Starplex

A pause. "Now, with the course I've described, will the darmats be able to grab us again?"

Thor shook his head. "Not at the speed we'll be going, unless they're all lying in wait for us just outside the exit."

"Rhombus," said Keith, "as soon as Lia

"It'll be at least thirty minutes before we can do that," said Lia

Keith pursed his lips, and thought about Rissa. If she were gone, it would take all the billions of years he had left to get over the loss.

He looked at the smudges of galactic light against the abyss. He didn't even know which direction to look in, which way to concentrate his thoughts. He felt incredibly small, insignificant, and lonely beyond belief.

There was nothing to focus on in the holo bubble — nothing sharp, nothing well defined. Just an abysswan ego-crashing emptiness.

Suddenly there was a strange sound like a dog's cough from his left; PHANTOM translated it as an expression of "absolute astonishment."

Keith turned to face Jag, and his mouth hung open as he stared at the Waldahud. He'd never seen Jag's fur do that before. "What's wrong?"

"I — I know where we are," said Jag.

Keith looked at him. "Yes?" "You're aware that the Milky Way and Andromeda have about forty smaller galaxies bound to them gravitationally, right?" said Jag.

"The Local Group," said Keith, irritated.

"Exactly," said Jag. "Well, I started off by trying to find some of the Local Group's distinctive features, such as superbright S Doradus in the Large Magellanic Cloud. But that didn't work. So I sorted the catalog of known extragalactic pulsars by distance — which corresponds to age, of course — and used their signature radio pulses to orient myself."

"Yes, yes," said Keith. "And?"

"And the closest galaxy to us right now is that one there."

Jag pointed beneath his feet to a fuzzy spot in the hologram.

"It's about five hundred thousand light-years from here. I have identified it as CGC 1008; it has several unique attributes."

"All right," said Keith, sharply. "We're half a million light-years from CGC 1008. Now, for us nonastrophysics types, how far is CGC 1008 from the Milky Way?"

Jag's barking was subdued, almost soft. "We are," said the translated voice, "six billion light-years from home."

"Six… billion?" asked Thor, turning to face Jag.

Jag lifted his upper shoulders. "That is correct," he said, his voice still soft.

"That's… staggering," said Keith:

Jag lifted his upper shoulders. "Six billion light-years. Sixty thousand times the Milky Way's own diameter. Twenty-seven hundred times the distance between the Milky Way and

Andromeda." He looked at Keith. "In terms you nonastrophys-ics types might use, one hell of a long way."

"Can we see the Milky Way from here?" asked Keith.

Jag made a gesture with his arms. "Oh, yes," he said, his barking still subdued. "Yes, indeed. Central Computer, magnify sector 112."

A border appeared around a portion of the holographic bubble. Jag left his workstation and walked toward it. He squinted for a moment, getting his bearings. "There," he said, pointing. "That one there.

And that's Andromeda next to it.

And this is M33, the third-largest member of the Local Group."

Rhombus's lights twinkled in confusion. "Boundless apologies, but that can't be right, good Jag. Those aren't spiral galaxies. They look more like disks." "I'm not mistaken," said Jag. "That is the Milky Way.

Since we are now six billion light-years from it, we are seeing it as it looked six billion years ago."

"Are you sure?" said Keith.

"I am positive. Once the pulsars had told me approximately where to look, it was easy enough to identify which galaxy was the Milky Way, which was Andromeda, and so on. The Magellanic Clouds are too young for any light from them to have reached this far out, but globular clusters contain almost exclusively ancient first-generation stars, and I've identified several specific globulars associated with both the Milky Way and Andromeda. I am sure of it — that simple disk of star is our home galaxy."

"But the Milky Way has spiral arms," said Lia

Jag turned to her. "Yes, without question, the Milky Way today has spiral arms. And, just as surely, I can now say that when it was six billion years younger, it did not have spiral

"How can that be?" asked Thor.

"That," said Jag, "is a vexing question. I confess that I would have expected a Milky Way even half its present age to still have arms."

"Okay," said Keith. "So the Milky Way gains spiral arms sometime in the interim."

"No, it is not okay," said Jag, his bark returning to its usual sharpness. "In fact, it has never made any sense. We've never had a good model for galactic spiral-arm formation. Most models are based on differential rotation — the fact that stars near the galactic center make several orbits around the core in the time it takes for those farther out to complete just one. But any arms that resulted because of that should be temporary phenomena, enduring at most for a billion years. Oh, we should see some spiral galaxies, but there is no way that three out of every four large galaxies should be spirals — which is the ratio we actually observe. Ellipticals should far outnumber spimls, but they do not."

"Obviously, then, there's a flaw with the theory," said Keith.

Jag lifted his upper shoulders. "Indeed. We astrophysics types have been limping by for centuries with something called 'the density-wave model' for explaining the abundance of spiral galaxies. It proposes a spiral-shaped disturbance that moves through the medium of a galactic disk, with stars getting caught up in it — or even being formed by it — as the wave rotates. But it has never been a satisfactory theory.

First, it fails to account for all the different types of spiral forms, and, second, we don't have a good answer as to what would cause these imagined density waves in the first place. Supernova explosions are sometimes cited, but it's just as easy to model such explosions canceling each other out as it is to get them to build up long-duration waves." He paused. "We've had other problems with our galaxy-formation models, too. Back in 1995, human astronomers discovered that distant galaxies, observed when they were only twenty percent of the current age of the universe, had rotational rates comparable to what the Milky Way has today — that's twice as fast. as they should have been rotating at that age, according to theory."

Keith thought for a second. "But if what we're seeing right now is correct, then spiral galaxies like ours must somehow form from simple disks, right?"

Another lift of the Waldahud's upper shoulders. "Perhaps.

Your Edwin Hubble proposed that galaxies each start as a simple sphere of stars, gradually spin out into a flat disk, then develop arms that open up more and more over time.

But although we now have observational proof that that sort of evolution does indeed happen" — he gestured at the disk of stars in the glowing frame — "we still don't have an explanation for why the evolution takes place, or why the spiral structures persist."

"But you say three quarters of all large galaxies are spirals?" asked Lia

"Wellll," said Jag, PHANTOM translating a hissing bark as a protracted word, "actually, we don't know much directly about the ratio of elliptical to nonelliptical galaxies in the universe at large. It's hard to make out structure in dim objects that are billions of light-years away. Locally, we see that there are many more spirals than there are ellipti-cals, and that spirals contain a preponderance of young blue stars, whereas our local ellipticals contain mostly old red stars. We've assumed, therefore, that any vastly distant galaxy that showed lots of blue light — after correcting for redshift, of course — was a spiral, and any that showed mostly red was an elliptical, but we really don't know that for sure."