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COSMOLOGY
Current cosmology is the product of the four massive scientific upheavals below · How well do recent cosmological theories maintain their commitment to rigorous Science?
Newton and Einstein
Matter and velocity bend space and time
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The Hubble Expansion
Space Constantly Expands at a Constant Rate
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Animate
The Cosmic Black Body
Deep scientific mysteries after the Big Bang
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The Penrose Improbability
One chance in 10^10^124, but it happened
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Stop I'm Dizzy
 
Cosmology is the scientific study of the cosmos, or the universe outside our solar system. The word 'cosmos' means 'orderly arrangement' in ancient Greek. Cosmologists, then, work to understand the universe's orderly arrangement by studying stars, nebulae (gas clouds in space), galaxies, and even space itself.

Many different explanations for the night sky and all it contains have been attempted since the time of the Greeks and before. Only since the advent of Scientific Method culminating in the 1500's have we learned to validate our explanations by using them to predict the universe's behavior. This respectfully-skeptical scientific approach has revolutionized life on Earth and has produced a clear and amazing picture of the sky—space—and its contents. But in the last 125 years or so, our scientific understanding of the universe has been upended not once but four times as new experimental discoveries and cosmological theories have come about. In each case, Careful Science has been key to these revolutions in our understanding. Explore each of these revolutions by selecting them above.
"Careful Science": As we look at the amazing results of Science, we'll also work to identify and apply the careful scientific thinking that is behind those amazing results. Look for this scientific-thinking content in yellow in each section.
The Man Behind Gravity. Sir Isaac Newton (1643-1727) took cosmology and much of science into the modern age. His gravitational equations flawlessly predicted the behavior of falling objects. Newton invented new mathematics and developed most of what is known today as classical physics. He constructed an extremely detailed picture of the universe where distance, speed and time scaled linearly throughout space, just as though you were measuring them with a nice straight ruler and a correct clock. And that picture worked perfectly.

The Tiniest Hint of Trouble. As scientists developed more precise instruments, however, Newton's equations began to show the tiniest of errors in their predictions. As thoroughly as Newton's laws had been tested, scientists never stopped practicing respectful skepticism and requiring those laws to meet ever more stringent levels of accuracy. They knew that the Scientific Method shows that every fully-correct theory always makes correct predictions, down to the last detail. Eventually, because of problems like failing to predict changes in Mercury's orbit, it became clear that something was slightly but definitely wrong with Newton's laws.

The Man Behind Spacetime. Then, in the beginning of the 20th century, Albert Einstein (1879-1955) developed two Theories of Relativity which corrected Newton's tiny errors. The cost, however, in our understanding of the universe was huge, far out of proportion to the small errors that were fixed. Everything scientists believed about space and time from Newton had to be thrown out. His neat, straight-line, perfect-time picture of the universe was totally wrong. Einstein showed that time passed at different rates for people traveling relative to each other, and he also showed that gravity did not even exist: instead, stars and galaxies distorted space itself by their mass, which caused the behavior we thought of as gravity. And he famously discovered that mass and energy are the same thing, related by the simple equation E=mc2. After Einstein, everything was in upheaval. What caused scientists to discover this new revolutionary view of the universe? They insisted that their theories make completely error-free predictions as Scientific Method demands.
The Universe is Expanding! Edwin Hubble (1889-1953), while not the first discoverer of the fact that the universe was expanding, showed that it was expanding in a way that would make Newton proud: linearly with time and distance as the red dashed line in his data[1] at right show. (It's important to note that the horizontal axis of Hubble's plot is not time but distance. See the animation above for an idea of expansion over time.) Although Einstein's General Theory of Relativity allowed for such an expansion, Hubble's work put this theory to the test and found it actually happening. Scientific Method requires that theories be falsifiable, that is, testable.

Scientists describe this as an expansion of space itself, rather than galaxies, stars and other objects expanding into a space that already exists, like an explosion. Instead, the expansion is literally the universe getting larger like an inflating balloon. But that doesn't mean that the objects in it are getting larger, only that the space they occupy grows bigger with time.

