Dark energy and the accelerating expanding Universe

Last night there was a Horizon programme on BBC entitled “The Mystery of Dark Energy”. In essence, it was about the discovery that not only was the Universe expanding, but it was doing so at an accelerating rate, as (at least initially) discovered using a particular type of supernova whose brightness is characteristic. They (more or less) laid down a challenge to think up reasons why this might be the case.

Well, I’m not (at least officially) a physicist (though I am a member of the American Institute of Physics through my membership of the the Acoustic Society of America). But I like a challenge. So here goes…

Much is made of the fact that matter bends spacetime, resulting in gravity. And presumably energy also bends spacetime (though the effect is generally small), because of the well-known relationship between energy and matter. In addition, electromagnetic radiation travels at the speed of light, faster than matter can possibly travel. A great deal of energy has been radiated over the duration of the Universe, and is still being radiated. But where is this energy now (in the sense of: where is the wavefront of the energy from long ago?). Clearly, we see from Earth the radiation from a cone of visibility of this energy, but surely most of the energy that has ever been radiated is now travelling (at the speed of light) towards the outer edge of the Universe (aside: I might suggest that the outer edge of the Universe is actually defined by where the energy has reached, combining energy and space in some way, but this may be a distraction from the point I’m trying to make.) So as time goes by, more energy tends to be at the very outermost edges of the Universe, and the bending of space caused by this concentration of energy causes the matter in the Universe to accelerate outwards.

Now, this may be (i) obvious and/or (ii) wrong. And I don’t have the capability (or the time, though even if I had the time, I still doubt whether I’d have the capability!) to put this into equations. But I think it’s comprehensible, and it might be nice to know why it’s wrong…


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