HS-ESS1-2NGSS*ScienceEarth and Space ScienceGrades 9-12
HS-ESS1-2: Explaining the Big Bang with Evidence from Light, Galaxies and Matter
In plain English: HS-ESS1-2 is the NGSS high school performance expectation that asks students to construct an explanation of the Big Bang theory from astronomical evidence: redshifted light from galaxies, the pattern of their motion, the cosmic microwave background and the observed mix of about 3/4 hydrogen and 1/4 helium. It is usually taught in Earth and Space Science.
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.
Clarification Statement: Emphasis is on the astronomical evidence of the red shift of light from galaxies as an indication that the universe is currently expanding, the cosmic microwave background as the remnant radiation from the Big Bang, and the observed composition of ordinary matter of the universe, primarily found in stars and interstellar gases (from the spectra of electromagnetic radiation from stars), which matches that predicted by the Big Bang theory (3/4 hydrogen and 1/4 helium).
Next Generation Science Standards · Disciplinary Core Idea: Earth's Place in the Universe · Official standard
Students build a written scientific explanation of the Big Bang theory, the idea that the universe was once extremely hot and dense and has been expanding and cooling ever since. They measure redshifts from spectra, turn them into speeds, graph speed against distance for galaxies, and then test the explanation against two more lines of evidence: the cosmic microwave background and the amounts of hydrogen and helium seen in stars and gas.
The NGSS Clarification Statement says: "Emphasis is on the astronomical evidence of the red shift of light from galaxies as an indication that the universe is currently expanding, the cosmic microwave background as the remnant radiation from the Big Bang, and the observed composition of ordinary matter of the universe, primarily found in stars and interstellar gases (from the spectra of electromagnetic radiation from stars), which matches that predicted by the Big Bang theory (3/4 hydrogen and 1/4 helium)." This performance expectation has no Assessment Boundary. Dark matter, dark energy and the details of the first seconds are left out on purpose; where they come up, they are marked as beyond this standard. Data sets labeled invented are made up for teaching but patterned on real measurements. Values used: speed of light 3.00 × 10⁵ km/s; rest wavelengths hydrogen-alpha 656.3 nm, hydrogen-beta 486.1 nm, helium 587.6 nm; Hubble constant 70 km/s per megaparsec (measurements give about 67-73); 1 megaparsec (Mpc) = 3.26 million light-years = 3.086 × 10¹⁹ km; 1 year = 3.156 × 10⁷ s.
Learning Objectives
By the end of this lesson, students will be able to:
Measure the redshift of a spectral line and convert it to a galaxy's recession speed with v ≈ cz
Analyze speed and distance data for galaxies and explain why a straight line through the origin is evidence that the universe is expanding
Explain the cosmic microwave background as radiation left over from a hot, dense early universe
Use spectra to identify hydrogen and helium and compare the observed mix of about 3/4 hydrogen and 1/4 helium by mass with the Big Bang prediction
Construct a claim-evidence-reasoning explanation of the Big Bang theory that uses all three lines of evidence and answers a rival claim
Prior Knowledge Required
Students should already be comfortable with:
Gravity and the structure of galaxies and the solar system MS-ESS1-2
Wavelength, frequency and the electromagnetic spectrum MS-PS4-2
Atoms of different elements having different masses; hydrogen and helium as the lightest elements
Play or imitate the sound of a car horn or siren passing by: the pitch drops as it moves away. Then show a photo of a spectrum from a diffraction grating.
Warm-Up Prompt
"The pitch of a siren is higher as it comes toward you and lower as it goes away. Light is also a wave. If a galaxy were moving away from us, what might happen to its light? What would you need to measure to find out?"
Record predictions. Many students expect the galaxy to look dimmer but not to change color; others say it would turn red. Keep the ideas on the board and come back to them after the first example.
