The deeper astronomers look into the night sky, the further back in time they see. The oldest observable light is the cosmic microwave background (CMB)—radiation left over from the Big Bang that was emitted when the Universe was just 380,000 years old. Atomic hydrogen formed at that time, and researchers can follow its activities in the early Universe by looking for signs of the radiation that it emitted or absorbed. Hydrogen does this at a characteristic 21-centimetre wavelength, and that radiation has stretched over time as the Universe has expanded. Evidence of that 21-cm signal charts the evolution of the Universe from the dark ages, before the first stars emerged, through to the galaxy-studded cosmos we see today.
Then read more from this 2019 Nature feature: The quest to unlock the secrets of the baby Universe.
Watch this next: The beginning of the universe, for beginners.
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