After more than 18 years in space and nine years following its historic encounter with Pluto, New Horizons is now over 5.4 billion miles (7.3 billion kilometers) away from Earth. In this remote region of the solar system, the spacecraft is far enough from the Sun to observe the darkest skies available, offering a unique perspective to measure the overall brightness of the universe.

Marc Postman, an astronomer at the Space Telescope Science Institute in Baltimore and lead author of a new paper published in 'The Astrophysical Journal', explained, "If you hold up your hand in deep space, how much light does the universe shine on it? We now have a good idea of just how dark space really is. The results show that the great majority of visible light we receive from the universe was generated in galaxies. Importantly, we also found that there is no evidence for significant levels of light produced by sources not presently known to astronomers."

These findings address a question that has puzzled scientists since the 1960s when astronomers Arno Penzias and Robert Wilson discovered pervasive microwave radiation, a relic from the universe's creation. Their discovery earned them the Nobel Prize. Subsequent observations uncovered backgrounds of X-rays, gamma rays, and infrared radiation filling the sky.

Detecting the cosmic optical background (COB), or the "ordinary" light, adds up all the light produced by galaxies throughout the universe's history. Prior to the Hubble and James Webb space telescopes, this background light was difficult to measure directly, especially from Earth or the inner solar system, where sunlight and interplanetary dust interfere.

"People have tried over and over to measure it directly, but in our part of the solar system, there's just too much sunlight and reflected interplanetary dust that scatters the light around into a hazy fog that obscures the faint light from the distant universe," said Tod Lauer, a New Horizons co-investigator, astronomer from the National Science Foundation NOIRLab in Tucson, Arizona, and a co-author of the paper. "All attempts to measure the strength of the COB from the inner solar system suffer from large uncertainties."

New Horizons, now deep in the Kuiper Belt and on its journey toward interstellar space, offers a solution. Last summer, from 57 times farther from the Sun than Earth, New Horizons used its Long Range Reconnaissance Imager (LORRI) to collect 24 separate imaging fields. Shielded from the Sun by the spacecraft's main body, LORRI's observations avoided the bright disk and core of the Milky Way and nearby stars.

The team used far-infrared data from the European Space Agency's Planck mission to calibrate the level of dust emissions against visible light. This correction allowed for a more accurate measurement of the COB, a technique that improved upon their 2021 observations where dust-scattered light was underestimated.

After accounting for all known light sources, including background stars and dust within the Milky Way, the researchers found that the remaining visible light matched the predicted intensity from galaxies over the past 12.6 billion years.

"The simplest interpretation is that the COB is completely due to galaxies," Lauer said. "Looking outside the galaxies, we find darkness there and nothing more."

New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado, highlighted the significance of this work: "This newly published work is an important contribution to fundamental cosmology, and really something that could only be done with a far-away spacecraft like New Horizons. And it shows that our current extended mission is making important scientific contributions far beyond the original intent of this planetary mission designed to make the first close spacecraft explorations of Pluto and Kuiper Belt objects."

Launched in January 2006, New Horizons first explored Pluto and its moons in July 2015, followed by a close-up study of the Kuiper Belt object Arrokoth in January 2019. Now in its second extended mission, New Horizons continues to study distant Kuiper Belt objects, the outer heliosphere of the Sun, and perform critical astrophysical observations from its unique vantage point.

Research Report:New Synoptic Observations of the Cosmic Optical Background with New Horizons

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