cosmic horizon is closer, so that less can be seen. As the universe ages, it’s possible to see more, as there’s been time for light from more distant regions to reach the observer.
However, in the older and more evolved universe, matter has collapsed to make gravitationally bound objects. This ‘muddies the waters’ of the cosmic pond, says Loeb, because you lose memory of initial conditions on small scales.
According to Loeb’s calculations, the perfect balance between the two was to be found just 500 million years after the Big Bang, just as the first stars and galaxies began to form.
It’s still possible to do this, by using surveys designed to detect 21-cm radio emissions originating more than 13 billion light years away.
“21-centimeter surveys are our best hope,” said Loeb. “By observing hydrogen at large distances, we can map how matter was distributed at the early times of interest.”
The accelerating universe means future cosmologists will have a harder time, as galaxies are pushed beyond our horizon so that light leaving them will never reach the Earth.
In addition, the scale of gravitationally unbound structures is growing larger and larger, meaning that they, too, will eventually stretch beyond our horizon. At some point between 10 and 100 times the universe’s current age, cosmologists will no longer be able to observe them.
“I’m glad to be a cosmologist at a cosmic time when we can still recover some of the clues about how the universe started,” says Loeb.
“If we want to learn about the very early universe, we’d better
look now before it is too late!”