![]() ![]() The shadow is created because the colder water in the galactic cloud absorbs the warmer microwave radiation on its way to Earth. 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. When astronomers observe the rainbow-like spectrum of a star, these absorption lines actually appear like darker, line-shaped shadows. This creates absorption lines in the spectrum – certain wavelengths at which the starlight near the surface is absorbed by the higher and cooler layers. Above the hotter, deeper layers of gas, there are usually cooler ones through which the light has to rush. When observing this cloud, an effect that researchers know from the sun and stars occurred. In this galaxy, there was a vast cloud of cold water vapour. The light we receive today from HFLS3 set out when the universe was just 880 million years old. Behind it is a “starburst galaxy” – a young Milky Way system experiencing a phase of violent star births. This facility of the Institut de Radioastronomie Millimétrique (IRAM) consists of twelve 15-metre antennas aimed at the object HFLS3. This is where the recent observation with NOEMA comes into play. This could provide information about the dark energy that is driving the cosmos apart. 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. It can be observed in the microwave range with radio telescopes or satellites. This background radiation emitted 380,000 years later has cooled to 2.728 Kelvin (−270.42☌). The universe has been expanding since the Big Bang. The cosmos became transparent so to speak. And the photons had a free path and spread out into space. It was then possible for stable atoms to be created. After about 380,000 years, this plasma had cooled down to 3000 Kelvin. The density and temperature decreased just as quickly, and the light particles (photons) lost increasingly more energy. At that time, a hot, dense fog of radiation and elementary particles wafted in space, which was rapidly expanding. The universe came into being around 13.8 billion years ago with the Big Bang. © ESA and the Planck Collaboration zoom-in panel: Dominik Riechers/University of Cologne image composition: Martina Markus/University of Cologne ![]()
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