Paul Steinhardt and Neil Turok, Princeton and Cambridge, respectively, explain their new cyclic model of the universe in THE ENDLESS UNIVERSE, 2007. Like most on this forum, I took the big bang and subsequent inflation as the best explanation for how this universe got started. I now see these guys have a great alternative THEORY; Here's a quick overview. Perhaps the biggest surprise is that by combining Heterotic M theory (with it's two parallel branes as envisioned by Witten and Horava), a cyclic universe, endlessly repeating, can be theorized in ten dimensional space between the branes. Dark energy plays three fascinating roles at different cycle stages: it speeds up the expansion rate, as currently observed, it acts as a stabilizer by acting as a shock absorber (analogous to an automatic pneumatic door close cylinder), and it decays over long periods, shutting itself off. Dark energy is dynamic! It means the universe does NOT have to expand forever. One of the things I clearly don't understand is how the second law of thermodynamics (entropy must increase) is not violated. But they say: So they envision a varying, evolving dark energy rather than patching together "inflation" at one stage with "dark energy" from another stage; and unlike inflationary theory, no fine tuning is required in their model, a big plus. So far, no experimental observations can detect any differences between their cyclic and the alternative inflationary model. I did not see any specific proposed experimental differences which could prove one model over the other.