Expanding Universe Teacher Guide

Introduction

Vera C. Rubin Observatory will detect millions of previously unknown supernovae and faint galaxies, providing a wealth of data that reveals that the Universe is expanding and that the rate of expansion is changing with time. Students examine the expansion rate of the Universe for different galaxies, and learn that the Universe appears to be expanding uniformly regardless of location. Together, these observations provide evidence to support the Big Bang theory.

Learning Outcomes

• Students can interpret a Hubble Plot to infer the relationship between a galaxy’s recessional velocity and its distance.
• Students can explain how the slope of a Hubble plot reveals the Universe is expanding.
• Students can demonstrate, through the use of a model, that Earth does not hold a unique position at the center of the Universe—rather, that every vantage point in the Universe appears as though it is the center.
• Students can use a Hubble plot to demonstrate that the expansion rate of the Universe has changed at different times in the history of the Universe.
• Students can infer that the expansion rate of the Universe is increasing from a Hubble plot that does not display a constant slope.
• Students can use their data to argue how the idea of expansion provides supporting evidence for the Big Bang theory.

Prerequisite Concepts

• Students should know that certain types of stars can explode, forming a supernova.
• Students should be familiar with the way that certain types of objects (in this case, Type Ia supernovae) can be used as standard candles to measure distances to galaxies.
• Students should know that an object that has redshifted light is moving away from the observer, and that the greater the redshift is for an object‘s light, the faster the object is moving away from the observer.
• Students should know that a distant galaxy looks as it did when both it and the Universe were much younger, because of the time it took light to travel from the galaxy to Earth.
• Students should be familiar with the Big Bang theory and the evidence used to support it.

Where This Fits In Your Teaching

• Hubble’s Law
• Cosmological redshift
• Doppler shift
• Distance ladder
• Big Bang theory
• Cosmological principle
• Dark energy
• Supernovae
• Lookback time
• Galaxies
• History of astronomy
• Standard candles

Suggested investigations which could come BEFORE this one:

• Unlocking the Distances to Galaxies

• Exploding Stars

Suggested investigation which could come AFTER this one:

Exploring the Observable Universe

See Related Rubin Observatory Investigations for more details.

Investigation Timing

Online component: 40- 60 minutes.

Standards

Three-dimensional lesson summary

Students construct Hubble plots and analyze models to provide evidence for the Universe's expansion rate, and determine it has no center, then students compare data presented at two different scales to quantify how the expansion rate changes over time.

Building towards:

HS-ESS1-2 Construct an explanation of the Big Bang theory based on astronomical evidence of light spectra, motion of distant galaxies, and composition of matter in the universe.

Science Literacy and Critical Thinking Skills

• Engaging in argument from evidence
• Analyzing and interpreting data

Background

The International Astronomical Union voted in 2018 to rename Hubble’s Law the Hubble-Lemaître Law to recognize the contribution of Belgian astronomer Georges Lemaître, who published a paper on the expansion of the Universe two years before Hubble published his work. Lemaitre’s paper was published in a journal that had limited circulation, and therefore was not widely read until it was translated into English several years after Hubble’s publication.

The Hubble-Lemaître Law did not introduce the idea that the Universe was expanding. The idea of expansion preceded their work, and was supported both by theory and observations (Einstein and Slipher). The entire progression of the work of the scientists referenced in the introduction to this investigation provides an outstanding example of science as a collaborative and progressive process. The Hubble-Lemaître Law provided evidence for the expansion of the Universe and launched the next set of inferences: the Universe had a beginning, the age of the Universe could be estimated from the slope of the Hubble plot (the Hubble Constant), the rate of expansion has changed over time, and expansion is accelerating.

At different times in the history of the Universe, different factors dominated the rate of expansion. Photons drove the rate of expansion before matter in the Universe took over. Today, dark energy dominates, which is why the expansion of the Universe is accelerating instead of expanding at a slowing rate. This accelerated expansion may be used to introduce a discussion of dark energy following this investigation. So the value of the Hubble “Constant” (the slope of the Hubble plot) isn’t constant, but has increased over time.

