Content Benchmark E.12.B.5
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Students know scientific evidence suggest that the universe is expanding. I/S

In the early 1920s, an astronomer at Mount Wilson Observatory named Edwin Hubble made a startling discovery. Hubble observed that the spectral signatures of almost all galaxies were redshifted, indicating that they were moving away from Earth.

Furthermore, the farther away the galaxy is, the greater its redshift. In other words, galaxies were moving away from Earth at a rate proportional to their distance from us. This relationship is now called Hubble’s Law and is an indication that the universe is expanding.

Figure 1: The farther the galaxies are from Earth (top closest, bottom farthest),
the more the absorption (black) lines are shift to the red
(from http://ganymede.nmsu.edu/astro/a110labs/labmanual/img116.png)

To learn more about redshift and Hubble’s Law, go to http://starchild.gsfc.nasa.gov/docs/StarChild/questions/redshift.html

Hubble’s Law is assumed to apply to all galaxies. In other words, all galaxies are moving away from every other galaxy and the farther galaxies are separated, the faster they are moving apart. With every galaxy moving away from every other, we can only conclude that the space comprising the universe must be expanding.

The expansion of the universe is one of three critical observations that support the model of our universe’s origin, called the Big Bang. According to this model, the universe began in a very small, hot, and dense state. At some point, this universe began to expand and rapidly cool. The expansion we observe today is a remnant of that original expansion at the Big Bang.

The other two observations are the presence of cosmic microwave background radiation, and the abundance of hydrogen, helium, and lithium in the universe. These three measurable signatures strongly support the notion that our universe evolved from a dense, nearly featureless hot gas, just as the Big Bang model predicts.

To learn more about the Big Bang, go to http://map.gsfc.nasa.gov/m_uni/uni_101bbtest.html

Interestingly, Hubble’s Law and expansion have long been used to estimate the age of the universe. Because of uncertainties in measuring the distance to galaxies, the age of the universe using Hubble’s Law ranged from 7 to 20 billion years. Recently, more precise measurements made using the cosmic microwave background radiation place the age of the universe at 13.7 billion years.

To learn more about measurements of the universe’s age, go to
http://imagine.gsfc.nasa.gov/docs/features/exhibit/tenyear/age.html

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Content Benchmark E.12.B.5

Students know scientific evidence suggest that the universe is expanding. I/S

Common misconceptions associated with this benchmark:

1. Students incorrectly think that expansion involves matter spreading out into empty, infinite space.

Common notions of our expanding universe suggest that it is expanding into something. But, that is not true. In fact, space itself is expanding, not the matter within space. The matter in the universe is gravitationally attracted and is bound together in galactic clusters. As space expands, the size of these galactic clusters does not correspondingly expand. Rather, the distance between these clusters increases.

For more details about the properties of the expanding universe, visit http://skyserver.sdss.org/dr1/en/astro/universe/universe.asp

2. Students are confused about the model of the universe’s origin, called the Big Bang.

While most students are aware of the term Big Bang and that it is model explaining origins, they have incorrect beliefs about how the Big Bang model works. Many students incorrectly believe that matter existed prior to the Big Bang. In other words, the Big Bang is incorrectly thought of as an explosion of matter into empty space.

The standard Big Bang model states that at some time in the distant past there was nothing. A process known as vacuum fluctuation created what astrophysicists call a singularity. From that singularity, which was smaller than the size of a pea, our universe was born. Prior to and at the exact time of the Big Bang, the Laws of Physics, as we know them, did not exist. Neither did matter. But, after the Big Bang, matter formed, and eventually, stars, galaxies, and stellar systems (with planets) were created, thanks to gravitational attraction and other processes. Students also confuse the universe with our Solar System and incorrectly think that the Big Bang instantly formed the Sun, Earth, and/or Solar System in an explosion.

To understand more about student misconceptions about the Big Bang, go to
http://aer.noao.edu/cgi-bin/article.pl?id=26

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Content Benchmark E.12.B.5

Students know scientific evidence suggest that the universe is expanding.  I/S

Sample Test Questions

1st Item Specification: Recognize the red shift effect and know that the most distant objects have the greatest degree of red shift.

Depth of Knowledge Level 1

1. The expansion of the universe was first deduced from
1. Edwin Hubble showing that more distant galaxies are moving away more rapidly.
2. Edwin Hubble showing that more distant galaxies are moving away more slowly.
3. Albert Einstein showing that space and matter expand at a decreasing rate.
4. Albert Einstein showing that space and matter expand at an increasing rate.

2. Hubble’s law states that the faster
1. a spiral galaxy’s rotational velocity, the less luminous it is.
2. a spiral galaxy’s rotational velocity, the nearer it is.
3. the recessional velocity of a galaxy, the farther away it is.
4. the recessional velocity of a galaxy, the more luminous it is.

