Maybe one in a thousand cultures made it through such bottlenecks.

That fortunate few developed self-sustaining colonies off their home worlds, and — forever immune to the eggs-in-one-basket accidents that could afflict a race bound to a single world — they started spreading. Or else they made machines, robots that could change worlds and rebuild themselves, and sent them off into space, and they started spreading.

Either way, from one in a thousand habitable worlds, a wave of colonization started to expand.

There were many different strategies. Sometimes generations of colonists diffused slowly from star to star, like a pollutant spreading into a dense liquid. Sometimes the spread was much faster, like a gas into a vacuum. Sometimes there was a kind of percolation, a lacy, fractal structure of exploitation leaving great unspoiled voids within.

It was a brutal business. Lesser species — even just a little behind in the race to evolve complexity and power — would simply be overrun, their worlds and stars consumed. And if a colonizing bubble from another species was encountered, there were often ferocious wars.

“It’s hard to believe that every damn species in the Galaxy behaves so badly,” Madeleine said sourly.

Malenfant grinned. “Why? This is how we are. And remember, the ones who expand across the stars are self-selecting. They grow, they consume, they aren’t too good at restraining themselves, because that’s the way they are. The ones who aren’t ruthless predatory expansionists stay at home, or get eaten.”

Anyhow, the details of the expansion didn’t seem to matter. In every case, after some generations of colonization, conflicts built up. Resource depletion within the settled bubble led to pressure on the colonies at the fringe. Or else the colonizers, their technological edge sharpened by the world-building frontier, would turn inward on their rich, sedentary cousins. Either way the cutting-edge colonizers were forced outward, farther and faster.

Before long, the frontier of colonization was spreading out at near light speed, and the increasingly depleted region within, its inhabitants having nowhere to go, was riven by wars and economic crisis.

So it would go on, over millennia, perhaps megayears.

And then came the collapse.

It happened over and over. None of the bubbles ever grew very large — no more than a few hundred light-years wide — before simply withering away, like a colony of bacteria frying under a sterilizing lamp. And one by one the stars would come out once more, shining cleanly out, as the red and green of technology and life dispersed.

“The Polynesian syndrome,” Madeleine said gloomily.

“But,” Malenfant growled, “it shouldn’t always be like this. Sooner or later one of those races has got to win the local wars, beat out its own internal demons, and conquer the Galaxy. But we know that not one has made it, across the billions of years of the Galaxy’s existence. And that is the Fermi paradox.”

YES, Cassiopeia said. BUT THE GALAXY IS NOT ALWAYS SO HOSPITABLE A PLACE.

Now a new image was overlaid on the swiveling Galaxy: a spark that flared, a bloom of lurid blue light that originated close to the crowded core. It illuminated the nearby stars for perhaps an eighth of the galactic disc around it. And then, as the Galaxy slowly turned, there was another spark — and another, then another, and another still. Most of these events originated near the Galaxy core: something to do with the crowding of the stars, then. A few sparks, more rare, came from farther out — the disc, or even the dim halo of orbiting stars that surrounded the Galaxy proper.

Each of these sparks caused devastation among any colonization bubbles nearby: a cessation of expansion, a restoring of starlight.

Death, on an interstellar scale.

Their virtual viewpoint changed, suddenly, swooping down into the plane of the Galaxy. As the spiral arms spread out above her, dissolving into individual stars that scattered over her head and out of sight, Madeleine cried out and clung to Malenfant. Now they swept inward, toward the Galaxy’s core, and she glimpsed structure beyond the billowing stars, sculptures of gas and light and energy.

Her attention came to rest, at last, on a pair of stars — small, fierce, angry. These stars were close, separated by no more than a few tens of their diameters. The two stars looped around each other on wild elliptical paths, taking just seconds to complete a revolution — like courting swallows, Madeleine thought — but the orbits changed rapidly, decaying as she watched, evolving into shallower ellipses, neat circles.

