Transcript 26.5 The Expanding Universe

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
When you observe a star that is ten lightyears away, you are seeing the star as it was
ten years ago, because light took ten years to
travel from the star to Earth.
Images of galaxies that are billions of lightyears away show how those galaxies looked
billions of years ago.
26.5 The Expanding Universe
Hubble’s Law
How do astronomers know that the universe
is expanding?
The observed red shift in the spectra of
galaxies shows that the universe is
expanding.
26.5 The Expanding Universe
Hubble’s Law
The Doppler effect can be used to determine
how fast stars or galaxies are approaching or
moving away from Earth.
• When a star or galaxy is approaching Earth, the
lines in its spectrum are shifted toward the
shorter (bluer) wavelengths.
• When the star or galaxy is moving away, the
lines in its spectrum shift toward the longer
(redder) wavelengths.
The larger the observed shift, the greater is
the speed.
26.5 The Expanding Universe
Hubble’s Law
In the mid-1920s, Edwin Hubble discovered
that the light from most galaxies undergoes a
red shift—that is, their light is shifted toward
the red wavelengths.
This red shift showed that nearly all galaxies
are getting farther away from Earth.
26.5 The Expanding Universe
Hubble’s Law
Hubble also found that more-distant galaxies
have greater red shifts.
This relationship, called Hubble’s Law, says
that the speed at which a galaxy is moving
away is proportional to its distance from us.
The most distant observed galaxies are
moving away at more than 90 percent of the
speed of light!
26.5 The Expanding Universe
Hubble’s Law
To visualize the movement of galaxies,
imagine the dough for a loaf of raisin bread as
it rises.
• The raisins represent individual galaxies or
groups of galaxies, and the dough represents
the universe.
• When the dough rises, the distance between
raisins increases, but the raisins stay the same
size.
• In the same way, the most distant galaxies are
moving away from us more quickly.
26.5 The Expanding Universe
Hubble’s Law
The raisins in this
rising bread dough
are all moving away
from one another as
the dough expands. In
a similar way, galaxies
move away from one
another as the
universe expands.
26.5 The Expanding Universe
Hubble’s Law
The space between the galaxies is expanding
in all directions. The universe as a whole is
becoming larger.
26.5 The Expanding Universe
Hubble’s Constant
Hubble’s law expresses the relationship
between the velocity that a galaxy is moving
away from Earth and its distance from us.
The ratio of these variables is a constant
called Hubble’s constant. Hubble’s constant
can be estimated by finding the slope of a
graph of velocity versus distance for a set of
galaxies.
26.5 The Expanding Universe
Hubble’s Constant
Hubble’s constant is one of the most
important and debated numbers in
astronomy. It expresses how fast the
universe is expanding, and can be used to
estimate the age of the universe.
26.5 The Expanding Universe
Properties of Gemstones
1. Using Tables Which galaxy is moving away
the fastest? Which galaxy is closest to Earth?
Answer:
26.5 The Expanding Universe
Properties of Gemstones
1. Using Tables Which galaxy is moving away
the fastest? Which galaxy is closest to Earth?
Answer: Galaxy 6; Galaxy 1
26.5 The Expanding Universe
Properties of Gemstones
2. Graphing Graph the data shown in the table. Place
velocity on the vertical axis and distance on the horizontal
axis. What is the general shape of the graph?
Answer:
26.5 The Expanding Universe
Properties of Gemstones
2. Graphing Graph the data shown in the table. Place
velocity on the vertical axis and distance on the horizontal
axis. What is the general shape of the graph?
Answer: The points on the graph fall close to a straight line.
26.5 The Expanding Universe
Properties of Gemstones
3. Calculating Estimate Hubble’s constant by measuring the
slope of your graph. (Hint: Draw a line through the data
points. Recall that Slope = Rise ÷ Run.)
Answer:
26.5 The Expanding Universe
Properties of Gemstones
3. Calculating Estimate Hubble’s constant by measuring the
slope of your graph. (Hint: Draw a line through the data
points. Recall that Slope = Rise ÷ Run.)
Answer: slope = (rise/run) =
[24,000 km/s – 4200 km/s]/[(1170 × 106 light-years) – (200 × 106 lightyears)]
= (19,800 km/s)/(970 × 106 light-years)
= (20.4 km/s)/(106 light-years)
Although estimates vary somewhat, astronomers estimate that the actual
value for Hubble’s constant is about (20.4 km/s)/(106 light-years)
26.5 The Expanding Universe
Properties of Gemstones
4. Inferring About how fast is a galaxy receding if its
distance is 2000 × 106 light-years? (Hint: Use your estimate
of Hubble’s constant from Question 3.)
Answer:
26.5 The Expanding Universe
Properties of Gemstones
4. Inferring About how fast is a galaxy receding if its
distance is 2000 × 106 light-years? (Hint: Use your estimate
of Hubble’s constant from Question 3.)
Answer: 41,000 km/s
26.5 The Expanding Universe
Properties of Gemstones
5. Predicting Use your value for Hubble’s constant to
estimate the distance of a galaxy that has a velocity of
30,000 km/s.
Answer:
26.5 The Expanding Universe
Properties of Gemstones
5. Predicting Use your value for Hubble’s constant to
estimate the distance of a galaxy that has a velocity of
30,000 km/s.
Answer: 1470 × 106 light-years
26.5 The Expanding Universe
The Big Bang Theory
What is the big bang theory, and what
evidence supports it?
Astronomers theorize that the universe
came into being at a single moment, in an
event called the big bang.
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
The 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.
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 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.
• Today this glow is called the cosmic microwave
background radiation.
• This 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 the universe is
expanding now, they can infer how long it has
been expanding.
If you traveled backward in time, all of the matter
in the universe would be at its starting point 13 to
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
How can dark matter be detected?
Dark matter cannot be seen directly, but its
presence can be detected by observing its
gravitational effects on visible matter.
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 of mass, the
universe will continue to expand.
26.5 The Expanding Universe
Continued Expansion
Much of the matter in the universe can’t be
seen by astronomers.
Dark matter is matter that does not give off
radiation.
Galaxies like ours may contain as much as
ten times more dark matter than visible
matter.
26.5 The Expanding Universe
Continued Expansion
There are many unanswered questions about
dark matter. Astronomers don’t know what it is
made of or how it is distributed through the
universe.
Much of the mass of the universe may be
composed of dark matter.
26.5 The Expanding Universe
Continued Expansion
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.
A mysterious force called dark energy is
theorized to be causing the rate of expansion
to increase.
If the 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.
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
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
1. 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
26.5 The Expanding Universe
Assessment Questions
1. 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