3. What can we conclude from the observation that nearly all galaxies are moving away from us?
1. The once-smaller universe is expanding in all directions.
2. The Sun and Earth are located at the center of the universe.
3. Everything in the universe is moving the same direction.
4. Massive black holes are drawing galaxies away from Earth.

4. We know that nearly all galaxies are moving away from us because we
1. watch them move across the sky, all in the same direction.
2. see that their spectra are red-shifted relative to a rest position.
3. observe a condensation trail behind them as they move.
4. detect their sonic booms through radio telescope observations.

5. An important line in the absorption spectrum of stars occurs at a wavelength of 656 nm for stars at rest. Imagine that you observe four stars (1-4) from Earth and discover that this absorption line is at the wavelength shown in the table below for each of the four stars.

Which of the stars is moving with the greatest velocity away from Earth?
1. Star 1
2. Star 2
3. Star 3
4. Star 4

Depth of Knowledge Level 2

6. Use the four spectra shown below for objects 1-4 to answer the next question. Note that one of the spectra is from an object at rest (not moving) and the remaining spectra come from objects that are all moving away from the observer. The left end of each spectrum is blue, and the right end of each spectrum is red.

 Which of the four objects, 1-4, is moving with the fastest speed?

1. Object 1
2. Object 2
3. Object 3
4. Object 4

7. Use the four spectra shown below for objects 1-4 to answer the next question. Note that one of the spectra is from an object at rest (not moving) and the remaining spectra come from objects that are all moving away from an Earth-based observer. The left end of each spectrum is blue, and the right end of each spectrum is red.

If these objects are all galaxies, which one is farthest from Earth?

1. Object 1
2. Object 2
3. Object 3
4. Object 4

8. An important line in the absorption spectrum of galaxies at rest occurs at a wavelength of 21.0 cm. Imagine that you observe four galaxies (1-4) from Earth and discover that this absorption line is at the wavelength shown in the table below for each of the four galaxies.

Which of the galaxies, 1-4, is farthest away from Earth?

1. Galaxy 1
2. Galaxy 2
3. Galaxy 3
4. Galaxy 4

9. The graph below shows the relationship between the distance and velocity of several galaxies.

If you observed a new galaxy moving with a velocity of 38,000 km/s, at what distance should it be located?

1. 60 Mpc
2. 500 Mpc
3. 600 Mpc
4. 28,000 Mpc

10. The graph below shows the relationship between the distance and velocity of several galaxies.

If you observed a new galaxy at a distance of 700 Mpc, how fast would you expect it to be moving?

1. 20 km/s
2. 1,000 km/s
3. 45,000 km/s
4. 57,000 km/s

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Content Benchmark E.12.B.5

Students know scientific evidence suggest that the universe is expanding.  I/S

Answers to Sample Test Questions

1. A, DOK level 1
2. C, DOK level 1
3. A, DOK level 1
4. B, DOK level 1
5. B, DOK level 1
6. C, DOK level 2
7. C, DOK level 2
8. D, DOK level 2
9. B, DOK level 2
10. D, DOK level 2

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Content Benchmark E.12.B.5

Students know scientific evidence suggest that the universe is expanding. I/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. The Sloan Digital Sky Survey Observatory
The Sloan Digital Sky Survey Observatory has created several activities dealing with astronomy. Two of these directly relate to the expanding universe. These activities require students to use computers, but may be adapted to “pen and paper.”

The first is the simpler of the two and is called The Universe: Your Cosmic Address. This activity relates the size and scale of the universe with evidence that it is expanding and is found at http://cas.sdss.org/dr5/en/proj/basic/universe.

The other activity is The Hubble Diagram, where students retrace the steps followed by Edwin Hubble to discover that the universe is expanding by using data to construct a graph showing Hubble’s Law. This activity is found at http://cas.sdss.org/dr5/en/proj/advanced/hubble.

2. Redshifted Spectra and Hubble’s Law
NASA’s Imagine the Universe site has a lesson that focuses on gamma ray bursts, but also explores redshift and Hubble’s Law. The site also has good background information for students on Hubble’s Law and is found at http://imagine.gsfc.nasa.gov/docs/teachers/lessons/swift_grb/how_far_cover.html

3. Expanding Universe Animation
NASA’s Chandra X-ray Observatory has developed an excellent visualization that can be used as a demonstration in the classroom. This animation shows the expansion history of the universe by modeling it as a two-dimensional grid of galaxies. This animation is available in several different formats and resolutions. To access these, scroll to the bottom of the following Web site:
http://chandra.harvard.edu/resources/animations/galaxy_clusters.html.

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