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

Is the Universe Homogeneous or Isotropic?

  1. Nov 26, 2008 #1
    I recently read an article about whether the http://dailyphysics.com/" [Broken] and/or isotropic. (the story is at the top - sorry I couldn't get the link to the permanent article to work here) “gargantuan ripples in the density of matter across the universe, known as baryon acoustic oscillations” is what the article says causes inhomogeneities in the universe, and that we can test for them.

    First of all, do you think the universe is homogeneous and/or isotropic? Why or why not?


    Second, how would we be able to realistically test these properties? Would we have to send a spaceship to the far reaches of the universe and measure the force of gravity between some set of standardized masses? Thoughts on this?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Nov 26, 2008 #2


    User Avatar
    Gold Member

    This one?


    As an alternative explanation of the dimming of distant supernovae it has recently been advocated that we live in a special place in the Universe near the centre of a large spherical void described by a Lemaitre-Tolman-Bondi (LTB) metric. In this scenario, the Universe is no longer homogeneous and isotropic, and the apparent late time acceleration is actually a consequence of spatial gradients. We propose in this paper a new observable, the normalized cosmic shear, written in terms of directly observable quantities, and calculable in arbitrary inhomogeneous cosmologies. This will allow future surveys to determine whether we live in a homogeneous universe or not. In this paper we also update our previous observational constraints from geometrical measures of the background cosmology. We include the Union Supernovae data set of 307 Type Ia supernovae, the CMB acoustic scale and the first measurement of the radial baryon acoustic oscillation scale. Even though the new data sets are significantly more constraining, LTB models -- albeit with slightly larger voids -- are still in excellent agreement with observations, at chi^2/d.o.f. = 307.7/(310-4)=1.005. Together with the paper we also publish the updated easyLTB code used for calculating the models and for comparing them to the observations.
  4. Nov 26, 2008 #3

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Isotropic, homogeneous Friedmann-Robertson-Walker models seem to model observations well on very large cosmological scales.

  5. Nov 27, 2008 #4
    Yeah that's the one, thanks.

    Jim Graber's link in that thread, George, to the animation of the formation of the acoustic peak is good. I'm not familiar with the comoving coordinate system that the page references, though. Does anyone know when this comoving coordinate system was first used? Is it only useful for examining the structure of the universe on a large scale?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook