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Cosmic Glow

January 11, 2021

The night sky, when viewed in directions away from the Milky Way, is a deep blackness, punctuated by a few dots of starlight. This blackness is a sight that's so familiar that we don't give it much thought. However, a simple thought experiment known as Olbers' paradox concludes that in an infinite, static universe, every patch of sky should appear as bright as a star. This thought experiment is named after German astronomer Heinrich Olbers (1758-1840), but the idea had been around for many centuries prior to his time.

Cosmas Indicopleustes of Alexandria, Egypt, who spent his later life as a hermit monk, wrote about the problem of an infinite number of stars in his c.550 Christian Topography (Χριστιανικὴ Τοπογραφὶα).[1] Cosmas wondered why an infinite number of stars didn't melt melt the crystal dome of heaven. If you haven't heard of Cosmas, it's no wonder. This book attacked the geocentric model of the universe and argued that the Earth was flat, contrary to what most natural philosophers believed at the time.

Plate 9 of the Christian Topography by Cosmas Indicopleustes

Plate 9 of the Christian Topography of Cosmas Indicopleustes, showing the Ptolemaic system.[1] Shown are the twelve signs of the Zodiac and the names of the Roman and Egyptian months. The names of the Roman months are written in Greek characters. (From ref. 1. Click for larger image.[1])

In an infinite and static universe, any line of sight will end in a star; so, the night sky should be bright, not dark. A good analogy for this is looking out from inside a huge forest. Although distant trees will appear smaller than closer trees, there will always be some tree blocking your view. This idea helped to reinforce the "island universe" concept in which our finite Milky Way galaxy was sitting inside an immense voids. Interstellar dust might initially hide the radiation of a population of infinite distant stars, but that dust will glow just as brightly given an infinite time, as in a static universe.

We don't see a bright night sky for several reasons, the most important of which is that expansion of the universe has redshifted much of its visible light to infrared and radio wavelengths. If we could see microwave frequencies, the sky would be seen to glow at the cosmic microwave background radiation of the universe of the Big Bang. This radiation peaks at 160.23 GHz, and it's equivalent to a black body temperature of about 2.73 K.

Hubble Ultra Deep Field image

The Hubble Ultra Deep Field image.

This image of a very small patch of sky in the constellation, Fornax, was created from images acquired from September 3, 2003, through January 16, 2004, and it illustrates the blackness of regions between galaxies at this high resolution.

The area of this image is 11.5 square arcminutes, which is somewhat smaller than a 10 millionth portion of the celestial sphere.

(NASA and European Space Agency image from Wikimedia Commons. Click for larger image.)

Measurement of the actual blackness of space is complicated by local foreground light sources. On Earth, there's Earth's atmosphere; and, in space, there is light that's reflected from interplanetary dust. Earth's atmosphere is several orders of magnitude brighter than the cosmic optical background (COB), and sunlight scattered from interplanetary dust particles in the Solar System, known as the Zodiacal light, adds additional background to measurements made in space from inside our Solar System. A huge astronomy research team has recently made a precise measurement of the cosmic optical background using an imager on the New Horizons spacecraft.[2-3] This is an improvement on a similar measurement in 2017.[4]

The research team consisted of members from the National Optical Infrared Astronomy Research Laboratory (Tucson, Arizona), the Space Telescope Science Institute (Baltimore, Maryland), Johns Hopkins University (Laurel, Maryland), the Southwest Research Institute (Boulder, Colorado), the University of California at Berkeley (Berkeley, California), the Massachusetts Institute of Technology (Cambridge, Massachusetts), the University of Central Florida (Orlando, Florida), the Jet Propulsion Laboratory (Pasadena, California), Lowell Observatory (Flagstaff, Arizona), the University of Colorado (Boulder, Colorado), the University of Victoria (Victoria, British Columbia), Washington University, (St. Louis, Missouri), the NASA Ames Research Center (Moffett Field, California), the NASA Goddard Space Flight Center (Greenbelt, Maryland), the Lunar and Planetary Institute (Houston, Texas), the SETI Institute (Mountain View, California), and the University of Virginia (Charlottesville, Virginia).[2]

The New Horizons spacecraft passed Pluto in July, 2015, and it's presently about 50 astronomical units (AU) from Earth, well away from most interfering optical background.[5] The 2017 study used the long range reconnaissance imager (LORRI) on New Horizons to measure the COB, and it found a value in the center of the visible light spectrum of about 5 nanowatts per square meter per steradian (nW/m2/sr), albeit with a large uncertainty (see figure).[4] The more recent measurement with the same spacecraft gave a better precision.[2-3]

cosmic optical background

The cosmic optical background (COB), as determined by ref. 4, is shown as both an upper limit (red) and a mean (red star). Previous results in the literature are shown, also.

