MGU Science Series — Astronomy and Cosmology Foundations
Part VI: Cosmology and the Future of Astronomy
Redshift · Hubble's Law · Expansion of Space · Evidence
Chapter 22 of 24
MGU Science Series  ·  Astronomy and Cosmology Foundations  ·  Library Textbook Edition

Chapter 22:
The Expanding
Universe

Galaxy redshifts, Hubble's Law, the expansion of space, the observable universe, lookback time, and the evidence for a universe that has been growing larger since its beginning

■ Science Level: Astronomy & Cosmology ■

■ 9 Sections  •  Part VI of 6 ■

Follows Chapter 21: Galaxy Clusters, Dark Matter, and Large-Scale Structure  ·  Prepares students for Chapter 23: The Big Bang

Chapter 22 — Front Matter

Purpose and Overview

"The universe is under no obligation to make sense to you."

— Neil deGrasse Tyson

For most of human history, people assumed the universe was eternal and unchanging. The stars were fixed. The galaxies — on the rare occasions anyone thought about them at all — were simply part of a static, everlasting backdrop. Even Albert Einstein, when he developed his theory of general relativity in 1915, introduced an artificial mathematical term into his equations specifically to hold the universe still. He assumed it must be static, and he adjusted his equations to match that assumption.

He later called it the greatest mistake of his life.

In the 1920s, astronomers began accumulating evidence that galaxies were not simply sitting still. They were moving — and not randomly, in every direction, as you might expect from objects scattered through space. They were moving away. Almost all of them. And the farther away a galaxy was, the faster it appeared to be receding.

This chapter explains how astronomers discovered the expanding universe, what that expansion actually means, and how it connects to the history, structure, and possible future of the cosmos. The expansion of the universe is not a background fact to memorize — it is a living inference drawn from careful observation, distance measurement, and the interpretation of light.

Opening Question

If you looked out at hundreds of galaxies in every direction and found that nearly all of them had their light shifted toward the red end of the spectrum — and that the farther away they were, the larger the shift — what would you conclude? By the end of this chapter, you will understand exactly what Hubble concluded, and why it changed our picture of the universe forever.

9Sections
12Key Terms
5+Lines of Evidence
13.8BYears of History

Learning Objectives

  1. 1Explain how Edwin Hubble established that spiral nebulae are distant galaxies, not gas clouds within the Milky Way.
  2. 2Describe how galaxy spectra reveal recession velocities through the Doppler effect and cosmological redshift.
  3. 3State Hubble's Law and use it to relate recession velocity and distance.
  4. 4Explain the Hubble constant and use it in calculations at an introductory level.
  5. 5Distinguish between galaxies moving through space and the expansion of space itself.
  6. 6Explain the raisin bread analogy and what it implies about the center — or absence of center — of expansion.
  7. 7Define the observable universe and explain why its radius exceeds the age of the universe in light-years.
  8. 8Connect lookback time to the observation that distant galaxies appear as they were in the past.
  9. 9Identify and correct common misconceptions about the expanding universe.
  10. 10Describe the multiple independent lines of evidence that support the expanding universe model.
Chapter 22 — Contents

Sections at a Glance

Sections 22.1 – 22.3
Discovering That Galaxies Are Moving
Beyond the Milky Way · Spectra and Recession · Hubble's 1929 Discovery
Section 22.1

The Universe Beyond the Milky Way

One Island Among Many

For centuries, astronomers debated whether the faint, fuzzy patches of light visible in telescopes — called nebulae — were clouds of gas within the Milky Way or distant systems of stars far beyond it. The question was not settled until the 1920s.

The critical work was done by American astronomer Edwin Hubble. Using the 100-inch Hooker Telescope at Mount Wilson Observatory in California — the largest telescope in the world at the time — Hubble identified individual Cepheid variable stars inside the Andromeda Nebula.

