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Cosmology can be a difficult discipline to get a handle on, as it is a field of study within physics that touches on many other areas. (Although, in truth, these days pretty much all fields of study within physics touch on many other areas.) What is cosmology? What do the people studying it (called cosmologists) actually do? What evidence is there to support their work?
Cosmology at a Glance
Cosmology is the discipline of science that studies the origin and eventual fate of the universe. It is most closely related to the specific fields of astronomy and astrophysics, though the last century has also brought cosmology closely in line with key insights from particle physics.
In other words, we reach a fascinating realization:
Our understanding of modern cosmology comes from connecting the behavior of the largest structures in our universe (planets, stars, galaxies, and galaxy clusters) together with those of the smallest structures in our universe (fundamental particles).
History of Cosmology
The study of cosmology is probably one of the oldest forms of speculative inquiry into nature, and it began at some point in history when an ancient human looked toward the heavens, asked questions such as the following:
- How did we come to be here?
- What is happening in the night sky?
- Are we alone in the universe?
- What are those shiny things in the sky?
You get the idea.
The ancients came up with some quite good attempts to explain these. Chief among these in the western scientific tradition is the physics of the ancient Greeks, who developed a comprehensive geocentric model of the universe which was refined over the centuries until the time of Ptolemy, at which point cosmology really didn't develop further for several centuries, except in some of the details about the speeds of the various components of the system.
The next major advance in this area came from Nicolaus Copernicus in 1543, when he published his astronomy book on his deathbed (anticipating that it would cause controversy with the Catholic Church), outlining the evidence for his heliocentric model of the solar system. The key insight that motivated this transformation in thinking was the notion that there was no real reason to assume that the Earth contains a fundamentally privileged position within the physical cosmos. This change in assumptions is known as the Copernican Principle. Copernicus' heliocentric model became even more popular and accepted based upon the work of Tycho Brahe, Galileo Galilei, and Johannes Kepler, who accumulated substantial experimental evidence in support of the Copernican heliocentric model.
It was Sir Isaac Newton who was able to bring all of these discoveries together into actually explaining the planetary motions, however. He had the intuition and insight to realize that the motion of objects falling to the earth was similar to the motion of objects orbiting the Earth (in essence, these objects are continually falling around the Earth). Since this motion was similar, he realized it was probably caused by the same force, which he called gravity. By careful observation and the development of new mathematics called calculus and his three laws of motion, Newton was able to create equations that described this motion in a variety of situations.
Though Newton's law of gravity worked at predicting the motion of the heavens, there was one problem… it wasn't exactly clear how it was working. The theory proposed that objects with mass attract each other across space, but Newton wasn't able to develop a scientific explanation for the mechanism that gravity used to achieve this. In order to explain the inexplicable, Newton relied on a generic appeal to God, basically, objects behave this way in response to God's perfect presence in the universe. To get a physical explanation would wait over two centuries, until the arrival of a genius whose intellect could eclipse even that of Newton.
General Relativity and the Big Bang
Newton's cosmology dominated science until the early twentieth century when Albert Einstein developed his theory of general relativity, which redefined the scientific understanding of gravity. In Einstein's new formulation, gravity was caused by the bending of 4-dimensional spacetime in response to the presence of a massive object, such as a planet, a star, or even a galaxy.
One of the interesting implications of this new formulation was that spacetime itself wasn't in equilibrium. In fairly short order, scientists realized that general relativity predicted that spacetime would either expand or contract. Believe Einstein believed that the universe was actually eternal, he introduced a cosmological constant into the theory, which provided a pressure that counteracted the expansion or contraction. However, when astronomer Edwin Hubble eventually discovered that the universe was in fact expanding, Einstein realized that he'd made a mistake and removed the cosmological constant from the theory.
If the universe was expanding, then the natural conclusion is that if you were to rewind the universe, you'd see that it must have begun in a tiny, dense clump of matter. This theory of how the universe began became called the Big Bang Theory. This was a controversial theory through the middle decades of the twentieth century, as it vied for dominance against Fred Hoyle's steady state theory. The discovery of the cosmic microwave background radiation in 1965, however, confirmed a prediction that had been made in relation to the big bang, so it became widely accepted among physicists.
Though he was proven wrong about the steady state theory, Hoyle is credited with the major developments in the theory of stellar nucleosynthesis, which is the theory that hydrogen and other light atoms are transformed into heavier atoms within the nuclear crucibles called stars, and spit out into the universe upon the star's death. These heavier atoms then go on to form into water, planets, and ultimately life on Earth, including humans! Thus, in the words of many awestruck cosmologists, we are all formed from stardust.
