# How far are we from the edge of the Universe? [duplicate]

Following the Big Bang the Universe continues to expand, presumably and roughly equally in all directions. It is understood that the Big Bang occurred 13.798 ± 0.037 billion years ago.

Is there any way for us to know how far we are away from the nearest edge of the expansion front of the Big Bang?

## 1 Answer

In the standard models of the Big Bang, there is no such thing as the "edge of the expansion front". The universe, as far as we can see and as far as standard cosmology assumes, is homogeneous and isotropic on the large scale, so there is no edge or anything analogous to an the shock front of an explosion.

According to the seven-year WMAP results [pdf], the proper distance to the surface of last scattering that emitted the cosmic microwave backgrounds is approximately $46.0\,\mathrm{Gly}$, which is basically as far as we can actually see. The true cosmological horizon slightly more distant that this, closer to $46.6\,\mathrm{Gly}$. What's beyond the horizon is not known.

Another cosmologically significant distance scale is the Hubble radius, at which the galaxies comoving with the Hubble flow recede from us at the speed of light. It is $c/H_0 = 13.9\pm0.3\,\mathrm{Gly}$.

• If the curvature of the geometry of the universe is such, then it's very probable that one looking far enough with a telescope would see his own back. Commented Jan 16, 2014 at 19:59
• @IstvánZachar: There's a cosmological horizon preventing that, but even ignoring that problem, if we're keeping the assumptions of homogeneity and isotropy, that would only be possible if the spatial curvature of the universe is positive. That's rather uncertain. (If those assumptions are left out, then then positive curvature is not required but there are many, many more possibilities.) Commented Jan 16, 2014 at 20:28
• Given that the scattering happened "here", when you say we are 46 Gly away from it, do you mean that as a measurement of its (relativistic) red shift, or something else? Commented Jan 18, 2014 at 17:43
• Just worth adding that the "cosmological principle" suggests that "what is beyond the horizon" is something that (in this epoch) looks like just where we are. Of course we are also peering back in time, so if we could "see" beyond the horizon we would not see something like our neighbourhood! The principle is, in its way, the ultimate extension of the Copernican principle and has, in general, provided to be a good basis on which to build cosmology. But it's also flawed in the sense that if everywhere looked the same there would be no concentrations of matter at all, just an even smear. Commented Jan 18, 2014 at 17:47
• @adrianmcmenamin: The scattering that produced the CMBR we see definitely happened "there", since light took over $13\,\mathrm{Ga}$ to reach us "here". Scattering also happened "here", but we don't see that light, so it's not relevant. And by $46\,\mathrm{Gly}$, I mean proper distance in the comoving frame. Additionally, in this context the "cosmological principle" winds up being "the universe is homogeneous and isotropic." It's only meant to apply on the large scale, so there's a kind of "averaging" involved. Commented Jan 18, 2014 at 19:45