Xeelee: An Omnibus: Raft, Timelike Infinity, Flux, Ring

by Stephen Baxter

  ‘Yes . . . But—’

  Spinner let out a mock groan. ‘But now you’re going to tell me how things just ain’t what they used to be, again, aren’t you?’

  ‘Well, it’s true, Spinner,’ Louise said angrily. ‘Look at it . . . Even from this distance, outside the Galaxy, you can see the handiwork of those damn photino birds.’

  The Galaxy contained two main classes of stars, Louise told Spinner. Population I stars, like the Sun, had evolved in the hydrogen-rich spiral arms, away from the centre. Some of these - like the blue supergiants - had been hundreds of times larger than the Sun, blazing out their energy in a short, insanely profligate youth. Population I stars tended to explode, enriching the interstellar medium - and later generations of stars - with the complex products of their nucleosynthesis.

  By contrast, Population II stars had formed in regions where hydrogen fuel was in scarce supply: in the old regions close to the core, or in the clusters outside the main disc. The II stars were more uniform in size, and - by the era of the earliest human astronomy - had already been old, characterized by jostling herds of red giants.

  ‘Look at that disc,’ Louise snapped. ‘I don’t suppose the damn birds had to do much to the dull, stable Population IIs; those things were half-dead already. But look - oh, look at the spiral arms . . .’

  Spinner saw how ragged the spirals were, disrupted by the blisters of yellow-red light which swelled across the lanes of dust.

  ‘Those blisters are supernova remnants,’ Louise said bitterly. ‘Spinner, not every star would respond as peacefully to the photino birds’ engineering as did our poor old Sun. A lot of the more spectacular, and beautiful, Population I stars would simply explode, tearing themselves apart . . . Probably the birds set off chain reactions of supernovae, with the wreckage of one star destabilizing another.’

  Spinner stared up at the wreckage of the disc, the muddled spiral arms.

  . . . We’re already forty thousand light-years below the disc, Spinner, her companion said. The light you’re seeing now left the stars forty millennia ago . . . Think of that. Forty thousand years before my birth, humans were still shivering on the edges of glaciers, making knives out of bits of stone. And the further we travel, with every second, the light is getting older: Spinner-of-Rope, you’re taking us through a hail of ancient light . . .

  Spinner laughed. ‘You should have been a poet.’

  ‘What?’

  ‘ . . . Tell me what’s coming next, Louise.’

  ‘All right. Spinner, do you know what a globular cluster is?’

  Spinner frowned. ‘I think so.’ She closed her eyes. ‘A stable ball of stars - perhaps a hundred thousand of them - orbiting around the main disc, in the Galactic halo.’

  ‘Right,’ Louise said. ‘They are Population II stars. And one particular cluster, called Omega Centauri, was one of the brightest clusters visible from old Earth.’

  Spinner thought that over. ‘Omega Centauri. That name means it was in the line-of-sight of the Centaurus constellation.’

  ‘Right.’

  ‘You mean—’

  ‘We’re heading right for it. Keep your eyes tight shut, Spinner-of-Rope.’

  Spinner turned, and looked ahead.

  Beyond the fragile cage, giant stars ballooned at her, dazzling her with their billowing silence.

  24

  Upright on their zero-gee scooters, Lieserl and Milpitas descended into the deep loading bay at the base of the Northern’s lifedome. Above Lieserl the maintenance bulkhead at the base of Deck Fifteen spread out, an improbable tangle of ducts, cables and tree roots.

  From the corner of her eye, Lieserl watched Milpitas curiously. He looked down at the drop beneath his feet with undisguised dread. Milpitas had been a starship traveller for a thousand years, but he was so obviously a gravity-well dweller. He visibly suffered in this zero-gee environment, his instincts quite unadapted to the fact that even if his scooter failed completely he’d simply drift through the air, perfectly safely.

  Beneath the thick layer of dank, empty air into which she was descending, the base of the Northern’s lifedome had been turned transparent. The base appeared to Lieserl as a pool of cool darkness - and there, pinned against the underside of the lifedome base, like some immense insect immersed in a pond, was the slender form of the Xeelee nightfighter which bore them through space. Its sycamore-seed wings looked somehow darker even than the emptiness between stars.

