Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Three questions

  1. Oct 2, 2008 #1
    What is the true definition of our universe in terms of spatial dimensions? Should the definition of universe include higher spatial dimensions?

    Is the expansion of our universe causing gravity; if our universe wasn't expanding, would we have the natural phenomenon of gravity?

    Is the shortest distance between two points in our universe a hyperbolic curve?
  2. jcsd
  3. Oct 2, 2008 #2


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    I don't know of any scientific reason to say the universe need more than 3 spatial dimensions.
    An example of a recent quantum spacetime model that is 3D spatially is in the Loll SciAm article in my sig.
    Some models use more than 3 spatial, and some models do not. Those using extra spatial dimensionality do not have a clear advantage.

    Who is to say? The object of cosmology is to fit math models to the data and get a model you can predict with. It is not intended to say what really "is", it is intended to predict as precisely as possible with the model that is as simple as possible.

    No gravity is not caused by expansion. If our universe were contracting instead of expanding, then Yes we would still experience gravity as usual.

    the standard cosmology model would allow a universe to be contracting, but that does not fit the data. Given the observed data, the model does not predict that the universe will ever start to contract.
    The geometry of 3D space can be quite irregular, likewise 4D spacetime. so one cannot say in advance some kind of curve will always work out to be the shortest. If you specify in advance some fixed geometry which space must have, like the "flat" vanilla space of Special Relativity (in which there is no gravity to bend it) or some curved but very uniform example, then you can talk about some kind of curve that would be the shortest. Otherwise I think not.
    Last edited: Oct 2, 2008
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?