The scatter of values on this logarithmic scale plot illustrates the difficulty of the COB measurement.

(Fig. 2 of Ref. 4, licensed under a Creative Commons Attribution 4.0 International License. Click for larger image.)

While New Horizons was between 42 and 45 AU from the Sun, they used its LORRI camera to measure the optical-band (400-900 nm wavelength) sky brightness at seven high galactic latitude fields.[2] The high galactic latitudes minimized the background light from our Milky Way galaxy. The average raw level brightness was 33.2±0.5 nW m−2sr−1, which is about ten times darker than a Hubble Space Telescope background.[2] After certain corrections,[2-3] there remained a diffuse flux component of unknown origin in the range of 8.8±4.9 (1.8 statistical error and 4.5 systematic error) nW m−2sr−1 to 11.9±4.6 (1.8 statistical error and 4.2 systematic error) nW m−2sr−1.[2]

Tod Lauer, an astronomer with the National Optical-Infrared Astronomy Research Laboratory and lead author of the study, is quoted by NPR as saying,
"The images were all of what you just simply call blank sky. There's a sprinkling of faint stars, there's a sprinkling of faint galaxies, but it looks random... What you want is a place that doesn't have many bright stars in the images or bright stars even outside the field that can scatter light back into the camera."[3]
One of the adjustments made to the light measurement was to subtract the portion that's suspected to arise from unseen galaxies, but there was still an excess.[3] The unexplained light was roughly equal to the light from the known galaxies.[2-3] The excess could be explained by unrecognized galaxies, more dust than expected, or an unknown phenomenon related to dark matter.[3] However, as Lauer explains, "Space is dark... it's still pretty dark."[3]

New Horizons spacecraft at Pluto (artist's conception)

An artist's conception of the New Horizons spacecraft at Pluto.

The long range reconnaissance imager (LORRI), with its large circular lens cover flipped open, can be seen at the upper portion of this image.

(NASA image from the Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.)


  1. Cosmas Indicopleustes, The Christian Topography, J. W. McCrindle, Trans., The Hakluyt Society, 1897.
  2. Tod R. Lauer, Marc Postman, Harold A. Weaver, John R. Spencer, S. Alan Stern, Marc W. Buie, Daniel D. Durda, Carey M. Lisse, A. R. Poppe, Richard P. Binzel, Daniel T. Britt, Bonnie J. Buratti, Andrew F. Cheng, W.M. Grundy, Mihaly Horanyi J.J. Kavelaars, Ivan R. Linscott, William B. McKinnon, Jeffrey M. Moore, J. I. Nuñez Catherine B. Olkin, Joel W. Parker, Simon B. Porter, Dennis C. Reuter, Stuart J. Robbins, Paul Schenk, Mark R. Showalter, Kelsi N. Singer, Anne. J. Verbiscer, and Leslie A. Young, "New Horizons Observations of the Cosmic Optical Background," arXiv, November 9, 2020.
  3. Nell Greenfieldboyce, "Scientists Discover Outer Space Isn't Pitch Black After All," NPR, November 18, 2020.
  4. Michael Zemcov, Poppy Immel, Chi Nguyen, Asantha Cooray, Carey M. Lisse, and Andrew R. Poppe, "Measurement of the cosmic optical background using the long range reconnaissance imager on New Horizons," Nature Communications, vol. 8, Article no. 15003, April 11, 2017, DOI: https://doi.org/10.1038/ncomms15003. This is an open access article with a PDF file here.
  5. New Horizons, NASA's Mission to Pluto and the Kuiper Belt, Where is New Horizons?

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