the figure 1.73 is approximate...given uncertainties, somewhere around 1.7. We see galaxies out beyond z = 7, of course, and the ancient background light has redshift 1090, so we see ancient matter emitting light, before it even formed stars and galaxies. But as of now, the standard model says, nothing that happens more than 15.7 billion lightyears away can affect us, from now on. And that 15.7 distance corresponds to redshift z = 1.73 In a curious way our "from now on" causally effective universe only extends to the socalled "cosmic event horizon" at z = 1.73. All the photons that will ever reach us from OUTSIDE that horizon are already on their way to us and already inside that range. If they were not, they would be destined to not ever reach us. Most of the galaxies we can see are outside the z = 1.73 range. And we can look forward to admiring these millions of beautiful galaxies for a great many more years. But if something happens today in one of them--like a supernova--we will never see it. Nothing that happens to them or in them from now on can affect us. There is a good article about the cosmic event horizon by Charles Lineweaver here http://arxiv.org/abs/0909.3983 also a new article by Nobelist George Smoot et al. http://arxiv.org/abs/1002.4278 ================== This leads to a curious question. Our model of the cosmos is the pair of Friedman equations. These run on "universe time" a universal time parameter t, and govern the evolution of the scalefactor a(t). What the model predicts will happen with a(t), what it says about future expansion, depends only on the present. That's just as we expect with any system of ordinary differential equations. Normally one thinks of the Friedman model as treating the whole boundless universe as an organic whole. But maybe we should modify the Friedman equations by introducing a boundary term representing the cosmic event horizon. This seems to be what the co-authors Smoot, Easson and Frampton are doing. I suppose it could be thought of as primarily a formal change: a change of interpretation or in how we think about the model. But right now I find it a bit puzzling. If other cosmologists go along with this, it will take some getting used to.