Protons and neutrons are baryons made of three quarks, while mesons derive from just two quarks. Quantum chromodynamics (QCD), the theory of how quarks interact with each other, allows five-quark configurations. The search for exotic baryons and mesons predates QCD, having started in the late 1960s. A revival of interest followed the prediction in 1997 of a five-quark resonance, soon being searched for by several experimental groups. The discovery (#3) and subsequent confirmation (#7,8) of a 1540 MeV resonance with strangeness S=+1—a pentaquark, no less—sparked a rush of experiments. Papers and talks describing these experiments are driving up the citation rates of the discovery papers. About a dozen papers announcing positive detection have already been published. Given that the pentaquarks produced by particle accelerators have a life expectancy of 10^-20 s, any detection seems truly remarkable.

The present position is indeed a deep puzzle, because at least 10 experiments have confirmed the resonance but others cannot see it. So what’s going on? There are many detailed pentaquark models, none of which examine the conflicting data. The contrast between positive and negative experiments is sharp because the positive experiments have better statistics that rule out explanations such as a statistical fluctuation. However, neither side has good statistics.

One possibility is that a specific production mechanism is at work here, a mechanism that is triggered in the positive experiments but absent in the negative ones. But analyzing this suggestion is difficult because many of the negative statements have been left at the rumor stage in scientific meetings and not confirmed in peer-reviewed papers.

In Hot Paper #10, Robert Jaffe and Frank Wilczek (MIT) show how the pentaquark can be understood as the bound state of one antiquark with two diquarks. This reduces the dynamical problem to three objects rather than five. The beauty of #10 lies in its prediction of the masses and properties of several further exotic pentaquarks, searches for which can provide a strong test of their model.

The five cosmology papers in this collection also cut to the heart of physics. Four (#1,2,4,5) address the enormous recent improvement in astronomers’ knowledge of the cosmological parameters, and the type of universe those parameters describe. Paper #1 and #2 are the mostly highly cited in space science. They present the results from the Wilkinson Microwave Anisotropy Probe, the highlights of which are the following: The age of the universe is 13.7 Gy; the first stars ignited 200 My after the Big Bang; and the content of the universe is 4% baryonic matter, 23% cold dark matter, and 73% dark energy.

Paper #5 is about supernova cosmology. Exploding stars known as Type Ia supernovae all have the same intrinsic brightness at maximum, so they are used as standard candles" for studying the dynamics of the universe. The apparent brightness of a Type Ia gives a reliable measure of its distance, and the velocity is found from the spectrum. By collecting data from supernovae scattered through the universe, astronomers have found that the universe is accelerating. This discovery sheds light on the dark energy that physicists suspect is driving the growing expansion.

These highly cited papers have brought cosmology to the stage reached 30 years ago by particle physicists: the cosmologists have at last converged on a standard model, in which there is agreement over the values of the parameters. Despite the standard models both communities face deep puzzles: why are there three families of matter? What is dark matter? What is dark energy?