Astronomers may have finally answered this long-standing question by exploiting the capabilities and remote position of NASA’s New Horizons spacecraft to make the most precise direct measurements ever made of the total amount of light generated by the Universe .
More than 18 years after launch and nine years after the historic exploration of Pluto, New Horizons is more than 8.7 billion kilometers from Earth, in a region of the Solar System far enough from the Sun to provide the darkest sky available to any existing telescope – and to provide a unique vantage point from which to measure the overall brightness of the distant Universe.
“If you raise your hand into deep space, how much light does the Universe shine on it?” asked Marc Postman, an astronomer at the Space Telescope Science Institute in Baltimore and lead author of a new paper detailing the research, published in
“Now, we have a good idea of how dark space really is. The results show that the vast majority of the 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 currently unknown to astronomers.”
The findings solve an enigma that has puzzled scientists since the 1960s, when astronomers Arno Penzias and Robert Wilson discovered that space is permeated by powerful microwave radiation that was predicted to remain itself.
This result led to the awarding of the Nobel Prize. Later, astronomers also discovered evidence of a background of X-rays, gamma rays, and infrared radiation also filling the sky.
The most accurate measurements
Detecting the “ordinary” (or visible) light background—more formally called the cosmic optical background, or COB—provided a way to collect all the light generated by galaxies throughout the life of the Universe before the Hubble Space Telescope and the James Webb Space Telescope at NASA to be able to directly see the faint galaxies in the background.
In the era of the Hubble and James Webb Telescopes, astronomers measure the COB to detect light that might come from sources other than these known galaxies. But measuring the total production of is extremely difficult from Earth or anywhere in the inner Solar System.
“People have tried many times to measure it directly, but in our part of the Solar System, there is too much sunlight and reflected interplanetary dust that scatters the light into a haze that obscures the faint light of the distant Universe,” said Tod Lauer, a New Horizons co-investigator, astronomer at the National Science Foundation NOIRLab in Tucson, Arizona, and co-author of the new paper. “All attempts to measure COB strength within the Solar System suffer from large uncertainties.”
Late last summer, from a distance 57 times farther from the Sun than Earth, New Horizons scanned the Universe with the Long Range Reconnaissance Imager (LORRI), gathering two dozen separate image fields, writes
How dark is space really?
LORRI was intentionally shielded from the sun by the main body of the spacecraft – preventing even the faintest sunlight from entering the sensitive chamber directly – and the target fields were positioned away from the bright disk and core of the Milky Way and bright stars nearby.
New Horizons observers used other far-infrared data taken by the European Space Agency’s Planck mission of fields with varying dust density to calibrate the level of this far-infrared emission to that of ordinary visible light.
This allowed them to accurately predict and correct for the presence of dust-scattered light from the Milky Way in the COB images—a way that was not available to them during a 2021 COB observing test with New Horizons, in which they underestimated the amount of light scattered by the dust and overestimated itself.
But this time, after taking into account all known sources of light, such as background stars and light scattered by the Milky Way’s thin dust clouds, the researchers found that the remaining level of visible light was entirely consistent with the intensity of light generated by all galaxies in the last 12.6 billion years.
“The simplest interpretation is that the COB is entirely due to galaxies,” Lauer said. “Looking outside the galaxies, we find there darkness and nothing else,” the researchers conclude.