Direct Instruction15-20 minutes
Spectra as evidence. Each element absorbs and emits light only at its own set of wavelengths, so a spectrum shows which elements are present. In the light of distant galaxies the whole pattern of lines is shifted to longer wavelengths, a redshift. The size of the shift is z = (λobserved - λrest) ÷ λrest, and for small z the recession speed is about v = cz. Show Diagram 2: every line moves by the same fraction, which is how astronomers know the pattern belongs to hydrogen and helium and not to some unknown element.
Motion of galaxies. When speeds are graphed against distance, farther galaxies move away faster, in proportion to distance (Hubble's law, v = H0d). The explanation is not that galaxies fly away from one center: space itself is stretching, so every galaxy sees every other distant galaxy moving away. Run the model backward and everything was once packed together in a hot, dense state.
Redshift of one galaxy
The hydrogen-alpha line, 656.3 nm in the lab, appears at 669.4 nm in a galaxy's spectrum. Find z and the recession speed.
Result: z = (669.4 - 656.3) ÷ 656.3 = 13.1 ÷ 656.3 ≈ 0.020. v ≈ cz = 3.00 × 10⁵ km/s × 0.020 ≈ 6,000 km/s, moving away (the shift is to longer wavelengths).
Speed is proportional to distance
Invented data for four galaxies: distances 20, 50, 90 and 140 Mpc; speeds 1,450, 3,480, 6,350 and 9,760 km/s. Is the pattern proportional?
Result: v ÷ d = 72.5, 69.6, 70.6 and 69.7 km/s per Mpc: nearly constant, so v is proportional to d with H0 ≈ 70 km/s per Mpc. A galaxy twice as far recedes about twice as fast.
An age from the expansion rate
If galaxies had always moved apart at today's rate, how long ago were they together? Use H0 = 70 km/s per Mpc and 1 Mpc = 3.086 × 10¹⁹ km.
Result: Time = d ÷ v = 1 ÷ H0 = 3.086 × 10¹⁹ km ÷ 70 km/s ≈ 4.4 × 10¹⁷ s ≈ 14 billion years. Combining this with other data gives today's best estimate, about 13.8 billion years.
The cosmic microwave background
Radio telescopes detect faint microwave radiation from every direction with a temperature of 2.725 K. The Big Bang model says it was released about 380,000 years after the Big Bang, when the universe had cooled to about 3,000 K. By what factor has the universe stretched since?
Result: Stretch factor ≈ 3,000 K ÷ 2.725 K ≈ 1,100. The radiation's wavelengths have been stretched about 1,100 times, which cooled a glow like a cool red star's into microwaves, just as the model predicts.
Hydrogen and helium by mass and by atoms
Spectra of stars and gas clouds show that ordinary matter is about 75% hydrogen and 25% helium by mass. A helium atom has about 4 times the mass of a hydrogen atom. How many hydrogen atoms are there for each helium atom?
Result: In 100 g: 75 g ÷ 1 = 75 units of hydrogen and 25 g ÷ 4 = 6.25 units of helium, so 75 ÷ 6.25 = 12 hydrogen atoms per helium atom (helium is about 7.7% of the atoms). Big Bang models predict close to this mix for the first few minutes.
After each example ask: "What would we observe if the universe were not expanding and had never been hot?" (no systematic redshift, no microwave glow from all directions, and no reason for so much helium in the oldest gas).
Guided Practice15-20 minutes
Pairs analyze the invented data below. The distances come from standard candles (stars and supernovae of known brightness). Pairs compute z and v for each galaxy, plot v against d, and find the slope. Diagram 1 shows the finished graph for checking.
Guided practice galaxies (invented)
Galaxy
Observed H-alpha (nm)
Distance (Mpc)
G1
658.5
15
G2
662.6
40
G3
666.1
65
G4
671.9
100
G5
676.0
130
G6
682.4
170
Find z for each galaxy from the 656.3 nm rest wavelength, then v = cz, rounded to the nearest 10 km/s.
