Bulk Flow

We called it a bulk flow because nobody wanted to call it anything. Give a thing a name and you’ve made it real. You’ve made it a thing you can argue with.

The data set was big. Sixteen hundred galaxy clusters, all of them full of the rich kind, the kind with a hundred galaxies apiece and a core that’s been sitting there relaxing, aristocrats a billion years old, with nothing better to do. We measured their peculiar velocities. Not the drift of expansion. The drift on top of the drift. And they were all moving. Same way. All of them.

Sixteen hundred clusters, a hundred galaxies apiece, a hundred billion suns each, streaming, all of them, in one direction, at a speed you don’t get by chance. Chandra’s, Planck’s, the deepest surveys, checked and rechecked until the plot looked like a wall you couldn’t argue with. Maybe it was chance. One in ten to the something. We all knew the something was big.

And the flow pointed at nothing. It pointed at an empty patch of sky. Every direction’s empty if you look right, but this patch was emptier than the rest, and it wasn’t a hole in the survey. It was a hole in everything we’d ever seen. The flow ran toward it, straight at it.

“No,” Geri said. Geri’s the one who finds the holes. “We don’t get to have a drain.”

Theories did what theories do. A void would do it, a huge underdensity ahead of us, pulling. A Great Attractor, a Shapley. Something we’d modeled a hundred times. Something we could argue with. We ran the numbers for a private universe that nobody else could see, just to have a number. The void would have to be so big it would show up in the microwave background as a cold spot the size of half the sky.

There was no cold spot.

So we ran it the other way. Push instead of pull. Something behind us, shoving the whole thing. What’s behind us is the same as everywhere else. That’s the thing about a universe that’s homogeneous and isotropic. Every direction is the same. There’s no special direction. There’s no center. The man with the best seat in the house has no way to know he’s got the best seat, because every seat is the best seat.

We had a direction now. We had a special direction. And a special direction makes a special place.

Io put it together. Io’s the geometry kid. She laid the flow on a map with the scale that says TINY on it and she laughed, the laugh that should have come first, and then she stopped. Because we had been standing in the flow the whole time, in the path of it, and it wasn’t flowing past us. It was flowing around us. Around. Like water around a stone.

A stone in the flow. Us.

“No,” I said. “Absolutely not.”

“Look,” she said. “Look at where the fastest clusters are. Look at where the slowest are. The flow isn’t uniform. It’s windward and leeward. There’s an object here.” She tapped the map. “In this position. In this patch of sky, at this distance, there’s an object. Massive enough to bend the global flow around it.”

“Avoided,” Geri said. “Nobody’s measured an aperture, nobody’s seen anything, nobody’s got a redshift for anything that size. It’s void.”

“It’s avoided,” Io said. “Avoided by everything we’ve looked at. Because it deflects around it.”

“You’re describing a Great Attractor,” Geri said.

“I’m not,” Io said. “A Great Attractor pulls. This deflects. This is a repeller. Not a hole in the sky, Geri. A whole that fills the sky. Everything we see is flowing away from it.”

We stood in the lab with the map full of arrows, all of them streaming away from a point that was behind us, all of them streaming away from a very old, a very patient, a very large. And from the pause she made before the last word, I knew the last word wasn’t going to be a name.

“It’s Earth, isn’t it,” I said.

It wasn’t a question. The fastest clusters were the ones that had been moving the longest. The nearest ones barely moved at all. The flow was uniform in comoving coordinates, which is the physicist’s way of saying it had been going on forever, and it had been, and the center of it, the thing it was flowing away from, was us. A point in a flow doesn’t have a size. It’s just a point. But between the point and the flow there’s a boundary, and a boundary is an interface, and an interface is a surface.

“And a surface is a place,” Io said.

There’s something behind us. Something that’s been pushing ever since there was an us, or a sky, or a surface to put a horizon on. You want to know how big? The flow that we’d been calling a mystery was the shadow. The thing making the shadow was bigger than every galaxy we’ve ever seen, combined, and older, and it was all behind us, all of it, in the one direction that has no clusters, no light, no anything. The direction I’d been avoiding, the one with my name on it.

And it was behind us because it had always been behind us. It’s not that we’re in the wrong place. There is no wrong place. We’re in the only place. The special place. The place it’s been going around. And if one day you want to know what it is, you’d have to. No. You wouldn’t. You’d have to look at it, and you can’t look at it, because you are it.

Io and I buried it. Of course we buried it. You don’t publish a paper that says we’re the center of the universe; you don’t even whisper it, because a whisper is a publication for a thing this size. We compressed the data. Took the fastest clusters out of the public sample. Fraction of them, not all. The statisticians would smell a kill. Just enough to smooth the flow until it looked like noise. The fewer fast ones you leave in, the more it looks like coincidence.

We shrank our own data. That’s how you hide a direction. You hide it by shrinking it until it’s smaller than the error bars. We made ourselves small. The universe didn’t notice.

But Geri found the hole we’d left. Not the one in the sky. The one in the sample. She walked into the office holding the plot, and she said, “You’ve been cooking the books. The fastest clusters are gone. The flow…” and stopped. And looked at the map. And looked at me.

She read us in about three seconds. Then she looked at the wall where the plot is pinned, the flow pointing at the empty patch, and she said, “It isn’t behind us.”

“It is,” I said.

“It is,” Io said.

Geri closed her eyes. Family. Colleagues. The two of them. She said: “How many did you pull?”

“Enough,” Io said.

“Enough to make it noise.”

“That was the idea.”

Geri looked at the hole one more time. She didn’t say what she saw. She didn’t have to. She printed her own name and initialed the plot. I signed it. We all signed it. We signed a plot like it was a contract, and in a way it was. We agreed, the three of us, on a thing we didn’t name.

We keep the candle in the window dark. The survey data is public and honest and wrong about the fastest clusters. The flow reads as a curiosity, a one-in-ten-to-the-something, a reason for a footnote. Io pushed the real numbers through a dark pool of the sky, a survey channel nobody watches, buried in the auxiliaries, the kind of data dump you’d need a reason to open. The kind nobody has a reason to open. The specialness of where we are is a secret kept by three people and a bad plot.

There’s a name for the direction. There’s a name for the thing in it. I don’t write them down. I don’t say them. I bury them the way you bury a thing that would kill you if it got out, except this is a thing that would kill everyone.

The dark pool of the sky has been open this whole time. Anyone could have looked. The hole is right there, and behind the hole is the thing that made it, and behind the thing is the reason we’ve been running, all of us, for every year there’s been an us. We aren’t moving. We’re being pushed. And the thing that pushes doesn’t need a name. It was here before names.