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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 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.
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/m
2/sr), albeit with a large
uncertainty (see figure).[4] The more recent measurement with the same spacecraft gave a better precision.[2-3]
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]
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.)
References:
- Cosmas Indicopleustes, The Christian Topography, J. W. McCrindle, Trans., The Hakluyt Society, 1897.
- 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.
- Nell Greenfieldboyce, "Scientists Discover Outer Space Isn't Pitch Black After All," NPR, November 18, 2020.
- 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.
- New Horizons, NASA's Mission to Pluto and the Kuiper Belt, Where is New Horizons?
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