How did the universe begin? How did it become the way it is? How will it evolve? What is its eventual fate? These questions, unanswerable as they may seem, are yielding to the science of astronomy. The most astounding discoveries of astronomy in the twentieth century were in the area of cosmology. At the beginning of the twenty-first century, these and related questions of cosmology are the hottest research topics in astronomy.
For some reason, this chapter is not divided into sections. I will need to refer to rough page groupings to call attention to specific ideas.
pp. 188–191. This introductory section tells how Edwin Hubble discovered the relationship now known as Hubble’s Law. The basic idea is that
This section tells how this fact was first uncovered; all subsequent observations have confirmed it.
pp. 192–193. Now this introduces another wrinkle to the law: Galaxies aren’t so much moving away from us as space itself is stretching. What’s the difference? Not much, at first glance; the galaxies get farther from us either way. One difference is that, with large enough distances, the separation between objects can increase faster than the speed of light. Another difference is that it is not necessary to define an unmoving “reference frame” relative to which everything else moves. See Fig 121 (p. 194) for an illustration.
pp. 194–195. In two paragraphs, the big bang model is introduced. You have surely already heard of this idea, but you may not know what makes astronomers take it seriously.
End of pp. 195. This describes a particularly significant electromagnetic spectrum, the cosmic background radiation (CBR).
To explain this particular spectrum, the book must briefly explain how objects radiate thermally. This does not take up much space in the book, but follow it carefully.
p. 196. The first paragraph explains how we think the CBR became the way it is.
pp. 196–197. This reports that the CBR is not actually perfectly uniform in all directions.
The book does not explain why this is a significant finding. We know that the universe is the way that it is, so naturally the way it was in the past had to have been such that it would lead to the way it is today. So think about this for yourself: why did astronomers go to the trouble to make an instrument precise enough to measure this tiny variation? What did detecting this tiny variation tell them?
pp. 197–198. Now dark matter is brought into the discussion.
pp. 198–199. This section describes possibilities for the “curvature” of the universe, which basically describes how parallel lines behave when extended for great distances.
Page 200’s first full paragraph discusses dark energy, which has shown itself only in careful measurements of distances to galaxies. This is new stuff, and is not as firmly established as dark matter or nearly as well as big bang theory.
pp. 200–204. This gives an abbreviated history of the universe from 0.0001 seconds to one billion years after the big bang (about 13 billion years ago).
pp. 204–206. Here you are introduced to the idea of cosmic inflation, which refers to a more dramatic expansion of spacetime than even the standard big bang model requires.
Don’t worry about quantum fluctuations and the birth of universes. This is, for now, pure speculation. In ten years, or maybe fifty, we will have a much better idea if the notion has merit.
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Where Does the Universe Go from Here? The question is simple enough, but astronomers and theoretical cosmologists take it very seriously and are trying to find the answer.
pp. 193–197. The key to predicting the fate of the universe lies in knowing how much mass is in the universe. Getting a handle on the elusive Dark Matter is a crucial piece of that puzzle. This section re-caps what you have already learned about dark matter.
pp. 198–199. This explains dark energy’s place in the mystery. This section gives the most accessible explanation I have found of why astronomers think that dark energy is am important part of the universe, all from an observation that the redshifts and distances of distant type 1a supernovas don’t agree with each other. This book refers to dark energy as the cosmological constant.
pp. 199–201. Now that the preliminaries are taken care of, we can read what’s in store for our universe. First, the Sun will change. The precise process is predicted to be different here and in our (more recent) main textbook, but the result is the end of all life on Earth either way.
Then the galaxy will be irreparably changed, then, in the far distant future, there will be no more stars, no matter how dim. Then what? What is left after there is nothing else to happen?
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Copyright © 2008, Richard Barrans
Revised: 2 November 2009. Maintained by Richard Barrans.
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