Does space expand equally in every direction, so that it draws a perfect sphere or is expansion asymmetric, so it creates "bubbles" (when seen from the outside)?

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    $\begingroup$ The current accepted model is that the universe is isotropic (see Cosmologinal Principle). $\endgroup$ – AccidentalFourierTransform Jan 8 '16 at 13:18
  • $\begingroup$ @AccidentalFourierTransform that should probably be an answer $\endgroup$ – David Z Jan 8 '16 at 13:36
  • $\begingroup$ You have to take into account the question if de Sitter metrics is influenced by Schwarzschild metrics, see this question. $\endgroup$ – Moonraker Jan 8 '16 at 13:41
  • $\begingroup$ The backreaction due to Schwarzschild metrics is neglected at the scales of the universe $\endgroup$ – Mikey Mike Jan 8 '16 at 14:01
  • $\begingroup$ Wouldn't the distribution of red shifted objects vary with direction if spatial expansion were assymetric? $\endgroup$ – Lewis Miller Jan 8 '16 at 14:42

The current accepted cosmological model is called $\mathrm{\Lambda{}CDM}$, and one of its basic assumptions is that the universe is isotropic. This assumption is refered to as Cosmological principle, and if true, then the rate of expansion must be equal in every direction (the "bubbles" you speak of exist at galactic scales, but not at cosmological scales: the universe, seen as a whole, is very smooth). In the model, the expansion rate (scale factor) is $a(t)$, which is position-independent. For more details, see Friedmann–Lemaître–Robertson–Walker metric.

The universe expands isotropically, but it doesn't mean that it "draws a perfect sphere": the universe does not expand from a point. For more details, see What is our location relative to the Big Bang?.


In the Friedmann Lemaitre Roberston Walker universe, the space-time is expanding expand equally in every direction of space. Cosmological studies suggest that the universe is flat and it is believed that matter structures have grown from the stage when they were in fact small deviations from local equilibrium (they were local instabilities) and by gravity effects they grew to big matter structures. The universe have developed in certain stages.

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    $\begingroup$ This doesn't answer the question. $\endgroup$ – pela Jan 8 '16 at 13:56

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