Performance Benchmark E.12.B.5
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://www.usask.ca/geology/classes/geol109and122/
109_122_A/Age_Universe.htm)
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
Performance 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
Performance Benchmark E.12.B.5
Students know scientific evidence suggest that the universe is expanding. I/S
Sample Test Questions
1. Which of the following best describes the expansion of the universe?
a. Expansion involves the spreading of matter into infinite and empty space.
b. As space expands, matter spreads out to fill up the empty space.
c. As space expands, the empty space between matter increases.
d. As space expands, matter continually is created to fill that space.
2. The expansion of the universe was first deduced from
a. Edwin Hubble showing that more distant galaxies are moving away more
rapidly.
b. Edwin Hubble showing decreasing redshift in galactic spectra with
increasing distance from Earth.
c. Albert Einstein introducing a cosmological constant in General Relativity
to overcome gravitational attraction.
d. Albert Einstein showing that space expands at an accelerated rate,
eventually reaching the speed of light.
3. Which of the following is the best description of the Big Bang Theory?
a. The Big Bang Theory is the scientific explanation of how the Sun, Earth,
and the rest of the Solar System were created at the beginning of time.
b. The Big Bang Theory states that pre-existing matter was compacted into a
hot and dense form and suddenly exploded into infinite, empty space.
c. The Big Bang Theory is just one of the ideas about the universe’s origin,
many of which are equally respected by the scientific community.
d. The Big Bang Theory is the prevailing model about the universe’s origin
and is supported by observations of the universe’s expansion.
Performance Benchmark E.12.B.5
Students know scientific evidence suggest that the universe is expanding. I/S
Answers to Sample Test Questions
1. (c)
2. (a)
3. (d)
Performance Benchmark E.12.B.5
Students know scientific evidence suggest that the universe is expanding. I/S
Intervention Strategies and Resources
The following list of intervention strategies and resources 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