Anyway, back to the evolution of the universe. As scientists gained more information about the universe and more carefully measured the cosmic microwave background radiation, there was a problem. As detailed measurements were taken of astronomical data, it became clear that concepts from quantum physics needed to play a stronger role in understanding the early phases and evolution of the universe. This field of theoretical cosmology, though still highly speculative, has grown quite fertile and is sometimes called quantum cosmology.
Quantum physics showed a universe that was pretty close to being uniform in energy and matter but wasn't completely uniform. However, any fluctuations in the early universe would have expanded greatly over the billions of years that the universe expanded… and the fluctuations were much smaller than one would expect. So cosmologists had to figure out a way to explain a non-uniform early universe, but one which had only extremely small fluctuations.
Enter Alan Guth, a particle physicist who tackled this problem in 1980 with the development of inflation theory. The fluctuations in the early universe were minor quantum fluctuations, but they rapidly expanded in the early universe due to an ultra-fast period of expansion. Astronomical observations since 1980 have supported the predictions of the inflation theory and it is now the consensus view among most cosmologists.
Mysteries of Modern Cosmology
Though cosmology has advanced much over the last century, there are still several open mysteries. In fact, two of the central mysteries in modern physics are the dominant problems in cosmology and astrophysics:
- Dark Matter - Some galaxies are moving in a way that cannot be fully explained based on the amount of matter that is observed within them (called "visible matter"), but which can be explained if there is an extra unseen matter within the galaxy. This extra matter, which is predicted to take up about 25% of the universe, based on most recent measurements, is called dark matter. In addition to astronomical observations, experiments on Earth such as the Cryogenic Dark Matter Search (CDMS) are trying to directly observe dark matter.
- Dark Energy - In 1998, astronomers attempted to detect the rate at which the universe was slowing down… but they found that it wasn't slowing down. In fact, the acceleration rate was speeding up. It seems that Einstein's cosmological constant was needed after all, but instead of holding the universe as a state of equilibrium it actually seems to be pushing the galaxies apart at a faster and faster rate as time goes on. It's unknown exactly what is causing this "repulsive gravity," but the name physicists have given to that substance is "dark energy." Astronomical observations predict that this dark energy makes up about 70% of the universe's substance.
There are some other suggestions to explain these unusual results, such as Modified Newtonian Dynamics (MOND) and variable speed of light cosmology, but these alternatives are considered fringe theories that are not accepted among many physicists in the field.
Origins of the Universe
It is worth noting that the big bang theory actually describes the way the universe has evolved since shortly after its creation, but cannot give any direct information about the actual origins of the universe.
This isn't to say that physics can tell us nothing about the origins of the universe. When physicists explore the smallest scale of space, they find that quantum physics results in the creation of virtual particles, as evidenced by the Casimir effect. In fact, inflation theory predicts that in the absence of any matter or energy, then spacetime would expand. Taken at face value, this, therefore, gives scientists a reasonable explanation for how the universe could initially come into being. If there were a true "nothing", no matter, no energy, no spacetime, then that nothing would be unstable and would begin generating matter, energy, and an expanding spacetime. This is the central thesis of books such as The Grand Design and A Universe From Nothing, which posit that the universe can be explained without reference to a supernatural creator deity.
Humanity's Role in Cosmology
It would be hard to over-emphasize the cosmological, philosophical, and perhaps even theological importance of recognizing that the Earth was not the center of the cosmos. In this sense, cosmology is one of the earliest fields that yielded evidence that was in conflict with the traditional religious worldview. In fact, every advance in cosmology has seemed to fly in the face of the most cherished assumptions that we'd like to make about how special humanity is as a species… at least in terms of cosmological history. This passage from The Grand Design by Stephen Hawking and Leonard Mlodinow eloquently lays out the transformation in thinking that has come from cosmology:
Nicolaus Copernicus' heliocentric model of the solar system is acknowledged as the first convincing scientific demonstration that we humans are not the focal point of the cosmos… We now realize that Copernicus' result is but one of a series of nested demotions overthrowing long-held assumptions regarding humanity's special status: we're not located at the center of the solar system, we're not located at the center of the galaxy, we're not located at the center of the universe, we're not even made of the dark ingredients constituting the vast majority of the universe's mass. Such cosmic downgrading… exemplifies what scientists now call the Copernican principle: in the grand scheme of things, everything we know points toward human beings not occupying a privileged position.