Openstax Astronomy textbook links:
The expansion of the Universe
Big Bang theory
Dark matter and dark energy

Links to Videos

These resources are from a teacher workshop on this investigation. They include a talk from a cosmologist (15 minute video) and the accompanying talk visuals (speaker slides).

Video: Keith Bechtol, "The Expanding Universe as Seen with Vera C. Rubin Observatory"
Speaker slides

The Hobermann Sphere video used as the phenomenon for this investigation may be revisited and used for discussion throughout the lesson.

Teacher Notes

1. Our investigations are designed so that students cannot proceed to the next page without answering each question. If you would like to quickly preview the entire investigation, you can use “educator mode” on the Start page. Enter the passphrase: 3ducatorMod3 to activate it.

2. The data in this investigation differ from that in a typical Hubble plot. Velocity data are usually derived from the spectroscopic redshift of galaxies. Rubin Observatory uses a different technique that produces a photometric redshift. The advantage of using this different technique is that it can be applied to many more and much fainter galaxies. Cepheid variable stars are usually used to measure distances to galaxies. This investigation substitutes a different type of standard candle, Type Ia supernovae, which are more precise than Cepheid variables.

3. The original Hubble plot shown on p. 1 of the investigation, shows three points that have a velocity less than zero (a negative velocity). Negative velocities can occur when nearby galaxies in the Local Group are approaching our galaxy. Hubble’s observations included many nearby galaxies, which accounts for the large scatter in the plot.

4. Hubble determined most of the galaxy distances for this first plot by using the brightest stars in galaxies or by the luminosity of the galaxies themselves. Although Hubble had previously used a Cepheid variable star to estimate the distance to the Andromeda Galaxy, it was only later that he and other astronomers began to use Cepheids to measure distances to other galaxies.

5. Distant galaxies with supernovae will appear only as faint smudges or dots, and often the supernova will outshine the galaxy itself. The galaxy and the supernova can be distinguished from each other because the galaxy appears in both the pre- and post-discovery images. Students may think that the galaxy itself is exploding, or that the supernova is outside of the galaxy. It is extremely rare that a star would be alone in space and not associated with a galaxy. You can assume that the supernova belongs to the galaxy.

   Galaxy image quality is not related to the distance of the galaxy. The low image quality of the galaxy data used in this investigation are due to the images being made from short exposures, which is sufficient for supernova detection. Well-focused galaxy images are produced by combining many exposures of the same image to reduce image noise and accumulate more light.
   

6. This investigation deals with the age of the Universe in a conceptual way rather than using the full model necessary to actually calculate the age of the Universe. The computation of a numerical age involves additional variables beyond the Hubble Constant. See this sample cosmology calculator for more information.

7. Students will struggle to infer that the Universe is expanding from the information in a Hubble plot.

   To help them understand, ask students to identify the direction of motion for the galaxies in the Hubble plot (all galaxies are moving away), then have them compare the speed of the galaxies that are far away to those that are much closer, and ask if the nearby galaxies will ever catch the distant galaxies. Then ask if these observations describe whether galaxies are getting closer together or farther apart from one another—and whether that means the Universe is getting bigger (expanding) or shrinking.

8. Students will have a difficult time understanding how the slope of the Hubble Plot is used to determine the expansion rate of the Universe.

   One way to help students understand this is to show them two Hubble plots with lines of different slope, and ask students, "In which plot do the recessional velocities of galaxies increase more quickly with distance?” Follow this with “In which case does the Universe expand faster: a Hubble plot that has a steep slope or one that has a shallow slope?” This sequence of questions sets students up to then be shown a Hubble plot that has a curve, indicating a changing slope (and an accelerated expansion). Students can then be asked “Where on the graph is the slope steepest and most shallow?” and, “Is the expansion rate faster for the steeper slope or the more shallow slope?”

