Arguably the most fascinating question in modern cosmology is why the universe is expanding at an accelerating rate. An original solution to this puzzle has been put forward by four theoretical physicists: Edward Kolb of Fermilab, Sabino Matarrese of the University of Padova, Alessio Notari of the University of Montreal, and Antonio Riotto of the Italian National Institute for Research in Nuclear and Subnuclear Physics (INFN)/Padova. Their study has been submitted to the journal Physical Review Letters.
In 1998, observations of distant supernovae provided detailed information about the expansion rate of the universe, demonstrating that it is accelerating. This can be interpreted as evidence of “dark energy”, a new component of the universe, representing some 70% of its total mass. (Of the rest, about 25% appears to be another mysterious component, dark matter, while only about 5% consists of the ordinary “baryonic” matter.) Other explanations include a modification of gravity at large distances and more exotic ideas, such as the presence of a dynamic scalar field referred to as “quintessence”.
Although the hypothesis of dark energy is fascinating and more appealing than the other explanations, it faces a serious problem. Attempts to calculate the amount of dark energy give answers much larger than its measured magnitude: more than 100 orders of magnitude larger, in fact.
Kolb and colleagues offer an alternative explanation, which they say is rather conservative. They propose no new ingredient for the universe; instead, their explanation is firmly rooted in inflation, an essential concept of modern cosmology, according to which the universe experienced an incredibly rapid expansion at a very early stage.
The new explanation, which the researchers refer to as the Super-Hubble Cold Dark Matter (SHCDM) model, considers what would happen if there were cosmological perturbations with very long wavelengths (“super-Hubble”) larger than the size of the observable universe. They show that a local observer would infer an expansion history of the universe that would depend on the time evolution of the perturbations, which in certain cases would lead to the observation of accelerated expansion. The origin of the long-wavelength perturbations is inflation, as, effectively, the visible universe is only a tiny part of the pre-inflation-era universe. The accelerating universe is therefore simply an impression due to our inability to see the full picture.
Of course, observation is the ultimate arbiter between theories. The SHCDM model predicts a different relationship between luminosity-distance and redshift than the dark-energy models do. While the two models are indistinguishable within current experimental precision, more precise cosmological observations in the future should be able to distinguish between them.
