![CMB Image source: University of California at Berkeley Cosmology Group CMB Image source: University of California at Berkeley Cosmology Group](http://www.timeone.ca/wp-content/uploads/2013/03/CMB-e1403876689320.jpg)
Classification: concept
Chapter: Looking Forward, Looking back
The low-energy photons from the 370,000 year-old universe stretched by a factor of 1000 by expansion of space in the 13,700,000 years they have been traveling are called the cosmic microwave background.