Maybe the universe isn’t so dark after all

If you follow popular presentations of cosmology, you may be under the impression that most of the matter and energy in the universe is unseen. So-called Dark matter and dark energy have been invoked by astrophysicists to explain observed gravitational interactions between galaxies. Calculations show that dark matter should make up 23% of the mass-energy density of the universe, with visible matter comprising less than 5%. The remainder is dark energy.

For many scientists dark matter and dark energy are bothersome hacks that make the accepted cosmological model fit the data. And they present as many awkward questions as they answer, especially when it comes to the formation of stars and galaxies, with dark energy providing a feedback mechanism that hinders stellar formation.

If one accepts the Occam’s razor principle…

“Entia non sunt multiplicanda praeter necessitatem”

or “entities must not be multiplied beyond necessity”, it would help to dispense with dark matter and dark energy, and explain large-scale observations of the cosmos without recourse to exotic and as yet undetected subatomic particles, and mysterious energy fields.

Maybe we can, if recent work by Durham University graduate student Utane Sawangwit and his supervisor Tom Shanks is verified with data from the European Space Agency’s Planck satellite, launched a year ago last month.

Planck is currently gathering data on the cosmic microwave background (CMB), an afterglow of the big bang which tells us much about the mass-energy distribution of the universe, and its evolution. Small-scale spatial irregularities within maps of the CMB are the result of tiny thermal fluctuations in the early universe which have grown with the expansion of the universe, and are intimately connected with the composition of the universe today.

Observations made a decade ago with NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) revealed ripples in the CMB, and this led to the current thinking on dark matter and dark energy. But creating maps of the CMB is a tricky business that requires complex statistical analysis, and some smoothing of the data. It is this CMB data smoothing which Sawangwit and Shanks find problematic when it comes to the accepted interpretation of astronomical observations.

Using objects that appear as point sources in radio telescope images, the researchers were able to test WMAP smoothing, and found that the effect of the data manipulation is larger than previously understood, which casts doubt on the size of the observed CMB ripples. If Sawangwit and Shanks are correct, then the CMB ripples are significantly smaller than estimated before, in which case dark matter and dark energy may not exist. Or at least there would be no need to invoke them.

“CMB observations are a powerful tool for cosmology and it is vital to check for systematic effects”, says Shanks. “If our results prove correct then it will become less likely that dark energy and exotic dark matter particles dominate the Universe. So the evidence that the Universe has a ʻDark Sideʼ will weaken!”

A universe without dark matter and dark energy may be no more than the wished-for result of an aesthetic prejudice in favour of relative simplicity and tidiness, but as a working principle Occam’s razor has served us well, and in science we do well to heed our gut instincts. Many advances have come about as a result of stripping away unnecessary complexity, and even if the standard model of cosmology – with some if not all of its dark matter and dark energy – survives these new tests, it is likely that the numbers will change, and we will cease talking of a universe dominated by something that cannot be seen and measured.

There remain many unknowns in our understanding of the physical universe, and it would surely improve public understanding if scientists and science communicators were a little more honest and up-front about them in their popular presentations. Dark matter and dark energy have to date been presented in the media as fact, even though they are questionable inferences based on data analyses that are highly complex and have known issues.

Admitting uncertainty need not undermine the public’s confidence in the scientific process. On the contrary, it should stimulate the popular imagination, and increase support for science.

Further reading

Sawangwit & Shanks, “Beam profile sensitivity of the WMAP CMB power spectrum”, MNRAS (in press, 2010)

Sawangwit et al., “Cross-correlating WMAP5 with 1.5 million LRGs: a new test for the ISW effect”, MNRAS 402, 2228 (2009)