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the discovery of galaxies

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before we began studying the universe in a scientific sense, we learned about galaxies which are its constituents

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but first people didn't know what they were

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they were just smudges in the sky, called "nebulae"

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first cataloged by Charles Messier

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before this, philosophers tried to address the question

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sometimes called "island universes", what today we call galaxies

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until 1920s, basic question: are these nebulae galaxies like the milky way or are they just some smudges within our own galaxy

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Shapley-Curtis debate

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inconclusive

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Shapley had the wrong answer, that the smudges were part of the Milky Way

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Curtis was advocating that there were many galaxies like the Milky Way

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articles were not based on solid experimental data

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1923 Hubble

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Mt. Wilson observatory

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found a variable star in Andromeda

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comparing the star to stars in our galaxy, he determined that it is much farther away than anything in our galaxy

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several 100 kiloparsecs away

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1 kiloparsec = 3261 light years

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so all of a sudden the picture changed from the milky way being the entire universe to it being a much, much bigger universe

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if you want to understand the universe at large in physical terms, the only interaction that actually matters is gravity

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because all other forces are short-range, except electromagnetic force, but there charges are mixed so well so that the net electromagnetic field is zero

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not so with gravity

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gravity cannot be cancelled or compensated

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this came in the form of the theory of relativity

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1917: Einstein

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that the universe would collapse on itself because of its gravity seemed like a natural thing

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proposed the cosmological constant which balanced this force of gravity

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turned out to be wrong

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1917: Willem De Sitter

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developed equations for an expanding universe

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the discovery of the expanding universe was probably one of the most important scientific discoveries of all time

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Vesto Melvin Slipher

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Arizona

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measured radio velocities of galaxies, later used by Hubble

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Knut Lundmark, 1924

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Carl Wirtz, 1923

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Edwin Hubble, 1929

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plotted distances to galaxies against velocities obtained by Slipher

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"The Hubble Diagram"

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shows that the first away galaxies were, the faster they were moving

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evidence for expanding universe

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space expands and carries galaxies apart

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Einstein regretted his cosmological constant theory

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failed to predict the expansion of the universe even though this was explicitly contained in his equations and is probably the greatest scientific prediction anyone could make

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modern relativistic cosmological models

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Alexander Friedman

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1922, Soviet Union, relativity based expanding universe model

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George Lemaitre

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1927, Belgium, look back, it all must have started with in a hot, dense state which he called the cosmic egg

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not taken too seriously because it was a bit of an extrapolation

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1930s relativistic cosmology models

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Edward Milne

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the main difference between the Milne model of an expanding universe, and Einstein's model of an expanding universe was that Milne did not assume a priori that the universe has a homogeneous matter distribution

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Arthur Eddington

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proved that theory of general relativity was right

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the Eddington limit, the natural limit to the luminosity of stars, or the radiation generated by accretion onto a compact object, is named after him

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Robertson and Walker

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developed mathematical basis of expanding universe