The Dark Night Sky

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The Dark Night Sky

The Dark Night Sky The Universe : Now, Then, and LaterOctober 30, 2004Plan II Parents Day 1


Introduction What do we know about the universeOn very large scales How do we know?Looking out is looking back• All that we see is glow• All that we see is nowThe mystery of the dark night skyOctober 30, 2004Plan II Parents Day 2


Even if we look at the sky with the HubbleSpace Telescope we will still find patchesof light against darkness.October 30, 2004Plan II Parents Day 4


If the Universe were infinite in age and extent,we would see a star in every direction.The sky would glowwith the light of abillion billion suns.starsOctober 30, 2004Plan II Parents Day 5


Olber’s Paradox In 1826, astronomer Heinrich Wilhelm Olberbecame the first to make the consequences of adark sky popularly understood. In his time it wasnot clear what was happening, so the word“paradox” was applied.Stationary UniverseOur UniverseOctober 30, 2004Plan II Parents Day 6


The Receding Universe In 1917 astronomer VestoSlipherobserved 25 of the nearest and brightestgalaxies, and measured the galaxiesrelative velocities using the Doppler Effect.October 30, 2004Plan II Parents Day 7


The Receding Universe Much to his surprise, Slipher discoveredthat 21 of the galaxies had their lightshifted to the red, implying that they aremoving away from us.October 30, 2004Plan II Parents Day 8

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