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People find their own ways of making sense of modern cosmology. Unless they are utterly unable to get comfortable and reject it whole hog (as some do quite vehemently 
). If you have a way of making your peace with cosmology, feel free to describe how you do it. Or try this recipe and see if it works for you.
PF Cosmo functions to some extent as a sort of lab where we encounter the problems people have had in adjusting to the emerging picture of the universe, so this recipe derives a lot from our common experience.
You won't believe how simple. Here's the recipe:
1. There's an idea of rest.
2. It's about geometry.
3. A revolution is occurring.
===========================
I owe you some explanation.
1. 1905 Special and 1915 General don't have a preferred idea of rest. But Cosmology does. An observer can be at rest with respect to background. In other words, with respect to the matter of the early universe. If you move too fast wrt the light coming from the matter of the early universe your nose will be roasted by the doppler hotspot ahead of you and your tail will be chilled by the doppler coldspot behind you. Or words to that effect. It's not an especially precise idea of rest because the background has fraction of a percent temperature fluctuation bumps, but it is a practical one.
2. When in doubt, throw space out. It is said that the Greek philosopher Aristotle did not believe that space had objective physical existence, but simply consisted of all the spatial relationships among things. If someone had asked him he might have said that spacetime doesn't exist either, but is merely the sum total of relationships among events. In other words don't worry about space or spacetime, think about geometry.
1915 GR is a proposed law to govern the evolution of geometry. We have no right to assume the angles of a triangle will sum to 180, or that distances between stationary objects will always remain the same. Geometry is dynamic and we affect it whenever we move massive objects around. Only very slightly, one must admit.
The practical notion of rest (point 1.) gives us an approximate but workable idea of simultaneity. Cosmology has a succession of present moments. So there is a collective pattern to how geometry has evolved thru time. Hubble law. A rough pattern of increasing distances.
The Friedmann metric, with a scale factor that evolves in time according to the two Friedmann equations.
3. Comes the revolution. It makes a difference that modern cosmology is in the midst of a revolution. I actually think you can't understand it without appreciating how hot the field is, and how fast it's changing. One thing this means is that alternative ideas are getting considered and discarded at a rapid pace. Inevitably there are disconsolate theorists whose pet proposals have fallen by the wayside. During any given 3 or 4 year period one will be able to discern a main direction in which research is going, or two or three major contending directions.
Yesterday I saw a preprint of a 4-page paper which I am confident will be important. It was about a completely new approach to early universe cosmology (this one based on Alain Connes noncommutative geometry version of the standard particle model.) The preprint appeared to have an unusual number of typos, as if the authors, whom we can call William and Mairi, were in reckless haste to post. The arxiv number is
http://arxiv.org/abs/0812.1657
(which could be remembered as December 1657)
One way to give a sense of the rapid change in cosmology is to quote from their introductory paragraphs. I'll do that in a separate post. Meanwhile, if you don't like my recipe, or want clarification, or prefer a different way of making sense of modern cosmology please feel free.
). If you have a way of making your peace with cosmology, feel free to describe how you do it. Or try this recipe and see if it works for you.PF Cosmo functions to some extent as a sort of lab where we encounter the problems people have had in adjusting to the emerging picture of the universe, so this recipe derives a lot from our common experience.
You won't believe how simple. Here's the recipe:
1. There's an idea of rest.
2. It's about geometry.
3. A revolution is occurring.
===========================
I owe you some explanation.
1. 1905 Special and 1915 General don't have a preferred idea of rest. But Cosmology does. An observer can be at rest with respect to background. In other words, with respect to the matter of the early universe. If you move too fast wrt the light coming from the matter of the early universe your nose will be roasted by the doppler hotspot ahead of you and your tail will be chilled by the doppler coldspot behind you. Or words to that effect.
It's not an especially precise idea of rest because the background has fraction of a percent temperature fluctuation bumps, but it is a practical one.2. When in doubt, throw space out. It is said that the Greek philosopher Aristotle did not believe that space had objective physical existence, but simply consisted of all the spatial relationships among things. If someone had asked him he might have said that spacetime doesn't exist either, but is merely the sum total of relationships among events. In other words don't worry about space or spacetime, think about geometry.
1915 GR is a proposed law to govern the evolution of geometry. We have no right to assume the angles of a triangle will sum to 180, or that distances between stationary objects will always remain the same. Geometry is dynamic and we affect it whenever we move massive objects around. Only very slightly, one must admit.
The practical notion of rest (point 1.) gives us an approximate but workable idea of simultaneity. Cosmology has a succession of present moments. So there is a collective pattern to how geometry has evolved thru time. Hubble law. A rough pattern of increasing distances.
The Friedmann metric, with a scale factor that evolves in time according to the two Friedmann equations.
3. Comes the revolution. It makes a difference that modern cosmology is in the midst of a revolution. I actually think you can't understand it without appreciating how hot the field is, and how fast it's changing. One thing this means is that alternative ideas are getting considered and discarded at a rapid pace. Inevitably there are disconsolate theorists whose pet proposals have fallen by the wayside. During any given 3 or 4 year period one will be able to discern a main direction in which research is going, or two or three major contending directions.
Yesterday I saw a preprint of a 4-page paper which I am confident will be important. It was about a completely new approach to early universe cosmology (this one based on Alain Connes noncommutative geometry version of the standard particle model.) The preprint appeared to have an unusual number of typos, as if the authors, whom we can call William and Mairi, were in reckless haste to post. The arxiv number is
http://arxiv.org/abs/0812.1657
(which could be remembered as December 1657)
One way to give a sense of the rapid change in cosmology is to quote from their introductory paragraphs. I'll do that in a separate post. Meanwhile, if you don't like my recipe, or want clarification, or prefer a different way of making sense of modern cosmology please feel free.
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