The Universe is deleting the evidence

The galaxies are not flying away through space. Space itself is getting bigger, and they are along for the ride. Pics/iStock

Not the theory - the evidence. The stuff we point at when we say this is how we know. What if the universe takes its own proof away, and the only reason we know anything at all is that we showed up early?

Let me tell you how I got here. A hundred years ago the night sky had smudges in it, and nobody was sure what they were. Clouds of gas inside our own galaxy, some said. In 1920 two American astronomers held a public debate about it and could not agree. They were not clouds. They were other galaxies - entire cities of stars, each holding billions of suns, at distances the mind is not built to hold.

Meanwhile, in Arizona, an astronomer called Vesto Slipher was doing something nobody thanked him for. Since 1912 he had been photographing the light from those smudges, one long exposure at a time. He kept finding the same odd thing. The light was stretched. You already know what stretched light means, even if nobody has ever told you. A train comes at you and its horn sounds high; it passes, and the sound drops. The sound stretches as the train pulls away. Light does something like it. The reason is not quite the same - but the clue is.

In 1927 a Belgian priest, Georges Lemaître, worked out what Slipher's numbers were saying. Two years later Edwin Hubble, with a bigger telescope and a far bigger personality, showed it beyond argument: the farther a galaxy sits, the more its light is stretched.

There is only one honest way to read that. The galaxies are not flying away through space. Space itself is getting bigger, and they are along for the ride. Now run it backwards. If everything is spreading apart today, then long ago it was all crushed together - hot, dense, small. That is the Big Bang. That is where 13.8 billion years comes from. That is where nearly everything I was taught comes from.

And notice what the whole thing rested on. Other galaxies, visible in the sky, available to be photographed by a patient man in Arizona.

What 1998 changed

In 1998 two rival teams set out to measure how fast the expansion is slowing down. Everybody expected slowing - gravity pulls things together; it should be braking.

It is not slowing. It is speeding up. Something is pushing everything apart, harder every year, and we do not know what. We call it dark energy, which is a confident-sounding name for an admission of ignorance. The consequence is easy to say and hard to feel.

Think of shouting to a friend on an airport travelator - except this one runs the wrong way, and keeps getting faster. For a while she hears you. Then comes a point where your voice will never reach her again. She has not vanished; she is standing right there. The floor between you is growing faster than sound can cross it.

The universe does this with light. Beyond a certain distance a galaxy's light is still travelling towards us and will never arrive. Not tonight, not in a billion years, not with a better telescope.

The last astronomer

So picture someone doing my job a hundred billion years from now. Our galaxy and its close neighbours are held together by gravity, so they will still be there, merged into one. Everything else will have gone - every other galaxy in the universe, over the edge and out of sight.

The Big Bang also left a faint hum that fills the whole sky today, our best evidence that everything started hot. By her time it will be stretched too thin for anything to pick up.

She looks. One galaxy, hers, and darkness. No stretched light, because nothing is far enough away to stretch. No hum. Nothing strange in the elements.

So she writes it down: one galaxy, alone, unchanging, always here. That is roughly what people believed in 1920 - except she will have no Slipher, no smudges, no way at all to find out she is wrong. Every measurement available to her will agree with her.

Two physicists, Lawrence Krauss and Robert Scherrer, laid this out in 2007, and what unsettles me is the generosity of their argument. They give the future everything: better instruments, sharper minds, more patience than we have. It loses anyway.

Here I owe you the part enthusiasm usually skips. This is an extrapolation, and a hundred billion years is long enough for assumptions to rot. If dark energy behaves differently than we think - and there are real hints it might - the timetable shifts.

The shape does not. Some evidence is already unreachable, and that much is not a forecast.

What survives

Last week in these pages I wrote that if all moments are equally real, nothing is truly lost. I still think that is right. But real and reachable are different things. The book may be complete; we only get to read the pages the light delivers, and the deliveries are stopping. So what would save her?

Not a telescope. Only a record. She could know the truth for the same reason you know about the Battle of Panipat - because someone wrote it down and someone else kept it.

Which is a fragile hope, and I know exactly how fragile, because of what happened to Lemaître. When his 1927 paper was translated into English in 1931, the paragraph with his calculation of the expansion rate was missing. For eighty years people muttered that Hubble had it removed to protect his own claim. Then in 2011 the astronomer Mario Livio went through the Royal Astronomical Society's archives and found the letter that settled it. Lemaître had done the translation himself, and cut the paragraph himself, because by then he thought it was old news.

He gave away his own discovery, and the proof of it sat in a box in London for eighty years until somebody bothered to look. That is the whole business in miniature. Evidence goes missing. Sometimes we lose it, sometimes we hand it over, and sometimes it survives only because a stranger goes digging.

We are the lucky ones. We arrived late enough to build the instruments, and early enough that the sky still has the answers in it. That is not a permanent condition. Go and look while it is.

Nishant Sahdev is a physicist at the University of North Carolina at Chapel Hill, US. He chases ideas that refuse to stay in laboratories, for your favourite Sunday mid-day.