The whole point of asking the question was to discuss the idea of expanding space. The cmbr confuses me because i can't make it make sense within a big bang expanding universe. All the explanations I have heard have shoehorned it in and made generalizations.. for example why its not really uniform and why if its not uniform it forms no pattern. Analogies work when using 2d examples like the balloon... but fall apart in 3.
It should be simple to explain.
I think it's a good idea to understand the different levels in variations of the CMBR.
Obviously first of all when examining the CMBR, you need to make sure you're looking in the right band of radiation, you need to account for the effects local objects such as the Sun, the Moon, the Milky Way and Earth itself, etc, etc. For example in the plane of the Milky Way the sky is a lot hotter in the band of the CMBR due to the fact there are a lot of stars giving off radiation in the galaxy.
The first thing noticeable about the the CMBR as viewed from Earth is not the same in all directions, it has a significant dipole (i.e. it has it's hottest spot at one end of the sky and it gets colder and colder in different directions until it reaches its coldest spot in the opposite direction). This dipole varies seasonally and is simply due to the result of the motion of the Earth around the Sun, the motion of the Sun in the Milk Way and the motion of the Milky Way itself relative to the frame in which the CMBR is isotropic (i.e. the same in all directions).
Once you factor out this dipole, the first thing to notice about the CMBR is it is very, very uniform. The pictures showing fluctuations in the CMBR from WMAP are essentially showing very small variations which actually require very sensitive equipment to pick up. In fact it is too uniform for the earlier and simpler big bang models because they say that opposite ends of the sky were not causally-connected in the early Universe (i.e. could affect each other) and therefore should not be in thermal equilibrium. The uniformity of the CMBR was the prime motivator for postulating "cosmic inflation" (i.e. that the early Universe went through a period of very rapid early expansion), which actually also solved a few other problems with the original big bang model.
The fluctuations in CMBR that can be seen on WMAP are essentially small and random. We know the Universe can't be completely uniform, otherwise things like planets, stars, galaxies, clusters, superclusters wouldn't exist, so it's no surprise the early Universe wasn't 100% uniform and is this is completely consistent with big bang/inflationary models. It is still possible that there are genuine anomalies in the CMBR (e.g. the proposed "The Axis of Evil"), but that we don't have sensitive enough data/analysis to truly distinguish these anomalies from the random noise of the CMBR. But at the moment all data is consistent with the small and random fluctuations predicted by modern mainstream big bang cosmology. When cosmologists look for anomalies in the CMBR, they are not looking for random variations, they are looking for departures from the Gaussian distribution of these variations, which indicated a departure from the standard model of cosmology.
Another observation of the CMBR is that it's temperature varies in space as well: the further away you look the hotter it seems to be at those points; but this too is exactly what standard big bang cosmology predicts. because the further away you look the further back in time you are also looking and so you are essentially observing the Universe at an earlier denser period when the CMBR was hotter.
The thing to realize about the analogies for the expansion/geometry of the Universe is that they are just analogies for mathematical models that are not so easy to grasp unless you are have some grounding in Lorentzian manifolds. Perhaps the best thing to know is that simply by assuming a uniform Universe governed by general relativity with realistic dynamics leads you pretty much inescapably to the conclusion that it is either expanding or contracting.
In the disputes on here over the nature of money, it is contended that banks create money out of nothing. They do this by taking a balance sheet and putting the same amount on either side - a debit and a credit that are equal and opposite. This balance sheet is another way of expressing nothing: eg: 3 - 3 = 0. Similarly, for each function of an electron running 'forwards' in time, there is another function for it to run 'backwards' in time as a positron. And you cannot have the one without the other.
