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How dark is the universe – optically?

John Healy

How dark is the universe? Optically – for visible light. Far away from star light and reflected dust.

Here’s an interesting perspective on electromagnetic (EM) cosmic energy density – just for optical wavelengths. And without any clarification of fuzzy bounds for a galaxy’s reach.

The entire electromagnetic spectrum is another story.

• Live Science > “Scientists confirm that most of the universe is ‘darkness and nothing more’” by Ben Turner (August 30, 2024) – NASA’s New Horizons spacecraft [currently far away in the Kuiper Belt] has made the most precise and direct measurements of the total amount of [visible] light produced by our universe.

The background of visible light added up over the universe’s lifespan (called the cosmic optical background or COB) is important to astronomers because it helps them to match the light coming from stars and the exteriors of black holes with that predicted by theory [by their models].

“The simplest interpretation is that the COB is completely due to galaxies,” Lauer [Tod Lauer, a New Horizons co-investigator and an astronomer at the National Science Foundation NOIRLab in Tucson, Arizona] said. “Looking outside [hmm] the galaxies, we find darkness [in the visible spectrum] there and nothing more.”

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One thought on “How dark is the universe – optically?”

  1. Here’re two images which contrast optical brightness with infrared.

    • APOD > “Seven Dusty Sisters” (2024 September 29)

    Pleiades shown in infrared light Infrared
    Pleiades shown in optical light Optical

    Image Credit: WISE, IRSA, NASA; Processing & Copyright: Francesco Antonucci

    Explanation: Is this really the famous Pleiades star cluster? Known for its iconic blue stars, the Pleiades is shown here in infrared light where the surrounding dust outshines the stars. Here, three infrared colors have been mapped into visual colors (R=24, G=12, B=4.6 microns).

    The base images were taken by NASA’s orbiting Wide Field Infrared Survey Explorer (WISE) spacecraft. Cataloged as M45 and nicknamed the Seven Sisters, the Pleiades star cluster is by chance situated in a passing dust cloud. The light and winds from the massive Pleiades stars preferentially repel smaller dust particles, causing the dust to become stratified into filaments, as seen.

    The featured image spans about 20 light years at the distance of the Pleiades, which lies about 450 light years distant toward the constellation of the Bull (Taurus).

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