THE inexplicably high temperature of the hydrogen floating between galaxies may have a simple explanation: the gas is being heated by cosmic rays zipping through space. If so, it will be bad news for cosmologists, since evidence of past events such as the birth of stars will be erased from the hydrogen.
The hydrogen that makes up most of the intergalactic medium should be just a few degrees above absolute zero. Instead, it is extremely hot, with temperatures of tens of thousands of degrees. It is often assumed that this gas was heated up during an epoch in the early…
