![]() The Pioneer measurements remain the most stringent constraints of the COB 23, and have uncertainties dominated by errors associated with subtracting galactic components including the integrated light from stars (ISL) and diffuse galactic light (DGL). These data have been used to measure both the decrease in the IPD light with heliocentric distance 19, diffuse light from the Galaxy 20, 21 and the brightness of the COB itself 22, 23 using the two IPP bands spanning 390–500 and 600–720 nm. Two exceptions to this are the early NASA probes Pioneer 10 and 11, which were instrumented with imaging photopolarimeters (IPPs) that returned measurements of the sky brightness ranging from 1 to 5.3 a.u. Though many planetary probes have had optical-wavelength cameras, they are rarely designed with the demands of extragalactic astronomical observations in mind. It is thus desirable to measure the COB from vantage points where the earth’s atmosphere and the light from IPD are not appreciable components of the diffuse sky brightness, such as the outer parts of our Solar system 17. Though progress has been made in carefully accounting for the atmosphere and Zodiacal light in the optical 7, 8 and into the near-IR 9, 10, 11, 12, 13, 14, as it is typically >100 times brighter than the COB small errors in this accountancy propagate to large errors on the COB 15, 16. Sunlight scattered from interplanetary dust (IPD) particles in the Solar system, known as Zodiacal light when viewed from the earth, also produces a large foreground to direct measurement of the COB from vantage points in the inner Solar system. The earth’s atmosphere is several orders of magnitude brighter than the COB, and accounting for the various relevant emission, absorption, and scattering effects is a daunting task. A comparison of the COB intensity to the surface brightness arising from known galaxy populations can reveal the presence of diffuse backgrounds produced by more exotic phenomena such as the decay of particle species outside the standard model or light from objects outside of galaxies 4, 5, 6.ĭirect photometric measurement of the COB has proven to be challenging. It is a powerful diagnostic of the emission from known astrophysical processes in galaxies including stellar nucleosynthesis, mass accretion onto black holes and the gravitational collapse of stars 1, 2, 3. The cosmic optical background (COB) is the summed emission from all sources outside of our Milky Way galaxy emitted at wavelengths roughly corresponding to those visible with the human eye. We conclude that a carefully performed survey with LORRI could yield uncertainties comparable to those from galaxy counting measurements. Here we analyse the data from the Long Range Reconnaissance Imager (LORRI) instrument on NASA’s New Horizons mission acquired during cruise phase outside the orbit of Jupiter, and find a statistical upper limit on the optical background’s brightness similar to the integrated light from galaxies. Observations from probes far from the Earth are not affected by these bright foregrounds. ![]() ![]() Measurement of the absolute brightness of this background is complicated by local foregrounds like the Earth’s atmosphere and sunlight reflected from local interplanetary dust, and large discrepancies in the inferred brightness of the optical background have resulted. The cosmic optical background is an important observable that constrains energy production in stars and more exotic physical processes in the universe, and provides a crucial cosmological benchmark against which to judge theories of structure formation.
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