the asinine cretin Posted May 31, 2012 Share Posted May 31, 2012 If the universe is dozens of orders of magnitude larger than the observable universe would it be reasonable to question homogeneity and isotropy? (My guess is that this would conflict with the theory that undergirds inflation in the first place.) What is the smallest actual size of the cosmological horizon allowed if we assume the inflationary epoch? Thanks, people. Link to comment Share on other sites More sharing options...
EMField Posted August 11, 2012 Share Posted August 11, 2012 (edited) It is endless, not orders of magnitude larger, 14-20 billion years distant is all you can observe because of the intervening dust and plasma. Not because that is the end. http://www.space.com...nly-bright.html And when we get a view through this dust and plasma, what do we see? http://news.sciencem...est-spiral.html A spiral galaxy where none has the right to be according to standard cosmology. How spiral galaxies are formed is already answered. http://farside.ph.utexas.edu/teaching/302l/lectures/node73.html Edited August 11, 2012 by EMField -1 Link to comment Share on other sites More sharing options...
imatfaal Posted August 13, 2012 Share Posted August 13, 2012 ! Moderator Note I split this question and response from the Cosmo Basics thread. Please use a new thread to ask new questions rather than branching a pre-existing question. BTW EMField - the observable universe is not a limit placed by technical boundaries. The observable universe is that portion of the universe that we could possibly see/measure - beyond that distance the background space is expanding such that even light could never cross the gap to us. In an expanding universe - especially one in which the expansion is accelerating - there are volumes of space that are forever cut off from us, as even at light speed the gap gets bigger over time and we can never receive any light or influence from those volumes. Link to comment Share on other sites More sharing options...
StringJunky Posted August 13, 2012 Share Posted August 13, 2012 ! Moderator Note I split this question and response from the Cosmo Basics thread. Please use a new thread to ask new questions rather than branching a pre-existing question. BTW EMField - the observable universe is not a limit placed by technical boundaries. The observable universe is that portion of the universe that we could possibly see/measure - beyond that distance the background space is expanding such that even light could never cross the gap to us. In an expanding universe - especially one in which the expansion is accelerating - there are volumes of space that are forever cut off from us, as even at light speed the gap gets bigger over time and we can never receive any light or influence from those volumes. Being causally disconnected beyond the observable universe I suppose that effectively means we are on an island surrounded by nothing. Link to comment Share on other sites More sharing options...
imatfaal Posted August 13, 2012 Share Posted August 13, 2012 The cosmological principle - homogeneity and isotropy - means at the very least that we assume that everything/observer in the (observable) universe is in the same situation as us; if you start limiting this (island surrounded by nothing) then you might have logical difficulties but I am not sure. Link to comment Share on other sites More sharing options...
michel123456 Posted August 13, 2012 Share Posted August 13, 2012 Being causally disconnected beyond the observable universe I suppose that effectively means we are on an island surrounded by nothing. An object at the limit of our observable universe can be causally connected to another object outside of our observable universe. Link to comment Share on other sites More sharing options...
StringJunky Posted August 13, 2012 Share Posted August 13, 2012 (edited) The cosmological principle - homogeneity and isotropy - means at the very least that we assume that everything/observer in the (observable) universe is in the same situation as us; if you start limiting this (island surrounded by nothing) then you might have logical difficulties but I am not sure. We can extrapolate a homogenous and isotropic universe beyond our visible horizon from the universe's past evolution even though we can't see or measure anything now....if that makes sense. I found a paper ,linked by Martin, a couple of years ago in answer to a question of mine that fast forwards the current expansion and where I originally learned of this "island universe" idea...it's by Lawrence Krauss and Robert Scherrer. Abstract: We demonstrate that as we extrapolate the current $\Lambda$CDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our "island universe" will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented. Edited August 13, 2012 by StringJunky 1 Link to comment Share on other sites More sharing options...
imatfaal Posted August 14, 2012 Share Posted August 14, 2012 We can extrapolate a homogenous and isotropic universe beyond our visible horizon from the universe's past evolution even though we can't see or measure anything now....if that makes sense. I found a paper ,linked by Martin, a couple of years ago in answer to a question of mine that fast forwards the current expansion and where I originally learned of this "island universe" idea...it's by Lawrence Krauss and Robert Scherrer. Abstract: We demonstrate that as we extrapolate the current $\Lambda$CDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our "island universe" will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented. What a strange coincidence - just yesterday I was listening to a TED talk by Brian Greene explaining exactly the same concept of future observers not being able to make same observations and thus deductions as we do now. He was tying it into the idea that some things can remain forever hidden (his funky extra dimensions for string theory to work) Link to comment Share on other sites More sharing options...
StringJunky Posted August 14, 2012 Share Posted August 14, 2012 What a strange coincidence - just yesterday I was listening to a TED talk by Brian Greene explaining exactly the same concept of future observers not being able to make same observations and thus deductions as we do now. He was tying it into the idea that some things can remain forever hidden (his funky extra dimensions for string theory to work) when I first read it I remember thinking it was of the utmost importance that we store our current cosmological data in catastrophe-proof storage in case there is a serious interruption in the status of humanity on Earth and if they recovered when the "island universe" was a reality then the truth is in storage to enlighten them. Is that paranoia? Link to comment Share on other sites More sharing options...
imatfaal Posted August 14, 2012 Share Posted August 14, 2012 when I first read it I remember thinking it was of the utmost importance that we store our current cosmological data in catastrophe-proof storage in case there is a serious interruption in the status of humanity on Earth and if they recovered when the "island universe" was a reality then the truth is in storage to enlighten them. Is that paranoia? No it is not paranoia. OTOH the time scale that physicists use are so extreme compared to "human" scales that we create a situation that is unimaginable. 10,000 years is a blink in astronomical / cosmological time but it takes us back to a pre-Homer, pre-Hammurabi etc world that has no real connection to our reality. The differences between London in 8000 bc and now are, in my parochial view, enormous and world-shattering - but in cosmological terms there is no difference. we struggle to store information n *10^3 years - can we envisage storing it for n * 10 ^ 9 years? 1 Link to comment Share on other sites More sharing options...
Alan McDougall Posted August 16, 2012 Share Posted August 16, 2012 Being causally disconnected beyond the observable universe I suppose that effectively means we are on an island surrounded by nothing. The surface of a sphere, is perfectly homogeneous and isotropic so it makes sense when deriving the geometry of the universe to start with that then generalize in a way that matches relativity. Thus is the universe flat or what. In an infinite universe things might appear homgeneous , but maybe not? Link to comment Share on other sites More sharing options...
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