Can the Ekpyrotic Cyclic Universe Model replace the Standard Model?

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:

  1. The laws of physic cannot demonstrate that the universe had a beginning;
  2. 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;
  3. 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;
  4. 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.

The ekpyrotic universe model is not new. It’s been mentioned in similar threads here since 2002.

One way to disprove the Ekpyrotic Cyclic Universe Model is to detect gravitational waves produced by the inflation postulated in the Standard Model. Telescopes appeared to identify such gravitational waves last year and two years ago, but the source was not cosmic inflation.

In the absence of such evidence, maybe the Ekpyrotic Cyclic Universe Model should be considered to have more explanatory power, shouldn’t it?

No, that time doesn’t come until the model has positive evidence for itself. As Steinhardthimself wrote:

They’ve had 15 years to work on it. What positive evidence do they have that makes it the preferred model rather than just another set of speculations?

To say the current standard comsological model has no problems (or perhaps more correctly, no unanswered questions) would not be correct, but I don’t particularly agree with the below list:

  1. The finitely-aged big bang universe comes from taking a simplified general relativistic model of the Universe and a set of very reasonable assumptions and finding that it can only be rewound a finite amount of time back and no more. Moreover the model makes observational predictions that have been borne out and Roger Penrose later demonstrated that the singular nature of such a model did not come from its simplified nature. That to me is as clear cut proof as there can be that a certain set of laws do indeed predict a Universe with a beginning. Of course the question still remains whether they are the complete set of physical laws (probably not tbh) or whether a new set of laws kicks in/become dominant just before or at the point of the predicted beginning which would allow us to extrapolate the Universe even further back in time.

  2. Whether and how quickly a certain cosmological model re-collapses depends on its energy density, pressure, curvature and current rate of expansion. In particular there is no energy density so great that the Universe must re-collapse. Or in other words this not a problem nor has it ever been a problem in big bang theory.

  3. Inflation requires an additional physical field (or fields) for which we have no alternative evidence for, which makes it in many ways rather ad-hoc However to explain the problems of bog standard GR cosmology, the dominance of a hitherto unobserved field in the early Universe is one of the more humdrum explanations. Indeed one of the reasons many find inflation attractive is that the required behaviour fits with a scalar quantum field. As I said pressure plays a role in the evolution of an expanding Universe and the inflaton field would’ve had very large (in proportion to its energy density) negative pressure in the early Universe which has the effect of accelerating expansion, so the inflaton field counteracts re-collapse (though as hinted above the presence of such a field is not required to avoid re-collapse).

  4. Energy conservation in cosmology is a tricky subject: my opinion is that if you want it you have to do some creative accounting. However the standard cosmological model predicts that after the inflationary era, the Universe broadly-speaking went through three eras: radiation dominated, matter dominated and dark energy-dominated. The radiation-dominated era lasted for tens of thousands of years and during which the expansion decelerated. During this era the deceleration of expansion would’ve been at its largest (depending on how you define decelerated expansion). Afterwards comes the matter dominated era that lasted a little short of 10 billion years, ending roughly 4 billion years ago (co-incidentally roughly when the first life appeared on Earth). During this era the expansion of the Universe was also decelerating, though not as much as during the radiation dominated era. Finally comes the present era, the dark energy-dominated era. This era is marked by the fact that the expansion of the Universe is accelerating. The reason that different forms of energy can dominate in the Universe in different epochs without them necessarily being coupled goes back to what I said about conservation of energy in cosmology needing some creative accounting.

Based on the observational evidence and its (relative) simplicity inflation looks like the best bet for the explanation of the early Universe, but the evidence is not so great that alternative like the ekpyrotic model can be ruled out.

Either way, the layman will remain equally confused. With the Ekpyrotic Model, he will have to accept the existence of 6 additional dimensions carefully hidden in the creases of 3-dimensional reality and yet another hyper-dimension along which the brane containing our universe glides and finds energy for its periodical expansions. With the Standard Model, he will have to live with mysterious concepts such as the Big Bang, cosmic inflation and dark energy.

I’ve read your explanations. My layman ‘expertise’ allows me simply to acknowledge them and not to make further comments. Like I said, I’ve read some popular science (Einstein, Hawking, Feynman, Barrow, Smolin, Bohm, Davies, Rees, Weinberg, Singh, Greene, to mention but a few :D) and I admire the physicists’ effort to offer a simple model of the universe, but when I hear of Big Bang, cosmic inflation and dark energy my guts tell me there’s something fishy with the Standard Model and my despondency worsens.

My feeling is we’re stuck and it’s not just me.