Plot the six points with distance on the horizontal axis and speed on the vertical axis.
Draw a best-fit line. Does it pass through the origin? Find its slope in km/s per Mpc.
Write a claim about the motion of galaxies and cite the slope and the origin as evidence.
The speeds come out near 1,010, 2,880, 4,480, 7,130, 9,010 and 11,930 km/s, and the best-fit slope is about 70 km/s per Mpc. Ask: "Why is a line through the origin, and not just any rising line, the evidence we want?" (A proportional pattern is what uniform stretching of space produces.)
Independent Practice10-15 minutes
Each student analyzes invented spectral measurements of gas clouds in five small galaxies. Oxygen is made only in stars, so gas with less oxygen has been changed less by stars. The oxygen column is the number of oxygen atoms per 100,000 hydrogen atoms.
Independent practice: helium in gas clouds (invented)
Oxygen per 100,000 H
2
4
6
8
10
Helium mass fraction
0.248
0.250
0.253
0.255
0.258
Students (1) graph helium fraction against oxygen, (2) extend the line to zero oxygen to estimate the helium that was present before any stars formed, and (3) compare the result with the Big Bang prediction of about 0.25. They finish with a claim that links the composition of matter to the Big Bang and explains why stars cannot account for most of the helium.
Closure5 minutes
Exit ticket: "Name the three lines of evidence for the Big Bang we used today. For each one, write one observation and what it tells us about the early universe." Collect and sort the responses into the three categories to plan the next lesson's review.
Differentiation Strategies
For Struggling Students
Give a calculation table with columns for λobserved, λobserved - λrest, z and v, so the redshift steps are done one at a time
Start the balloon model before the redshift math, so students see recession growing with distance before they compute it
Provide a claim-evidence-reasoning frame with one row for each line of evidence
For Advanced Students
Ask students to explain why the 1 ÷ H0 estimate is only approximate, since the expansion rate has changed over time (details beyond this standard)
Have students compare the Big Bang with the steady-state model and write which observations each model predicts
Ask students to research how the first detection of the cosmic microwave background in 1965 changed the debate between the two models
Assessment Guidance
What to Look For
Check that students compute redshift as a fraction of the rest wavelength, not as a difference in nanometers alone, and that they read a proportional speed-distance graph as uniform expansion rather than an explosion from a center. Look for explanations that use all three lines of evidence (light spectra, galaxy motion, composition) and that name the cosmic microwave background as leftover radiation, not light from stars. Listen for students who think the Big Bang happened at one place in space or that the helium was all made in stars.
02
Classroom Activities
3 Activities
1
Balloon Universe: Measuring Expansion
20 minPairs
Pairs mark dots on a partly inflated balloon, measure the distance from a home dot to four others, inflate it further over a counted 10 seconds, and measure again. They compute each dot's recession speed and compare the pattern with Hubble's law.
Procedure
Use latex-free balloons (allergy safety). Inflate to about 12 cm across, pinch the neck, and draw a home dot and dots A-D at different distances.
Measure each distance from home along the surface with a flexible tape measure.
Inflate steadily for 10 seconds and measure again. Speed = change in distance ÷ 10 s.
Repeat with a different home dot to test whether the pattern depends on where you stand.
Sample Data (invented)
From the home dot, before: A 2 cm, B 4 cm, C 6 cm, D 8 cm. After 10 s: A 3 cm, B 6 cm, C 9 cm, D 12 cm. Speeds: 0.1, 0.2, 0.3 and 0.4 cm/s.
Discussion Questions
In the sample data, speed ÷ distance is 0.05 per second for every dot. What does that match in the galaxy data?
Did the pattern change when you switched home dots? What does that suggest about a center of expansion?
Name two ways the balloon is a poor model of the universe (for example, the dots themselves grow, and the balloon has an inside and an outside).
