Cosmology and our View of the World
Everything out of Nothing? - Our sense of Place Lead: Eberhard Möbius
Summary by Andrew Middleton
Everything out of Nothing?
The class began with a refresher of Olbers’ paradox, a postulate that the night sky should be observed as an infinite number of tiny stars that completely light up the sky if the universe itself were in fact infinite in all directions and eternal. By virtue of our mostly black sky, this would seem to indicate a finite universe. This conclusion came to be challenged, however, by advances made since the paper was published in 1826.
In the early twentieth century, Edwin Hubble noticed that galaxies seemed to travel away from Earth at faster speeds the farther away they were. Hubble noticed that the signature wavelengths of radiation emitted from familiar atoms within distant objects were recognized as having been shifted to a lower frequency. This shift to the red end of the electromagnetic spectrum is a result of the Doppler Effect, the same phenomenon observed when an ambulance drives by and the siren’s pitch seems to suddenly drop when it passes by. Hubble noticed that this shift increased (indicating a greater speed away from Earth) with greater distance in an even proportion. By observing how much light reaches Earth from a distant supernova that emits a known amount of radiation, one can approximate the distance the supernova is from Earth just as one can ascertain that one lamp post appears dimmer at a greater distance than a lamppost of equal intensity nearby. Plotting speed away from Earth on one axis of a graph and the distance from Earth on the other, Hubble noticed that most galaxies and distance objects formed a more or less linear relationship. The slope of the graph, known as Hubble’s Constant, is a hotly contested value in the field of astronomy but has been used to draw the conclusion that if objects are moving apart, there must have been a time when they were all at a single center from which they radiated outward. Using Hubble’s Constant, astronomers can extrapolate the time at which this occurred assuming the current speed of the universe’s expansion remains constant (which, unfortunately for this line of reasoning, it is not). This is a very good way of explaining the effects astronomers see today.
Since this “Big Bang,” an estimated 13 to 15 billion years ago, the universe has expanded and has done so at an accelerating rate in its more recent history. Using the analogy of German Stollen bread, expanding space is represented by dough that separates raisins which are analogous to clumps of matter. The expanding universe can be described by the dough’s rising and expanding, causing the raisins to move away from each other at a rate as a function of the amount of dough between them. The more dough that separates the raisins, the faster and farther the raisins move away from each other. This simplistic edible model is a visualization of a more complex Inflation Model, a demonstration that Professor Davis pointed out only works in zero gravity and that, unlike the universe, has an edge. Having none, an infinite universe is still difficult to comprehend.
During the past billions of years, the universe has increased in volume but energy has not been added or removed since. As a result, the immense amount of energy packed into a small volume long ago has diffused over the vastness of space as that vastness has increased. Professor Moebius demonstrated the opposite reaction by compacting matter in a test tube with a plunger, concentrating the matter and heat that lead to the ignition of a small wad of cotton on the bottom. The current universe can be said to exist in a state more consistent with the extended plunger with matter spread out, while the burning cotton represents a much earlier and hotter period in the universe’s history.
One significant piece of evidence that supported the Big Bang theory was the discovery of the remains of that energy that has diffused throughout the universe since the initial cosmic explosion. The Cosmic Microwave Background Radiation (CMBR) has been observed at a frequency very similar to what earlier scientists predicted. In addition, Helium is observed to be in a proportion with Hydrogen which is explained by the Big Bang theory, contributing greatly to the credibility of the theory.
Some problems that the Big Bang Theory has faced are the Flatness, Horizon and Matter problems. “The Flatness Problem” is that the universe appears to be very close to being “flat” in four dimensional geometry. Since it had to be so incredibly precisely “flat” at the beginning of the universe this was a problem, because standard Big Bang theory doesn’t offer an explanation. It just would be a “fine tuning.”
The Inflation Model solves this problem because space has expanded and smoothed these wrinkles out. This explains why, even after billions of years, the universe has not expanded so much that its heat is incredibly thin and diffuse and why the universe hasn’t collapsed back in on itself. “The Horizon Problem” considers why the universe is uniformly inundated by background radiation even though its immense size prevents opposite “ends” of the universe from communicating. Sending a signal across the universe at the speed of light, the universal speed limit, would be a futile gesture given the enormity of space. This too, however, can be explained using Inflationary Theory. “The Matter Problem” postulates why so much matter exists in the universe as opposed to anti matter. Because the two substances annihilate each other instantly on contact in a 1:1 ratio, it must be the case that there was a great deal more matter in the early universe than anti matter and that matter somehow “won” to be dominant.
“The Flatness Problem” is a bit more complicated. Just as Earth’s mass demands a certain velocity for a mass to escape its gravity, so too must the universe’s mass have a rate of expansion greater than this critical velocity if, from a relativistic perspective, it is to expand without collapsing in on itself. If the universe’s mass density is less than the critical mass density then the universe would exhibit a saddle-like or hyperbolic geometry. If the mass density were greater than the critical mass density then the universe could be described as a hyper sphere. If the mass density of the universe were equal to the critical mass density then the Universe can be said to exhibit flatness. The flat geometric model of the universe assumed that universal flatness projected all the way back to the Big Bang and predicted that the observations of CMBR clumps would be of a certain angular size. When this prediction was confirmed within a high degree of accuracy by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite team, it lent a great deal of credibility to the Inflation Model. The Inflation Model states that, after a period of fast inflation ended, the early universe was perfectly flat and remains so today. This theory is supported by the CMBR observations that are consistent with a flat universe like that which the Inflation Model requires.
Because the universe seems so accommodating to life, two variations on an Anthropic Prinicple have been created to explain the preset conditions of the universe that have allowed life to exist. For example, had the gravitational constant been weaker or stronger in relation to nuclear forces, matter may not have accreted and atoms may never have been formed. The universe would have collapsed back in on itself far before the time it took for life on Earth to develop or heavy elements would not even have been formed. The Strong Anthropic Principle suggests that the universe has too many seemingly improbable aspects that are essential to life as we know it. This implies that the universe is simply too perfect for life to have arisen by chance and that it may have been set up for life. An opposing alternate argument, the Weak Anthropic Principle, rebuts the former by suggesting that life in our universe is possible because life in our universe won the “cosmic jackpot.” Of all of the countless universes in a great multiverse, most are sterile and have properties and laws that make life there impossible. We are living in one of the lucky ones that are just right to allow life to form. By chance, our universe is capable of supporting life and countless others must be sterile in the multiverse of theoretical alternate universes, slightly or vastly different than ours.
Naturally, this leaves a number of unsolved philosophical questions that our class left open for exploration on another day.