Beyond the glare of the Milky Way, you can see the faint glow of distant galaxies through a powerful telescope. The dense clusters of galaxies are separated by huge voids hundreds of millions of light-years across. In their quest to understand our universe's history, scientists have observed millions of galaxies since the 1980s.
There is more to this structure than meets the eye. Hydrogen atoms emit radio waves with a 21-centimeter wavelength, and because hydrogen gas clouds tend to cluster around galaxies, patterns in this radio emission reflect matter. The first detection of these telltale patterns was reported in a recent preprint paper.
The result is an important first step towards a full map of the cosmic web using hydrogen radio emissions.
After the big bang, no stars existed to illuminate all the hydrogen gas. The densest parts of the country gave birth to stars, while the less dense parts gave birth to clusters.
The broad strokes of this story were understood by cosmologists by the 1990s. In 1998 they were shocked to discover that the expansion of the universe began after eight billion years of coasting. One important open question is whether the dark energy is a constant or whether it is a dynamic field with a strength that changes.
Maps of the web may point to an answer. The expansion of the universe stretches the wavelength of the ancient light towards the red end of the visible spectrum, and it takes longer to reach us. Astronomers can make three-dimensional maps of the cosmic web using precise redshift measurements based on the unique fingerprints of atoms in the universe. The maps contain a lot of information about the evolution of large-scale structure.
The most recent survey, called the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), cataloged the positions and redshifts of half a million galaxies and as many quasars. The eBOSS team used the catalog to create a map that spans more than 11 billion years and covers about 15 percent of the sky. Follow-up surveys are underway.
Despite their successes, galaxy surveys have limitations. Telescopes have to first look at the sky to pick out the best ones to include in the survey. State-of-the-art surveys demand expensive spectrometers with thousands of moving parts.
Hydrogen intensity mapping could prove to be a cheaper and faster way to map the universe. Radio waves from distant gas clouds are redshifted just like visible light. Astronomers can use radio telescopes to measure the intensity of radio emission across the sky at many different wavelengths at the same time. A professor of astronomy at the University of KwaZulu-Natal in South Africa, who is not affiliated, says that intensity mapping telescopes are an order of magnitude cheaper than comparable spectroscopic instruments.
Intensity mapping has its own challenges. The main challenge is that the signal is small, and the Milky Way is a strong radio source. Careful analysis and telescope modeling are required to remove the imprint of the web.
There are four radio telescopes with no moving parts, each resembling a snowboarding half-pipe made of chicken wire. The telescopes sweep out a low-resolution map of the northern hemisphere when Earth rotates. The resulting 3-D map is composed of voxels with 30 million light-years on a side and 10 million light-years deep. Adding up the radio emission from all the hydrogen in each voxel allows the astronomer to pick up faint signals they would not otherwise see. The structure of individual voxels is irrelevant for these studies because of the effects of dark energy.
Scientific American newsletters are free to sign up for.
The first traces of the cosmic web in hydrogen were found in Australia and West Virginia by Chang and others. The large areas needed for a more complete view of the sky can't be mapped with these telescopes because they collect light from a small region. The facilities are in high demand and only a small portion of their observations can be devoted to 21- cm observations. The first results from a radio telescope that was specifically designed to map the cosmic web were derived from data collected in 2019. This allowed the researchers to better control systematic errors, and they didn't have to compete with other astronomer for telescope time. The project's data goes back as far as nine billion years, a billion years deeper into the past than previous radio measurements.
The researchers used a technique called stacking to study the correlations between the data and the maps from the eBOSS survey. The regions of more intense radio emission overlap with the positions of known galaxies and quasars. He says that the result is an important milestone because it gives the researchers a baseline from which to improve.
The team is working on a stand-alone map without the help of the eBOSS catalog. It plans to look for correlations in the distribution of hydrogen gas on longer distance scales, for which teasing apart the signal from foreground emission becomes especially challenging. Such correlations are the remnants of sound waves that traveled through the primordial universe. The characteristic scale of these oscillations is roughly 500 million light-years in the present-day universe. The team can use baryon acoustic oscillations to measure other distances in its maps in search of deviations from standard cosmology, such as changes in the strength of dark energy.
Richard Shaw, a research scientist at the University of British Columbia, who co-led the analysis with Siegel, emphasizes that this is just the beginning.
All Rights Reserved © 2024
