Exploring the Observable Universe Teacher Guide
This investigation explores the evolution of the large-scale structure of the Universe by viewing the distribution of galaxies at different redshifts.
Vera C. Rubin Observatory will greatly contribute to the refinement of our model of the large-scale structure of the Universe by discovering billions of previously undetected galaxies. These refinements may validate existing theories about the fundamental nature and interactions of matter and energy, or they may uncover new questions to investigate.
Students begin by exploring how cosmological redshift affects galaxy color, and how color can be used to determine distances and lookback time. Students then consider how light is used to define the observable Universe. Finally, they devise an explanation for what forces drive the observed changes in the large-scale structure of the Universe over time.
- Students interpret the redness-distance plots of galaxies to show that the Universe is expanding (cosmological redshift).
- Students make observations of galaxy distributions to determine that the Universe is the same in all directions (the cosmological principle).
- Students calculate a minimum size for the observable Universe using galaxy redshift data.
- Students analyze galaxy concentration maps to determine how the large scale structure of the Universe has become less homogenous (more clumpy) over time due to gravitational interactions.
- Students should know the definition of light-year (ly) as a distance unit.
- Students should be able to distinguish between Doppler shift (due to a galaxy’s recessional velocity) and cosmological redshift (due to the expansion of the Universe.
- Students should understand look back time: the image of a galaxy with a look back time of one million years shows what the galaxy looked like one million years ago.
- Students should be familiar with the way mass and distance affect gravitational forces (Newton’s Laws).
- Students should be familiar with the Law of Conservation of Mass.
- Students should be familiar with the Hubble-LeMaitre Law as supporting evidence for the expansion of the Universe.
Where This Fits In Your Teaching
- expansion of the Universe
- cosmological principle
- observable Universe
- large-scale structure
- evolution of the Universe
- Hubble’s Law
- cosmological redshift
- Big Bang theory
- lookback time
- distance ladder
- Newton’s Laws of Motion
- conservation of mass
- How many galaxies are in the Universe?
- What can we measure and what can’t we measure about the Universe?
- How can galaxy redshifts be used to view the Universe at different times in its history?
- How can the size of the observable Universe (in light-years)far exceed the age of the Universe (in years)?
- How has the large-scale structure of the Universe changed over time?
- How can we use our understanding of gravitational interactions and Newton’s Laws of Motion to account for the changes in the large-scale structure?
- What is the distance to the farthest galaxies that we can observe?
Suggested investigation which could come BEFORE this one:
See Related Rubin Observatory Investigations for more details.
Core activity: 45-90 minutes
Three-dimensional lesson summary
Students explore the relationships between galaxy properties and distances by analyzing images and scatter plots. Students then compare concentration maps to observe the changing patterns in galaxy distributions over various times in the history of the Universe, and construct an explanation for the observed changes that uses their understanding of gravitational interactions and conservation of energy and matter.
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 and Engineering Practice
Analyzing and interpreting Data
Science and Engineering Practice
Disciplinary Core Idea
ESS1.A: The Universe and Its Stars
PS2.B: Types of Interactions
Forces at a distance are explained by gravitational fields permeating space that can transfer energy through space.
Scale, Proportion, and Quantity
Energy and Matter
The total amount of energy and matter in closed systems is conserved.
Stability and Change
Connections to Engineering
Interdependence of Science, Engineering, and Technology
Science and engineering complement each other in the cycle known as research and development (R&D). Many R&D projects may involve scientists, engineers, and others with wide ranges of expertise.
Connections to Nature of Science
Scientific Knowledge Assumes an Order and Consistency in Natural Systems
Science Literacy and Critical Thinking Skills
- Analyzing and interpreting data
- Constructing explanations
- Engaging in argument using evidence
Since light has a finite speed, the amount of time it takes light to travel to Earth (lookback time) we see distant objects as they were in the past. A galaxy with a lookback time of two billion years appears as it was two billion years ago.
Due to the expansion of the Universe, the actual distance to a galaxy derived from cosmological redshift is greater than its light travel time.
We can only characterize the part of the Universe that we can detect (the observable Universe) based on the age of the Universe and light travel time. We cannot observe the entire Universe.
The cosmological principle makes the assertion that the Universe looks the same in all directions, and the average density of matter is also the same when viewed on large scales. Observations support this assumption.
