New Horizons has provided groundbreaking insights into the nature of deep space, revealing that true darkness exists beyond the influence of zodiacal light. Four years ago, astronomers realized they could finally search for a cosmic optical background, leading to significant theories about the origins of light in the universe:

• Residual light from the Big Bang, now in microwave wavelengths.
• Light from stars, distributed across galaxies, clusters, and groups.
• Reflected light off neutral matter clouds.
• Infrared light generated by heated matter, contributing to a cosmic infrared background.

Theoretically, the only expected cosmic optical background should arise from starlight confined to galaxies and their clusters, along with minimal additional reflected light. However, measurements taken from Earth and within our Solar System are hindered by stray light from the Sun reflecting off interplanetary particles.

Five spacecraft, including New Horizons, have ventured far enough from Earth to significantly reduce the interference from zodiacal light, allowing for the exploration of deep space's darkness. Unlike the other four spacecraft, New Horizons is equipped with the necessary tools to conduct these critical measurements.

Imagine witnessing the universe's evolution from the beginning of the Big Bang. Initially, the universe was filled with a dense, energetic mix of particles and radiation, including photons with extremely high energies. As the universe expanded and cooled, light wavelengths stretched, shifting from ultraviolet to infrared as neutral atoms formed.

Once neutral atoms emerged, gravitational contractions began, leading to the formation of stars that reignited the universe's illumination with visible light. Today, it is estimated that over two sextillion stars exist, organized within galaxies and larger structures.

Several processes within these matter collections also emit visible light:

• Black holes accrete material, heating it to emit visible light.
• Stars radiate ultraviolet light, shifted into the visible spectrum by cosmic expansion.
• Starlight heats neutral grains like dust, causing them to emit light.
• Scattered starlight reflects off clouds of matter, generating additional optical radiation.

Primarily, stars in galaxies and black holes contribute the most to the universe's optical light. A critical test of this theory would involve launching a telescope beyond the Solar System to measure the faint light from external sources, thereby assessing the cosmic optical background. Matching the total detected light to predictions from galaxies would support current cosmological models; discrepancies could suggest unknown sources of light.

Astronomers have conducted extensive surveys of faint galaxies across various distances using advanced techniques from observatories like JWST and Hubble. However, these telescopes are still too close to the Sun, risking contamination from scattered light from the Milky Way and zodiacal dust. To explore interstellar space accurately, these sources of "light pollution" must be eliminated.

Observations taken during Earth's shadow might help, but zodiacal light persists and can still contaminate measurements. Observing outside the ecliptic plane reduces zodiacal light exposure, but the background brightness is still overwhelming. The most effective approach is to place instruments far from the Sun, where the density of interplanetary dust is negligible.

At last, New Horizons, situated around 57 Astronomical Units from the Sun, serves as an ideal observatory for studying the cosmic optical background. This spacecraft, equipped with the Long-Range Reconnaissance Imager (LORRI), is free from zodiacal light interference, allowing it to make precise measurements of the cosmic optical background.

A 2021 study using New Horizons analyzed various factors impacting observations, including:

• Camera noise.
• Scattered sunlight.
• Off-axis starlight.
• Reflections from spacecraft thrust.
• Instrumental effects.

By focusing on regions far from the Milky Way's plane, researchers quantified the light observed and compared it with predictions from stars and galaxies to discern excess light in the cosmic optical background.

This initial study estimated a cosmic optical background intensity of 15.9 ± 4.2 nanowatts per square meter per steradian, approximately double the anticipated light from unresolved stars and galaxies, suggesting unexpected additional sources of light in the universe.

However, potential sources of foreground light, such as diffuse galactic light, could have contaminated these findings. The new analysis from 2024 revisits earlier data and seeks to minimize these contaminants.

The latest findings indicate that the diffuse galactic light contribution is greater than previously believed, affecting the significance of the anomaly detected in 2021. The refined analysis reveals that the excess light attributed to the cosmic optical background is now half of what was originally suggested.

As the new paper states:

> "While our observations can accommodate a modest [cosmic optical background] anomaly relative to the amplitude of the IGL, we cannot falsify the simpler hypothesis that the [cosmic optical background] is due entirely to the known population of galaxies."

The significance of the latest results indicates that the cosmic optical background is largely accounted for by known sources, with the remaining anomaly suggesting a chance of around 15% that it could be an error. The authors conclude that there is minimal evidence for excess light beyond current astrophysical understanding.

The latest study utilized more fields than previous analyses, improving the robustness and calibration of the results. Enhanced understanding of background noise and its effects on measurements also contributed to refining the analysis.

The contributions from scattered galactic light were also re-evaluated, leading to a better understanding of the optical signals from interstellar dust. A superior calibration method and the increased number of observed fields provided more accurate estimates of the cosmic optical background.

In conclusion, the revised analysis suggests that the cosmic optical background is significantly smaller than earlier estimates, aligning closely with predictions from known astrophysical sources. The findings emphasize the importance of accurate measurements and the ongoing quest to understand the universe's true nature.

The research led by the New Horizons team marks a significant step in our understanding of cosmic light and the universe's structure, reaffirming the need for precise observational techniques in the pursuit of knowledge.

> Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, Treknology, and The Littlest Girl Goes Inside An Atom. His first National Geographic book, Infinite Cosmos, releases October 8th!