9. Students may struggle to reason how the slope of the Hubble plot and expansion rate are related to the age of the Universe.

   Show graphs with different slopes and ask, “In which graph is the expansion rate of the Universe fastest? Do you think a fast expanding Universe will get to its present size in less time or more time than a Universe that is expanding slowly? So will a fast expanding Universe result in a Universe with a younger or older age?”

10. Students may know that the Hubble plot can be used to estimate the age of the Universe, but not understand how this is determined.

    Remind students that the Hubble Constant represents the slope of the graph. It is this number that is used to determine the age of the Universe. Next, point out that the age of the Universe is inversely related to the Hubble Constant.

Common Student Ideas

1. The expansion of the Universe is due to the expansion of space.

   Bridge to learning: Space does not expand, like a fabric or something that can be stretched (a misconception introduced by most models). Likewise, the Universe does not expand "into" anything, and since there is no space "outside" it. Instead, the scale that is used to define space changes. For this reason, it may be helpful to talk about the spacetime of the Universe expanding rather than saying that space is expanding.
   

   In this video from JPL of the Expansion of the Universe, a series of waves are sketched on an elastic band. Visualize that there are points on either end of the sketched waves. Imagine there is a term that defines the distance between those two points. As the band is stretched, the term remains the same, but its scale has changed.

2. Earth (or our galaxy) is at the center of the Universe.

   Bridge to learning: Constructing a Hubble Plot places our galaxy (and Earth) at the origin of the graph. This may be interpreted by students to mean that our galaxy occupies a unique position at the center of the Universe. It’s important to show that the same perspective would be seen by observers in other galaxies as well. This can be addressed by working through questions that use the galaxy scrambler interactive in this investigation. Having students view a Hubble plot based off of the observations from other galaxies can be a powerful way to help them understand that all locations all see the same expansion away from their location, so therefore no location is the center of the Universe.

3. The Universe is not really expanding (or not expanding everywhere) because some galaxies (like the Andromeda Galaxy) show a blueshift.

   Bridge to learning: The gravitational attraction of our Local Group of galaxies (a galaxy cluster) keeps our local galaxies close together even as the Universe expands. In fact, the local gravitational attraction is so strong that galaxies in clusters commonly merge. As a result, some galaxies in the Local Group have velocities that show a component of their motion towards the Milky Way galaxy (a blueshift).

4. Students confuse the recessional velocity of a galaxy with the expansion rate of the Universe.

   Bridge to learning: A steep slope for a Hubble plot indicates that expansion rate for the Universe is “fast,” whereas a galaxy with a fast recessional velocity (from the observer) is plotted toward the top of the graph. Have students compare the value of two points at very different distances from the observer to see that the galaxies’ recessional velocities are different and the slope of the graph at the two points (1) would be the same, if the graph is a straight line (constant expansion) or (2) different for the case when the graph is curved and indicating that the Universe’s expansion is accelerating.

5. Students interpret the recessional velocities (redshifts) of galaxies as Doppler shifts instead of cosmological redshift. The difference is that in the first case, galaxy velocities are due to physical motions through a static volume of space, whereas cosmological redshift attributes the motions to the expansion of space itself. Although each galaxy does have a velocity, the cosmological redshift component of its motion far exceeds any minor contribution from a galaxy’s velocity.

   Bridge to learning: The difference is that in the first case, galaxy velocities are due to their physical motions through a static volume of space, whereas cosmological redshift attributes the motions to the expansion of spacetime itself. Although each galaxy does have a velocity, the cosmological redshift component of its motion far exceeds any minor contribution from a galaxy’s velocity, with the exception of galaxies very close to the Milky Way Galaxy.

   Use any of the commonly available models (inflating a balloon, the raisin bread analogy, the rubber band exercise) to demonstrate the expansion of space and discuss the difference.
   

