Astronomers are capturing radio signals from the “dark ages” of space

An international team of astronomers from Montreal and India has captured a radio signal that was emitted an unimaginably long time ago, 8.8 billion years ago. They were able to pick up the extremely faint radiation using the Giant Metrewave Radio Telescope (GMRT) in Pune, India, the world’s largest radio telescope for wavelengths in the meter range. It assumes neutral hydrogen; each hydrogen atom emits only a small amount of radiation, but the huge clouds of hydrogen in space glow as a result. Neutral hydrogen does not glow in visible light, but its radiation can be picked up with radio telescopes.

Hydrogen is by far the most abundant element in space. On the one hand, it is the main component of the stars; on the other hand, however, large amounts of hydrogen float through the cosmos in huge clouds. The element exists mainly in the form of individual atoms – the so-called atomic hydrogen H – or in the form of molecules – molecular hydrogen H₂ – Before.

Atomic hydrogen has only existed since space was 380,000 years old. It used to be too hot in the cosmos, which is why hydrogen only existed in ionized form. Then, through the expansion-related cooling of space, protons and electrons united, first to atomic hydrogen and then—after further cooling—to hydrogen molecules, each consisting of two atoms. However, stars were formed from molecular hydrogen, which in turn broke down molecular hydrogen into atomic hydrogen and these individual atoms into ionized hydrogen with their ultraviolet radiation.

Hydrogen in atomic form is mainly found in the dense regions of galaxies. The radiation mentioned at the beginning is released when this neutral hydrogen changes its quantum state; the spin of the electron changes from parallel to antiparallel. This so-called spin-flip transition releases an amount of energy that corresponds to a frequency of 1420 megahertz and has a wavelength of 21 centimeters. The radiation is therefore also called the 21 cm line or HI line.

As early as 1944, the Dutch astrophysicist Hendrik Christoffel van de Hulst calculated that the electron of a hydrogen atom should emit a characteristic radiation when the direction of rotation is reversed. Colin Stanley Gum, Frank John Kerr and Gart then recognized the importance of the 21 cm line for astronomy Westerhout 1951. Only by observing the 21 cm signal is it possible to map the clouds in the Milky Way and the gas in other galaxies. This is also relevant for estimating the mass of galaxies and determining the motion of numerous objects.

The astronomers also hope to use the 21-cm signal to gain new insights into space’s distant past, the so-called Dark Ages, and subsequent eras – such as the Reionization Age. The Dark Ages began about 400,000 years after the Big Bang—when the Universe had cooled enough to form stable, neutral hydrogen—and lasted until the first stars lit up and reionization started.

Arnab Chakraborty of McGill University in Montreal and Nirupam Roy of the Indian Institute of Science (IISc) in Bangalore have now succeeded in looking so deeply into the cosmic past. The 21 cm signal they picked up came from the star-forming galaxy SDSSJ0826+5630 and was emitted when the Universe, now 13.7 billion years old, was only 4.9 billion years old. “That’s the equivalent of looking back in time 8.8 billion years,” Chakraborty said in a statement from McGill University.

The astronomers were also able to measure the gas composition in the galaxy. They found that the atomic mass of the gas content in this particular galaxy is almost twice the total mass of the stars visible to us. They published their results in the Monthly Notices of the Royal Astronomical Society.

The deep view into space was made possible by an effect known as “gravity lensing”. A huge mass located between the observer and the observed object bends spacetime so that the light coming from the object behind is deflected towards the mass. As Nirupam Roy puts it:

According to the study’s authors, this demonstrates the possibility of observing distant galaxies in similar situations with gravitational lenses. It also opens up exciting new possibilities for studying the cosmic evolution of stars and galaxies with today’s low-frequency radio telescopes.