  The Planner turned to her stiffly and smiled. ‘You look - uncomfortable - on that scooter.’

  She suppressed a grin. Me? ‘Uncomfortable? Not really.’ She clicked her fingers and her scooter disappeared. She smiled at Milpitas, feeling mischievous. She did a back flip in the air, rolling twice; the clear floor beneath her wheeled across her vision.

  She finished up falling alongside Milpitas once more. ‘I don’t feel uncomfortable, ’ she said. ‘Just - well, a little foolish. Sometimes I feel these Virtual masks Mark sets up for me are a little forced.’

  Milpitas had turned away from her antics, his face pale; he gripped the handles of his scooter so hard his knuckles were white.

  Hastily she called subvocally for the return of her Virtual scooter. I’m sorry,’ she said, sincerely. ‘I guess I shouldn’t have done that.’

  She saw how the sweat glistened on the patchwork scars of his brow, but he determinedly held himself erect on his scooter. ‘Don’t apologize,’ he said primly. ‘We’re here on an inspection tour . . . to consider the disposition of the ship, not my well-being.’

  So, after that brief moment of human frailty, Milpitas was back in his shell. She turned away, vaguely disappointed.

  They were approaching the base of the loading bay, now. Lieserl could see the twin small jets of her scooter reflected in the clear floor; like attracting stars, she converged with her own image - in fact it was an image of an image, she thought wryly; the processors which sustained her were doing a good job with their Virtual reality creation today.

  Milpitas, with a tense flick of his bony, scarred wrist, levelled off and began to sail parallel to the surface. Lieserl followed, a few feet behind.

  Beneath the dome base, the Xeelee nightfighter spread its construction-material wings, huge, dormant.

  ‘Good morning, Spinner-of-Rope,’ Louise said.

  Spinner stretched. Allowing herself to wake up slowly, she sucked fortified fruit juice from her helmet nipples and let the environment suit clean her skin with blasts of ultrasonics; she felt a warm trickle of urine enter her catheter.

  She grunted in reply to Louise.

  It was Spinner’s tenth day in the nightfighter cage.

  She loosened her restraints and looked around - and found herself staring into intergalactic emptiness. In the distance were patches of muddy light which could have been galaxies, or clusters of galaxies - so remote that even at the ‘fighter’s immense speed of three million light-years a day, she could make out no discernible movement.

  Spinner slumped back into her couch. ‘Lethe. Another day in the middle of this grey, lifeless desert,’ she said sourly.

  Louise - watching, Spinner knew, from her encampment on the Northern’s forest Deck - laughed, sounding sympathetic. ‘But today should be a little more interesting than most, Spinner-of-Rope. We’ve reached a milestone. Or rather, a mega-light-year-stone . . .’

  ‘We have?’

  ‘After ten days, we’ve come thirty million light-years from Sol. Spinner, we’ve reached the centre of the Virgo Cluster - the supercluster of galaxies of which our Galaxy is a member. Way behind you is a little patch of light: that’s the Local Group - three million light-years across, the small cluster dominated by our Galaxy and the Andromeda galaxy. And to your left, at about eleven o’clock, you’ll see the centre of the Virgo Cluster itself: that massive group of several thousand bright galaxies. They used to be bright, anyway . . .’

  Spinner made out the central galaxy group. It was a grey, grainy cloud of light. ‘Fascin
ating.’

  ‘Oh, come on, Spinner. Look, we’re making an epic journey here - we’re travelling so far we’re making progress through the large-scale structure of spacetime. You can’t fail to be - well, uplifted.’

  ‘But I can’t see any of it, Louise,’ Spinner said fretfully.

  Louise was silent for a moment. Then she said, ‘All right, Spinner. I’ll show you where you are.’

  A ball of brilliant white light, expanding rapidly to about a foot across, appeared a few yards in front of the ‘fighter cage.

  Spinner slouched in her couch and folded her arms. ‘Another educational Virtual display, Louise?’

  ‘Bear with me, Spinner-of-Rope. Look at this. Here’s the Universe, expanding from the Big Bang - as it was after perhaps three hundred thousand years. The cosmos is a soup of radiation and matter - a mixture of the dark and light variants.

