Revolutions in science don’t come that often, but the book De revolutionibus orbium coelestium (On the revolutions of the heavenly spheres) published in 1543 certainly caused one. The work by Nicolas Copernicus overthrew the ‘geocentric’ model of the solar system where Earth is at the centre, and suggested an alternative view whereby Earth revolves around the Sun.

No-one disputes the fact that the sun is at the centre of our solar system, and no-one also seems to dispute the idea that we are not at the centre of the universe. Indeed, this is encapsulated in a principle known as the ‘Copernican Principle’ that states that the Earth is not in any specially favoured position and is taken as a fait accompli among researchers. But how can we test it? Two independent teams of physicists think they know how, and argue their cases in back-to-back papers in the journal Physical Review Letters.

In the first paper, Robert Caldwell from Dartmouth College and Albert Stebbins from Fermi National Laboratory in the US explain how the Cosmic Microwave Background (CMB) radiation spectrum — an all pervasive sea of microwave radiation originating just 380 000 years after the Big Bang — could be used to test whether the Copernican Principle stands (Phys. Rev. Lett. 100 191302).

Cosmic acceleration and dark energy

Cosmologists like Caldwell and Stebbins are interested in the Copernican Principle because it plays an important role in the interpretation of the observational evidence for cosmic acceleration and dark energy. If the Copernican Principle is invalid, then there may not be any need for exotic dark energy and in order to explain the observation for the acceleration of the universe, we would need to be living at the center of a ‘void’. This void would then leave a distortion in the CMB away from being a black body. “This is so fundamental that we need to test it”, explained Caldwell.

I would bet my house now that the results will come out null so the Copernican Principle is valid on the scales we observe Paul Steinhardt, Princeton University

To measure if this holds, the team propose to measure the black body nature of the CMB more precisely than before. The void would lead to large anisotropies of scattered light coming from the CMB giving a slight deviation in the CMB from being a black body. But if we see further evidence that the CMB is a black body then it will be evidence that the Copernican Principle holds.

However, it is already accepted that the CMB is a black body. Indeed, the Nobel Prize in Physics was awarded to John Mather and George Smoot in 2006 who showed the CMB is a black body and is also anisotropic. The Nobel Prize winning work came from data collected on NASA’s Cosmic Background Explorer (COBE), which housed the Far Infrared Absolute Spectrophotometer (FIRAS) which recorded the perfect black body spectrum.

Caldwell and Stebbins think the next NASA missions such as the Absolute Spectrum Polarimeter will be able to detect possible deviations from the black body behaviour of the CMB which COBE with FIRAS was not sensitive too. “We need to measure the CMB at different frequencies, which previous missions were not able to do” said Caldwell.

Another test

In a separate paper, Jean-Philippe Uzan from the Pierre and Marie Curie University in France along with Chris Clarkson and George Ellis from the University of Cape Town in South Africa suggest another way to test the Copernican Principle (Phys. Rev. Lett. 100 191303). Their scheme involves measuring the red-shift of galaxies — the shift in wavelength of light to longer wavelengths due to a speedup — very precisely over time to see if there are changes. The team argues that this red-shift data can be combined with measurements of the distance of the galaxies to infer if the universe is spatially homogeneous — which is a tenant of the Copernican Principle.

However, it seems one of the cornerstones of cosmology is not about to be quickly overturned. “I would bet my house now that the results will come out null so the Copernican Principle is valid on the scales we observe,” says Paul Steinhardt a cosmologist at Princeton University, “But I think the experiments should be done.”