Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

How long before the expansion of the universe causes our earth to drift away from the sun?

Category: Space      Published: May 4, 2013

Public Domain Image, source: NASA/JPL-Caltech.

The expansion of the universe does not affect the relative position of astronomical bodies within galaxies. It is true that the universe is expanding, but this does not alter the distance between the earth and the sun. It also does not affect the distance between atoms. The expansion of the universe is partly caused by the Big Bang, and partly caused by dark energy. This expansion should not be thought of as stars flying away from each other in a static spacetime fabric. Instead, the stars are more or less static relative to a spacetime fabric that is itself expanding. The question is often asked, "Where is the center of the universe's expansion?" This question only makes sense if all the stars were flying away from some central point. Because the expansion is space itself, there is no center.

The book "Universe" by Martin Rees states,

Several notable features have been established about the universe's expansion. First, although all distant galaxies are moving away, neither Earth nor any other point in space is at the center of the universe. Rather, everything is receding from everything else, and there is no center. Second, at a local scale, gravity dominates over cosmological expansion and holds matter together. The scale at which this happens is surprisingly large – even entire clusters of galaxies resist expansion and hold together. Third, it is incorrect to think of galaxies and galaxy clusters moving away from each other ‘through' space. A more accurate picture is that of space itself expanding and carrying objects with it.

Think of an infinite sheet of paper with a grid of one-inch squares drawn on its surface, and another infinite sheet with a grid of two-inch squares. The second sheet is expanded relative to the first, but there is no center of expansion. Solar systems do not expand despite existing in an expanding universe because of the binding force of gravity. In fact, even galaxies have enough gravity to withstand expansion. Only when you get to the level where mutual gravitational attraction is negligible; the inter-galactic level; does the expansion of the universe become evident. Similarly, the electrons in atoms are not drifting apart despite the expansion of the universe. Everything on earth, from your hand to a ruler, is not expanding. This fact is the reason we can detect the expansion of the universe in the first place. If our rulers were expanding at the same rate that the galaxies are drifting apart, we would never have discovered the expansion of the universe.

This expression that gravity locally overcomes the universe's expansion is somewhat oversimplified. Spacetime on the cosmological scale is quite complicated. A more accurate statement would be that anywhere near matter (in galaxy groups), spacetime curves so as to cause objects to be attracted, and we call this attraction gravity; but far away from matter (in between galaxy groups) spacetime naturally expands of its own accord.

Topics: cosmic expansion, earth drift, expanding universe, expansion of the universe, metric expansion