The cosmic microwave background (CMB) is the first light that freely shone in the universe and it is a goldmine of information for understanding the cosmos. But it's also full of mysteries. One of these is that the universe seems hotter on one side than the other. So far scientists have assumed that this difference is not real, believing it to be an apparent consequence of the movement of the Solar System, and now there's a way to test this for sure.
Cosmologists Siavash Yasini and Elena Pierpaoli from The University of Southern California Dornsife were working on a related study when they realized something important. In a paper published in Physical Review Letters, the researchers showed that the average temperature across the sky won’t be exactly the same if one end of the CMB is, in fact, hotter than the other.
"We think that one side of the CMB only looks hotter because we are moving towards it, and the opposite side looks colder because we are moving away from it," said Yasini, a PhD student in physics and astronomy, in a statement.
It is assumed that the difference in temperature, namely a dipole, is a Doppler effect. The wavelength of light is changed because the Solar System rushes around the Milky Way, with the light ahead appearing bluer and the light behind appearing redder. The observation and assumption have been key to estimating the speed of the Solar System.
"If there is an intrinsic dipole in the CMB – that is, if one side of the sky is actually partially hotter than the opposite side – the velocity we assign the Solar System with respect to the CMB would be incorrect," Yasini added.
Other measurements might be affected too. If the dipole is real, the distance of galaxies might have to be corrected and obviously, there would be a discussion on how such temperature difference arises. One of the most striking features of the CMB is its general uniformity, with the temperature differing by a tiny fraction of a degree.
The next generation of surveys to study the cosmic microwave background will be capable of taking measurements that can be used to test if the dipole is real or not.
"Now that we have a mathematical basis for finding an answer, it just remains to make the observations," Pierpaoli concluded.