Cepheid Variable Stars as Distance Tools

Cepheid variable stars pulsate in brightness with regular periods. Astronomer Henrietta Swan Leavitt discovered a precise relationship between a Cepheid's pulsation period and its intrinsic luminosity — a star that takes longer to complete one brightness cycle is genuinely more luminous than one with a shorter cycle.

The method: measure the period → infer the true luminosity → compare to apparent brightness → calculate distance.

When Hubble measured the Cepheids in Andromeda, he found they were far dimmer than their periods suggested — not because they were intrinsically faint, but because they were enormously far away. His calculation placed Andromeda at roughly 900,000 light-years from Earth (the modern measurement is about 2.5 million light-years). This was vastly beyond the size of the Milky Way. Andromeda was not a gas cloud inside our galaxy. It was a separate galaxy in its own right — an island universe, comparable in scale to our own Milky Way.

Subsequent observations confirmed that the universe contains not just two galaxies but hundreds of billions of them, distributed across observable space at distances ranging from hundreds of thousands to billions of light-years.

Why This Matters for Expansion

Once astronomers understood that galaxies were real, distinct structures spread across enormous distances, a new question became possible to ask: Are they moving? And if so, how? To answer that question, they turned to spectra.

Practice 22.1Concept Check
  1. What is a Cepheid variable star, and why is the period-luminosity relationship useful for measuring distances?
  2. Before Hubble's work, what were the two competing explanations for what spiral nebulae were? Which did Hubble's measurements support?
  3. Why was the discovery that Andromeda is a separate galaxy important for the study of the expanding universe?
Section 22.2

Galaxy Distances and Redshifts

Reading Motion from Light

Recall from Chapter 7 that the Doppler effect shifts the wavelengths of light emitted by a moving source. When a source moves toward an observer, its light is compressed to shorter wavelengths — shifted toward the blue end of the spectrum, called blueshift. When a source moves away, its light is stretched to longer wavelengths — shifted toward the red end of the spectrum, called redshift.

By examining the absorption lines in a galaxy's spectrum and comparing their positions to known laboratory wavelengths, astronomers can measure how much the light has been shifted. This gives them a recession velocity — the speed at which the galaxy appears to be moving away.

Spectral ShiftDirection of MotionWhat Astronomers Observe
RedshiftSource moving away from observerAbsorption lines shifted to longer wavelengths
BlueshiftSource moving toward observerAbsorption lines shifted to shorter wavelengths
No shiftNo motion along line of sightLines at laboratory (rest) wavelengths

What Early Measurements Showed

In the early 1910s and 1920s, astronomer Vesto Slipher at Lowell Observatory systematically measured the spectra of spiral nebulae. The results were puzzling: the vast majority showed redshifts, not blueshifts. Of the dozens of nebulae Slipher measured, roughly 90 percent were receding.

This was not what a static universe would predict. If galaxies were simply moving in random directions through space, you would expect roughly equal numbers of redshifts and blueshifts. Something more systematic was happening.

A Few Exceptions

A few nearby objects — including the Andromeda Galaxy — showed blueshift, indicating approach. These exceptions are real: nearby galaxies can have individual motions that override the general expansion pattern. Hubble's Law applies most reliably to distant galaxies. The more distant a galaxy, the more the expansion signal dominates over its individual motion.

Practice 22.2Spectra and Recession
  1. A galaxy's calcium absorption line is normally at 393 nm in the laboratory. You observe it at 412 nm. Is this a redshift or a blueshift? What does it tell you about the galaxy's motion?
  2. Why was the fact that roughly 90 percent of spiral nebulae showed redshift surprising if the universe were static?
  3. Explain in your own words why the Andromeda Galaxy shows blueshift even though the universe is expanding.
Section 22.3

Hubble's Discovery

A Simple and Astonishing Pattern

Once Hubble established that the spiral nebulae were distant galaxies, and once Cepheid distances gave him reliable measurements for some of them, he was in a position to look for a pattern. Working with his colleague Milton Humason, Hubble compiled a dataset of galaxy distances and recession velocities.

