Astronomers have produced one of the most accurate, comprehensive cosmic maps ever made, revealing a brilliant “sea of light” that permeated the early universe.
Unlike other universal maps, this 3D representation is composed of light emitted by a single element: hydrogen, the simplest and most abundant element in the universe, which emits large quantities of a specific wavelength of light when it becomes excited by energy from nearby stars.
The new research, described in a paper published March 3 in The Astrophysical Journal, is part of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), a sky survey that aims to illuminate how dark energy and gravity shape the universe. The researchers can now compare their simulations with this new data, gathered with the Hobby-Eberly Telescope at the McDonald Observatory in Texas, to assess how cosmological models differ from observations.
An exciting way to see the baby universe
When hydrogen atoms are bombarded by stellar radiation, they become excited and emit Lyman-alpha light, a specific wavelength in the ultraviolet part of the electromagnetic spectrum.
Massive, bright galaxies are easier to detect, but fainter galactic structures and the immense interstellar gas clouds that form stars and galaxies have remained largely undetected — until now.
To reveal the sea of light that permeated the fledgling cosmos, the researchers used a technique called line-intensity mapping, which focuses on the telltale wavelengths, or signature spectral emissions, given off by different elements. Astronomers can therefore use line mapping to chart the concentration and distribution of those specific elements throughout the cosmos, forming a map of the luminous galaxies and glowing gas clouds lit up by excited hydrogen atoms.
Cosmology is about zooming out
When studying individual galaxies, stars or other discrete celestial objects, astronomers analyze their characteristics by zooming in. Cosmology, however, requires zooming out. Accordingly, the HETDEX survey doesn’t observe individual galaxies but rather the combined light from every object in a designated region of the sky. As a result, astronomers can gather integrated data from a multitude of galaxies and intergalactic gas clouds simultaneously.
“Imagine you’re in a plane looking down,” study co-author Julian Muñoz, a theoretical cosmologist at The University of Texas at Austin, said in a statement. “The ‘traditional’ way to do galaxy surveys is like mapping the brightest cities only: you learn where the big population centers are, but you miss everyone that lives in the suburbs and small towns. Intensity mapping is like viewing the same scene through a smudged plane window: you get a blurrier picture, but you capture all the light and not just the brightest spots.”
In the quest to understand dark energy and chart more than 1 million bright galaxies, HETDEX “has gathered more than 600 million spectra over an area equivalent to more than 2,000 full moons, creating an unprecedented dataset,” the researchers said in a different statement.
A golden age of cosmic mapping
The mapping method made possible by HETDEX offers another way to examine cosmology’s driving forces and how mass is distributed throughout the universe.
“These new 3D maps allow us to study how galaxies cluster together,” study co-author Karl Gebhardt, a professor of astrophysics at The University of Texas at Austin, told Live Science via email. “The culprit that causes galaxies to come together is gravity. So by studying the clustering properties, we are understanding the properties of gravity and how much mass exists,” Gebhardt explained.
Seeing galactic structures as a collective is invaluable for measuring large-scale density fluctuations across the cosmos to explore the influence of dark energy, the mysterious entity that appears to be accelerating the universe’s expansion.
Unsurprisingly, detecting the signals from ancient galaxies is hard, “but excluding the faint signal from everything else — faint galaxies in the foreground, noise from the detector, artifacts produced by the analysis techniques, scattered light sources like the moon, weak absorption/emission lines from the Earth’s atmosphere, is even harder,” study co-author Robin Ciardullo, a professor of astronomy and astrophysics at Penn State and the observing manager of HETDEX, told Live Science via email.
The next step is to improve noise-reduction techniques and separate the desired signals from the numerous astronomical and Earthly contaminants. The researchers can then use fainter sources and lower-mass objects as tracers of cosmic evolution to more robustly constrain gravity models.
“The Hobby-Eberly is a pioneering telescope,” Muñoz said. “And with new, complementary instruments coming online, we’re entering a golden age for mapping the cosmos.”
