Nancy Huang

Apr 17th 2023

Tuning in to a Radio Signal From Space That’s 9 Billion Years Old


Whether it’s photos from a bygone era or descriptions of a faraway land, people are fascinated by distant times and places. Astronomers are no different, always trying to peer into the far reaches of the distant past.

This is exactly what researchers from the Indian Institute of Science and McGill University have done — they recently used a radio signal from space to gain new knowledge about a star-forming galaxy that’s nearly 9 billion light years away, as reported in the Monthly Notices of the Royal Astronomical Society. To put that in perspective, the Big Bang occurred about 13.7 billion years ago, and our solar system formed a mere 4.5 billion years ago.

The previous record for this type of radio signal from space was “just” 5 billion light years away, as described in The Astrophysical Journal Letters, and was achieved by using a state-of-the-art telescope for a long period of time. In contrast, the method used for the new record is compatible with a modest amount of time on less sophisticated telescopes, which makes additional discoveries more likely. The secret is one weird trick that Einstein described back in 1916 involving radio signals emitted by hydrogen and the use of a nearby galaxy to magnify the signal.

Hydrogen and the 21-Centimeter Line

About 400,000 years after the Big Bang, protons and electrons bonded to neutrons to form neutral hydrogen, which would eventually give rise to the first stars and galaxies. The energy from ultraviolet light emitted by newly formed stars can strip the electrons from neutral hydrogen, creating ionized hydrogen. When ionized hydrogen returns to its neutral state, it emits a radio wave with a characteristic wavelength of 21 centimeters, according to Big Think. Astronomers call this the “21-centimeter line,” or the “hydrogen line,” and use it to learn about early stars and the evolution of galaxies over cosmic time.

Unfortunately, radio waves have long wavelengths and low intensity, so the 21-centimeter line is often drowned out before it can travel through the vast distances of space. Thus, most of the hydrogen radio signals that have been successfully detected come from nearby — like from within the Milky Way Galaxy, which is 100,000 light years across, and the neighboring Andromeda galaxy, which is about 2.5 million years away.

Einstein’s Weird Trick

To amplify these radio waves, the record-breaking research made use of Albert Einstein’s theory of general relativity. Einstein postulated that the curving of spacetime causes light to bend as it passes by objects of enormous mass, such as a black hole or a galaxy. The greater the mass, the more extreme the curvature.

When light traveling through the universe encounters a massive object, each ray takes a different path and bends around it. This can make a single object appear at multiple points in the sky — which is evidence that this “gravitational lensing” has occurred — and can magnify the signal so it can be more easily detected by telescopes on Earth. Radio waves, such as the 21-centimeter line, are a form of light and can therefore be magnified by gravitational lensing.

The aforementioned researchers started with nearby galaxies that could serve as a strong lens for satellites located on Earth. Then, they looked for signals that were being magnified. The best result came from a gigantic galaxy that was relatively close to Earth at 1.7 billion light years away. It was magnifying the signal from a galaxy behind it (named SDSSJ0826+5630) by a factor of 30.

Determining Distance Traveled

When a wave of light travels across the ever-expanding universe, its wavelength becomes longer and its frequency is reduced. This is called “redshift.” The higher the redshift, the farther a wave has been traveling.

In the study, the nearby galaxy that provided gravitational lensing had a redshift of 0.13. The previous record holder that was 4.1 billion light years away had a redshift of 0.376. In the most recent finding, the 21-centimeter line had a redshift of 1.3, which corresponds to a travel time of nearly 9 billion years. The wavelength of the 21-centimeter line had more than doubled to 48 centimeters.

The magnified 21-centimeter line allowed astronomers to calculate how mass was distributed in this distant galaxy from nearly 9 billion years ago. Compared to the stars visible from Earth, this galaxy had a much larger percentage of mass floating around compared to mass in the stars. The significance of this finding is not immediately clear, but the clever technique of gravitational lensing will allow astronomers to gather more information from the early days of the universe. This will help us understand the evolution of stars and galaxies and how the early universe led to the cosmos that currently surround us.

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