He plotted recession velocity on the vertical axis against distance on the horizontal axis. The result, published in 1929, was a clear, positive relationship: the more distant a galaxy, the faster it was receding.

A galaxy twice as far away receded at roughly twice the speed. A galaxy ten times farther away receded at roughly ten times the speed. The recession velocity appeared to be proportional to distance.

The Core Finding

Recession velocity is proportional to distance. This relationship — now called Hubble's Law — was not predicted by any existing model of the universe. It was not what astronomers expected. It changed cosmology forever.

What This Pattern Means

At first, some astronomers tried to interpret the redshifts as ordinary Doppler motion — galaxies actually flying away through static space. But there was a problem. If galaxies were flying outward from some central point, then the Milky Way would have to occupy a special location at or near the center. There would be a preferred center of the universe.

This is deeply uncomfortable for scientific cosmology, which generally assumes that our location in the universe is not special. This assumption is called the cosmological principle: the universe looks roughly the same from any location, and no observer occupies a privileged position.

The resolution came from a different way of thinking about the data. Rather than interpreting the recession as galaxies moving through space, astronomers recognized that the pattern was exactly what you would expect if space itself is expanding.

Practice 22.3Hubble's Discovery
  1. Describe what Hubble's 1929 graph showed. What was on each axis, and what was the pattern?
  2. A student says: "Because all galaxies are moving away from us, the Milky Way must be at the center of the universe." Explain the error in this reasoning using the cosmological principle.
  3. Why did the proportional relationship between recession velocity and distance suggest something deeper than random galaxy motions?
Sections 22.4 – 22.7
Hubble's Law, Expanding Space, and Cosmic Time
Hubble's Law · Space Expansion · Observable Universe · Lookback Time
Section 22.4

Hubble's Law at an Introductory Level

The Mathematical Relationship

Hubble's observation can be expressed as a simple equation:

Hubble's Law

v = H₀ × d

v = recession velocity of a galaxy (km/s)

d = distance to the galaxy (megaparsecs; 1 Mpc ≈ 3.26 million light-years)

H₀ = the Hubble constant (~67–73 km/s per megaparsec)

The Hubble constant describes the current rate of expansion: for every additional megaparsec of distance, a galaxy recedes approximately 70 km/s faster.

DistanceApproximate Recession Velocity (H₀ = 70)
10 Mpc700 km/s
100 Mpc7,000 km/s
1,000 Mpc70,000 km/s (~23% speed of light)

Using Hubble's Law as a Distance Indicator

Hubble's Law has a practical use: if you can measure a galaxy's redshift (which is relatively straightforward with a spectrum), you can estimate its distance. This is how astronomers measure the distances to very remote galaxies that are too far away for Cepheid variables or other direct methods.

Worked Example 22.4.1
Using Hubble's Law to find distance
  • ProblemA galaxy's spectrum shows its hydrogen absorption lines are shifted to longer wavelengths by 3 percent. This corresponds to a recession velocity of about 9,000 km/s. Using H₀ = 70 km/s/Mpc, find the galaxy's distance.
  • ThinkRearrange Hubble's Law to solve for distance: d = v / H₀.
  • Solve d = v / H₀ d = 9,000 km/s ÷ 70 km/s/Mpc d ≈ 129 Mpc
  • CheckConvert: 129 Mpc × 3.26 million ly/Mpc ≈ 420 million light-years. A large distance — reasonable for a galaxy bright enough to observe its spectrum. ✓
Answer: The galaxy is approximately 129 megaparsecs (about 420 million light-years) away.
Worked Example 22.4.2
Using Hubble's Law to find recession velocity
  • ProblemA galaxy cluster is measured at 500 Mpc. Using H₀ = 70 km/s/Mpc, what is its recession velocity?
  • ThinkApply Hubble's Law directly: v = H₀ × d.
  • Solve v = H₀ × d v = 70 × 500 v = 35,000 km/s
  • Check35,000 km/s is roughly 12% of the speed of light (300,000 km/s). Plausible for a moderately distant galaxy cluster. ✓
Answer: 35,000 km/s, approximately 12 percent of the speed of light.
The Hubble Tension

The precise value of H₀ is actively debated. Different measurement methods — using Cepheids and supernovae in the nearby universe, or using the cosmic microwave background from the early universe — yield values that disagree slightly but persistently. This discrepancy, called the Hubble tension, is one of the open problems of modern cosmology. The values cluster between roughly 67 and 73 km/s/Mpc.

