Re: Does general theory of relativity still apply to our universe?

Good question! The latest observations of the cosmic microwave background, from experiments such as BOOMERanG and MAXIMA, do suggest that the Universe has a "flat" or Euclidean geometry. This simply means that our intuition about space really will apply on the largest scales - for example, parallel lines never meet or diverge, the angles of a triangle add up to 180 degrees, and the circumference of a circle is 2*PI times the radius of the circle. However, this does not contradict general relativity!

First, on smaller scales than cosmological, we still need Einstein's theory. General relativity claims that matter "warps" nearby space-time. For example, the sun warps the space around it so that the planets "flow" around in their orbits. Newton's laws give a close approximation to the orbits, but to precisely describe Mercury's orbit, we do need general relativity. We also need general relativity to correctly describe very massive, small systems, such as black holes and neutron stars. These very massive objects warp spacetime so severely that the full machinery of general relativity is necessary to understand nearby space.

But, as described above, the latest results show that on the largest scales, "normal" geometry works fine and we needn't worry about the curvature of space. How can we understand this in terms of energy curving spacetime? It turns out that Einstein's theory predicts that if there were no energy in the universe, the universe would be "open." This means that parallel lines would diverge, getting farther apart as they got longer (as happens on a saddle shape). Adding energy to the universe warps space in such a way that the divergence of parallel lines gets smaller - in fact, if we add enough energy, parallel lines will eventually meet (as happens with lines of longitude on the earth). So if we add just the right amount of energy, we can cancel the divergence of the lines exactly, without making them meet. Thus, the recent observations are showing us that the universe contains exactly enough energy to make a flat universe.

So, we see that we can still describe a flat universe in general relativistic terms. The next question to ask is whether we need to use general relativity. If all the energy of the universe were in "normal" matter, like stars, dust clouds, people, etc., then we would not need to use general relativity to describe the cosmological state of the universe. However, it turns out that this is not the case. Current observations suggest that there is only about 30% as much "normal" matter out there as is needed to make the universe flat. The remaining energy is in the form of a "cosmological constant" or vacuum energy. This is energy, first postulated by Einstein, that has the strange effect of acting like antigravity - i.e., it pushes other matter away rather than pulling it in. General relativity is necessary to describe this energy. So if the current observations hold up, it will be required in any discussion of the universe, even though the universe has a Euclidean geometry!

For more information on the BOOMERanG experiment, click here. For a description of why recent supernova results suggest the existence of a cosmological constant, click here. For some more discussion on the implications of a flat universe, click here.