74 lines
3.6 KiB
Plaintext
74 lines
3.6 KiB
Plaintext
.NH
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a cosmic mystery story: beginnings
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.PP
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.ft CW
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Contrary to the book, I'll describe things chronogically in the summary.
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.ft
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1784: first observation of Cepheid variable star, which are stars whose brightness varies over some regular period.
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1908-1912: Henrietta Swan Leavitt discovers a relation between Cepheid variable stars' brightness and period of their variation.
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And this leads to knowing the distance between these stars: we now can make wild approximations on astonomic distances between us and these stars.
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.ft CW
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.ps 8
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.vs 9p
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If one could determine the distance to a single Cepheid of a known period, then measuring the brightness of other Cepheids of the same period would allow one to determine the distance to these other stars.
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.vs
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.ps
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.ft
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Determining the distance between us and stars always has been a challenge in astronomy.
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The observed brightness of stars goes down inversely with the square of the distance to the star.
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.FOOTNOTE1
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The light spreads out uniformly over a sphere whose area increases as the square of the distance.
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Thus since the light is spread out over a bigger sphere, the intensity of the light observed at any point decreases inversely with the area of the sphere.
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.FOOTNOTE2
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1916, general theory of relativity: a decade-long struggle to create a new theory of gravity by Albert Einstein.
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This work is also about space and time, and explains not only how objects move in the universe, but also how the universe itself might evolve.
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1923-1924, with the period-luminosity relation and the measurement of Cepheid variable stars, Hubble determines that the distance with some Cepheids was too great to be inside our Milky Way.
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The universe contains
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.I "at least"
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another galaxy.
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He identifies a first galaxy (NGC 6822) in 1925, then the Triangulum galaxy (M33) in 1926, and Andromeda (M31) in 1929.
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1925: Hubble publishes his study on spiral
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.I nebulae ,
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where he identified Cepheid variable stars in them (including the
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.I nebulae
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we currently know as Andromeda).
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1925: Mount Wilson 100-inch Hooker telescope.
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A. Einstein publishes his work on the
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.I "general theory of relativity"
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in 1916 but this doesn't match with observation and what we
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.I thought
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the universe was at the time.
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The scientific community still imagined the universe to be static, eternal and composed of a single galaxy (our Milky Way) surrounded by a vast, dark, infinite empty space.
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This idea was consistent with the observations.
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On the contrary, the theories of Newton and Einstein were both inconsistent with the observations since gravitation was thought to be an attractive force: objects should then always collapse into each other.
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.EQ
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delim $$
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.EN
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However, his theory is able to predict the orbit of Mercury slightly better than before with Newton's theory of gravity.
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The new theory fixes a small difference between observation and theoretical results.
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.FOOTNOTE1
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The planet doesn't come back to its initial position after an ellipse around the sun.
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There is a slight precession of the perihelion of Mercury: 43 arc seconds (only $1 over 100$ of a degree) per century.
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.FOOTNOTE2
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.EQ
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delim off
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.EN
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The first person to suggest the universe was expanding was Georges Lemaître in 1927, while solving the Einstein's equations for general relativity (which leads to this conclusion).
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Then in 1930 he proposed that the universe began in a very small point called
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.I "Primeval Atom" .
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This wasn't accepted by the scientific community right away: actual observations were provided by Edwin Hubble.
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Current state of knowledge: expansion of the universe started 13.72 billion years ago.
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Our galaxy is one of the about 400 billion other galaxies in the observable universe.
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