WHAT'S HOT IN... PHYSICS , Mar./Apr. 2008

The top paper for this period deals with the implications for cosmology from the first three years of observations with the Wilkinson Microwave Anisotropy Probe (WMAP), which soared aloft on June 30, 2001. WMAP audits the cosmic microwave background (CMB) radiation to give us a balance sheet of the assets in the universe.

The results in paper #1 mark the halfway stage of this six-year mission, which has already led to a dramatic improvement in the measurement of six key cosmological parameters, as well as providing watertight evidence in favor of a simple cosmological model. The high citation rate reflects the fact that almost every paper in cosmology these days has to cite #1 in its introductory section. The report is now the frame of reference for all cosmological investigations.

The concordance model of big-bang cosmology is a stellar achievement. It stands on four pillars of wisdom. Its key observational successes are its ability to account for: the accelerating expansion of the universe, the cosmic microwave background radiation, the creation of the light elements in the early universe, and the origin of large-scale structure in the universe.

Two distinguishing features of the model include a so-called lambda (L ) term that represents the dark energy causing an acceleration of the expansion, and the presence of cold dark matter (CDM) that is non-baryonic and is dark because it cannot emit photons. These two contributors to the heart of darkness that is our universe account for almost 96% of its mass-energy.

Cosmologists happily play around in parameter space. Their standard model has six fundamental parameters that can only be determined by observation. Three of these parameters are easy to understand: the density of matter, the density of baryons, and the rate of expansion (the Hubble constant). But a further three are less intuitive. They concern properties of the cosmic microwave background, which is truly a fossil from the moment the universe became transparent to its own radiation. And just like palaeontologists, observational cosmologists are able to track down the environmental conditions at the time of the fossil’s formation. Hot Paper #1 dramatically shrinks the volume of parameter space that describes our universe.

WMAP scores on large-scale structure. By measuring the polarization in the CMB it is possible to look at the amplitude of the fluctuations of density in the universe that produced the first galaxies. That is a real breakthrough in our understanding of the origin of structure.

However, WMAP is not the only show in town. The Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey have mapped the large-scale distribution of matter and galaxies in exquisite detail. The cosmological information from these sources is in many respects orthogonal to WMAP so the concordance model is not heavily dependent on the assumptions of a single mission.

Paper #1 demonstrates that the standard model has survived another rigorous set of tests. Errors in the data are reduced threefold. The data are so constraining that there is little wriggle room for the basic L CDM model. By combining WMAP with other measurements it is possible to place significant limits on the geometry of the universe, which is nearly flat (almost no spatial curvature), the nature of dark energy, and a mechanism to generate the primordial ripples that ultimately became gravitational tidal waves producing galaxies.

It is now abundantly clear that this new cosmology requires new physics. Dark matter and dark energy nowhere feature in the palette used so creatively by particle physicists for whom the next big story might be detecting the Higgs boson at the Large Hadron Collider. But the cosmologists have work in progress as well. WMAP data are not a good fit for present ideas on structure formation in the universe.

What will WMAP do in the second half of its mission? Further observations will test the inflationary paradigm that holds that the flatness of the universe is due to an exponential expansion driven by energy in the vacuum. Better still, WMAP still has the capacity to probe directly the physics of the first moments of the big bang.