Originally posted by: silverpig
f95toli has the basics of it. My degree is in astrophysics and one of the mission scientists for WMAP was a prof of mine (there's also a theorist who collaborates with them as well and he taught me cosmology), so this should be fairly accurate.
WMAP is basically just a very sensitive directional thermometer which measures the difference in temperature between any two points in the sky. It basically sits out in space, spinning around measuring temperature differences and changing the angle between it's sensors. The result is a fairly famous map of the temperature of the cosmic microwave background.
You may have seen it before.
What happens next is a bit of math. If you're familiar with fourier transforms, they take a fourier transform in spherical harmonics of the pattern over the sky. If you're not familiar with fourier transforms, this mathematical operation basically tells you what the dominant size of the features of the map is. Large, slow spatial variations lead to low angular frequency signals, and small spotty noise like variations lead to high angular frequency signals. Basically you get something like this:
CMB power spectrum
Multipole moment on the bottom axis is basically "size of features" and starts from big on the left to small on the right. Anisotropy power basically tells you how prominent features of a given angular size (multipole moment) are. Spherical harmonics (multipole moments) are used because they have the mathematically simple property of being orthogonal.
What this tells you is if you look at the WMAP map of the sky, most of the features are about 1 degree in size (top horizontal axis).
Whoop-dee-doo right? Well theoretical cosmologists can simulate the dynamics of the universe assuming certain abundances of certain types of "stuff" with different equations of state. This is called the Friedmann equation and it relates the curvature of the universe to the amount of stuff in it. Lots of stuff means lots of gravity and that means a closed universe. Lots of um, other stuff (vacuum energy) means an open universe. A nice balance means a flat universe. Different types of stuff will affect the dynamics of the universe in different ways at different times.
So what is this stuff? Well there are 4 types that fit so far. Matter, dark matter, radiation, and {vacuum energy, quintessence, whatever you want to call it}. Matter is the self gravitating clumpy stuff we know and love. Dark matter is similar to matter except it is very weakly interacting. It gravitates but goes through things. Radiation is the standard stuff we also know and love. The other thing is something that drives the expansion of space.
So you take this equation, mix in varying amounts of this stuff, make sure it adds to 1 (this is a condition of a flat universe), pack it all together into something very small, add some quantum fluctuations, let your simulation rip and see what happens.
The quantum fluctuations of the small universe provide "seeds" of different energy density. Imagine a pool of water. Normally it should be perfectly flat if you leave it alone, but you throw in a handful of sand, nudge the pool a bit and you get ripples and waves. The quantum fluctuations basically start your pool off with some of these kinds of ripples and waves.
As time goes by the waves will travel along the surface of the pool and interact. This is a very simple analogy because the pool contains only one thing: water. However, the universe contains 4 types of "fluid".
So imagine the pool with the ripples in the water. Now add some clumpy matter like stuff. Maybe cork bits with some tiny magnetic centers. Now add some dark matter like stuff. I guess you'd have the same magnetic cork bits, but they could pass through everything. Radiation could analogously be described as making everything vibrate. The vacuum energy is the pool becoming larger. All of these different things add their own vibrations to the mix and the system evolves over time. Pretty soon it equilibrates and you take a snapshot at a given time. It is then possible to figure out how much of each type of stuff is in the pool by looking at the wave pattern you see.
To quit with the analogy, in the hot plasma of the early universe all 4 types of stuff were present. They drove the dynamics of the plasma and produced wave patterns. The universe was actually so dense and hot that photons travelled very short distances similarly to how they travel in the sun (ever hear that it takes x-million years for a photon created in the center of the sun to reach the surface?) Consequently, the waves of the early universe were largely sound waves. Sound waves travel characteristically in the medium in which they travel. This can tell you properties of the early universe. What THIS means is say you have a hot dense area of the universe next to a cooler less dense area (set up by the initial quantum fluctuations remember). Physical sound waves will travel from the higher pressure area to the lower pressure area as the primary method of causal contact. Again, the dynamics of this fluid will depend largely on the constituents.
So how do we see it? Well the universe expanded. As it expanded it cooled. As it cooled protons started to capture electrons. The plasma turned into a gas. Plasmas are opaque (ie the sun), whereas gases are largely transparent (air). As the universe was all in thermal equilibrium (it was a plasma for about 200 000 years I think the number is), this plasma/gas phase transition happened almost simultaneously all over the universe. The minute temperature differences due to density fluctuations seeded by the quantum nature of the early universe and evolved through the specific dynamics of the fluid of the early universe were then frozen out. The photons were then able to travel freely without interaction through the universe, carrying information about the temperature (and thus density) of the piece of space they originated in. These photons are the CMB photons and form a black body spectrum at 2.73 (or so) Kelvin.
So how do you get the age from this? Well you know the amount of different types of things in the early universe from the size of the fluctuations that evolved in the plasma. You then let your simulation run until the photons reach a temperature of 2.73 Kelvin and see how long it takes
Disclaimer: I wrote this all in one shot from memory. If it's unclear I'll try to answer questions and fix errors.
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