1. 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.

2. When you observe a star that is ten light-years 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 light-years away show how those galaxies looked billions of years ago.

3. 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.

4. The larger the observed shift, the greater is the speed.
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.

5. 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.

6. 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!

7. 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.

8. 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.

9. Hubble’s Law
The space between the galaxies is expanding in all directions. The universe as a whole is becoming larger.

10. 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.

11. 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.

12. Properties of Gemstones
Using Tables Which galaxy is moving away the fastest? Which galaxy is closest to Earth? Answer:

13. Properties of Gemstones
Using Tables Which galaxy is moving away the fastest? Which galaxy is closest to Earth? Answer: Galaxy 6; Galaxy 1

14. Properties of Gemstones
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:

15. Properties of Gemstones
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.

16. Properties of Gemstones
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:

17. Properties of Gemstones
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 light-years)] = (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)

18. Properties of Gemstones
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:

19. Properties of Gemstones
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

20. Properties of Gemstones
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:

21. Properties of Gemstones
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

22. 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.

23. 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.

24. The universe expanded and cooled down after the big bang.
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.

25. The universe began with the big bang 13.7 billion years ago.
The Big Bang Theory
Big bang occurred 13.7 billion years ago.
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.
First stars and galaxies formed 200 million years after big bang.
Solar system formed 4.6 billion years ago.
Earth today

26. Evidence for the Theory
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.

27. 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.

28. 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.

29. 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.

30. 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.

31. 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.

32. 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.

33. 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.

34. According to Hubble’s Law,
Assessment Questions
According to Hubble’s Law,
the apparent wavelength of light is dependent on relative motion compared to the observer.
the amount of matter in a large region of the universe is constant over long periods of time.
the age of a star is proportional to the red shift of its spectrum.
galaxies are moving away from Earth at speeds proportional to their distance from Earth.

35. According to Hubble’s Law,
Assessment Questions
According to Hubble’s Law,
the apparent wavelength of light is dependent on relative motion compared to the observer.
the amount of matter in a large region of the universe is constant over long periods of time.
the age of a star is proportional to the red shift of its spectrum.
galaxies are moving away from Earth at speeds proportional to their distance from Earth. ANS: D

36. How is dark matter in the universe detected?
Assessment Questions
How is dark matter in the universe detected?
direct observation
its effect on electromagnetic interactions
its gravitational effects on matter
very faint background radiation in the universe

37. How is dark matter in the universe detected?
Assessment Questions
How is dark matter in the universe detected?
direct observation
its effect on electromagnetic interactions
its gravitational effects on matter
very faint background radiation in the universe
ANS: C

38. Assessment Questions
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

39. Assessment Questions
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