Practice 22.4Hubble's Law Calculations
  1. Using H₀ = 70 km/s/Mpc, find the recession velocity of a galaxy 300 Mpc away.
  2. A galaxy has a recession velocity of 14,000 km/s. Using H₀ = 70 km/s/Mpc, find its distance in megaparsecs.
  3. Galaxy A is 50 Mpc away. Galaxy B is 200 Mpc away. What are their recession velocities? What is the ratio of their recession velocities? What does this ratio tell you about Hubble's Law?
  4. The Andromeda Galaxy is about 0.78 Mpc away and is approaching us at about 110 km/s. What recession velocity does Hubble's Law predict at this distance? Why doesn't Andromeda follow Hubble's Law?
Section 22.5

Expansion of Space

Rethinking What the Recession Means

Hubble's Law describes a precise pattern: recession velocity is proportional to distance. But what does this pattern actually mean physically?

The answer requires a shift in thinking. The recession of galaxies is not primarily galaxies moving through space the way a thrown ball moves through air. It is the expansion of space itself — the fabric of the universe is stretching, and galaxies are being carried apart like raisins in a rising loaf of bread.

The Raisin Bread Analogy

Imagine raisins distributed through bread dough before baking. As the bread rises, the dough expands in all directions. From any single raisin, every other raisin moves away — and raisins that were farther away to begin with recede faster, because there is more expanding dough between them.

This is exactly the pattern Hubble observed. No raisin is the center. Every raisin sees the same pattern. Similarly, no galaxy is the center of the expanding universe.

Cosmological Redshift

This reframing also changes how we interpret the redshift of distant galaxies. The light from a remote galaxy is not Doppler-shifted because the galaxy is flying away through space. It is stretched because the universe was smaller when the light was emitted and has expanded while the light was traveling. By the time the light reaches us, the wavelengths have been stretched — redshifted — in proportion to how much the universe has expanded since the light departed.

This is called cosmological redshift, and it is subtly different from ordinary Doppler redshift. For very distant galaxies — at high redshifts — the distinction becomes important for calculating distances accurately.

Recession Faster Than Light?

Because expansion is the stretching of space — not motion through space — recession velocities can in principle exceed the speed of light for very distant objects. This does not violate the rule that nothing can move through space faster than light. Space itself is not a physical object moving somewhere; it is the arena in which motion occurs, and that arena can expand at any rate. Objects cannot move through space faster than light; space itself has no such restriction.

Practice 22.5Expansion of Space
  1. Describe the raisin bread analogy in your own words. What does the dough represent? What do the raisins represent?
  2. Using the raisin bread analogy, explain why there is no single center of expansion.
  3. What is cosmological redshift? How is it different from ordinary Doppler redshift?
  4. A distant quasar has a recession velocity of 180,000 km/s — 60 percent of the speed of light. Does this violate the rule that nothing can travel faster than light? Explain your reasoning.
Section 22.6

The Observable Universe

Why We Cannot See Everything

If the universe is expanding, and if light takes time to travel, then there is a fundamental limit to how far we can observe. We can only see objects from which light has had enough time to reach us since the universe began.

The universe is approximately 13.8 billion years old. Naively, you might think this means we can see out to 13.8 billion light-years in every direction. But because the universe has been expanding while that light was traveling, the actual radius of the observable universe is larger. The current estimate is roughly 46 billion light-years.

