We know that deep space is dark, but just how dark is it? Or, put another way, how bright is it? And how much of that brightness comes from galaxies? Astronomers have gotten a clearer answer to those questions, thanks to observations sent back from billions of miles away.
Nine years after its history-making flyby of Pluto, NASA’s New Horizons spacecraft measured the brightness of the distant universe from a vantage point in deep, dark space.
“If you hold up your hand in deep space, how much light does the universe shine on it?” Marc Postman, an astronomer at the Space Telescope Science Institute in Baltimore, asked today in a news release. “We now have a good idea of just how dark space really is.”
Postman, who’s the lead author of a new study published in the Astrophysical Journal, said 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,” he said.
The newly published findings address a puzzle that astronomers have been trying to piece together for decades — ever since the 1960s, when they discovered the existence of cosmic microwave background radiation, also known as the afterglow of the Big Bang. Over the years, astronomers have detected additional background radiation in X-ray, gamma-ray and infrared wavelengths.
So, what about the light level in optical wavelengths? The kind of light we can see with our eyes? It turns out that the cosmic optical background, or COB, is hard to measure in our celestial neighborhood.
“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 study co-author Tod Lauer, an astronomer from the National Science Foundation’s NOIRLab. “All attempts to measure the strength of the COB from the inner solar system suffer from large uncertainties.”
The New Horizons probe is in a better position to do the job. It’s currently more than 5.4 billion miles (7.3 billion kilometers) from Earth, far from the worst of the interplanetary dust. To produce the data for the newly published study, New Horizons’ Long Range Reconnaissance Imager, or LORRI, was positioned to be shielded from the sun as it collected brightness measurements in a range of imaging fields. Those target fields were selected to minimize the glow of the Milky Way and the glare of nearby stars.
This wasn’t the New Horizons team’s first attempt to make this sort of measurement. In 2021, the team conducted a test run, but the results underestimated the amount of dust-scattered light and overestimated the excess light coming from deep space.
This time around, astronomers were able to use other data collected in far-infrared wavelengths by the European Space Agency’s Planck spacecraft. The far-infrared data helped the scientists on the New Horizons team calibrate the data that LORRI collected.
The fully calibrated results matched what would be expected if you added up the intensity of light generated by all of the visible universe’s galaxies over the past 12.6 billion years.
If you want to get geeky about it, the figures from the New Horizons team work out to 11.16 nanowatts per square meter per steradian at a wavelength of 0.608 microns. That’s actually a little higher than the estimated integrated intensity from background galaxies. However, when the margins of error are taken into account, the anomalous gap is “not significantly different from zero,” the researchers reported.
“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, a planetary scientist at the Southwest Research Institute, said the newly published study represents an important contribution to cosmology “that could only be done with a faraway 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,” Stern said.
A year ago, Stern and other planetary scientists worried that NASA might disband the original New Horizons science team — but last September, the space agency announced that the mission could continue studying its environs in the Kuiper Belt, a broad ring of icy objects on the solar system’s edge.
“Its extended operations will continue until the spacecraft exits the Kuiper Belt, expected in 2028-2029,” Nicola Fox, NASA’s associate administrator for science, said at the time.
New Horizons’ focus was also broadened to include multidisciplinary research — for example, studying the sun’s influence on the edge of the solar system. And measuring the brightness of the cosmos beyond.
In addition to Postman, Lauer and Stern, the authors of the study published by the Astrophysical Journal, “New Synoptic Observations of the Cosmic Optical Background With New Horizons,” include Joel Parker, John Spencer, Harold Weaver, J. Michael Shull, Pontus Brandt, Steven Conard and G. Randall Gladstone.
