In the far future humanity has begun to colonise the galaxy. Re-engineered into a clade of related speicies (both coporeal and pure software), humanity has realised that it is alone, and the only other life discovered so far are simple forms. Worlds have been evacuated to protect a single species of microbes, in the hope that eventually they'll evolve to intelligence). Immortal and, near enough, all powerful, human civilisation is stagnating. There are no challenges - even colonising a new world is a matter of depositing a few packages on a barren world and waiting for a few thousand years. On a remote space station a researcher (in a millimetres-high body) is exploring the ramifications of an obscure section of a successor theory to loop quantum gravity, by radically modifying the shape of space-time.
Unfortunately (or is it fortunately?) the experiment creates a whole new region of space-time that starts expanding into ours at half the speed of light. The nuovo-vacuum is a new universe that's eating ours from the inside out. Six hundred years later and there's a plan to try and stop the expansion - or even reverse it, using engineered space-time viruses. But then a set of accidental discoveries change everything, and drag us along on a great imaginative travelogue. The universe behind the border is not only complex, built of deeply entagled quantum phenomena - it also appears to contain life. In fact, lots of life. And some of it may well be intelligent.
This is Egan-lite. The science is rigorous, and the ideas enticing, but it lacks the bravado and depth of Permutation City or Diaspora. Instead of Egan's customary philosophical debate we follow cardboard characters through repetitive discussions and arguments (and rare for Egan, even acts of violence). It's only in the last 70 or so pages of the novel, as we explore the inside of the nuovo-vacuum, that we see Egan catch fire - finally producing some of the enticing descriptions of the indescribable that only good SF can provide.
Worth reading? It's hard to say. Egan has always been strongest at short lengths, and the final section might have been best seen as a novella rather than a section of a slow (and in some places, downright tedious) novel.
Stick with the shorts.