393 Slipher and Expanding Universe

In the early 20th century, astronomers marveled at the abundant and beautiful “spiral nebulae” that flecked the night sky. But they had few clues about the nature of these lovely objects. Were they new Solar Systems forming within our own Milky Way? Or were they galaxies in their own right, separated from our own galaxy […]

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447 Evidence for Big Bang

Just a few minutes after the Big Bang, the universe was a hot dense soup of hydrogen nuclei (protons), helium nuclei, electrons, and electromagnetic radiation (light) in the form of photons. But the photons were essentially trapped in the burgeoning universe—they could travel only short distances before being absorbed and then re-emitted by some particle […]

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