Digging Up the Early Universe

I wrote another column for Discover (the actual magazine), which is now available online. It’s about how far back in cosmological time we can push our knowledge on the basis of actual data, not mere theory.

Of course we literally look back in time every time we peer into a telescope, since it takes time for light to travel to us from distant objects. But there’s an earliest moment we can possibly see using light — the moment of recombination, about 380,000 years after the Big Bang, when electrons hooked up with protons and other nuclei to form atoms. Earlier than that, the electrons were floating around freely, bumping into photons, and generally making the universe opaque.

So we have to be a bit more clever. And we have been: using the fact that the early universe was a nuclear fusion reactor, and observing the surviving abundances of light elements to pin down what conditions were like at that time. This technique gets us within seconds of the Big Bang. But if things break just right — the dark matter turns out to be a weakly-interacting particle, whose properties we can study here on Earth — we might be able to push the data-informed era much earlier back than that.

Think about what that means: Sitting here on Earth, cosmologists extrapolated our understanding back 13.7 billion years, to a few seconds after the universe began. We used that understanding to make predictions about the current universe—and we were right. We may not know for sure whether it will rain tomorrow, but we do know exactly how protons and neutrons bounced around like Super Balls in the nuclear inferno of the Big Bang. This will surely go down as one of the most impressive accomplishments of the human intellect.

And yet cosmologists want to do better still. The goal is to discover relics that predate even Big Bang Nucleosynthesis. At the moment that’s not quite possible, but there is one promising candidate: dark matter, the dense but unseen stuff that holds galaxies together.

Roughly speaking, if we get lucky, we could learn about conditions in the universe about 1/10,000th of a second after the Big Bang. We’d like to go even much earlier than that, but let’s not forget to be impressed at how well we’ve already done.