Cosmic microwaves get polarized
Sep 19, 2002
The polarization of the cosmic microwave background - the microwave ‘echo’ of the big bang - has been detected for the first time. John Carlstrom of the University of Chicago and colleagues used the Degree Angular Scale Interferometer (DASI) at the South Pole to make the measurements, which agree with predictions and provide further support for the standard big bang plus ‘inflation’ model of cosmology. “What is unique about polarization is that it directly measures the dynamics in the early universe,” says Carlstrom, who presented the results at the COSMO-02 conference in Chicago today.
Cosmologists believe that the photons created in the big bang were scattered by free electrons in the early universe. But after 300 000 years, the universe had cooled enough for atoms to form, and there were no longer any free electrons to scatter the photons. This means that the properties of the photons - which have been stretched to microwave wavelengths by the expansion of the universe - reflect the properties of the universe at the so-called time of last scattering.
Recent studies of the spatial variation – or anisotropy – in the temperature of the cosmic microwave background have shown how matter was distributed when the universe was 300 000 years old, and have confirmed that space is flat.
But now cosmic microwaves are set to reveal more. When the photons from the big bang were scattered by free electrons, they would have become polarized in a way that would provide details about the dynamics of the early universe. Astrophysicists were convinced that this polarization would still be present in the cosmic microwave background – and now DASI has spotted it after monitoring the microwave signal from the sky above the South Pole for 200 days.
The measurements also agree with the predictions of the standard model of cosmology, in which the big bang was followed by an extremely short period of very rapid expansion known as ‘inflation’. One mysterious feature of this model is that ordinary matter accounts for less than 5% of the total mass and energy of the universe. The vast majority comes in the form of ‘dark energy’, which is needed to explain why the expansion of the universe continues to accelerate against the influence of gravity. “Polarization is predicted. It has been detected and It is in line with theoretical predictions,” says Carlstrom. “We’re stuck with this preposterous universe.”
“Polarization is going to triple the amount of information that we get from the cosmic microwave background,” says John Kovac, another member of the DASI team. “It's like going from the picture on a black-and-white TV to color.”
Future studies to measure the polarization even more accurately are already planned. “Detection of the polarization opens a new door to exploring the earliest moments and answering the deep questions before us,” says Michael Turner, also of the University of Chicago.
About the author
Katie Pennicott is a science writer living in Bristol.