IS OUR spot in the universe somehow special? It is an age-old question, but now there鈥檚 a way to answer it once and for all 鈥� as long as we鈥檙e patient.
Convention has it that our neck of the woods is very ordinary, based on what鈥檚 known as the Copernican principle, which states that the cosmos is pretty much the same wherever you go. It suggests space-time is expanding at the same rate in every part of the universe, meaning that the distribution of matter is roughly the same.
Although this makes sense theoretically, nobody has managed to test this assumption definitively. Previous attempts have rested on further assumptions, such as the nature of gravity or how matter is distributed through the universe 鈥� in other words, they rely on trusting our cosmological models, which were built on the Copernican principle.
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Now Jean-Philippe Uzan of the Paris Astrophysics Institute in France, working with Chris Clarkson and George Ellis of the University of Cape Town in South Africa, have a test that gets around this problem. All you have to do is measure the light from objects such as quasars, and see if these measurements change over time. The paper will be published in Physical Review Letters.
Light shifts towards the red end of the spectrum on its journey to Earth because the expansion of the intervening space causes the light鈥檚 wavelength to increase. Uzan and colleagues say that measuring the 鈥渞ed shift鈥� of various objects over a period of, say, 10 years would show whether the Copernican principle is false. If different parts of space are expanding at different rates, then the red shifts of two objects that match initially, for instance, would be out of kilter 10 years later.
If this is true, the case for dark energy would start to look shaky. Dark energy is thought to be accelerating the expansion of the universe. But if expansion rates differ over areas of space rather than time, it would scupper dark energy models that assume acceleration is uniform.
Ruth Durrer of the University of Geneva in Switzerland likes the idea. 鈥淚t could have a profound effect on the interpretation of the apparent acceleration,鈥� Durrer says. 鈥淚t could help us get rid of dark energy.鈥�
We might be in for a bit of a wait before the test can be done, however. The only way to measure red shift accurately enough will be by using the CODEX spectrograph at the European Extremely Large Telescope, which won鈥檛 be fired up for at least 10 years.
Durrer says it would be quicker to measure 鈥渂aryon acoustic oscillations鈥�, sound waves that were generated in the plasma of the primordial universe. Like red shift, measuring the oscillations at various points of the sky over time would show whether the universe was being stretched faster or slower in different areas. However, Uzan points out that this method is less direct than measuring actual objects.
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