The hot Research Fronts listed in the table to the right each have at least 8 core papers. The table is ranked by "newness," according to the average age of the core papers. Fields with the highest percentage of recent core papers, those from 2000 and 2001, merit priority. Our second table, below, lists the high-impact space science papers of 1999, 2000, and 2001, ranked (in each respective year) by total citations.

Among the Research Fronts, the appearance of stellar evolution at #1 is astonishing. This chestnut is already a century old, and in the decades 1970 to 2000 seemed eclipsed by high-energy astrophysics and cosmology. The core papers in this group are mainly devoted to the formation of the earliest stars in the universe, the stars that released the first visible light into the cosmos. At issue in this field is the origin of structure formation in the universe, and the formation of the first galaxies. The earliest stars may predate the first galaxies.

Colliding brane cosmology provides the second big hitter. The inflationary hot Big Bang has enjoyed enormous popularity, but is now challenged by a model universe with no Big Bang at all! This ekpyrotic cosmology posits that our universe is the result of a collision between two higher-dimension branes. The name derives the ancient Greek stoic philosopher’s notion that the universe was cyclically destroyed and re-created by fire. Instead of a big bang, the universe started in a Big Clap, a fiery crash of two branes, and the theory provides an interesting alternative to inflation, which is why it is hot.

Cosmological themes continue, with a cluster of papers on dark energy. Here the theorists are grappling with the acceleration of the expanding universe, which is driven by a "fifth force" in physics, quintessence. The mass equivalent of this force field could account for 70% of the mass of the universe; clearly this is an attention -grabbing statistic. And at #9 is another renaissance: Do the fundamental constants of physics vary with time? In the 1930s this was a large question in cosmology, which faded away for 60 years. Now it is back with a vengeance, with both the speed of light and the fine structure constant accused of varying with time!

Observational astronomy jumps in at #5, with a superb cohort stimulated by two space observatories: Chandra, and XMM-Newton. The core interest here is the distribution of X-ray-emitting gas in large clusters of galaxies. The high-energy astrophysics theme continues with compelling results on quasars (#8) from the Sloan Digital Sky Survey (#6). The last Science Watch survey of space science (11[1]: 1-2, January/February 2000), covering the years 1996 to 1998, predicted that X-ray astronomy would soar in the high-impact rankings once results from the new observatories came on stream.

In the high-impact papers, #1 for 1999 stands head and shoulders above the rest, with 1,112 citations as of late November 2003. This is the foundation report on the accelerating universe, presenting the observational evidence for cosmic repulsion. The "lambda" in the title is a symbol first introduced by Einstein into relativistic models of the universe with an anti-gravity term. He later removed it, as his "worst mistake." "No mistake at all!" cries #1: type Ia supernovae, the best standard candles for distance measurement, show clear evidence for a runaway universe.

Observations of structure in the cosmic microwave background (CMB) win the top-notch slot in 2000 with the first report from the BOOMERanG experiment, a long balloon flight over Antarctica that lofted a microwave telescope to high altitudes. The paper opens a thrilling new field of "precision cosmology" in which minute changes in the CMB across the sky are employed to measure the fundamental parameters of our universe. The observations fit standard inflationary cosmology with a flat (think Euclid!) geometry. In our table, paper #5 in 2000, together with #3 and #6 in 2001, continue the quest. Paper #3 performs a spectacular synthesis of results from several investigations to conclude that our universe has non-baryonic dark matter and dark energy. Galaxies, stars, and atoms are the froth on the mass-energy budget of the (invisible) universe.

Paper #2 for 2001 will surely garner far more than 200 citations in the long run. Lead author John Bahcall (Institute for Advanced Study, Princeton, New Jersey) recently scooped the Gold Medal of the Royal Astronomical Society for his work enshrined in this paper, which clarifies all the conundrums long associated with solar neutrinos. Neutrino oscillations now account for the historic discrepancies between observed and predicted solar neutrino counts.

Related features:

Dr. John N. Bahcall is listed in the rankings of the Special Topic Neutrinos, in the Top 25 Papers (#3, #8, #9, and #16), and also is among the Top 25 Authors in both number of papers and cites per paper.

Read a classic Science Watch^® interview with John N. Bahcall.

Physics correspondent Dr. Simon Mitton is Senior Fellow,
St. Edmund’s College, University of Cambridge, U.K.