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Basic 'our universe' question

  1. Nov 20, 2009 #1
    relatively new to physics/astrophysics/astronomy/cosmology, and can't get enough of it..

    as such, i've started reading as much as i can on the topic, and have one lingering question;

    when we talk about 'the universe', are we talking about our 'observable universe'? that is, the universe we can measure, from it's beginning singularity to it's ever expanding growth?

    if so, is the reason as simple as we need a finite universe, or a model, in order to apply our equations to, base assumptions/theories around etc?

    and finally, if that is the case, what is the bigger universe picture? a bunch of disconnected universes going through their respective cycles? if so, how does that work? do they overlap?

    Thanks guys
  2. jcsd
  3. Nov 20, 2009 #2
    There are a plethora of theories on the universe so depending who you're reading they may use the words a bit differently, normally they make it quite clear though.

    I'm pretty sure mainstream cosmology uses the term 'universe' to mean everything within the universe (even outside what's observable.) If they are strictly speaking of the observable universe then they say 'within the observable universe' or something to that effect. Multi-verse theories come in a large variety most have no visible impact on our universe so they can't be proven/disproven (falsifiable) so I think that most mainstream scientists dismiss Multi-verse scenarios as pointless to talk about (they don't say it doesn't exist or that it does exist unless of course the theory in particular has some observable consequences and can be falsified in which case they may take an interest).

    I don't think multi-verse theories are dependant on our universe being infinite/finite, regardless of this there can exist other universes outside of our own. (Not entirely sure about this though so probably best to wait for more informed answers ;P)
  4. Nov 20, 2009 #3


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    I think it is an assumption of mainstream cosmology that the laws of physics are everywhere the same. Therefore, when we write down cosmological equations, we assume they apply outside of our observable universe.

    I know for a fact that when one does cosmology with a flat or open FRW model, the metric assumes space extends infinitely in all directions, even though our observable universe obviously does not. Then we sort of brush aside the question of the "size" of the universe and just consider changes in the scale factor.

    Also, sure, there are portions of our universe that are completely disconnected from our observable universe. Specifically, they are not causally connected. I.e an event taking place at point X cannot affect us here on Earth. In particular to the cosmology we live in, now dominated by dark energy, some regions are doomed to forever be outside of the realm of what we can interact with. If you want to call these "separate" universes, I think that's more an issue of semantics. Again, standard cosmology assumes the physics are the same in these disconnected parts, and so they're all part of the same structure, even if we cannot possibly interact with them.

    Uhh, I hope that wasn't too confusing.
  5. Nov 21, 2009 #4


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    The observable universe, by definition, includes all observable effects - even those emanating from undetected sources. Ascribing them as originating from regions outside the observable universe appears to violate causality and the laws of thermodynamics.
    Last edited: Nov 21, 2009
  6. Nov 21, 2009 #5
    A good working definition of the universe 'as a whole' is the totality of galaxies causally connected to the galaxies which we observe. We observe a small distribution of galaxies in the universe. An observer in a galaxy on the edge of our observable portion of the universe sees a different distribution, and so on. Visualize a field of daisies, and imagine yourself living in a single one of those daisies.

    Not necessarily finite. The geometry of general relativity gives us an infinite universe (as defined above) which could be imagined to live within a finite 'size'. If that is hard to imagine, think of fractal geometry or something similar; imagine time and space racing inevitably away from us on all sides.

    You might read up on the Landspace of string theory or other multiverse theories if you want some ideas of possible 'bigger pictures'.
  7. Dec 25, 2009 #6
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