Introduction ! Universe
—
defined by astronomers as: " All of space " All of the matter and
energy within it ! Cosmology: the scientific study of the Universe. " Structure " History
What we thought about the Universe ! Three thousand years ago, people looked up and
knew: " The stars are fixed relative to each other. " All the stars rotate about a fixed point. "
The planets move against the background of stars. ! They did not know that Earth is a planet.
! They did not know how
͞
heavenly bodies
͟
move.
What Is the Structure of the Universe? ! Up until the end of the Middle Ages (ca. 476-1400
C.E.) it was thought that the Universe was geocentric. " Earth sat at the center of the
universe. " The moon, planets, and stars circled a motionless Earth. " Ptolemy (100
–
170 C.E.)
proposed equations to predict the movements of planets.
What Is the Structure of the Universe? The heliocentric model gained acceptance during the
Renaissance (15th century). " The sun lay at center of the universe. " Earth and planets
orbited the sun. ! Renaissance: a new age of discovery in 1400s Europe " Copernicus &
Galileo
—
studies led to acceptance that Earth and planets orbit the sun " Newton
—
planet
motion was explained by Theory of Gravity ! Modern Era: Neither the Earth nor the Sun are
at the center of the Universe, and everything is in motion
Stars and Galaxies ! Stars are immense balls of incandescent gas. " Gravity binds stars
together into vast galaxies. " Over 100 billion galaxies exist in the visible universe. ! Our Solar
System is on an arm of the Milky Way galaxy. " Our sun is just one of 300 billion stars in the
Milky Way.
Distance to Celestial Objects ! The dimensions of the Universe are staggering! ! We must
consider huge expanses of space and time. " The sun is
͞
only
͟
150 million km away. " A light-
year is the distance light travels in one year. #equal to about 10 trillion km. " The closest star
beyond our sun is over 4 light-years away. " Our Milky Way galaxy is 120,000 light-years
across. " Edge of the visible universe: >13 billion light-years away!
The vastness of the Universe is staggering! " Earth is a planet orbiting a star on the arm of a
galaxy. " The sun and over 300 billion stars form the Milky Way. " Over 100 billion galaxies
exist in the visible universe. ! Where did all this
͞
stuff
͟
come from? " We don
’
t know! But we
have scientific evidence for what happened the instant after it appeared.
Big Bang Theory 13.8 x 109 years ago, all mass and energy was contained in an infinitesimally
small point. It was unimaginably hot, and expanding unimaginable rapidly (like an explosion),
and cooling. The modern Universe is still expanding.
The Doppler Effect
Light from Stars Hydrogen atoms in a star absorb certain specific wavelengths of light. We
see these wavelengths as dark lines on a light spectrum.
Expanding Universe Hydrogen light observed from distant galaxies, shifted to longer
wavelengths or towards the red of the light spectrum
–
͞
Red Shift
͟
. Hence, the hydrogen light
spectrum from distant galaxies is recorded on Earth at different wavelengths which are
characteristic of the retreat velocity of the galaxy.
