he Physics Top Ten is ablaze with a remarkable claim that the universe will expand eternally, according to new data on the luminosities of distant supernovae. If the observations reported in paper #4 are confirmed, then its conclusions will overturn half a century of cosmological thought. A measure of the turmoil being wrought is the citation tally to date of 227, more than any other physics paper in this period.

The standard Big Bang cosmology sets the origin of the universe at approximately 15 billion years ago, when an initial fireball triggered an expansion that has continued ever since. The central question in cosmology asks if this expansion will coast along indefinitely or decelerate or accelerate. Gravitational forces should restrain the expansion, both through ordinary matter such as stars and galaxies and the postulated cold dark matter, invoked to provide extra restraining ballast. In contrast to these decelerating agents, which in exotic forms such as dark matter enthuse today’s cosmologists, Einstein proposed, early in the 20th century, a form of negative energy. Expressed as a cosmological constant in the field equations, this provides both the driving force of the expansion and the means of speeding it up. Cosmologists have shunned this mathematical fudge, perhaps because observations of distant objects such as galaxies and quasars could not tease out the subtle velocity differences that discriminate between uniform expansion and outright acceleration.

To map the measure of the universe cosmologists use "standard candles." Their trick is to find a class of objects whose absolute luminosity is known precisely. The observed brightness of a standard candle depends only on its distance and the geometry of spacetime. What should be a simple observational procedure is extremely demanding because well-calibrated standard candles are rare. Quasars, the brightest of the most remote objects, cover a huge range of intrinsic luminosity but there is no means of getting a fix on their absolute luminosities, so they are useless.

The standard candle employed in paper #4 is the type Ia supernova (SNe Ia), caused by the thermonuclear explosion of a carbon-oxygen white dwarf in a binary system. Studies of nearby SNe Ia have shown that their maximum brightness covers a small range, making them ideal as a standard candle. But to use the SNe Ia for cosmology you first have to find high-redshift objects. The authors of #4 collaborate as the High Redshift Supernova Search Team, which has made imaginative use of the world’s largest telescopes to discover and probe SNe Ia in the redshift range 0.3 ≤ z ≤ 0.6. Paper #4 presents 10 new SNe Ia, which expands the high redshift data bank to a point where real cosmology can be done.

Alexei Filippenko’s team finds that the distances to high-redshift supernovae are 10% to 15% further away than expected in a universe without a cosmological constant. The observations favor a universe with a low mass density–no cold dark matter! But crucially the data favor eternal expansion as the fate of the universe at a very high level of statistical uncertainty. This is a new paradigm, and those for and against will now join battle.

As luck would have it, the Top Ten has two new papers (#7, #10) on the possibility of the extra spacetime dimensions advocated in #1,#2, and #6: what are the cosmological explanations? Both papers embrace the speculation that the only way to explain why gravity is such a weak force is to accept that there are extra dimensions in which gravity alone operates. In #7, Csaba Csaki, University of California, Berkeley, and colleagues consider 5-dimensional theories with localized gravity. They find that tweaking theory so that it explains the weakness of gravity is unsatisfactory because the model then predicts a contracting universe with a life time of just a few thousand years. Clearly this is wrong.

Paper #10 continues the recent idea that spacetime has an extra dimension. Within the extra dimension and also the old dimensions, these authors add a cosmological constant. Their considerations allow the universe to expand normally during nucleosynthesis (the First Three Minutes) but faster than that in earlier epochs.

The cosmological flavor to the Top Ten continues with newcomer #9 from the big-hitting neutrino physicists of the Super-Kamiokande collaboration. The big story in neutrino physics is oscillations: a neutrino flux starts out with one flavor and mutates to another. This group found evidence for oscillations by observing atmospheric neutrinos. Now they have turned the experiment upside down and found similar results from neutrinos that have travelled through the Earth to reach the detectors. This result supports evidence for neutrino oscillations, which imply that the neutrino has mass. 

Dr. Simon Mitton is the Senior Fellow of
St Edmund’s College, University of Cambridge, UK