Until quite recently, astronomy depended solely on electromagnetic waves. Although the detection of gravitational waves in 2016 marked a significant shift, scientists had by then established key theories within the electromagnetic spectrum.
A key theoretical model divided the electromagnetic spectrum into three segments according to their wavelengths: infrared, optical, and ultraviolet. For astronomers, each segment represented distinct physical processes, and observing them individually provided insights into those specific phenomena, regardless of the information offered by the other spectral regions.
This trend was particularly noticeable during investigations into galaxies, where researchers utilized both infrared and optical wavelengths to examine various facets of their development and activity. Nonetheless, Christian Kragh Jespersen from Princeton’s Department of Astrophysics along with his team believes they've uncovered an anomaly that challenges the conventional electromagnetic spectrum understanding—suggesting a connection between what we typically see via optical and infrared observations.
That is the straightforward title. new paper The researchers published arXiv A preprint server details their impressive achievement. Using optical data gathered by the Sloan Digital Sky Survey (SDSS), they forecasted the infrared measurements of galaxies observed by the Wide-field Infrared Survey Explorer (WISE).
Most studies in galaxy astronomy operate under the belief that various “parts” of a galaxy (such as its supermassive black hole or stars at the tips of its spiral arms) can be considered distinct because they radiate at different frequencies. This distinction is condensed into a metric known as the spectral energy distribution, or SED, which serves as a fitting parameter. These values are often employed to define the characteristics and physical attributes of the galaxies being analyzed.
Regrettably, based on the examination presented in this document, the premise upon which SED codes for existing galaxies are built—that different parts of a galaxy can be considered separate entities—turns out to be incorrect. The authors state, “Our findings suggest that [infrared-emitting and optically-emitting] mechanisms are likely closely intertwined.” Whether this revelation comes as a shock or not, it clearly contradicts the foundational principles of contemporary SED modeling approaches.
In order to substantiate their argument, the researchers examined information gathered by the SDSS and WISE. Their study encompassed more than 500,000 distinct galaxies. Following the calibration and verification of an algorithm, they utilized the optical observations obtained through SDSS to forecast the infrared measurements captured by WISE for every single galaxy. This task became simpler due to pre-existing indices correlating the WISE and SDSS data on a galactic level.
The outcomes were impressive—the algorithm managed to forecast the infrared values using only optical inputs with minimal interference. To strengthen their argument, the researchers employed two SED modeling tools: CIGALE and prospector. However, both significantly erred when trying to determine the accurate figures, enabling the authors to annotate several charts illustrating the poor accuracy of these predictions as “Overconfident and biased.”
Conversely, their own data aligned perfectly with the observations from the WISE database. The authors deserve recognition for acknowledging certain weaknesses in their argument, including the difference in aperture sizes between WISE and SDSS, which might affect the accuracy of their algorithm. Nonetheless, the abundance of evidence strongly suggests a straightforward conclusion: the foundational premise known as the "separability" assumption, central to comprehending the physical characteristics of galaxies, appears flawed.
The paper is currently available as a preprint. arXiv And it hasn’t been published in a peer-reviewed journal yet. However, should it be accepted, it has the potential to substantially alter our comprehension of the universe and, even more crucially, the frameworks through which we interpret it.
More information: Christian K. Jespersen and colleagues state that there is a connection between the optical and infrared regions. arXiv (2025). DOI: 10.48550/arxiv.2503.03816
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