Why 46 Billion, Not 13.8 Billion?
Resolving the apparent paradox
  • PuzzleThe universe is 13.8 billion years old, but we can see objects 46 billion light-years away. How is this possible?
  • AnswerThe light from very distant objects left those objects when the universe was much smaller and when those objects were much closer to us. While the light traveled, the universe expanded. The objects were carried to their current positions by expansion. We are seeing them as they were long ago, but they are now much farther away.
  • ImplicationDistance in an expanding universe requires careful definition. The 46-billion-light-year radius is the current distance to objects whose ancient light is just now arriving.

Beyond the Observable Universe

The observable universe is not the same as the entire universe. There may be — almost certainly are — regions of space beyond our observable horizon, but light from those regions has not yet had time to reach us. We have no direct information about them. Cosmologists believe the universe extends far beyond the observable horizon, and may be infinite. We simply cannot know, because we are limited to what light can carry to us.

Practice 22.6The Observable Universe
  1. What is the approximate radius of the observable universe? Why is this larger than 13.8 billion light-years?
  2. Is the edge of the observable universe a physical wall or boundary? What is it?
  3. Express the radius of the observable universe (46 billion light-years) in meters using scientific notation. Use 1 light-year ≈ 9.46 × 10¹⁵ m.
  4. Does the existence of an observable universe horizon mean the universe itself is finite? Explain.
Section 22.7

Lookback Time and Cosmic History

Seeing the Past

Every time you observe a distant object, you are looking back in time. The light from an object 1 million light-years away left that object 1 million years ago. The light from an object 1 billion light-years away left 1 billion years ago. The light from the most distant detectable galaxies left when the universe was only a few hundred million years old.

This is both a limitation and an extraordinary opportunity. Astronomers cannot watch a single galaxy evolve over billions of years. But they can observe many galaxies at many different distances, and because distance corresponds to lookback time, they can effectively watch cosmic history unfold by studying galaxies across a range of redshifts.

Redshift RangeApproximate Lookback TimeWhat We See
z < 0.5 (nearby)Less than ~5 billion years agoSettled galaxies; organized spiral and elliptical structures; lower star formation rates
z ≈ 1–3 (moderate)Roughly 6–11 billion years agoPeak era of star formation; vigorous galaxy growth and interaction
z ≈ 6–10 (distant)Roughly 12–13 billion years agoEarliest galaxies; smaller, less structured; actively forming stars at high rates
Telescopes as Time Machines

The James Webb Space Telescope, launched in 2021, was designed specifically to observe galaxies at very high redshifts — seeing the universe as it was only a few hundred million years after the Big Bang. In this sense, larger, more powerful telescopes do not just see farther in space; they see farther back in time.

Practice 22.7Lookback Time and Galaxy Evolution
  1. A galaxy is 500 million light-years away. What is the lookback time to this galaxy? What does that mean in practical terms?
  2. Why do distant galaxies look systematically different from nearby galaxies? What does this tell us about the universe?
  3. Explain why more powerful telescopes do not just see farther in space but also farther back in time.
  4. At redshift z ≈ 2, astronomers observe galaxies with very high star-formation rates — much higher than in nearby galaxies. What does this suggest about when most of the stars in the universe were formed?
Sections 22.8 – 22.9
Misconceptions, Evidence, and the Robustness of the Model
What Expansion Does and Does Not Mean · Five Lines of Evidence
Section 22.8

What Expansion Does and Does Not Mean

The expansion of the universe is one of the most frequently misunderstood concepts in all of science. Several important clarifications deserve careful attention.

Misconception 1 — The Big Bang was an explosion into empty space
"The Big Bang was a giant explosion at a specific point in space, with matter flying outward into a pre-existing void."

Correction: The Big Bang was the beginning of space itself. The entire early universe — not just a central region — was hot and dense. There was no pre-existing empty space for it to expand into. The universe expanded from a hot, dense state everywhere simultaneously. (Chapter 23 develops this further.)