If the universe is getting larger linearly as time goes forward, it must have been smaller in the past. The natural question is then, if we look far enough back in time, then, does the universe's size ever go to zero? The answer to that appears to be yes. A size of zero means zero existence, however, and since the universe now exists, scientists soon identified the moment when the universe went from zero size to existence. That's the Big Bang that started it all.
The Noise that Won the Nobel Prize. In 1964, two scientists named Robert Wilson and Arno Penzias accidentally discovered[2] something called the Cosmic Microwave Background, abbreviated the "CMB". Their measurements of the night sky contained low-level noise that they could not explain or eliminate. Soon they concluded that this noise was coming from the universe itself, and they were right. This discovery produced an upheaval that scientists still haven't adequately resolved.

Here's what the CMB seems to tell us. When our 14-billion-year-old universe was very young, about 380,000 years old, it became cool enough that it went from opaque to transparent. At that moment, the light from all the universe's collisions and particle interactions flashed out in all directions. That pent-up light energy carried with it a snapshot of the core attributes of the early universe: its density, its temperature, its uniformity. That flash of light appears today as the noise that Penzias and Wilson discovered. The CMB reveals that the early universe had properties that just make no sense given what scientists know of basic physics. These include an almost perfectly constant equilibrium "black body" temperature everywhere, a uniform density of matter available only if gravity does not operate, and perhaps most amazing of all, perfectly-matched properties across vast spaces far too distant to have any physical connection with each other.

Many important questions that go to the core of Science arise in connection with the Cosmic Microwave Background. How far do we let our theories go as they try to explain results that appear to defy the known laws of the universe? How do we treat effects like the near-constant temperature of the early universe which appear to have no reasonable cause? What do we do with actual data that seem to suggest our most basic assumptions (such as that the laws of the universe don't switch on and off) aren't correct? If unfalsifiable, untestable theories are all that's left to explain these data, must at least one of them be considered correct for lack of any other option? If no theory fits with the data and we don't have any other theory, must we conclude the data are wrong even though those data continue to be verified? What's a good Scientist to do?
The Unbelievably Improbable Big Bang.  In the first three sections we have already described the huge upheavals in our understanding of the universe brought on by Einstein, Hubble and Penzias & Wilson's discovery of the CMB. Finally we have Sir Roger Penrose. Penrose has been a peer of, and frequent collaborator with, the late Stephen Hawking. His ideas are nothing if not unconstrained scientific imagination, but they are also absolutely solid, unimpeachable, yet frequently revolutionary theoretical constructions. Roger Penrose, then, has calculated the value of a simple property of the Big Bang and from that property has determined the probability of our specific universe. The results are our fourth and final cosmological upheaval.

Space prohibits a detailed explanation here, but the outline goes like this. We start with something called "entropy". Entropy is the orderliness of a system; for example, thanks to my wife, our house is orderly. Except, that is, for my study. Anywhere else, if you swap the locations of any two items in the house, we'll immediately know. This says that our house (or most of it) has very low entropy: it's very well-ordered. My study, however, is so messy that you could swap dozens of items and I would never know. It's a high-entropy place, highly-disordered.

Penrose has compared the universe's beginning low entropy at the Big Bang to its final maximum entropy value, like comparing my well-organized study at first to my messy study today. The chance of my study spontaneously returning to that first neat state is very, very low. Similarly, Penrose's entropy calculations showed that at the Big Bang the universe was in a state so highly-ordered that it could hardly be imagined. As for a number, the chance of our universe's initial Big Bang state happening at random was the almost infinitely-tiny 1 in 1010124.[3] Needless to say, this has driven cosmologists into a fury of highly-creative explanations. In this case as well as others, should untestable, unfalsifiable explanations for this absurdly-high orderliness be considered actual scientific theories? This goes against one of those fundamental principles of Scientific Method. And if we allow unfalsifiable theories to be considered scientific here, why not everywhere? Is there any need to do any experimental validation of theories anymore in that case?
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