A few wisps of gas circled the two stars. Each star seemed to glow blue, but the gas around them was reddish. Farther out she saw a lacy veil of color, filmy gas that billowed against the crowded background star clouds.

“Neutron stars,” Malenfant said. “A neutron star binary, in fact. That blue glow is synchrotron radiation, Madeleine. Electrons dragged at enormous speeds by the stars’ powerful magnetic fields…”

The Gaijin said, PERHAPS FIFTY PERCENT OF ALL THE STARS IN THE GALAXY ARE LOCKED IN BINARY SYSTEMS — SYSTEMS CONTAINING TWO STARS, OR PERHAPS MORE. AND SOME OF THESE STARS ARE GIANTS, DOOMED TO A RAPID EVOLUTION.

Malenfant grunted. “Supernovae.”

MOST SUCH EXPLOSIONS SEPARATE THE RESULTANT REMNANT STARS. ONE IN A HUNDRED PAIRS REMAIN BOUND, EVEN AFTER A SUPERNOVA EXPLOSION. THE PAIRED NEUTRON STARS CIRCLE EACH OTHER RAPIDLY. THEY SHED ENERGY BY GRAVITATIONAL RADIATION — RIPPLES IN SPACETIME.

The two stars were growing closer now, their energy ebbing away. The spinning became more rapid, the stars moving too fast for her to see. When the stars were no more than their own diameter apart, disruption began. Great gouts of shining material were torn from the surface of each star and thrown out into an immense glowing disc that obscured her view.

At last the stars touched. They imploded in a flash of light.

A shock wave pulsed through the debris disc, churning and scattering the material, a ferocious fount of energy. But the disc collapsed back on the impact site almost immediately, within seconds, save for a few wisps that dispersed slowly, cooling.

“Has to form a black hole,” Malenfant muttered. “Two neutron stars… too massive to form anything less. This is a gamma-ray burster. We’ve been observing them all over the sky since the 1960s. We sent up spacecraft to monitor illegal nuclear weapons tests beyond the atmosphere. Instead, we saw these.”

THERE IS INDEED A BURST OF GAMMA RAYS — VERY HIGH-ENERGY PHOTONS. THEN COMES A PULSE OF HIGH-ENERGY PARTICLES, COSMIC RAYS, HURLED OUT OF THE DISC OF COLLAPSING MATTER, FOLLOWING THE GAMMA RAYS AT A LITTLE LESS THAN LIGHT SPEED.

THESE EVENTS ARE HIGHLY DESTRUCTIVE.

A NEARBY PLANET WOULD RECEIVE — IN A FEW SECONDS, MOSTLY IN THE FORM OF GAMMA RAYS — SOME ONE-TENTH ITS ANNUAL ENERGY INPUT FROM ITS SUN. BUT THE GAMMA-RAY SHOWER IS ONLY THE PRECURSOR TO THE COSMIC RAY CASCADES, WHICH CAN LAST MONTHS. BATTERING INTO AN ATMOSPHERE, THE RAYS CREATE A SHOWER OF MUONS — HIGH-ENERGY SUBATOMIC PARTICLES. THE MUONS HAVE A GREAT DEAL OF PENETRATING POWER. EVEN HUNDREDS OF METERS OF WATER OR ROCK WOULD NOT BE A SUFFICIENT SHIELD AGAINST THEM.

“I saw what these things can do, Madeleine,” Malenfant said. “It would be like a nearby supernova going off. The ozone layer would be screwed by the gamma rays. Protein structures would break down. Acid rain. Disruption of the biosphere—”

A COLLAPSE IS OFTEN SUFFICIENT TO STERILIZE A REGION PERHAPS A THOUSAND LIGHT-YEARS WIDE. IN OUR OWN GALAXY, WE EXPECT ONE SUCH EVENT EVERY FEW TENS OF THOUSANDS OF YEARS — MOST OF THEM IN THE CROWDED GALAXY CORE.

Madeleine watched as the Galaxy image was restored, and bursts erupted from the crowded core, over and over.