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    • CommentRowNumber1.
    • CommentAuthorUrs
    • CommentTimeFeb 7th 2012
    • (edited Feb 7th 2012)

    One day I hope to find the time to make universe a disambiguation page that also points to the physical meaning of the term.

    For the time being: just a stub for observable universe, containing nothing but one link to a video. But one worth linking to.

    • CommentRowNumber2.
    • CommentAuthorzskoda
    • CommentTimeFeb 7th 2012

    I added link from universe.

    For a physical notion of universe see observable universe.

    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeMar 24th 2014

    Added some actual text to observable universe. Right now it reads as follows:


    On the very largest scales observable in astrophysical experiment, the cosmos is well described by an FRW model with cosmological constant Λ\Lambda, with plenty of dark matter and with primordial cosmic inflation (called the Λ\Lambda-CDM “concordance model of cosmology”). This and everything on smaller scales is the observable universe.

    There has never been a reason to assume that beyond this cosmic horizon visible to us today, the cosmos would not extend further. In fact in the simple FRW models with positive cosmological constant, the spacetime manifold is not a closed manifold and extends indefinitely beyond our observable horizon. This is directly analogous to the ancient mariner who would stand on the deck of his ship and see a few miles of ocean around him. That was the world within his horizon, but there was no telling what lied beyond.

    Therefore it is important to distinguish the observable universe from the universe as such. Unfortunately, intellectual laziness tended to ignore this distinction and at some point some people who said “universe” to mean just the stretch of our cosmic horizon felt the need to have a new term for what whatever may lie beyond. For better or worse, that new term has become wide-spread these days and is “multiverse”. The main scientific observation that goes with this is the observation that cosmic inflation – for which there is by now excellent and ever increasing experimental evidence that indeed it happened – does, at least by the simple method which is currently used to model it, naturally predict a large ambient space in which local regions stochastically undergo inflationary expansion. This scenario of chaotic inflation has in fact been singled out as the most likely one (within the standard model) by the bicep2-experiment in 2014.

    In any case, as far as the observable universe is concerned, two facts are worth noticing:

    1. the observable universe alone is already pretty darn large, see the various discussion of its scales below;

    2. Giordano Bruno’s old intuition remains plausible even at the largest scales: possibly the full xyz-verse beyond our present horizon is vastly larger still… and we may never know.

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