The large-scale structure of the Universe describes the distinct structure seen in the distribution of matter in the Universe on very large scales (beyond the scale of an individual galaxy or galaxy cluster). The structure changes over time, but in every direction it changes the same way over the same time scales. The large-scale structure has evolved from having a smooth appearance with an even distribution of galaxies in the early history of the Universe to having larger voids and galaxy clumps in recent times.
Openstax Astronomy textbook links:
Observations of distant galaxies
The cosmological principle
The formation and evolution of large-scale structure
Links to Videos and Visualizations
Here is a recently released (2021) interactive visualization of about 30,000 galaxies based on data collected by the Dark Energy Spectroscopic Instrument (DESI) in the span of 5 hours.
These resources are from a teacher workshop on this investigation:
Video: Eric Morganson, "How Astronomy's Biggest Dataset Will Change Your Universe"
Many students struggle with the premise of the cosmological principle, because they may focus on variations on a small scale instead of reasoning about whether these variations are seen on the large scale. They may see a small concentration of galaxies or one image that has a color variation in one spot, and use this as evidence that the Universe is not uniform. Encourage them to focus on the structure and variation that is on the large scale.
Distances given in light-years in this investigation are calculated from the photometric redshift value using current cosmological models, and reflect the distance from Earth at which the object would be today.
The most accurate way to determine a galaxy’s redshift is by taking a spectrum of the galaxy’s light, but this is not possible for very distant galaxies due to their dimness and their sheer number. Rubin Observatory uses an alternative technique called photometric redshift. It is not as precise, but it’s the only practical way to determine the redshift for the billions of faint galaxies Rubin Observatory will detect.
Photometric redshift is derived from measuring the broadband flux (brightness) of a galaxy through multiple filters to construct a shape that mimics the shape of a spectrum. To see how this works, look at Figure 1 below. The amount of light passed by the filters u, g, r, i, and z is shown in grey.
Figure 1. An example of photometric redshift: the amount of light passed through filters (in grey) is used to create a shape that resembles a model galaxy spectrum. It is then compared with galaxy model spectra at various redshifts (in blue, green and red) to find the best match. Source: http://ogrisel.github.io/scikit-learn.org/sklearn-tutorial/auto_examples/tutorial/plot_sdss_filters.html
A model galaxy spectrum with a known redshift (such as the three colored spectra models in Figure 1) is matched to the curve produced by the amount of light collected through the filters. These models are built using a set of initial conditions, such as the star formation history of the galaxy, its mass, and the amount of dust it contains.
Although less precise, Rubin Observatory photometric redshifts over the range 0.3 < z < 3.0 generally have errors of less than 2% when compared with redshifts determined from spectra. Only 10% of this sample will have redshift errors larger than 6%.
The redness-distance plots in this investigation contain galaxy data only to a cutoff distance of 14 gigalight-years (Gly). This is an intentional cutoff, and does not represent the entire range of data. The educational goal is for students to see the trend that more distant galaxies are redder, and the trend is clear up to 14 Gly. Beyond that distance, different rest frame ultraviolet spectral features start to move through these bands and make the trend less obvious. When Rubin Observatory data are available, galaxies at greater distances can be incorporated. Note: until Rubin Observatory data are available, we will use a simple photometric ratio of two bands (i/z) as a proxy for redness. Rubin Observatory will produce its own definition of redness in the future.
Common Student Ideas
Students may think that the redshift of the light we receive from galaxies means that the galaxies themselves have turned red.
Bridge to learning: A student that has this misconception is not taking into account that the observer sees light that is redshifted due to the motion between observer and the galaxy. If you were to view the galaxy at rest (no relative motion) you would see that the galaxy is actually giving off light that is "unshifted" and has not changed color.
Students may not understand how distances to galaxies in the Universe (measured in light-years) can exceed the actual age of the Universe (in years).
In order to make sense of this idea, first clarify that light-years is a measure of distance, not time. Then ask students to imagine a static Universe that is not expanding. If a galaxy's light in that Universe has traveled for 1 billion years, then it would be 1 billion light years away. But what about if the Universe was expanding over that 1 billion years? Then the actual distance to the galaxy would be more than 1 billion light years away when its light reaches Earth.