Modification for Distance Learning
Students mark dots on a strip of elastic or a rubber band, stretch it to 1.5 times its length, measure, and post their table and one sentence about the pattern.
2
Spectrum Detectives
25 minGroups of 3-4
The teacher first shows hydrogen and helium gas tubes through diffraction-grating glasses so that every student sees that each gas has its own line pattern. Groups then receive five invented galaxy spectrum cards and work out how each galaxy is moving.
Teacher Demonstration and Safety
Only the teacher handles the gas tubes and power supply: the supply uses high voltage and the tubes get hot. Turn the room lights down and keep students at least 1 m away.
Students sketch the bright lines they see for hydrogen and for helium and note that the patterns differ.
Galaxy Cards (invented)
Galaxy P: H-alpha at 655.6 nm, distance 0.8 Mpc
Galaxy Q: H-alpha at 660.7 nm, distance 29 Mpc
Galaxy R: H-alpha at 670.1 nm, distance 90 Mpc
Galaxy S: H-alpha at 687.5 nm, distance 205 Mpc
Galaxy T: H-alpha at 702.2 nm, distance 300 Mpc
Discussion Questions
Only Galaxy P is blueshifted, and it is also the nearest. Suggest a reason, and explain whether one blueshifted neighbor overturns the pattern shown by Galaxies Q-T.
Rank the galaxies by redshift and by distance. Are the two rankings the same?
For Galaxies Q-T, is v ÷ d close to the same value each time? What value?
Challenge Variation
Give each group a card with two unlabeled lines shifted by the same fraction. Groups decide which elements the lines belong to by testing which pair of rest wavelengths gives the same z for both lines.
3
Evidence Gallery: Big Bang or Steady State?
25 minGroups of 3-4
Groups visit four stations, each with one invented data set patterned on real observations, and record whether the Big Bang model, the steady-state model (the universe has always looked the same, with new matter appearing as it expands) or both can explain it. They end by writing a group explanation.
Station Data (invented)
Station 1, microwave sky: background temperature measured in six directions: 2.72549, 2.72547, 2.72550, 2.72548, 2.72551 and 2.72548 K.
Station 2, galaxy motion: Diagram 1 and the guided practice table.
Station 3, helium mass fraction in five places: a young star cluster 0.27, the sun's surface layers 0.25, a nearby gas cloud 0.28, a distant small galaxy 0.25, an old star cluster 0.26.
Station 4, a rival claim card: "Galaxies move apart, but new matter forms in the gaps, so the universe never had a beginning."
Discussion Questions
At Station 1 the six readings differ by at most 0.00004 K. Why is a glow this even from every direction hard to explain without a hot, dense early universe?
At Station 3 no helium fraction is below 0.25. What would you expect if all helium had been made in stars over time?
Which station can both models explain? Which stations favor the Big Bang?
Modification for Distance Learning
Post each station as a slide. Groups use a shared document with one column per model and one row per station, then post a final paragraph.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: Speed Against Distance for Six Galaxies (Invented Data)
The guided practice galaxies, drawn to scale. Speeds come from redshifts and distances from standard candles. The points lie close to a straight line through the origin with a slope of about 70 km/s per Mpc, so speed is proportional to distance: the pattern expected if space is expanding uniformly.
Diagram 2: The Same Line Pattern, Shifted Toward the Red
Top: hydrogen and helium lines measured in the lab. Bottom: the same pattern in an invented galaxy spectrum with z = 0.05. Every line moves by the same fraction of its wavelength (486.1 nm to 510.4 nm, 587.6 nm to 617.0 nm, 656.3 nm to 689.1 nm), which identifies the elements and gives the redshift at once.
04
Homework Assignment
~30 min
HS-ESS1-2 Homework: Building the Case for the Big Bang
Directions: Data sets are invented for practice. Use c = 3.00 × 10⁵ km/s, rest wavelengths H-alpha 656.3 nm, H-beta 486.1 nm and helium 587.6 nm, 1 Mpc = 3.086 × 10¹⁹ km and 1 year = 3.156 × 10⁷ s. Show calculations with units, and end each problem with a claim that names your evidence.