Misconception 2 — Galaxies are flying through space like shrapnel
"Galaxies are actually rocketing outward through space from a central point."

Correction: The recession of galaxies is primarily the stretching of space itself, not individual galaxies moving through static space. This distinction matters especially at large distances, where recession velocities can exceed the speed of light.

Misconception 3 — Expansion means everything is getting bigger
"Since the universe is expanding, atoms, planets, and galaxies are also gradually growing larger."

Correction: Ordinary matter — atoms, planets, stars, and galaxies — is held together by forces much stronger than the large-scale expansion. Gravity, electromagnetism, and nuclear forces keep structures intact. Only on the largest scales, across the voids between galaxy clusters, does expansion dominate. You are not expanding.

Misconception 4 — The Milky Way is at the center
"Because all galaxies are moving away from us, the Milky Way must be at the center of the universe."

Correction: As the raisin bread analogy shows, every raisin (galaxy) sees all others receding. No location is the center. The Milky Way is not special; observers in any galaxy would see the same pattern.

Misconception 5 — There is an edge we could reach
"The observable universe horizon is a physical edge or boundary you could fly toward."

Correction: The cosmic horizon is not a physical boundary. It is simply the limit of what light has had time to bring us. There is very likely more universe beyond the horizon; we just cannot observe it. There is no physical wall at the horizon.

Practice 22.8Correcting Misconceptions

For each misconception, write a corrected explanation in your own words.

  1. A student says: "If the universe started with the Big Bang, there must be a center where it exploded from." How would you respond?
  2. A student says: "If space is expanding, our solar system must be getting slightly bigger every year." Is this correct? Explain why or why not.
  3. A student says: "Nothing can travel faster than light, but recession velocities can exceed the speed of light. That's a contradiction." Resolve this apparent contradiction.
  4. A student says: "The fact that we can see 46 billion light-years away proves the universe is at least 46 billion years old." Explain what is wrong with this reasoning.
Section 22.9

Evidence for an Expanding Universe

The conclusion that the universe is expanding rests not on a single observation but on multiple independent lines of evidence that converge on the same picture.

#Line of EvidenceWhat It Shows
1Galaxy redshifts and Hubble's LawRecession velocity proportional to distance; confirmed across hundreds of thousands of galaxies spanning billions of light-years
2Cosmic microwave background (CMB)Faint microwave radiation pervading all of space — the cooled remnant of the hot early universe; precisely predicted by the expanding universe model
3Abundance of light elementsThe expanding universe model predicts specific ratios of hydrogen, helium, and lithium formed in the early universe; measured abundances match closely
4Ages of oldest starsThe Hubble constant gives an estimated age for the universe (~13.8 billion years); independent measurements of oldest stars yield consistent ages
5Galaxy evolution with redshiftDistant (high-redshift) galaxies look systematically younger and less evolved than nearby galaxies — exactly as expansion and cosmic aging predict
A Robust and Well-Tested Model

No single piece of evidence would be sufficient to establish the expanding universe. But five or more independent lines of evidence — from different measurement techniques and different physical phenomena — all pointing to the same conclusion: that is the hallmark of a robust scientific model.

The expanding universe is not a speculation. It is one of the best-supported conclusions in all of science.

Practice 22.9Evidence and Reasoning
  1. Name five independent lines of evidence that support the expanding universe model. For each, give a one-sentence explanation of what it shows.
  2. A student argues: "Redshift could be caused by something other than expansion — maybe light just loses energy as it travels." What evidence would help evaluate this alternative? What observations might distinguish between 'tired light' and genuine expansion?
  3. Hubble's original 1929 dataset contained significant errors in the distances he used. Yet his conclusion has held up remarkably well. What does this tell you about scientific discoveries built on imperfect data?
  4. Why is it significant that five different measurement methods using different physical phenomena all point to the same conclusion about the expanding universe?
Chapter 22 — Back Matter

