How constant is our changeable Universe?
23 April 2003Constants govern the way our Universe behaves. We regard them as fixed, unchanging quantities. However, how can we be sure that constant really means constant? Modern theories require the values of universal constants to slowly change during the course of cosmic history. Is this the case? ESA is preparing to search for (small) changes in the 'constants of nature'.
The forces of nature interact to make the Universe the way it is. There are four fundamental forces: gravity, which holds us on Earth and draws celestial objects into orbits; electromagnetism, which gives us electricity and magnetism; and lastly the strong and the weak nuclear forces, which govern the behaviour of atomic nuclei. Each of these forces has a different strength, encoded by the constants of nature.
Experts call the gravitational constant the 'Big G', whereas the fine-structure constant governs the electromagnetic force. New evidence shows that both of these have changed by small amounts during cosmic history. ESA's star mapper, Gaia, will set limits on the variation of Big G. Similarly, ESA's Planck mission will look at the fine-structure constant by examining minute variations in the very first light emitted in the Universe. Carlos Martins works in the Centro de Astrofisica da Universidade do Porto, Portugal, and the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, in the United Kingdom. He analyses images of this very first light - known to astronomers as the cosmic microwave background - to set limits on the variation of the fine-structure constant.
"We expect the fine-structure constant to increase with time," explains Martins, "At the moment, the cosmic microwave background data is consistent with a variation of less than 4%. NASA's recent spacecraft WMAP returned data that was more impressive than previous findings." However impressive it may have been, it did not provide the definitive answers that Martins and others wanted. Only ESA's Planck mission will give those. "Planck will detect variations as small as 0.1% of the present value of the constant. When we combine the other data sets (taken by other telescopes and missions), we will be able to improve that even further."
Why is this variation so important? Scientists believe that you can explain the four forces by a single - but so-far elusive - unifying mathematical description. "Our currently preferred unification theories require the constants to vary with time," says Martins.
These current theories, collectively known as string or M-theory, suggest that our Universe exists in more than the familiar three dimensions. As the Universe expands, the extra dimensions change slightly and our perception of universal 'constants' alters, making them appear to change.
"Searching for the variation of fundamental constants is a very good way to judge models that use additional dimensions," says Martins. Planck is the ultimate test of the various string theories. Martins explains, "Planck will be very sensitive to any variation of the fine-structure constant. If it finds no changes, all theories that suggest the Universe has extra dimensions will be in trouble."
Once Planck launches in 2007, we will start to find out just how constant our Universe's constants are.