tombombodil wrote:All the answers you have given involve a sub-question I've always been curious about. How do physicists "observe" things like the CMBR? What instruments and methods do you use to judge the Age of the universe?
So the big thing to acknowledge is that virtually all observations in physics are made through instruments, and its been that way for decades at least. The further back you go, the more physics was performed with direct observation with the human senses, but even then instruments have been important to observation for centuries. Nowadays, there is very little on the forefront of research that can even be perceived with human senses, and even when it can be, it is highly rare that people actually do so. I'll get more into the implications of that in relation to the later parts of your question.
Limiting ourselves for purposes of discussion to astrophysics, the bulk of observations are of electromagnetic radiation (radio, microwave, infrared, visible light, ultraviolet, x-rays, or gamma rays). The light can be characterized in terms of spectrum (what wavelengths are present), brightness (how much of each wavelength is coming at us per second), and polarization (how is the electric field in the electromagnetic way oriented). And of course we look at the variations of each of these from point to point on the sky and from moment to moment.
Depending on what you are observing, some of these are more important than others. For the CMBR, spectrum and polarization vary for different points on the sky. Brightness doesn't vary that much by direction, and none of them really vary over time.
Different instruments have different strengths. There's often some sort of trade-off between the different properties you might want to study in designing an instrument. For example, to get good spectral resolution, you might need to give up on some of the angular resolution (eyesight analogy: someone with excellent color sense but blurry vision).
Usually, observations are made with a variety of instruments. That way you can have some observations that are extremely precise in one way or another, combined with others that are less precise in any single area but are of decent precision in all areas. You can use these different observations to build up a fairly coherent picture of whatever you're studying.
Astrophysical observations are divided into surveys and targeted observations. Surveys sweep over the whole sky or a significant portion of it. They're generally less precise overall but give a good view of the big picture. Targeted observations spend more time on a single portion of the sky than is possible in a survey (assuming you ever want your survey to be completed). This gives the most precise picture possible of the areas you study, at the cost that you are only studying a small fraction of the sky.
I believe most of the observations of the CMBR are part of all sky surveys, since the variation over the whole sky is one of the most important pieces of information in understanding the early universe. However, there are almost certainly some long targeted observations of some parts of the sky to try to get a more detailed picture.
I'm not really knowledgeable on microwave detector physics. I worked at the opposite end of the spectrum. If you're interested in more specifics for microwave observations, I can do some research, but I figured that some generalities would do for a start.
As for how this information is used to calculate things like the age of the universe, that is of course very highly dependent on the theories that are used to interpret the data in the first place. The broad strokes for this particular question are that cosmologists take the distance/red-shift relationship that they observe and interpret it as indicating that the universe is expanding. They then run the clock backwards; based on the inferred motions, there was a time in the past when everything we can see was scrunched down into a very small volume. How long ago was that?
How certain are we (and how are we certain) that these measurements/observations are accurate or that we're even interpreting them correctly?
When the scientists give an uncertainty in a calculation, like the ones I quoted for the age of the universe, those uncertainties are all based on the quality of the measurements. There is always the understanding that these are under the assumption that the underlying theory is correct. It has to be that way, because there is really no way to quantify the possibility that the theory is wrong.
The scientists usually have an extremely good idea of how well they can trust their measurements. In the construction of any instrument (at least when done correctly) there is a lot of testing to characterize the behavior of the instrument before it ever sees "first light." You calibrate the instrument against sources you understand very well, so that you know what the data will mean when you use it to observe the sources you seek to understand. You also collect data with it blocked off from any source at all, so you understand the instrument noise that could obscure a signal.
There's always the possibility of something going wrong, of course. That's why science is so big on repetition of observation. It has to be acknowledged that the picture that gets presented in grade school science class that every
observation gets repeated by another scientist and every
result confirmed is just not true. But interesting results lead to more observations as people try to understand them better. There will generally be a steady supply of eyes on a problem that generates interest.
As for how we can be certain that the interpretation is correct, here we start to get into philosophy of science. Another lie from grade school science class is that a theory can eventually be proven and become a law. Theories can not be proven, if by that we mean that we have eliminated any possibility of error. We can only say that only say that a theory has been tested and not proven false. (As a side note, no, laws aren't proven either. Also, laws are actually components of theories. Saying that a theory may someday become a law is something like saying your house may someday become a floor.)
If we for a moment take skepticism to a ridiculous extreme, there has never been an observation of the cosmic microwave background. People have built instruments, and those instruments are observed to behave a certain way under certain conditions. To assert even that microwaves exist, let alone that microwave radiation permeates the entire universe and that radiation has its source in the early history of the universe, is to assert knowledge of something that is really only an interpretation of the observed behavior of our instruments.
Of course you can't do science this way. You probably can't even live this way. This level of skepticism is a short step from leading you to pure solipsism. At a certain point, although we must concede the logical possibility that a future observation could overturn our theories at any level, some of our interpretations have stood up to sufficient scrutiny that we just believe them.
We often use the word "theory" as if it denoted one simple proposition and its consequences, but real theories aren't really so tidy. Modern cosmological theory is a synthesis of a lot of different concepts and observations of varying levels of maturity.
We gain confidence in the interpretations of science through their repeated success. Predictions based on the interpretation are confirmed when someone performs the observations to check. Seemingly different lines of inquiry converge on the same result. The game of theoretical science is to try to come up with explanations that get more out of the theory than was put into it and to do so in a way that can be objectively tested.
To take this out of the abstract, let's look at the status of some of the concepts in cosmology I've already talked about.
Dark energy is, as I said, primarily a shorthand name for ignorance at this point. There are trial explanations that people are working on, but so far they haven't resulted in much testable.
We hope that one day Dark Energy will be a shorthand name for a good explanation of he observations that expansion is accelerating. Right now, these observations are in their second decade now, and have been confirmed by multiple groups. They have a fairly solid status. No one expects that they will be overturned, but they're probably young enough that were they overturned it would be a surprise but not a shock.
The general phenomenon that the universe is expanding is probably not in any doubt by any professional physicist. It is based on a wide variety of independent measurements. To cast much doubt on the observations, we would need to cast doubt on other mature areas of inquiry: the Doppler effect, atomic spectra, stellar models. It also has the benefit of being predicted by general relativity, a theory that has been successfully tested on much smaller scales than the cosmological.
The cosmic microwave background is one of the big triumphs of Big Bang cosmology. It was predicted before it was ever detected. If the universe was hot and dense in the distant past, there had to be thermal radiation still around from that time. The one-time rival explanation for an expanding universe, the steady state model, does not predict the CMBR, so its existence becomes a vexing problem under that model. Furthermore, steady state cosmology can't provide a similar successful prediction. As a result, the number of working physicists that pursue steady state instead of big bang cosmology, if not zero has become vanishingly small.
Science in general is not a set of isolated hypotheses that stand or fall on their own. Instead, things are interconnected. Many questions are far from settled, but whatever the answer, it will be tied to a web of other results