A University of California, Riverside, study has expanded our knowledge of the structure of the universe and supports the existence of a new physical effect, such as a particle that has never been observed before.
The astrophysicists used the “Lyman-Alpha Forest,” a powerful tool for mapping the distribution of hydrogen in the universe — and, indirectly, dark matter. The “forest,” a series of absorption lines in the spectra of distant quasars and galaxies, is so named because in graphs it appears like a dense tangle of saplings. To analyze the forest, the scientists used a new model and simulations, which allowed them to reconstruct the distribution of matter, including dark matter, over a vast portion of the universe.
Published in the Journal of Cosmology and Astroparticle Physics, the study confirms a “tension” or discrepancy between observations and theoretical predictions about the universe, suggesting the possible existence of a new particle.
Simeon Bird, an associate professor of physics and astronomy and the study’s senior author, explained that each of the peaks in the wavelength distribution graph observed in the Lyman-Alpha Forest represents a sudden drop in “light” at a specific and narrow wavelength, serving as a “map” of the matter that the light encountered on its journey to us.
“It’s somewhat like shadow puppetry, where we guess the character placed between the light and the screen based on its silhouette,” he said. “The ‘shadow’ of hydrogen molecules, suspended at vast distances between us and the light projected by intense luminous sources even farther away, is well recognized by astrophysicists.”
The images used are called spectrograms, Bird explained. They are decompositions of light.
“It’s like a kind of very fine-grained rainbow,” Bird said.
When sunlight passes through a prism (or a water droplet), the light is split into its “ingredients,” the colors of the rainbow. In spectrograms of light coming from cosmic sources like quasars, the same kind of splitting happens, but almost always some frequencies go missing, visible as black bands where the light is absent, as if something had cast a shadow.
“These are the atoms and molecules that the light has encountered along the way,” Bird said. “Since each type of atom has a specific way of absorbing light, leaving a sort of signature in the spectrogram, it is possible to trace their presence, especially that of hydrogen, the most abundant element in the universe.”
According to Bird, the hydrogen is useful because it’s like a tracer of dark matter.
“We still don’t know what dark matter is and we’ve never seen it, but we are certain it exists in great abundance—greater than that of normal matter,” he said.
When Bird and his colleagues used hydrogen to track dark matter indirectly, they found it was like “sticking dye into a stream of water.”
“The dye will follow where the water goes,” Bird said. “Dark matter gravitates so it has a gravitational potential. The hydrogen gas falls into it, and you use it as a tracer of the dark matter. Where it is denser there’s more dark matter. You can think of the hydrogen as the dye and the dark matter as the water.”
The work by Bird and his colleagues does more than simply monitor dark matter. Current studies of the cosmos report some so-called “tensions” or discrepancies between observations and theoretical predictions.
“One of the current tensions is the number of galaxies on small scales and at low redshifts,” Bird said. “The low redshift universe is the one relatively close to us. Currently, there are two hypotheses to explain the discrepancy between observations and expectations: first, there exists a new physical effect, such as a never-before-seen particle, or, second, that something strange is happening with supermassive black holes inside galaxies. The black holes are stunting the growth of galaxies in some way and are messing up our structure calculations.”
Bird cautioned that the tension is not yet at the level of certainty needed to claim a detection.
“It’s not completely convincing yet,” Bird said. “But if this holds up in later data sets, then it is much more likely to be a new particle or some new type of physics, rather than the black holes messing up our calculations.”
Bird’s coauthors on the paper are M. A. Fernandez and Ming-Feng Ho.
The title of the research paper is “Cosmological Constraints from the eBOSS Lyman-α Forest using the PRIYA Simulations.”
The research was funded by the National Science Foundation (NSF) and NASA, with the original survey funded by the Department of Energy. Simulations were run using NSF Frontera and additional modeling was done on UCR’s High-Performance Computing Center.
