Our Lumpy Universe

June 6, 2014

Children of my generation were faced with quite a few inconveniences associated with growing up in the 1950s. Aside from having to walk miles to school, both ways uphill, in the snow, and working our math homework problems with chalk on the back of a coal shovel, we had lumpy pancakes.

Food processors weren't consumer items until well into the 1970s, but blenders were available long before then. However, when a newlywed's gift blender broke, it was seldom replaced, leading to the poignant First World tragedy of lumpy pancakes.

Because of atomic and molecular forces, things in nature, such as the ingredients in pancake batter, tend to agglomerate and are seldom homogeneous. Gravitational forces have that same affect in large scale structures such as our Milky Way Galaxy. When distance measurements of galaxies using cosmological redshift became routine in 1989, Margaret Geller and John Huchra of the Harvard–Smithsonian Center for Astrophysics discovered the first large-scale structure of the universe.[1]

A Hubble Space Telescope. image of a pair of spiral galaxies, known as 2MASX J00482185-2507365. (Image: NASA, ESA, and The Hubble Heritage Team (STScI/AURA), via Wikimedia Commons.)

This structure, known as the Great Wall (more properly, the CfA2 Great Wall, named after the Center for Astrophysics, which was responsible for both the redshift survey and the analysis), is a 500 million light-year's wide shell of galaxies just 16 million light years thick about 200 million light-years distant from Earth. The extent of this shell, which may be the boundary of a giant "bubble," might be larger, but our own galaxy prevents further observations.[1]

A much larger wall, the Sloan Great Wall, named after the Sloan Digital Sky Survey, was discovered in 2003. This wall was observed to extend 1.38 billion light-years, which is nearly three times larger than the CfA2 Great Wall. For comparison, the diameter of the observable universe is about 93 billion light-years.

A representation of the 2dF Galaxy Redshift Survey, showing the Sloan Great Wall. (Modified Wikimedia Commons image by Willem Schaap.)

Nor is the Sloan Great Wall the last wall found. The farther we look, the more walls we find, the last being the Hercules–Corona Borealis Great Wall, measuring more than ten billion light-years across, or more than 10% of the diameter of the observable universe. The discovery was made by mapping gamma ray bursts.

Data such as these suggest that the universe is far from homogeneous, and it consists, instead, of immense voids with galaxies defining their boundaries. It's as if the universe is actually a foam, and the galaxies are aligned as a "cosmic web." It's conjectured that the matter we see is aligned with a superstructure of dark matter that gravitationally attracts the normal matter. Simulations show that the voids might contain faint filaments of galaxies (see figure).[2-3]

A simulation showing faint filaments of galaxies within a cosmic void. (International Centre for Radio Astronomy Research (ICRAR) image by Cunnama, Power, Newton and Cui)[3]

Do such inhomogeneities average out when we get to the scale of the entire universe? Analysis of the cosmic microwave background radiation (CMBR) indicates that the universe as a whole might be anisotropic, divided in half by a poetically named "Axis of Evil."[4-6] The evil, if it scales with the degree of anisotropy, is actually quite small. The temperature variation found in the CMBR is just a few millionths of a degree out of 2.73 kelvin.

This anisotropy might be connected with something in the early universe, but there are other possibilities. It might be caused by an object between us and the CMBR photons, and this possibility is buttressed by the fact that the "Axis of Evil" is closely aligned with the ecliptic, the plane of planetary orbits in the Solar System (see figure).

Anomalies in the Cosmic Microwave Background Radiation, as detected by the Planck spacecraft. There is slightly higher average temperatures (red) in the southern ecliptic hemisphere and slightly lower average temperatures (blue) in the northern ecliptic hemisphere. The line marks the ecliptic, which is closely associated with the "Axis of Evil." A surprisingly cold patch is circled. (ESA and the Planck Collaboration image, used with permission.)

It might also be that we are seeing random fluctuations in the observable Universe, which is a small part of the entire universe.[6] You will see variation by looking too closely at anything.

References:

1. Margaret J. Geller and John P. Huchra, "Mapping the Universe," Science, vol. 246, no. 4932 (November 17, 1989), pp. 897-903.
2. E. Tempel, R. S. Stoica, V. J. Martínez, L. J. Liivamägi, G. Castellan and E. Saar, "Detecting filamentary pattern in the cosmic web: a catalogue of filaments for the SDSS," MNRAS, vol. 438, no. 4 (March 11, 2014), pp. 3465-3482.
3. These aren't the voids you're looking for, International Centre for Radio Astronomy Research Press Release, March 10, 2014.
4. Kate Land and Joäo Magueijo, "Examination of Evidence for a Preferred Axis in the Cosmic Radiation Anisotropy," Phys. Rev. Lett., vol. 95, no. 7 (August 11, 2005), Document No. 071301 [4 pages].
5. Kate Land and Joao Magueijo, "The axis of evil," arXiv Preprint Server, February 22, 2005.
6. Matthew Francis, "Is the lopsided Universe telling us we need new theories?" Arstechnica, March 10, 2014.
7. Craig J Copi, Multipole Vectors Web Site, Case Western-Reserve University.