November 24, 2005
Mysteries at Universe's Largest Observable Scales
by Dragan Huterer
Mysteries at Universe's Largest Observable Scales
Cosmological principle states that the universe is homogeneous and isotropic on its largest scales. Nevertheless, this fundamental premise has only begun to be precision tested recently, with the advent of first large-scale maps of the cosmic microwave background anisotropy and galaxy surveys. Extraordinary full-sky maps produced by the WMAP experiment, in particular, are revolutionizing our ability to test the isotropy of the universe on its largest scales. Performing these tests is important because universe at large scales probes a variety of physical processes, and in particular can be affected by the mysterious dark energy. KICP fellow Dragan Huterer and his collaborators proposed a useful new basis for representing the CMB anisotropy, and furthermore found statistically significant and completely unexpected correlations of the CMB quadrupole and octopole with the the geometry and direction of motion of the solar system. KICP members Chris Gordon, Wayne Hu and Tom Crawford, together with Huterer, then investigated the possible instrumental and cosmological explanations for the observed alignments and proposed a class of models that could explain them.
<b>Multipole Vectors</b><br /> Dragan Huterer and his collaborators Craig Copi and Glenn Starkman from Case Western Reserve University found an alternative basis for representing the CMB anisotropy. In their approach, anisotropy at each multipole L is represented by L unit vectors, plus an overall constant, forming an equivalent irreducible representation of the rotation group SO(3). Multipole vectors were found to have a number of nice properties, and in particular they probe different aspects of gaussianity and isotropy than the usual spherical harmonics. (Incidentally, it was later found that multipole vectors were previously discussed by J.C. Maxwell in his Treatise on Electricity and Magnetism 113 years ago!). Copi, Huterer and Starkman also found a convenient algorithm to compute the multipole vectors starting from the usual harmonic decomposition of the CMB temperature. One particular advantage of multipole vectors is that they naturally define planes, and are therefore well suited to test planarity of the CMB anisotropy pattern.
<b>Large Angle CMB Anomalies</b><br> WMAP team's first year data confirmed the finding first found by the COBE satellite in early 1990s that the power at scales greater than about 60 degrees is essentially zero, in conflict with the prediction of the standard cosmological model. No convincing explanation for this lack of power has been found as of yet. In addition, several other intriguing correlations have been found. Copi, Huterer and Starkman, together with Dominik Schwarz from Bielefeld University in Germany, found statistically significant and completely unexpected correlations of the CMB quadrupole and octopole with the ecliptic plane. In particular, planes defined by the quadrupole and octopole are perpendicular to the ecliptic plane. (While this result was obtained using the statistics of the multipole vectors and normals they define, it is general and does not depend on using the multipole vector approach.) Furthermore, the ecliptic plane carefully separates stronger from weaker extrema of the quadrupole-octopole temperature anisotropy map, running within a couple of degrees of the null-contour between a maximum and a minimum over more than 120 degrees of the sky. These findings are complemented by those from other research groups, indicating that the CMB power in the south ecliptic hemisphere is larger than in the north, and that multipoles 4-7 also show alignments with an axis which is close to the dipole and equinox directions. Origin of these alignments is currently not understood, nor is it particularly clear if and how the various alignments are related.
<b>The Quest for an Explanation</b><br /> Understanding the origin of CMB anomalies is clearly important, as the observed alignments of power at large scales are inconsistent with predictions of standard cosmological theory. Regardless of whether the anomalies are caused by an instrumental, astrophysical or cosmological mechanism, many cosmological results from the CMB would need to be reconsidered as they depend on the information from large angular scales. KICP members Chris Gordon, Wayne Hu and Tom Crawford, together with Huterer, recently investigated the possible instrumental and cosmological explanations for the observed alignments. They showed that a large class of models considered in the literature, where anomalous power is added on top of the intrinsic CMB, is unlikely to explain the alignments. They further identified the general mechanism of 'spontaneous isotropy breaking', where statistical isotropy is spontaneously broken in the fluctuations observed from a given spatial position but not in the fundamental theory, that could explain the alignments. The search for explanations is ongoing, and the KICP team is working on including the effect of subtle systematics, taking careful account of the CMB foregrounds, and proposing theoretical models that can realize spontaneous isotropy breaking naturally. We also eagerly expect new data, such as further WMAP temperature and polarization maps and observations and, in a few years, those from the PLANCK satellite. The mystery of the CMB anisotropy alignments on largest observable scales continues, and recent results have only whetted the appetite of cosmologists to explain the observed anomalies.
<b>Further Reading</b><br /> <i>Multipole Vectors - a new representation of the CMB sky and evidence for statistical anisotropy or non-Gaussianity at 2 <= L <= 8</i>, C. Copi, D. Huterer and G. Starkman, Phys. Rev. D, 70, 043515 (2004) <br /><br /> <i>Is the Large-Scale Microwave Background Cosmic?</i>, D. Schwarz, G. Starkman, D. Huterer and C. Copi, Phys. Rev. Lett., 93, 221301 (2004) <br ><br /> <i>On the large-angle anomalies of the microwave sky</i> C. Copi, D. Huterer, D. Schwarz and G. Starkman, MNRAS in press; astro-ph/0508047 <br /><br /> <i>Spontaneous Symmetry Breaking: A Mechanism for CMB Multipole Alignments</i>, C. Gordon, W. Hu, D. Huterer and T. Crawford, Phys. Rev. D, 72, 103003 (2005) <br /><br /> <i>Is the Universe out of Tune?</i>, G. Starkman and D. Schwarz, Scientific American (August 2005)