‘True, although you never got that far, did you? You saw that kernels were associated with Hatch emplacements, of course. But, Stef, you never understood how the presence of kernels facilitates the setting up of a Hatch in the first place. You never even discovered the process by trial and error, as did the Romans, the Incas.

‘Stef, there is actually only one kind of technology here. Kernels are Hatches; a Hatch is a specialised form of kernel. The Hatches emerge when a kernel field is perturbed by an energetic event – I imagine it is almost an organic process, a self-selection, as a single dominant tree will emerge from a grove of saplings.’

‘Maybe. But what about the energy? For all the decades I spent studying those beasts we never came close to understanding where that energy came from.’

‘True,’ said Earthshine. ‘And I was never allowed access to kernels and Hatches to study them for myself. I had to rely on your work, at second hand. How much time was wasted!’

‘We guessed stellar cores, supernovas, gamma ray bursters, quasars—’

‘Wrong, wrong and wrong again. Remember, Stef, both kernels and Hatches are forms of wormholes. As we have experienced ourselves, a wormhole can link events separated by space and by time. We walked through a Hatch from the Mars of Inguill’s Inca era, the human age, to – this, a world light years distant and well over three billion years separated in time.’

‘Yes,’ said Stef. ‘But these wormholes aren’t as they were predicted by our own science, by relativity. They were rips in space and time held open by impossible kinds of antigravity … You could have travelled faster that light through Einstein’s wormholes. And you could have dragged such a wormhole around with a sublight ship to make a functioning time machine. But this is different. Kernel/Hatch wormholes are sublight. But they can link different universes. And so you could connect the present of one universe to the past or the future of another …’ The pieces of the puzzle moved around inside her head. ‘That’s it. If you’re right about the nature of the multiverse, then all the universes in our local ensemble share the same future, if you look ahead far enough …’

‘They all must face the End Time,’ the ColU said.

Earthshine said, ‘And that, Stef, is the answer to where a kernel’s energy comes from. Not from some quasar, from some point distant in space. It comes from a point distant in time—’

‘The future.’ Stef saw it now. ‘The End Time itself.’

‘Yes. You have it. The End Time will be a hugely energetic event. The Dreamers have tapped into that very energy, using the kernels, in order to build their Hatch network. Now, we multicellular toy-creatures are allowed to play with the technologies, to build our kernel-driven starships and to wage our wars, but—’

‘But it’s all secondary to the true purpose,’ Stef said. ‘Which is for the Dreamer worlds to be linked to each other. You know, my father saw this, right at the beginning. He sensed that whoever was giving us kernels – he never lived to learn about Hatches – had some agenda of their own. He was wary about that.’

‘He was right to be. Humans, however,’ Earthshine said softly, ‘could never resist such deadly toys. Even if they were powered by the energies of Ragnarok itself.

‘So the Hatch network spread. So the worlds were linked, as never before; so they learned and grew.

‘But that’s not the end of the story. For even this was not enough. The time left, mere billions of years, seemed horribly short to such minds as the Dreamers. And so, having intervened several times before in the destiny of life in the galaxy, now they intervened again. Seeking to find a way to have us serve their needs even more completely …’

As the final months passed Titus Valerius led many expeditions back to the nearside of Per Ardua. Given the gravity-tunnel network, the terminator was only days away; they always needed supplies, so why not travel back?

Titus didn’t retrace the journey that they had made to get here every time. He and his companions took the chance to explore the rest of the branching gravity-train system that fanned out across the dark face of Per Ardua, and to study different regions of the terminator and the edge of the star-facing side. This amounted to a kind of inspection of the tunnel system itself, of course, and Titus did report a few breakages, even collapses, times they had had to come back the way they’d travelled and find another route. The tunnel system was tremendously ancient and wonderfully robust – Stef joked in the silence of her head that it had kept working almost to the end of the universe itself – but nothing was perfect, it seemed.

Titus never forgot his primary purpose. Each time he returned he would faithfully deliver a sled full of root vegetables and fruit, plus anything exotic he found, such as, once, what looked and tasted like peaches.

But he also brought home specimens he thought might be of interest to the ColU or Earthshine. The ColU had specifically asked for samples of stems of any kind, the rod-like forms that had once been the fundamental unit of complex life on Per Ardua. And once Titus brought back a miniature stromatolite, a cylinder maybe a metre and a half tall, half a metre wide. He and Chu dragged this thing home strapped to the bed of the sled with ropes.

They had already given over part of the dome to a ‘Per Ardua garden’, where the ground-up rock floor had been laced with native soil, and the ColU was growing his stem samples and other native forms. Here they planted the stromatolite, bedding it deep in the worked ground. Not even the ColU had any experience of transplanting stromatolites before, and the little community spent some days fretting over the health of its new arrival before the stromatolite seemed to flourish, with its bronze-coloured carapace acquiring a new sheen. It was another example of the integration of life, Stef supposed, of living beings from different stars working together: humans from Earth tending a stromatolite from Per Ardua.

And it was the lack of time in this doomed universe for integration, of biospheres and cultures and minds, that had driven the Dreamers to attempt their most radical rebuilding.

‘Even humans had such fantasies,’ Earthshine said. ‘Of cultures crossing the stars and coming together. Perhaps there would be conflict at first, but in the end there would be integration. A galaxy united under a common civilisation – imagine it.’

‘I remember some of the scientists’ dreams,’ Stef said. ‘Perhaps if mind could encompass the universe it could change its destiny. Save it from a Big Crunch, or a Big Rip. Make the universe better than nature intended.’

‘Or at least,’ the ColU said, ‘mind, by filling the universe, could observe it. And thereby make its existence worthwhile.’

‘But there is no time for any of this,’ Earthshine said now. ‘No time! Not in a universe with such a short lifespan, and constrained by lightspeed. Even a single galaxy is too large, the Dreamers concluded, to be united in such a time. The Dreamers grew restless – though that’s an odd word to apply to billion-year-old minds. They wanted more time. But there was no more to be had, not in the future.’

‘Ah.’ Stef nodded. ‘I think I see where this is going. To gain more time, they started to reach, not into the future – but into the past.’

‘You have it. Remember, the Dreamers were becoming masters of wormhole technology; they had kernels and Hatches. By tapping the End Time event itself they had an effectively infinite energy supply. Now they began to reach out, not across time and space in this universe, but to other universes entirely. Universes with different histories.’

Stef laughed. ‘Of course. I see it now. Suppose you’re dissatisfied that humans in my reality sheaf, the UN-China culture, didn’t even start to work with Hatches until the twenty-second century. You wish it had been earlier. Well, then, you simply pluck another copy of the universe from the tree of possible realities, one where we did get to the Hatches earlier.’