# An Inflating Bubble in Chaotic Inflation

The animation at left shows a small region of space which starts to inflate because it is a peak of the initially random inflaton field. The small red line segment shows the size of the observable Universe, which ends up being much smaller than the inflating bubble. As a result, the observable Universe appears to be homogenous, isotropic and flat even though Universe is actually inhomogeneous, anisotropic and curved. The initial state could be curled up into a ball before the bubble starts to grow in which case the Universe would actually be finite even though inflation has made the bubble it contains millions of times bigger than the observable Universe.

The image on the right shows how the inflating bubble evolves over 13 time steps covering a factor of 64 in linear expansion. The y-axis in these diagrams is not time - this is not a space-time diagram. Instead, the y-axis is a non-physical coordinate used to make enough room for the great expansion of the size of the bubble. A solid red line segment indicating the maximum allowable size of the observable Universe is shown, but since we actually think that inflation expanded the size of the Universe by a factor of more than 1,000,000,000,000,000,000,000,000,000,000 instead of the illustrated factor of 64, the actual size of the observable Universe should be much smaller relative to the size of the bubble than what I can illustrate here. Even so, a flat approximation to the Universe shown by the dashed red line drawn as a chord across the observable Universe is an excellent match to all the observations. The large deviations between the dashed part of the red curve and the black curve are of course outside the observable Universe and thus not observable. Calculations in the flat model are much simpler and very accurate over the observable Universe. In the chaotic inflation model there are many such inflating bubbles randomly distributed through space. Each bubble is effectively a Universe in its own right. There is no possibility of communicating from one bubble to another, since all communications have to be along the curve because the y-axis is not a real spatial dimension in this graph. Light can only travel a short distance along the curve -- this distance determines the size of our observable Universe. Thus the existence and properties of any other bubbles are more the subject of metaphysics than astrophysics. But if a model makes distinctive predictions about our observable Universe, which are confirmed by precise observations, then the model should be taken seriously even if it predicts unobservable other bubbles.