New Horizons Measures the Background Light of the Universe

Think about background radiation and most people immediately think of the cosmic background radiation and stories of pigeon excrement during its discovery. That’s for another day though. Turns out that the universe has several background radiations, such as infrared and even gravitational wave backgrounds. NASA’s New Horizons is far enough out of the Solar System now that it’s in the perfect place to measure the cosmic optical background (COB). Most of this light comes from the stars in galaxies, but astronomers have always wondered if there are other sources of light filling our night sky. New Horizons has an answer. No!

Ok lets talk pigeon excrement.  Back in 1965 two telecommunication engineers were exploring signal interference at the Bell Laboratory. Penzias and Wilson detected a faint ‘hum’ in all directions and initially put it down to pigeon excrement as they nested in the horn of the radio receiver. Instead, what they had discovered was the cosmic background radiation, the faint glow that permeates the entire universe and is the thermal radiation left over from the Big Bang. Studying it allows us to understand more about the Universe when it was 380,000 years old. 

In the late 80’s a different type of background radiation was detected; the infrared background radiation. It consists of the diffuse infrared glow that fills the universe coming from numerous sources throughout the history of the universe. It is mostly from thermal emissions from dust grains heated by stellar radiation. In addition to this is the gravity wave background although this has yet to be detected. 

Another hotly debated background is the cosmic optical background (COB), a diffuse light which originates from stars and galaxies and spans the whole of the visible spectrum. There has been gathering momentum in its study however with observations from Hubble Space Telescope and the Spitzer Infrared Telescope. The studies however revealed that a large contribution to a general background optical glow come from faint unresolved galaxies. The study of the COB allows us to explore the total energy output of the universe, about galaxy and star formation across the history of the cosmos. 

The detection of the COB is a challenging one however with Earth based instruments or even those in Earth orbit plagued by interference. The zodiacal light for example is the result of sunlight scattered by interplanetary dust, it is dominant in the inner solar system  and makes studies of the COB difficult. The New Horizon probe is ideally positioned out beyond the orbit of Pluto over 8 billion kilometres away from interference. On board New Horizons is the LORRI (Long Range Reconnaissance Imager) camera which was identified as an ideal platform to begin a search. 

Using images from the LORRI camera, a team of astronomers led by Marc Postman from the Space Telescope Science Institute attempted to measure the COB over the range 0.4 to 0.9 micrometers. The images were from high galactic latitudes to ensure no diffuse light from the Milky Way or scattered light from bright stars. Isolating the COB contribution to the total sky brightness levels required digitally subtracting the scattered light from bright stars and galaxies and from faint stars within the field that were fainter than that detectable by LORRI. Interestingly, the results showed that, based on the estimated galaxy counts in the sampled regions the COB is the result of light from all the galaxies within our observable region of the universe.

Source : New Synoptic Observations of the Cosmic Optical Background with New Horizons