New research published in the journal Monthly Notices of the Royal Astronomical Society explains that the accelerating expansion of the Universe may not be real, but could simply be an apparent effect.
The new study, conducted by a group at the University of Canterbury in Christchurch, New Zealand, identifies the fit of Type Ia supernovae to a model universe without any dark energy to be very slightly better than the fit to the regular dark energy model.
A computer-simulated image depicting one possible scenario of how light sources are distributed in the cosmic web. Credit: Andrew Pontzen and Fabio Governato / Wikimedia Commons (CC BY 2.0)
Generally, dark energy is assumed to produce roughly 70% of the existing material content of the Universe. However, this mysterious quantity is basically considered to be a place-holder for unknown physics.
Existing models of the Universe need this dark energy term in order to explain the observed acceleration in the rate at which the Universe is expanding. Scientists base this assumption on measurements of the distances to supernova explosions in aloof galaxies, which appear to be more far away than they should be if the Universe’s expansion were not accelerating.
However, in 2016 the aspect of how statistically significant this signature of cosmic acceleration is, has been a much debated topic. The earlier debate pitted the standard Lambda Cold Dark Matter (ΛCDM) cosmology against an empty universe whose expansion neither decelerates nor accelerates. However, both of these models assume a simplified 100 year old cosmic expansion law—Friedmann's equation.
Friedmann's equation assumes an expansion just like that of a featureless soup, without a complicating structure. However, the present Universe in fact comprises of a complex cosmic web of galaxy clusters in filaments and sheets that surround and thread massive empty voids.
Prof David Wiltshire, who headed the study from the University of Canterbury in Christchurch, said,
“The past debate missed an essential point; if dark energy does not exist then a likely alternative is that the average expansion law does not follow Friedmann's equation.”
Instead of comparing the typical ΛCDM cosmological model with an empty universe, the new study draws a comparison between the fit of supernova data in ΛCDM to a different model, known as the ‘timescape cosmology’. This has no dark energy. As an alternative, clocks carried by observers in galaxies vary from the clock that best describes average expansion after the lumpiness of structure in the Universe becomes noteworthy. Whether or not one understands that accelerating expansion then relies significantly on the clock used.
The timescape cosmology was studied to provide a somewhat better fit to the largest supernova data catalogue than the ΛCDM cosmology. Unfortunately the statistical evidence is not yet powerful enough to rule definitively in favor of one model or the other, but future missions such as the European Space Agency’s Euclid satellite will indeed have the power to differentiate between the standard cosmology and other models, and also aid scientists in deciding whether dark energy is real or not.
Deciding that requires more data and also a better understanding of the properties of supernovae which presently limit the precision with which they can be employed for measuring distances. On that score, the new study demonstrates major unexpected effects which are missed when only one expansion law is applied. Therefore, even as a toy model, the timescape cosmology provides a powerful tool to test the present understanding, and throws new light on the most intense cosmic questions.