Part 1: Light Spectra (Problems 1-2)
The hydrogen-beta line in a galaxy's spectrum appears at 501.0 nm. (a) Is the light redshifted or blueshifted? (b) Find z and the galaxy's speed. (c) At what wavelength should its hydrogen-alpha line appear? Explain why checking a second line makes the evidence stronger.
A distant gas cloud shows three bright lines at 500.5 nm, 605.0 nm and 675.7 nm. (a) Divide each by the rest wavelengths of H-beta, helium and H-alpha. (b) What do the three ratios tell you about the cloud's composition and its motion? (c) Why could a line at 605.0 nm not be identified from the lab spectrum alone?
Part 2: Motion of Distant Galaxies (Problems 3-4)
Five galaxies have distances of 30, 60, 110, 150 and 200 Mpc and speeds of 2,250, 4,380, 8,030, 11,100 and 14,600 km/s. (a) Compute v ÷ d for each. (b) Estimate H0. (c) Use 1 ÷ H0 to estimate how long ago the galaxies were together, in billions of years. (d) State a claim about the universe that these data support.
A student writes: "All galaxies are moving away from us, so the Milky Way must be at the center of the Big Bang." Galaxies A, B, C and D sit in a row 10 Mpc apart (A at 0, B at 10, C at 20, D at 30 Mpc). The universe then stretches by 10%. (a) Find the new distances from A and from B to every other galaxy. (b) Compute how far each galaxy moved away from A, then from B. (c) Use your results to critique the student's claim.
Part 3: Composition and the Microwave Background (Problems 5-6)
A sample of the oldest known gas contains 92 hydrogen atoms for every 8 helium atoms. A helium atom has about 4 times the mass of a hydrogen atom. (a) What fraction of the gas's mass is helium? (b) Compare your answer with the Big Bang prediction of about 1/4. (c) Explain why this sample is better evidence than gas in a young star cluster.
Wien's law says a glowing object's brightest wavelength is λ = 2.898 × 10⁻³ m·K ÷ T. (a) Find the brightest wavelength of the background radiation today, at 2.725 K, and when it was released, at about 3,000 K. (b) Name the part of the spectrum each falls in. (c) Explain how this supports the claim that the radiation is left over from a hot early universe.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Spectra and Redshift
Redshift found as a fraction of the rest wavelength, speed from v = cz, and elements identified from matching ratios
Correct method with one arithmetic or unit error, or elements not identified
Galaxy Motion Data
Computes v ÷ d, estimates H0 and the time scale, and explains why proportionality means uniform expansion
Computes the values but does not interpret the pattern
Composition and Background
Converts between atom counts and mass fractions and explains both as evidence of a hot, dense early universe
Correct numbers with a weak or missing link to the Big Bang
Explanation and Critique
Claims cite specific data and answer the rival or flawed claim directly
Claim stated but evidence is general or the critique is missing
05
Quiz: 20 Questions
Interactive, with answers
Instructions
Data sets are invented. Use c = 3.00 × 10⁵ km/s, H-alpha 656.3 nm, H-beta 486.1 nm, helium 587.6 nm, H0 = 70 km/s per Mpc unless a question gives another value, and 1 Mpc = 3.086 × 10¹⁹ km. Your score updates as you answer, and Reset quiz clears everything so you or your students can try again.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
A galaxy's spectrum shows the familiar hydrogen line pattern, but every line sits at a longer wavelength than in the lab. What does this tell astronomers?
Answer: C
A shift of the whole pattern to longer wavelengths is a redshift, the sign of motion away from us. Choice B describes a blueshift, a shift to shorter wavelengths. Choice A is wrong because the pattern still matches hydrogen; only its position moved.