Common Student Questions
If this is where the galaxy is today, where will it be tomorrow? What about in 100 years? Where was the galaxy when it emitted its light?
Due to the vast distances in space, the galaxies will appear to be at the same measured distances, whether it’s tomorrow or 100 years from now. If a galaxy is a million light years away, in 100 years there will not be a noticeable change.
Although astronomers do have ways to determine the original position of a galaxy, most are focused on the overall, average properties of the Universe - its critical density, the value of the Hubble constant, and how both of these things are changing. They have developed cosmological calculators with variables that simulate these observed conditions in the Universe. Here’s a sample cosmological calculator.
If we can already see all the way back to the edge of the Universe, then why do we need to build bigger telescopes? How is Rubin Observatory special, if it is not going to see farther than other telescopes?
Think of this as filling in the blanks on an incomplete picture. Current telescopes only see the biggest and brightest objects, but Rubin Observatory will see billions of faint objects. By studying the nature of these objects and how their distribution contributes to the large-scale structure of the Universe, astronomers will have better insights into how galaxies evolved and merged, and the role of dark matter and dark energy in the evolution of the Universe.
If we wait longer, will we see the light from more galaxies? If we can see all the way to the cosmic microwave background now, will these new galaxies appear before or after the cosmic microwave background?
Yes, there are galaxies that will appear in the future, but their light will not be traveling from distances greater than our observable Universe. The light from new galaxies that appears in the future will be from galaxies that are still forming today, just as there are other objects in the Universe that have more recently begun to emit light, such as supernovae.
There are no galaxies beyond the distance of the cosmic microwave background, since the energy from that event happened at a time before the galaxies formed. All galaxies in the Universe are closer to Earth than the cosmic microwave background.
How big is the Universe in total? Is it infinite? How many galaxies are in it?
No one knows for certain the size of the Universe, since we can only observe galaxies at distances limited by the travel time of light (our observable Universe). However, astronomers have made estimates of the mass and density of the Universe, and these measurements suggest a Universe that is flat. A flat Universe predicts that the Universe is infinite in size.
Current estimates indicate that the radius of the Universe in total is at least 250 times the radius of the observable Universe, meaning it is at least 15 million times the observed volume. The estimates also predict there are at least 15 million times more galaxies than the estimated two trillion galaxies in our observable Universe.
Some astronomers think that our Universe may very well be infinite in size, with an infinite number of galaxies in it. Astronomers have different theories about the size of the Universe based on different calculations, but one thing they agree on is that it's either infinite, or really really big.
Students explore deep field images of galaxies with an interactive tool that displays the selected galaxies on scatter plots of apparent brightness vs. distance, then color vs. distance. This helps to visualize the trends between apparent brightness, redness and distance. These observations are then connected with prior knowledge about how redshift, color and the expansion of the Universe are related.
Students compare deep field images and scatter plots with each other to discover that all directions in the Universe contain similar distributions of galaxy distances and colors. This is further reinforced when students compare their own data with a much larger Rubin Observatory data set.
Students learn how to interpret concentration maps by first working with a population density map example. Then they use an interactive series of concentration maps to observe changes in the large structure of the Universe at different times.
This investigation presents a series of abstract ideas that can be very challenging to comprehend, especially for those who have not yet fully developed abstract reasoning skills (typically under age 16). Working in groups and encouraging a generous amount of discussion may help. Questions in the “Reflect and Discuss” section are designed to encourage students to discuss their observations and interpretations with each other to better clarify these concepts. In the process, students will be asked to draw on cross-curricular connections about how gravitational interactions of matter operate on large scales in the Universe, and how the expansion of the Universe and light travel time defines the Observable Universe.
Due to the complex nature of the ideas presented, “Putting it all Together” is designed as a guided reflection narrative, to help students with organizing and interpreting their observations.
Ideas for Further Study
- Some voids are smaller than others. Is there a relationship between the size of the void and the amount of its surrounding galaxies clustered into walls, bubbles, sheets?
- Are all voids growing over time? If not, can you discover a pattern about the ones that grow vs. the ones that seem to stay the same size or shrink? What might account for what you observe?
- Recent studies suggest that our galaxy may be located in a void. How would you go about verifying this with Rubin Observatory data?