6. All things are expanding away from each other—all galaxies, stars within a galaxy, etc.

   Bridge to learning: Expansion is only observed at large distances, where the expansion of spacetime is significant. Within the Solar System or a cluster of galaxies, gravity or curved spacetime keeps objects gravitationally bound to each other and not expanding.

Common Student Questions

1. How can you know that an observer in another galaxy would also observe all other galaxies moving away from them? You can’t go to those galaxies and make measurements from their locations, so how can you be sure that this is what an observer in another galaxy would see?

   Our observations of the redshifts of light from galaxies allows us to develop a model of how the Universe is expanding. Using this model, we can make predictions about how other locations in the Universe will observe expansion. Our observations tell us that the Universe is expanding uniformly. As space expands it causes galaxies to appear to move away from one another. No matter where you are in the Universe, you will observe galaxies moving away from you, with nearby galaxies moving away more slowly (since there is less space time expanding between them) than far away galaxies (which have a greater amount of space time between them). So, observers in all galaxies will make the same observations regarding the motion of galaxies and the expansion of the Universe.
   

2. How can we know there is no center to the Universe?

   Since all locations observe expansion away from them, then no one location is the central point for expansion, and therefore the Universe has no center. (see Note 1 for more explanation).
   

3. What causes the Universe to expand?

   No one knows why the Universe is expanding, but the theory of general relativity mathematically supports the role of gravity and its relationship to spacetime, predicting the result that we see.

   https://www.universetoday.com/116229/whats-causing-the-universe-to-expand/

4. Why does the rate of expansion change over time, and what is causing the acceleration?

   This is an unresolved question. Astronomers have given the name “dark energy” to whatever may be causing this acceleration, but no one really knows yet what it is. What we observe is that dark energy acts to oppose gravitational attraction on large scales and makes the entire Universe expand more quickly, and it seems to be uniform throughout the Universe.

   https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy

Differentiation

Students use an interactive tool to locate a supernova in a galaxy, and receive feedback when they have correctly identified the supernova. The plotting tool comes with graphing assists to help students accurately plot points and fit a line to their data.

The galaxy scrambler tool is designed to test students’ ideas about the homogeneity of the Universe, and to address student ideas that the Universe has a center, and that expansion rates can be different for different locations in the Universe.

In the "Reflect and Discuss" section, students are asked if the data from this investigation provide supporting evidence for the Big Bang Theory. This is an opportunity for class discussions, group whiteboarding, or creating concept maps or graphic organizers to interrelate the data and observations with theory. Students can demonstrate how data support the theory, as well as reveals areas of weakness.

An advanced option is described in the Ideas for Further Study section of this teacher guide.

Related Rubin Observatory Investigations

Students learn how to distinguish between two different types of supernovae in the Exploding Stars investigation, then use a Type Ia supernova light curve to calculate the distance to a galaxy.

In "Unlocking the Distances to Galaxies", students analyze light curves of Cepheid variable stars and use the period-luminosity relation to determine distances to galaxies.

In Exploring the Observable Universe students use the redshift and position of galaxies to explore how the large scale structure of the Universe has changed over time.

Ideas for Further Study

• Hubble’s original plot (shown on page 1 of the investigation) shows three points that have a velocity less than zero (a negative velocity). Ask students to reason what this means (they are blueshifting/approaching). What could that indicate about these galaxies’ positions in space?

• Examine the slope of the graph produced by the large volume of Rubin Observatory data. Where does the slope of the curve change? What time (years before present) does that translate to? Ask students to read about what astronomers think may have caused this change.

• Calculate the age of the Universe based on your determination of the value of the Hubble Constant (H). The units for the Hubble Constant are km/sec/Mpc. The age in years (t) is inversely related to the Hubble Constant: t = 1/H. Start by using your determined value for the Hubble Constant. First use dimensional analysis to convert megaparsecs (Mpc) to km and then cancel the units. Finally, convert seconds to years.