On Cyclic Universes

How old is the universe? Did it have a beginning or is it eternal? There is no more fundamental a question in cosmology than the question of the origins of the universe we see around us, nor one more vexing to theoretical physicists.

The most commonly accepted answer to the above question is the Big Bang model, which states that the universe began, actually started from "nothing," 14 billion years ago. Before this time, the entirety of the universe was shoved into a "point" so small and so dense that the laws of physics, the notion of the dimensionality of space and the flow of time are incomprehensible to us as we understand them today. Why do we think this? Simple. We just rewind what we see happening before us today. Edwin Hubble discovered that the universe was expanding. Not just expanding, but expanding at an accelerating rate. In fact, the further out we look from where we are in the universe today, the faster cosmological structures like galaxies appear to be receding from us. If the universe is expanding, the thinking goes, then at one point in time, everything must have been close together. Using Einstein's theory of general relativity, which explains gravity and it's effect on objects that occupy space-time, theorists can rewind the clock back to the point where it all began. Well, almost to that point. It turns out we can only see back to within 300,000 years of the bang itself. Before this era, the universe was still so energetic and dense that visible light couldn't travel freely about the universe. It wasn't until it began to cool and expand that light began to move from one place to another, transferring information about separated points in space to each other. Before this time, we rely on general relativity to explain what was happening given certain expected energy levels and masses present in the early universe. The equations are reliable and have let us know much about the time before us, but at a certain point, they begin to break down. If you rewind back far enough, well past that 300,000 year limit, the equations begin to break down. The assumption is that this point in time is the beginning and the expansion that followed is all there really is to the universe - a rather unsatisfying conclusion to say the least (even if it may be true). Could there have been more?

Paul Steinhardt at Princeton University certainly thinks there's room for more. In a 2007 article in Seed Magazine, he phrases it this way:

The big bang is formally defined as the moment when the equations say that the temperature and density of the universe became infinite, and it is impossible to extrapolate back any further. Concluding that this represents the beginning of all space and time is suspect, however, as Einstein himself once pointed out. Properly construed, finding that the temperature and density become infinite is an indication that the mathematical equations underlying general relativity have become invalid, not that this is when the universe began.

There are many models for eternal universes out there right now. One of the more interesting ideas put forward by theoretical physicist and mathematician Roger Penrose is a model of a cyclic universe that is eternal and oscillating. The argument goes something like this: the universe has always existed and is eternal and we are living in one of an infinite number of permutations and lives of the universe. The basis for this belief stems back to the days of Einstein, who himself played with the idea of an oscillating universe in the 1930s. He postulated that the universe has existed forever and that each cycle begins with a bang and an outward expansion, followed inevitably by gravity getting a hold of the expansion and slowing it down before reversing it altogether, collapsing the universe back down to a single point for the process to begin again. This is a very simplistic explanation, and there are far more sophisticated explanations for a Big Bounce theory of the universe, but you get the basic idea.

The question is: is there any evidence of such cycles? The light barrier mentioned earlier is certainly an obstacle to seeing back past this point for confirmation of the universe's other lives. It may very well be impossible to know anything for sure about the universe before the big bang, however, Roger Penrose thinks he's found a clue in the cosmic microwave background radiation, the remnant radiation of the earliest times in the universe discovered  in 1964 and which NASA's WMAP space probe has been dutifully mapping. The CMB shows a relatively uniform energy prevalent throughout the observable universe that is a relic of a time when the universe was small and uniform. After expansion and cooling, minor variations in the CMB provided a structure around which galaxies eventually formed. Penrose and colleague Vahe Gurzadyan recently published findings of a series of concentric circular patterns in the CMB that they argue are a sign of pre-Big Bang activity. The circles are areas of small temperature variation between one spot and its neighboring spot in the universe. What could have caused these structures is a mystery, but Penrose has suggested that they are the result of collisions of supermassive black holes causing the creation of uniform spheres of of gravitational waves that radiated outward from the collision and left an imprint on the CMB straight through to the current era.

Ripple Patterns in the WMAP CMB data noticed by Penrose

Other scientists remain skeptical. Earlier last month, physicists Hans Eriksen and Ingunn Wehus conducted a study of the same data examined by Penrose and his colleague and found the exat same evidence of concentric circles. They then built thousands of random simulations of expansion models from the Big Bang to the present and found that the circles showed up in the random models as well. What does this mean? Erikson and Wehus think that it may just be part of the structure of the universe for some reason. No matter how the CMB varies in the models, it still produces the circles. Moreover, they may simply be evidence of processes that occurred in very early times, before the universe expanded and settled and allowed light to travel freely throughout, hiding the evidence of causation. Sounds reasonable enough. I don't have a PhD in math or physics, but it would seem to me that anything that occurred before the Big Bang would in a sense be erased by the condensation of the universe that the cyclic models suggest. Any patterns created by gravitational waves due to colliding supermassive black holes would be erased as the fabric of space were warped back down to a point and a new expansionary phase would create new patterns.

Nevertheless, the idea of a cyclic universe is intriguing, even if we don't have solid evidence of its existence. For one thing, it gets around the sticky point of what caused the Big Bang in the first place if absolutely nothing existed before by simply asserting that everything that currently exists has always existed. Forever. The universe is eternal and we're just in one of its infinite phases. Cyclic theories aren't the only eternal models of the universe, however. It has rivals with names like baby universes and eternal expansion, that are equally intriguing and probably worth covering in future posts.