Stars, Galaxies, and the Universe
Section 4: The Big Bang Theory
Preview
• Key Ideas
• Hubble’s Observations
• A Theory Emerges
• Big Bang Theory
• Universal Expansion
• A Universe of Surprises
• Maps In Action
Section 4
Stars, Galaxies, and the Universe
Section 4
Hubble’s Observations
• cosmology the study of the origin, properties,
processes, and evolution of the universe
• Cosmologists and astronomers can use the light given
off by an entire galaxy to create the spectrum for that
galaxy.
• Edwin Hubble used galactic spectra to uncover new
information about our universe.
Stars, Galaxies, and the Universe
Section 4
Hubble’s Observations, continued
Measuring Red Shifts
• Hubble found that the spectra of galaxies, except for the
few closest to Earth, were shifted toward the red end of
the spectrum.
• Hubble determined the speed at which the galaxies were
moving away from Earth.
• Hubble found that the most distant galaxies showed the
greatest red shift and thus were moving away from Earth
the fastest.
Stars, Galaxies, and the Universe
Section 4
Hubble’s Observations, continued
The Expanding Universe
• Using Hubble’s observations, astronomers have been
able to determine that the universe is expanding.
• The expanding universe can be thought of as a raisin
cake rising in the oven. If you were able to sit on one
raisin, you would see all the other raisins moving away
from you.
• Similarly, galaxies in the universe are moving farther
away from each other due to the expansion of the
universe.
Stars, Galaxies, and the Universe
Section 4
A Theory Emerges
• Although cosmologists have proposed several different
theories to explain the expansion of the universe, the
current and most widely accepted is the big bang theory.
• big bang theory the theory that all matter and energy in
the universe was compressed into an extremely small
volume that 3 to 15 billion years ago exploded and
began expanding in all directions
• By the mid-20th century, almost all astronomers and
cosmologists accepted the big bang theory.
Stars, Galaxies, and the Universe
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A Theory Emerges, continued
Cosmic Background Radiation
• cosmic background radiation radiation uniformly
detected from every direction in space; considered a
remnant of the big bang
• Astronomers believe that cosmic background radiation
formed shortly after the big bang.
• The background radiation has cooled after the big bang,
and is now about 270 °C below zero.
Stars, Galaxies, and the Universe
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A Theory Emerges, continued
Ripples in Space
• Maps of cosmic background radiation over the whole sky
look very smooth. But on satellite maps that show where
temperatures differ from the average background
temperature, “ripples” become apparent.
• These ripples are irregularities caused by small
fluctuations in the distribution of matter in the early
universe.
• The ripples are thought to indicate the first stages in the
formation of the universe’s first galaxies.
Stars, Galaxies, and the Universe
A Theory Emerges, continued
Timeline of the Big Bang
Section 4
Stars, Galaxies, and the Universe
Big Bang Theory
Click below to watch the Visual Concept.
Section 4
Stars, Galaxies, and the Universe
Universal Expansion
Click below to watch the Visual Concept.
Section 4
Stars, Galaxies, and the Universe
Section 4
A Universe of Surprises
Dark Matter
• Analysis of the ripples in the cosmic background
radiation suggests that the matter that humans, the
planets, the stars and the matter between the stars
makes up only 4% of the universe.
• About 23% of the universe is made up of a type of matter
that does not give off light but that has gravity. This type
of matter is called dark matter.
Stars, Galaxies, and the Universe
Section 4
A Universe of Surprises, continued
Dark Energy
• Most of the universe is made up of an unknown material
called dark energy.
• Scientists think that dark energy acts as a force that
opposes gravity. Many scientists think that some form of
undetectable dark energy is pushing galaxies apart.
• Because of dark energy, the universe is not only
expanding, but the rate of expansion also seems to be
accelerating.