Question 2 of 20 · Multiple Choice
A galaxy's H-alpha line is observed at 662.9 nm. About how fast is the galaxy moving, and in which direction?
Answer: A
z = (662.9 - 656.3) ÷ 656.3 = 6.6 ÷ 656.3 ≈ 0.010, and v ≈ 0.010 × 3.00 × 10⁵ ≈ 3,000 km/s, away because the line is shifted to longer wavelengths. Choice B has the right size but the wrong direction. Choices C and D misplace the decimal point in z.
Question 3 of 20 · Multiple Choice
Which pattern in galaxy data is the main evidence that the universe is expanding?
Answer: D
Speed proportional to distance is what uniform stretching of space produces. Choice B reverses the observed trend. Choice C would suggest no overall expansion.
Question 4 of 20 · Multiple Choice
A galaxy is 250 Mpc away. About how fast should it recede, using H0 = 70 km/s per Mpc?
Answer: B
v = H0d = 70 × 250 = 17,500 km/s. Choice A divides 250 by 70 instead of multiplying. Choice C adds 70 and 250. Choice D drops a factor of ten.
Question 5 of 20 · Multiple Choice
A galaxy recedes at 11,200 km/s. About how far away is it?
Answer: C
d = v ÷ H0 = 11,200 ÷ 70 = 160 Mpc. Choice A multiplies instead of dividing. Choice D divides 70 by 11,200, and Choice B is off by a factor of ten.
Question 6 of 20 · Multiple Choice
Astronomers in any galaxy would see almost all other distant galaxies moving away from them. What does this imply?
Answer: B
If space stretches uniformly, every observer sees the same proportional pattern, so no galaxy is at a center. Choice A would require us to be in a special place, which the balloon and stretch models show is not needed. Choice C pictures an explosion into empty space, a common misreading of the Big Bang.
Question 7 of 20 · Multiple Choice
Suppose a team measures H0 = 65 km/s per Mpc. About what time does 1 ÷ H0 give for the age of the expansion?
Answer: D
1 ÷ H0 = 3.086 × 10¹⁹ km ÷ 65 km/s ≈ 4.7 × 10¹⁷ s ≈ 15 billion years. Choice A is the age of the solar system, not the universe. Choice C is off by a factor of ten.
Question 8 of 20 · Multiple Choice
What is the cosmic microwave background?
Answer: A
It is remnant radiation from the early universe, now stretched to microwave wavelengths and a temperature near 2.7 K. Choice B is wrong because the glow comes evenly from all directions, not from where stars are concentrated. Choice D confuses it with the sun, which is a single direction.
Question 9 of 20 · Multiple Choice
Why is the cosmic microwave background strong evidence for the Big Bang rather than for a universe that has always looked the same?
Answer: C
The Big Bang model predicted a leftover glow, cooled by expansion, before it was detected in 1965; the steady-state model has no natural source for it. Choice A is false: the glow is almost the same in every direction. Choice D contradicts the redshift evidence.
Question 10 of 20 · Multiple Choice
Spectra of a gas cloud show that each 1,000 kg of its gas contains about 740 kg of hydrogen, 250 kg of helium and 10 kg of heavier elements. How does this compare with the Big Bang prediction?
Answer: A
740 ÷ 1,000 = 74% hydrogen and 250 ÷ 1,000 = 25% helium, close to the predicted 3/4 and 1/4 by mass; the 1% of heavier elements comes from stars. Choice B misstates the prediction. Choice D is wrong because spectral lines are exactly how composition is measured.
Question 11 of 20 · Multiple Choice
Why does the amount of helium in the universe support the Big Bang theory?
Answer: D
Stars make some helium, but gas with almost no star-made elements still has close to 25% helium, so most helium must be older than the stars. Choice A is false: helium is the second most abundant element. Choice B ignores that the same fraction appears in distant galaxies.
