When I was a kid I had an immense fear of infinity. I would lie awake at night picturing a vast desert that stretched out infinitely in all directions, devoid of any objects save for prickly cacti scattered about the empty landscape. I imagined that was what death would be like: a long stretch of nothingness that was completely silent forever and always.
Religious teachings indoctrinated in me compounded my fears with images of eternal fires in hell and never ending streams of honey in heaven, and I couldn’t decide which was worse. It seemed to me that anything that stretched on forever without change was fundamentally disturbing and ultimately horrifying.
The origins of the universe according to creationism were equally unsatisfying to my mind. For the universe to appear suddenly according to the will of a divine entity contrasted with the slowness of natural processes that unfolded around me. These processes seemed to go through cycles gradually, like seasons.
I remember reading about the Big Bang for the first time in Stephen Hawking’s A Brief History of Time, when I was still in high school. Once again, I was disturbed by all the infinities that the theory seemed to imply. For the universe to spring out of nowhere with an infinite density and infinite temperature was just mind-boggling. Of course, I later learned that the Big Bang theory does not actually propose that such a universe existed, but Einstein’s equations of general relativity, on which the theory is based, run into such infinities, known as a singularity, when time is extrapolated all the way back to zero.
At moments just after t=0, these infinities are dispelled. In just fractions of a second, the Big Bang theory paints a picture of the universe as a hot, dense soup, with elementary particles such as electrons, protons, neutrons and photons zipping around in an opaque haze, with matter and anti-matter rapidly interacting and annihilating each other, followed by creation of new particles. The prevalence of matter over anti-matter presumably tipped the scales, which is why matter exists today. After a few hundred thousand years, neutral entities such as the hydrogen atom formed. This was an important development, as these entities absorbed much less light, allowing for light to roam more freely in a relatively transparent universe. It is particularly this wandering light from the early universe, known as the Cosmic Microwave Background Radiation, that has served as the most convincing experimental proof of the Big Bang theory, and helps us date the birth of our universe to about 14 billion years ago.
Einstein himself was unhappy about the singularities his equations ran into, and chalked them up to inadequacies of general relativity in dealing with the quantum phenomena of the early universe. He hoped a grand unified theory would do away with all the infinities, and worked towards this goal in the latter part of his career, albeit to no avail. To this day, physicists are actively seeking a plausible theory of quantum gravity that may resolve the singularity problems. In the meantime, we have to live with the fact that the Big Bang model breaks down at t=0, and has no explanation of what happened at or before that time.
One implication of the Big Bang theory is that the universe is expanding, which is also supported experimentally by the observation of red-shifts from distant galaxies. Namely, galaxies are moving away from us, and the further away they are the faster they are receding, according to the Hubble Constant. Since our vantage point is not special in the universe, this means the universe is being pulled apart everywhere. To explain the tendency of galaxies to move away from each other despite gravitational forces, the notion of dark energy was introduced. Today, dark energy is estimated to account for 68.3% of the mass-energy total of the universe. Along with dark matter, a completely unknown form of matter required to explain gravitational forces in galaxies, 94.1% of the universe is composed of dark energy and dark mass. We have no clue what they are, as we currently have no way to probe them.
Considering that the universe is expanding now, can we assume that it will continue to expand forever? The fate of the universe, according to the Big Bang framework, hinges on whether the mass density of the universe is greater or less than a certain critical value. In one case, the universe expands ceaselessly, while the stars in the galaxies run out of interstellar gases to light their furnaces, eventually leading to their deaths in the form of white dwarves, neutron stars, or black holes, depending on their size. Black holes will swallow dead stars and each other and eventually evaporate, in an endlessly cooling universe with a temperature approaching absolute zero – known as the Big Freeze. This is quite a bleak scenario indeed.
An alternative scenario, still consistent with Big Bang theory, is that the universe will expand up to a certain point, then contract again towards a high-density, high-temperature state, known as the Big Crunch. In several cosmological models, the contraction of the universe never actually reaches zero volume, and instead bounces back into another state of expansion in a so-called Big Bounce. The beauty of such models is that they never have to deal with singularities at t=0, and they never have to say farewell to the stars once and for all. It would just be good-bye, for now.
One model that embodies the Big Bounce scenario is known as quantum loop theory. In this theory, spacetime itself is quantized into tiny ‘atoms’ that weave together and define the fabric of space and time that permeate all matter in the universe. Since the units of spacetime are discrete, speculated to be on the order of the Planck length, or 10-35m, the universe can never contract to volumes smaller than a certain size, hence keeping the density of the universe and its temperature finite even at the time of utmost contraction.
Quantum loop theory is one of many models that agree with Big Bang theory while allowing for the possibility of a cyclic universe. Ekpyrotic theory, rooted in string theory, describes the Big Bang as a result of a collision between branes, multi-dimensional generalizations of strings, where a burst of dark energy drives the expansion of the universe. According to this theory, the dark energy is eventually depleted, allowing for the universe to contract according to gravitational forces, which in turn can result in subsequent collisions of branes. Peter Lynds has proposed a universe that avoids violating the second law of thermodynamics by reversing the arrow of time when the contracting universe approaches a singularity, hence expanding again and causing the universe to go through cycles, which he believes to be exact repeats.
Friedrich Nietzsche was perhaps the most poetic champion of an exactly repeating universe. In his own words:
“Whoever thou mayest be, beloved stranger, whom I meet here for the first time, avail thyself of this happy hour and of the stillness around us, and above us, and let me tell thee something of the thought which has suddenly risen before me like a star which would fain shed down its rays upon thee and every one, as befits the nature of light. Fellow man! Your whole life, like a sandglass, will always be reversed and will ever run out again, a long minute of time will elapse until all those conditions out of which you were evolved return in the wheel of the cosmic process. And then you will find every pain and every pleasure, every friend and every enemy, every hope and every error, every blade of grass and every ray of sunshine once more, and the whole fabric of things which make up your life. This ring in which you are but a grain will glitter afresh forever. And in every one of these cycles of human life there will be one hour where, for the first time one man, and then many, will perceive the mighty thought of the eternal recurrence of all things—and for mankind this is always the hour of Noon.”