I had a quick read through the paper. I don't think they are actually proposing a new theory here, but are using supernova studies to check one already suggeste

I had a quick read through the paper. I don't think they are actually proposing a new theory here, but are using supernova studies to check one already suggested. The problem is measurements of supernova show the universe is expanding at an accelerating rate. This was not predicted by any theories, so to explain it we invented something called dark energy. This dark energy is somehow pushing the universe apart and counteracting gravity. We have no real idea of what dark energy is. Lot of things have been suggested but we don't have any strong evidence to back them up. This is another idea, and as before, there's no real evidence to back it up. Essentially they are saying the universe could be filled with tachyons, which are hypothetical particles stuck moving faster than light. If so, according to some solutions of general relativity, they'd create a kind of accelerating expansion just as we see. What's new here is some analysis of supernova data which they say is consistent with their theory. That's a quick overview, but the gist is it's a new explanation for dark energy that isn't completely impossible but doesn't have a lot of evidence to support it.