26.5 The Expanding Universe
•Absorption lines of
a galaxy shift
toward the blue
end of the
spectrum when it
moves toward
Earth.
•The lines shift to
the red end of the
spectrum when a
galaxy moves
away from Earth.
26.5 The Expanding Universe
Hubble’s Law
•The Doppler effect can be used to determine how fast
stars or galaxies are approaching/moving away from Earth
• When a star or galaxy is approaching Earth, its spectrum
lines are shifted toward the shorter (bluer) wavelengths.
• When the star or galaxy is moving away, its spectrum
lines shift toward the longer (redder) wavelengths.
–The larger the observed shift, the greater is the speed.
•Edwin Hubble discovered that light from most galaxies
undergoes a red shift
–Showed that nearly all galaxies are getting farther away
from Earth.
26.5 The Expanding Universe
Hubble’s Law
•Hubble’s Law: the speed at which a galaxy is moving
away is proportional to its distance from us
–Space between galaxies is expanding in all directions
–The universe as a whole is becoming larger
•To visualize the movement of galaxies, imagine the
dough for a loaf of raisin bread as it rises.
• Raisins = individual galaxies or groups of galaxies
• Dough = the universe
• Distance between raisins increases, but they stay the same
size
• Most distant galaxies are moving away from us more quickly
26.5 The Expanding Universe
Hubble’s Constant
•Hubble’s constant is one of the most important and
debated numbers in astronomy:
–Ratio of velocity to distance in Hubble’s Law
–Can be estimated by finding slope of a graph of velocity vs
distance for a set of galaxies.
–Expresses how fast the universe is expanding
–Can be used to estimate the age of the universe
26.5 The Expanding Universe
The Big Bang Theory
• Astronomers theorize that the universe came into
being at a single moment, in an event called the big
bang.
–Motion of galaxies indicates that the universe is expanding
uniformly.
–The big bang theory states that the universe began in an
instant, billions of years ago, in an enormous explosion.
–The existence of cosmic microwave background radiation
and the red shift in the spectra of distant galaxies strongly
support the big bang theory.
26.5 The Expanding Universe
The Big Bang Theory
After the Big Bang
The universe expanded and cooled down after the big
bang.
• After a few hundred thousand years of expansion, the
universe was cool enough for atoms to form.
• Gravity pulled atoms together into gas clouds that
eventually evolved into stars in young galaxies.
• The sun and solar system formed about 4.6 billion years
ago, when the universe was about two thirds of its
present size.
26.5 The Expanding Universe
The Big Bang Theory
•The universe began with the
big bang about 13.7 billion
years ago.
•The first stars and galaxies
formed 200 million years later.
•The solar system, and Earth,
formed about 9 billion years
after the big bang.
Big bang
occurred 13.7
billion years ago.
First stars and
galaxies formed
200 million years
after big bang.
Solar system
formed 4.6 billion
years ago.
Earth today
26.5 The Expanding Universe
The Big Bang Theory
Evidence for the Theory
In 1965, Arno Penzias and Robert Wilson, using a
radio telescope, noticed a faint distant glow in every
direction.
• This glow is called the cosmic microwave background
radiation.
• The glow is energy produced during the big bang, still
traveling throughout the universe.
26.5 The Expanding Universe
The Big Bang Theory
•The big bang theory describes how the expansion
and cooling of the universe over time could have led
to the present universe of stars and galaxies.
•It offers the best current scientific explanation of the
expansion of the observable universe.
•Variations of the theory continue to be proposed and
are being tested with new observations.
26.5 The Expanding Universe
The Big Bang Theory
Age of the Universe
•Since astronomers know how fast it is expanding
now, they can infer how long the universe has been
expanding.
•If you traveled backward in time, all of the matter in
the universe would be at its starting point about 13 14 billion years ago.
–Recent measurements of the microwave background
radiation have led to a more precise age.
–Astronomers now estimate that the universe is 13.7 billion
years old.
26.5 The Expanding Universe
Continued Expansion
•To have a gravitational force strong enough to reverse
the expansion, there must be sufficient mass in the
universe.
–If there is less than this amount, the universe will continue
expanding.
–Much matter in the universe can’t be seen by astronomers.
•Dark matter: matter that does not give off radiation
–Cannot be seen directly
–Can be detected by observing gravitational effects on
visible matter
•Galaxies like ours may contain as much as 10 times
more dark matter than visible matter.
26.5 The Expanding Universe
Continued Expansion
•Astronomers don’t know what dark matter is made of
or how it is distributed through the universe.
–Much of the mass of the universe may be dark matter.
•In the past few years, astronomers have discovered
that the rate of expansion of the universe may be
increasing.
–Galaxies appear to be moving apart faster now than
expected. The reason for this is uncertain.
–Dark energy is theorized to be causing the rate of
expansion to increase.
–If expansion is accelerating, it’s likely that the universe will
expand forever.
26.5 The Expanding Universe
Assessment Questions
1. According to Hubble’s Law,
a. the apparent wavelength of light is dependent on
relative motion compared to the observer.
b. the amount of matter in a large region of the universe is
constant over long periods of time.
c. the age of a star is proportional to the red shift of its
spectrum.
d. galaxies are moving away from Earth at speeds
proportional to their distance from Earth.
ANS:
D
26.5 The Expanding Universe
Assessment Questions
2. How is dark matter in the universe detected?
a.
b.
c.
d.
direct observation
its effect on electromagnetic interactions
its gravitational effects on matter
very faint background radiation in the universe
ANS: C
26.5 The Expanding Universe
Assessment Questions
3. Based on the size of the universe and its rate of
expansion, the big bang is calculated to have
occurred between 13 and 14 billion years ago.
True
False
ANS: T