I’ve always had trouble absorbing the standard model. As a layman, I learned it from popular science literature and struggled with the theory of a Big Bang that turned nothing into everything and with the idea of an additional inflation that came out of nowhere but allowed the universe to take the shape we see today. At this point, the universe should continue to cool down and its expansion should slow down. About twenty years ago, however, it became apparent that the expansion rate is actually accelerating, which made me even more suspicious of the Big Bang theory.
It was difficult for me to accept the cyclic model as well. In the cyclic model, time is endless, the universe is endless and the life of the universe is cyclic. There are periods of evolution from hot to cold, from dense to under-dense, from hot radiation to nowadays structure, and eventually to an empty universe, when energy is re-injected and a new period of expansion and cooling ensues. But the cyclic models that I found twenty years ago included the idea of a post-expansion Big Crunch, when the density and temperature of the universe rises to an infinite value and the usual laws of physics cannot really be applied. Plus, every cycle is longer than the one before due to higher entropy density, which prevented those models from being genuinely cyclic since an initial zero-duration cycle resulted as a logical consequence.
However, a new cyclic model has recently put forth, one that relies on string theory, branes and extra dimensions, and it may not only be easier to digest but also explain things more adequately.
In the standard model we can see a beginning of time (nicknamed Big Bang), a spectacular evolution from virtually nothing to everything, and a vast expansion and cooling of matter and energy taking 15 billion years. To explain the structure and expansion of the universe, the standard model has to include the Big Bang model and the inflationary theory. The latter shows why the universe is flat and homogenous, and how slight inhomogeneities have led to the formation of galaxies.
There are problems with the current standard model:
- The laws of physic cannot demonstrate that the universe had a beginning;
- Temperature and density must have diverged. Temperature could have been infinite but energy density couldn’t have been too high because it would have caused the universe to re-collapse before expanding through the so-called inflation;
- The subsequent inflation that led to the accelerated expansion of the universe was caused by some mysterious energy – moreover, all this energy was contained within the initial universe and yet its density did not lead to a re-collapse;
- The inflation energy decayed into a hot gas of matter and radiation, atoms formed, the universe became transparent and the first stars and galaxies appeared, which should be followed by a deceleration of the universe’s expansion (with matter being the dominant form of energy), which goes against the observations that show the expansion rate is increasing and dark energy is the main component of the universe.
The cyclic model proposed by Justin Khoury, Burt Ovrut, Paul Steinhardt and Neil Turok does not include a Big Bang or an inflation stage. There are recurring stages, which include a sort of Bang when temperature and density are at their highest but do not diverge as they do in the standard model. The initial temperature is not infinite – instead, it is situated below the Plank energy scale, but it can still evaporate atoms and nuclei into their fundamental components. An inflation phase is not necessary because radiation ensues naturally. The dark energy we witness today does not come as a surprise; instead, it turns out to be the driving force of the universe’s entire evolution.