The early cooling of the universe
The shadow of a cosmic water cloud reveals the temperature of the young universe
A telescope in the French Alps has allowed researchers to peer deep into the past of the universe. For the first time, they were able to observe an extremely distant hydrogen cloud that shadows the cosmic background radiation created shortly after the Big Bang. The shadow is created because the colder water absorbs the warmer background radiation on its way to Earth. This provides information about the temperature of the cosmos just 880 million years after the Big Bang. To measure the early history of the universe, an international team used the Northern Extended Millimetre Array (NOEMA), the most powerful radio telescope in the northern hemisphere.
The universe came into being around 13.8 billion years ago with the Big Bang. At that time, a hot, dense fog of radiation and elementary particles wafted in space, which was rapidly expanding. The density and temperature decreased just as quickly, and the light particles (photons) lost increasingly more energy. After about 380,000 years, this plasma had cooled down to 3000 Kelvin. It was then possible for stable atoms to be created. And the photons had a free path and spread out into space. The cosmos became transparent so to speak.
The universe has been expanding since the Big Bang. This background radiation emitted 380,000 years later has cooled to 2.728 Kelvin (−270.42°C). It can be observed in the microwave range with radio telescopes or satellites. But how exactly did this cooling process take place? If we could measure the temperature at different times in cosmic history, we could reconstruct the expansion history of the universe. This could provide information about the dark energy that is driving the cosmos apart.
This is where the recent observation with NOEMA comes into play. This facility of the Institut de Radioastronomie Millimétrique (IRAM) consists of twelve 15-metre antennas aimed at the object HFLS3. Behind it is a “starburst galaxy” – a young Milky Way system experiencing a phase of violent star births. The light we receive today from HFLS3 set out when the universe was just 880 million years old. In this galaxy, there was a vast cloud of cold water vapour.
When observing this cloud, an effect that researchers know from the sun and stars occurred. Above the hotter, deeper layers of gas, there are usually cooler ones through which the light has to rush. This creates absorption lines in the spectrum – certain wavelengths at which the starlight near the surface is absorbed by the higher and cooler layers. When astronomers observe the rainbow-like spectrum of a star, these absorption lines actually appear like darker, line-shaped shadows.
In the case of the starburst galaxy HFLS3, the cosmic background radiation acts like a light source that is located behind the galaxy from the observer’s point of view. The shadow is created because the colder water in the galactic cloud absorbs the warmer microwave radiation on its way to Earth. Because the temperature of the water can be extrapolated from other observable properties of the galaxy, the difference indicates the temperature of the cosmic background radiation at that time. It is about six times higher than in today’s universe.
Based on their observations, the astronomers concluded that the background radiation must have had a temperature of between 16.4 and 30.2 Kelvin at that time. This is consistent with the temperature of 20 Kelvin predicted by current cosmological models for 880 million years after the Big Bang. In view of the direct connection between the cooling of the background radiation and the expansion history of the universe, this is an important indication that these models are consistent.
“The discovery not only provides evidence of cooling but also shows us that the universe had some specific physical properties that no longer exist today”, says Dominik Riechers from the Institute of Astrophysics at the University of Cologne, lead author of the paper published in Nature.
According to Riechers, this provides unique insight into the young universe. “About 1.5 billion years after the Big Bang, the microwave background was too cold to observe this effect”. If a galaxy with otherwise identical properties to HFLS3 existed today, the water shadow would not be observable because the necessary temperature contrast would no longer exist.
“This is an important milestone that not only confirms the expected cooling trend for a much earlier epoch than previously possible but could also have direct implications for the nature of elusive dark energy”, says Axel Weiß from the Max Planck Institute for Radio Astronomy in Bonn, second author of the paper. “We see an expanding universe in which the density of dark energy does not change”.
Dark energy is thought to be one cause of the accelerated expansion of the universe over the past billion years. However, the properties of dark energy remain poorly understood because they cannot be directly observed with the facilities and instruments currently available. However, these properties influence the development of cosmic expansion and thus the cooling rate of the universe from the Big Bang to the present day.
After tracking down a cold water cloud at such a great distance, the team has now set out to find many more in the sky. The aim here is to map the cooling of the cosmic background radiation in the first 1.5 billion years of the history of the universe. “Thanks to the new technique made possible by the NOEMA interferometer, we can now study physical processes in the early universe that have eluded us until now”, says co-author Fabian Walter from the Max Planck Institute for Astronomy in Heidelberg.
NOEMA project scientist Roberto Neri adds: “Our team is pursuing this project by studying the environment of other galaxies”. With the expected improvements in precision from the analyses of larger samples of water clouds, it remains to be seen whether our current, fundamental understanding of dark energy will hold up.