  ‘The temperature is still too high for atoms to form. So the baryonic matter forms a plasma. But plasma is quite opaque to radiation, so the pressure of the radiation stops the matter from clumping together. There are no stars, no planets, no galaxies.’

  Abruptly the Virtual Universe expanded to double its size, and turned clear; a flash of light flooded out over Spinner’s face, making her blink.

  ‘Now the temperature has fallen below three thousand degrees,’ Louise said. ‘Suddenly the electrons can combine with nuclei, to form atoms - and atoms don’t interact strongly with photons. So the Universe is transparent for the first time, Spinner. The radiation, free to fly unhindered across space, will never interact with matter again. And in fact we can still see the primordial radiation today - if we care to look, its wavelength greatly stretched by the expansion of the Universe - as the cosmic background microwave radiation.

  ‘But the key point is, Spinner, that after this decoupling the radiation could no longer stop the matter from clumping together.’

  The model Universe was now a cloud of swarming, jostling particles.

  ‘It looks like a mist,’ Spinner said.

  ‘Right. Think of it as like a dew, Spinner. It’s spread out thin and uniform: on average there’s one hydrogen atom in a space the size of one of our transport pods. And at this point the expansion of the Universe is pushing the dewdrops still further apart. But now, the structures of matter - the galaxies, the clusters and superclusters of galaxies - are ready to coalesce; they’ll condense out like dewdrops on a spider web.’

  Spinner smiled. ‘Some spider. But where’s the web?’

  The ball of mist was filled, now, by a fine tracery of lines; the toy Universe looked like a cracked, glass sphere. ‘Here’s the web, Spinner,’ Louise said. ‘You’re looking at cosmic strings. Strings are defects in spacetime—’

  ‘I know about string,’ Spinner said. ‘The Xeelee used strings - and domain walls - in the construction of the nightfighter.’

  ‘Right. But these strings formed naturally. They are remnants of the phase transitions of the early Universe, remnants left over after the decomposition of the GUT unified superforce which came out of the singularity . . . Cosmic strings are residual traces of the ultrahigh, symmetric vacuum of the GUT epoch, embedded in the “empty space” of our Universe - like residual lines of liquid water in solid ice. And the strings are superconducting; as they move through the primordial magnetic fields, huge currents - of a hundred billion billion amps or more - are induced in the strings . . .’

  The strings writhed, like slow, interconnected snakes, across space. The particles of mist, representing the uniform matter distribution, began to drift towards the strings. They coalesced in narrow columns around the strings, and in thin sheets in the wake of the strings.

  ‘It’s beautiful,’ Spinner said.

  ‘The strings are moving at close to lightspeed,’ Louise said. ‘They leave behind them flat wakes - planes towards which matter is attracted, at several miles a second. Structure starts to form in the wakes, so we get a pattern of threads and sheets of baryonic matter surrounding voids . . .’

  Now the baryonic matter, coalescing around the string structure, imploded under its own gravity. Tiny Virtual galaxies - charming, gem-like - twinkled to life, threaded along the webbing of cosmic string.

  ‘And there’s more,’ Louise said. ‘Look at this.’

  Now there was a loop of cosmic string, twisting in space and oscillating wildly.

  ‘String loops can form, when strings cross each other,’ Louise said. ‘But they’re unstable. When loops form they decay away rapidly . . . unless they are stabilized, as the Xeelee have made stable their nightfighter wings. Now: remember I told you that the strings are superconducting threads, carrying immense electrical currents? When the strings decay, all that electromagnetic energy has to go somewhere . . .’

  Abruptly the loop shrank, precipitately, and once again light blasted into Spinner’s face.

  Spinner lifted her hand to her faceplate. ‘I wish you’d stop doing that,’ she said.

  ‘Sorry. But watch, Spinner. See what’s happened?’

  Spinner dropped her hand and blinked dazzled eyes.

  The explosion of the loop of string had blown out a huge hole, in the middle of the mesh of galaxy threads.

  Spinner nodded. ‘I get it. There’s a pulse of electromagnetic energy, which blows a bubble in the clouds of matter.’

  ‘Not quite,’ Louise said. ‘Spinner, remember that dark matter is transparent to photons - to electromagnetic radiation. So the loop’s electromagnetic pulse blows out just the baryonic matter; it leaves a hole, filled by dark matter but scoured clean of star stuff.