Question 12 of 20 · Multiple Choice
A gas has a helium mass fraction of 0.28 and a hydrogen mass fraction of 0.72. A helium atom has about 4 times the mass of a hydrogen atom. About how many hydrogen atoms are there for each helium atom?
Answer: C
Hydrogen: 0.72 ÷ 1 = 0.72; helium: 0.28 ÷ 4 = 0.07. The ratio is 0.72 ÷ 0.07 ≈ 10. Choice A divides the mass fractions without accounting for atom mass. Choice D multiplies the helium share by 4 instead of dividing (0.72 ÷ 1.12 ≈ 0.64).
Question 13 of 20 · Multiple Choice
How do astronomers know which elements are in a galaxy millions of light-years away?
Answer: D
Each element has its own set of spectral lines, like a fingerprint, and the whole set shifts by the same fraction when the source moves. Choice A is impossible at these distances. Choice B is not enough, because different mixtures can look the same color.
Question 14 of 20 · Multiple Choice
A galaxy in our Local Group, about 0.9 Mpc away, is blueshifted. Does this disprove the expansion of the universe?
Answer: B
Hubble's law describes the overall pattern for distant galaxies; nearby galaxies bound by gravity can move toward each other. Choice C overstates the law: individual galaxies also have their own motions. Choice D is false: a blueshift means motion toward us.
Question 15 of 20 · Short Answer
A gas cloud's spectrum has lines at 525.0 nm, 634.6 nm and 708.8 nm. Identify the elements, find z and the recession speed, and estimate the distance with H0 = 70 km/s per Mpc.
Dividing by 486.1, 587.6 and 656.3 nm gives 1.080 each time, so the lines are H-beta, helium and H-alpha: the cloud contains hydrogen and helium. z = 0.080, v ≈ 0.080 × 3.00 × 10⁵ = 24,000 km/s away, and d ≈ 24,000 ÷ 70 ≈ 340 Mpc.
Question 16 of 20 · Short Answer
Galaxy A is 80 Mpc away and recedes at 5,700 km/s. Galaxy B is 240 Mpc away and recedes at 16,900 km/s. Do they fit Hubble's law? Estimate H0 from them.
v ÷ d = 5,700 ÷ 80 ≈ 71.3 and 16,900 ÷ 240 ≈ 70.4 km/s per Mpc. The ratios are nearly equal, so yes, speed is proportional to distance, with H0 ≈ 71 km/s per Mpc. Galaxy B is 3 times as far and recedes about 3 times as fast.
Question 17 of 20 · Short Answer
The background radiation is stretched along with space, and its temperature falls in proportion as its wavelengths grow. If the universe expands until all distances are twice today's, what will the background temperature be? Explain.
Doubling distances doubles the wavelengths, so the temperature halves: 2.725 K ÷ 2 ≈ 1.36 K. This is the same reasoning run forward that, run backward, shows the early universe was far hotter.
Question 18 of 20 · Short Answer
Two gas clouds are measured. Cloud X has 12 oxygen atoms per 100,000 hydrogen atoms and a helium mass fraction of 0.262; Cloud Y has 4 oxygen atoms and 0.252. Stars add about 0.00125 to the helium fraction for each unit of oxygen. Estimate the helium each cloud had before stars changed it, and explain what the result is evidence for.
Cloud X: 0.262 - 12 × 0.00125 = 0.262 - 0.015 = 0.247. Cloud Y: 0.252 - 4 × 0.00125 = 0.252 - 0.005 = 0.247. Both clouds started with the same helium fraction, close to 1/4, before stars added any. That starting helium must be older than the stars, and it matches the amount Big Bang models predict for the first minutes of the universe.
Question 19 of 20 · Short Answer
A classmate says the redshift evidence alone proves the Big Bang. Explain why the cosmic microwave background and the hydrogen-helium mix make the explanation stronger.
