Since the Copernican Revolution removed humanity from the center of the Universe, two of the most fundamental principles guiding cosmologists have been the principles of cosmological homogeneity (or the Copernican Principle) and of cosmological isotropy. Observations show that these approximately hold true in the region of space we can observe, but new data from the Wilkinson Microwave Anisotropy Probe yield provocative indications that the principle of isotropy may need to be re-examined.

The Cosmic Microwave Background radiation is one of the cleanest probes available for studying the large scale structure of the Universe. According to the Big Bang picture, the CMB originated at the moment when the expanding Universe first became cool enough for the plasma of free electrons and protons to form neutral hydrogen and thus to become transparent to photons - the so-called recombination epoch - which happened when the universe was only a few hundred thousand years old. If the Universe were exactly homogeneous and isotropic, the CMB would be exactly the same in every direction on the sky - but then there would be no galaxies, stars, planets, or cosmologists! The small inhomogeneities which eventually formed into these objects are also present in the CMB, where they are seen as small fluctuations in temperature about a mean value of 2.725 K. Statistically speaking, these fluctuations should be uniformly distributed over the sky if the Universe is isotropic, and to a large degree, they are.

While the data are generally considered to be in very good agreement with the standard view, they contain anomalies on large angular scales that indicate there may be a preferred direction to the universe. In particular, Eriksen et al. have shown that the fluctuations show more power in one hemisphere than in the other, for a specifically determined coordinate frame. Motivated by this result, A. J. Banday and T. R. Jaffe of the MPA, along with collaborators H. K. Eriksen and F. K. Hansen in Oslo, Norway, and K. M. Górski in Pasadena, USA, have investigated a specific class of cosmological models which are homogeneous, but admit anisotropy.

Homogeneous models that include shear (anisotropic expansion) and vorticity (global rotation) are known as Bianchi type VII_h models. CMB photons propagate along geodesics that are rotating about the symmetry axis and "stretched" (or redshifted) due to the shear expansion. The radiation will then appear hotter or colder depending on the particular path the photons have taken, resulting in an additional anisotropy in the observed CMB in the form of a spiral pattern (see Figure 1 for examples).

The researchers have compared these models to the data and shown that the asymmetry may be due to a combination of shear (along a particular axis) and vorticity (about that axis). The data and the matching Bianchi model are shown in Figure 2 (a) and (b) respectively. If this anisotropic Bianchi component is removed from the CMB data (Figure 2 (c)), the power asymmetry and several related statistical anomalies are corrected.

Unfortunately, such a model is inconsistent with both the theory of inflation and with the measured total energy density of the Universe. However, the researchers stress that a pragmatic approach should be taken in the interpretation of the results. In particular,
i) the Bianchi model provides a means to statistically quantify deviations from isotropy, and
ii) the best-fit model provides a template temperature pattern which other models may need to reproduce in order to explain the observed anomalies.

Ultimately, a viable theory should be developed to self-consistently explain the observations on all angular scales, and make independently verifiable predictions for other cosmological probes. Nevertheless, the current results do suggest that the long-held assumption of cosmological isotropy may need to be abandoned.

T. R. Jaffe, A. J. Banday

Further Information:

Wilkinson Microwave Anisotropy Probe home page

Ned Wright's Cosmology Tutorial

Note on Bianchi models (PostScript)

T. R. Jaffe, A. J. Banday, H. K. Eriksen, K. M. Górski, & F. K. Hansen, Evidence of Vorticity and Shear at Large Angular Scales in the WMAP Data: A Violation of Cosmological Isotropy?, 2005, ApJL, 629, L1, (astro-ph/0503213).

H. K. Eriksen, F. K. Hansen; A. J. Banday, K. M. Górski, & P. B. Lilje, Asymmetries in the Cosmic Microwave Background Anisotropy Field, 2004, ApJ 605, 14 (astro-ph/0307507).