  ‘Spinner, all this cosmic engineering induced by the strings - the primordial seeds - has left us with a fractal structure. Fractal means the foam has the same general structure at all scales. It looks the same, no matter how far out or how close in you study. Our Galaxy is part of a small cluster - the Local Group - which, together with several other clusters, is part of a supercluster called the Virgo Cluster . . . which in turn—’

  ‘I get the idea,’ Spinner said.

  ‘The baryonic matter is clustered in filaments and sheets, around huge voids filled only with dark matter. It’s like a froth, Spinner - and it’s a very active froth, like an ocean’s surface, perhaps; the strings are whipping through space at near lightspeed, and so there are huge movements, currents in the foam.’

  ‘Louise, you said you’d show me where I am.’

  ‘All right, Spinner . . .’

  Below the glistening glass the curves of the nightfighter rippled like some immense sculpture. There was Xeelee construction material only feet away from her now, and Lieserl had an urge to reach out and stroke it, as if the ‘fighter were some immense, caged animal. But the material was separated from her both by the base of the lifedome and by a layer of hard vacuum - and, she thought ruefully, by a layer of unreality which only Mark Wu and his gadgets could breach.

  ‘You’re thoughtful,’ Milpitas said.

  She rubbed her chin. ‘I was thinking how very alive this Xeelee ship looks. Not like a piece of technology at all. This is like some immense ocean beast, trapped beneath a frozen surface; it’s as if I can see muscles beneath that skin of construction material.’

  Milpitas grunted. ‘It’s an attractive image,’ he said dryly. ‘Although I’m not entirely sure how helpful it is.’

  Lieserl glanced up at the maintenance layer, a fifth of a mile above her, with its tangle of tree roots and plumbing conduits. ‘Look at that primitive mess up there, by contrast . . . Lethe’s waters, Milpitas, this was a starship designed to last a thousand years. Some of that design looks as if it predates the Romans.’ She sighed. ‘You know, I caught a few glimpses of human technology, as we advanced over the years after the Northern’s launch. Obviously, we got better with time. But we always - always - ended up carrying our damn plumbing with us. I don’t think humans ever, in their long history, ever came close to matching the simple perfection of this one Xeelee artifact, this nightfighter.’<
br />
  Milpitas dipped closer to the transparent base surface and peered through it, intent. ‘Perhaps you are right. But does that imply we should bow down and worship the Xeelee and all their works?’

  ‘No,’ she said coldly. ‘But it does imply that the Xeelee were smarter than we ever were, or could have become.’

  She saw his eyebrows rise, through a fraction of an inch; otherwise he didn’t reply.

  Now they were close to the rim of the base, near the transparent, curving wall of the loading bay. Here, the broad shoulders of the ’fighter nestled against the underside of the base; thick bands curled from the base around the ‘fighter’s curves and out of sight, hugging the ‘fighter against the lifedome.

  Milpitas leaned over the control bar of his scooter, peering at the attaching bands. He seemed quite fearless, Lieserl thought with some amusement, now that he was only a few feet above the lifedome base: close to the floor of his rigid, gravity-dominated mental universe.

  She allowed herself to sail smoothly along the lines of the Xeelee ship. Shoulders - yes, that was a good label for this part of the ‘fighter, at the root of the wings; here, so close to the ship, she had a real sense of being carried, on the broad, strong shoulders of some giant of construction material.

  Milpitas straightened up from his inspection.

  ‘So how’s the engineering?’ she asked.

  ‘Fine,’ he said, without looking up. ‘That is, within tolerance limits . . . The creep is minimal today.’

  ‘Creep?’

  He studied her. ‘Perhaps you’re not aware of the problems we faced, fixing the lifedome to this nightfighter. Lieserl, Xeelee construction material is effectively frictionless, and it is harder than any material substance known to us. It’s impervious even to exotic matter . . . You know we’ve speculated its manufacture may have violated the Pauli Exclusion Principle—’

  ‘I heard about that.’

  ‘So when we came to attach the lifedome, we couldn’t simply nail a superstructure to the nightfighter. No known adhesive would adhere to the construction material either. So, instead, we constructed a loose cage around the ‘fighter.’