Redshifts show the universe is expanding now, but other models, such as steady state, also include expansion. The microwave background shows the universe was once hot and dense, and the hydrogen-helium mix matches what a hot early universe would make in its first minutes. Three independent lines of evidence that agree are much harder to explain any other way.
Question 20 of 20 · Short Answer
Write a claim-evidence-reasoning explanation of the Big Bang theory in five or six sentences, using all three lines of evidence from this lesson.
Model answer: Claim: the universe began in a hot, dense state about 13.8 billion years ago and has been expanding and cooling since. Evidence 1: spectral lines of distant galaxies are redshifted, and their speeds are proportional to their distances, so space is stretching. Evidence 2: a nearly uniform microwave glow at about 2.7 K comes from every direction, the cooled leftover of the hot early universe. Evidence 3: spectra of stars and gas show about 3/4 hydrogen and 1/4 helium by mass, matching what Big Bang models predict. Reasoning: running the expansion backward gives a hot, dense beginning, and only that beginning explains all three observations together. Rubric: full credit gives a clear claim, one specific observation for each line of evidence, and reasoning that links them.
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does HS-ESS1-2 mean?
HS-ESS1-2 asks students to construct an explanation of the Big Bang theory from astronomical evidence: light spectra, the motion of distant galaxies and the composition of matter. The code means High School, Earth and Space Science, core idea 1 (Earth's Place in the Universe), performance expectation 2. The goal is an evidence-based explanation, not a memorized timeline.
Is HS-ESS1-2 taught in Earth science or physics?
It is usually taught in Earth and Space Science, in an astronomy or cosmology unit. Some schools cover it in Physics, since redshift connects to waves and the Doppler effect, or in an integrated science course.
Was the Big Bang an explosion?
Not in the everyday sense. An explosion throws matter outward from one point into empty space. The Big Bang theory describes space itself expanding everywhere at once from a hot, dense state. Galaxies are carried apart by that stretching rather than flying through space.
Is the Big Bang just a theory?
In science, a theory is a well-tested explanation supported by many independent lines of evidence. The Big Bang theory explains redshifts, the microwave background and the hydrogen-helium mix together, and it has survived decades of new measurements. Scientists still refine details, such as the exact expansion rate.
Does HS-ESS1-2 include dark matter and dark energy?
No. The standard focuses on ordinary matter and on three lines of evidence. Dark matter and the accelerating expansion are real research topics but are beyond this standard; teachers can mention them as open questions without assessing them.
What happened before the Big Bang?
Science does not have a tested answer. The Big Bang theory describes how the universe changed from a hot, dense state onward. Ideas about what came before, if that question even has meaning, are speculative and beyond this standard.
Can students observe spectra in a school lab?
Yes. Diffraction-grating glasses show the separate bright lines of hydrogen, helium and other gases when the teacher runs gas discharge tubes, and a phone camera behind a grating can record them. Redshifts of real galaxies come from telescope data, which many observatories publish for classroom use.
How does HS-ESS1-2 connect to HS-ESS1-3?
HS-ESS1-2 explains where hydrogen and helium came from: the first minutes of the universe. HS-ESS1-3 continues the story with how stars later fuse those light elements into heavier ones, such as carbon, oxygen and iron.
What math do students need for HS-ESS1-2?
Ratios and percents for redshift and composition, slope for the speed-distance graph, scientific notation, and unit conversions for the age estimate. All of it is middle school or early high school math; the challenge is interpreting what the numbers show.
Why do astronomers disagree about the exact value of the Hubble constant?
Different methods give slightly different values, roughly 67 to 73 km/s per megaparsec, and the gap is larger than their stated uncertainties. This is an active research question. It changes the age estimate by about a billion years but does not change the evidence that the universe is expanding.
07
Related Standards
5 standards
These standards connect to HS-ESS1-2: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
MS-ESS1-2Prerequisite
Model the role of gravity in the motions within galaxies and the solar system
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