Anomalous Alignments in the Cosmic Microwave Background

I just answered a phone call for my parents. A very distant relative somewhere from the Northsea coast, who I vaguely remember having met sometime middle of the last century. She apologized that we didn't get a Christmas card this year (not that anybody had noticed) because her husband broke an arm (a-humm). When asked, I told her I turned 30 this year, upon which she exclaimed: OH MY GOD!

Now I feel really old. What helps in such a situation is to look at a picture of something even older. Like the cosmic microwave background (CMB). Since we've lately heard a lot about the high angular moments that allow us to determine the parameters of the LambdaCDM model, and since I'm already feeling low, let me instead tell you something about the low angular moments.

Basics

Back then when the universe was young and only 300,000 years old, radiation decoupled from matter and since then, photons could travel almost undisturbed. The CMB shows the temperature, or the inverse wavelength, of the microwaves that we receive on earth from this early times. This afterglow carries information about the conditions in the early universe which can help us understand the origin of the structures that we see today, and the processes that were important in this era.

We can draw the temperature of the received microwave signals on a map similar to how we draw a map of the earth. However, for the CMB, the orientation of the map is chosen such that the plane of the Milky Way falls on the equator.

The top figure to the right shows the temperature in a scale in which blue is 0 Kelvin and red is 4 Kelvin. What we see is the mean temperature of the CMB of 2.725 Kelvin. On this rough scale it looks very uniform.
The middle image is the same map displayed in a finer scale such that blue corresponds to 2.721 Kelvin and red is 2.729 Kelvin. The "yin-yang" pattern is the dipole that results from the motion of the sun relative to the rest frame of the CMB.
The bottom figure shows the microwave sky after the subtraction of the dipole. On this map, the hot regions, shown in red, are only 0.0002 Kelvin hotter than the cold regions, shown in blue. The red band in the middle is dominated by emissions from the Milky Way. (source)

What is usually shown in the CMB pictures is thus not the absolute temperature, but the differences between measurements taken in different directions, this is also called anisotropy - the deviations from isotropy.

An useful orientation in these maps is the ecliptic, that is the apparent path of the sun. If you draw it onto the sky map, the ecliptic will look like a lying S. The maximum and the minimum on that curve the are the equinoxes^*. On the days when the sun is on the equinox, there are 12 hours each of daylight and dark.

Advanced

The temperature anisotropies in the CMB have been measured with very high accuracy by the WMAP mission (Wilkinson Microwave Anisotropy Probe). The full map contains a lot of very small structures, you find a picture here (notice the scale!).

If one wants to analyse this data, one takes all these fluctuations apart in less messy shapes, called multipole moments. This is a procedure that essentially decomposes the whole picture into simpler ones that add up to the full one. For scientific purposes one uses very specific shapes that are defined though functions, the so called spherical harmonics. These are labeled by two numbers one of which is called the multipole, and usually denoted with l. The first multipole is the monopole, followed by the dipole, the quadrupole, the octopole, and here I've exhausted my Latin.

The lower the multipole moment is, the simpler is its shape. You can look at a picture of the lowest multipole moments here. To describe how you have decomposed the full picture into the multipoles, you will need to specify the axis of these things. The higher the moment is, the more axis you have to specify.

The pretty picture below shows the octopole moment that is extracted from the CMB (the three year Internal Linear Combination map ILC123). Again, red indicates the hotter, and blue the colder areas.

The solid line is the ecliptic plane and the dashed line is the supergalactic plane. Also shown are the directions of the equinoxes (EQX), the dipole due to our motion through the universe, the north and south ecliptic poles (NEP and SEP) and north and south supergalactic poles (NSGP and SSGP).

Pros

The dipole moment is defined through one axis that intersects the sky sphere. For the quadrupole moment one needs to specify two axis. These span a plane, the plane has a normal which defines another axis (its the cross-product of the other two). The octopole comes with three further axis, and correspondingly three normals.

In the above figure, the quadrupole vectors are plotted as the solid red symbols. (Different symbols are results from different maps, the ILC123 analysis is shown as triangles.) The octopole vectors are plotted as the solid magenta symbols for each map. The open symbols of the same shapes and color are for the normal vectors for each map (see here for what the other symbols mean).

As you can see in the figure, the normals of the quadrupole and the octopole are quite close together and clump in the South-South-West. This anomalous alignment is unlikely at the 99.9% CL [3]. But even more puzzling is that they are aligned with the direction of the cosmological dipole and the the equinoxes at a level inconsistent with Gaussian random, statistically isotropic skies at 99.95%CL [3]. To put it it less technical, this means the probability that this happens just by coincidence is very small.

Also remarkable is how the ecliptic plane carefully separates the weaker power in the northern ecliptic hemisphere from the stronger power in the southern ecliptic hemisphere. This is known as the north-south asymmetry. The confusing thing about these results is that there is absolutely no reason why the CMB - if we understand it correctly - should be correlated with any features of our local solar system. These correlations have been confirmed in three year WMAP data[2].

There are several attempts to explain these anomalies, e.g. instead of using the standard Friedmann-Robertson-Walker metric, one can consider anisotropic or inhomogeneous models, foreground effects, lensing effects, quantum gravitational imprints, non-trivial topologies of the universe, modifications of the gravitational potential that the background photons might experience (Rees-Sciama/Integrated Sachs Wolfe effect), scattering of the CMB (Sunyaev-Zel'dovich effect), etc.

So far, it is not clear how the measurements can be convincingly explained...

Conclusions

I've learned quite a lot while writing this post. Maybe getting older isn't all that bad.

People like you and I, though mortal of course like everyone else, do not grow old no matter how long we live...[We] never cease to stand like curious children before the great mystery into which we were born.

~Albert Einstein, in a letter to Otto Juliusburger

Further reading

[1] On the large-angle anomalies of the microwave sky
Authors: C. J. Copi, D. Huterer, D. J. Schwarz, G. D. Starkman
astro-ph/0508047

[2] Can extragalactic foregrounds explain the large-angle CMB anomalies?
Authors: Aleksandar Rakic, Syksy Rasanen, Dominik J. Schwarz
astro-ph/0609188

[3] Mysteries on Universe's Largest Observable Scales
Author: Dragan Huterer
astro-ph/0608318

[4] The axis of evil
Authors: Kate Land, Joao Magueijo
astro-ph/0502237

See also: Multipole Vector Information

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Footnote: Is there somebody who could explain me why these are the extrema on the curve when the equatorial plane is defined through the Milky Way? Is this just coincidence?

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TAGS: PHYSICS, COSMOLOGY, CMB