General Caption: Image 1: A diagram of the observable universe. Image 2: A cartoon observable universe embedded in a much larger surrounding outer universe.
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However, the farther you look out, the further back in cosmic time you look because of the finite vacuum light speed c = 2.99792458*10**5 km/s ≅ 3*10**5 km/s. So we see the observable universe as it looked further in the past, the farther out we look. Astronomers have this great advantage over historians: we can literally see the past.
For all larger astronomical distance scales, see astronomical_distances_larger.html.
Since Hubble constant value changes a bit with every new measurement, it is convenient to write H_length in terms of the reduced Hubble constant h_70 = H_0/(70 (km/s)/Mpc) as we have done above.
The favored Hubble constant value circa 2018 is 67.74(46) (see Wikipedia: Λ-CDM model parameters). Revisions by a few percent are possible.
There is no reason to believe the particle horizon defines the boundary of the whole universe---we don't have to follow Aristotle (384--322 BCE) anymore.
So the observable universe is very probably embedded in a much larger universe which is much the same as the observable universe for a long way. This probability is because the observable universe shows no signs of a boundary and has homogeneity and isotropy: i.e., it obeys the cosmological principle.
However, well beyond the particle horizon according to the multiverse paradigm, there could be boundary between our pocket universe and a false-vacuum background universe with other pocket universes.