Chapter Summary

Key Terms Review

Hubble's Law
The observational relationship stating that recession velocity is proportional to distance: v = H₀ × d.
Hubble constant (H₀)
The proportionality constant in Hubble's Law, expressing the current rate of expansion. Approximately 67–73 km/s per megaparsec.
recession velocity
The speed at which a distant galaxy is moving away from us, inferred from the redshift of its light.
cosmological redshift
The stretching of light to longer wavelengths caused by the expansion of space between the emitting object and the observer; distinct from ordinary Doppler redshift.
Cepheid variable star
A star that pulsates in brightness with a regular period related to its intrinsic luminosity, making Cepheids useful as standard candles for measuring distances.
period-luminosity relationship
Henrietta Swan Leavitt's discovery that the pulsation period of a Cepheid variable is directly related to its intrinsic luminosity.
observable universe
The spherical region from which light has had time to reach Earth since the Big Bang; radius approximately 46 billion light-years.
cosmic horizon
The boundary of the observable universe; not a physical edge, but the limit of what light has had time to bring us.
lookback time
The time in the past at which the light we are observing left its source; equal to the travel time of the light.
cosmological principle
The assumption that the universe is roughly uniform and that no location is special or privileged.
Hubble tension
The discrepancy between different methods of measuring the Hubble constant; an active area of research in modern cosmology.
cosmic microwave background
The faint microwave radiation pervading all of space — the cooled remnant of the hot early universe; key evidence for the Big Bang model.

Diagram Review

Diagram 22-A
Hubble's Original 1929 Data

Study a graph with distance on the horizontal axis (megaparsecs) and recession velocity on the vertical axis (km/s). Note that the points show scatter but follow a clear upward trend. The best-fit line has a positive slope — this slope is the Hubble constant. For each additional megaparsec of distance, recession velocity increases by approximately 70 km/s. Practice sketching the best-fit line and reading the slope.

Diagram 22-B
The Raisin Bread Model of Expansion

Two diagrams side by side: the loaf before rising and after rising. Raisins are labeled A, B, C, D. Arrows show how each raisin has moved. From raisin A's perspective, all others have receded; raisins twice as far away have receded twice as much. Note that no raisin is the center — every raisin sees the same pattern. This illustrates why there is no center to the expanding universe.

Diagram 22-C
Lookback Time and Galaxy Evolution

A timeline from the Big Bang (left) to the present (right), with markers at 1 billion, 5 billion, and 13 billion years. At each marker, a representative galaxy shows how structure evolves: early galaxies are irregular and compact; later galaxies are larger and more organized. Arrows indicate that observing at greater distances corresponds to observing earlier cosmic times.

Diagram 22-D
The Observable Universe Horizon

A circle centered on the Milky Way labeled "Observable Universe Horizon (~46 billion light-years)." Inside: example galaxies at 1 billion, 5 billion, 13 billion, and 40 billion light-years with approximate lookback times. Outside the circle: "Light from here has not yet reached us." A note clarifies that this is not a physical wall.

Scale Review

  1. Practice 22-1. A galaxy has a recession velocity of 14,000 km/s. Using H₀ = 70 km/s/Mpc, calculate its distance in megaparsecs and in light-years (1 Mpc ≈ 3.26 million ly).
  2. Practice 22-2. Galaxy A is 50 Mpc away. Galaxy B is 200 Mpc away. Using Hubble's Law (H₀ = 70), what are their recession velocities? What is the ratio of their recession velocities?
  3. Practice 22-3. The Andromeda Galaxy is about 0.78 Mpc away and is approaching us at about 110 km/s. What recession velocity does Hubble's Law predict for a galaxy at this distance? Why does Andromeda not follow Hubble's Law? What does this tell us about Hubble's Law and nearby galaxies?
  4. Practice 22-4. If the observable universe has a radius of 46 billion light-years, what is its diameter? Express your answer in light-years and in meters. Use 1 light-year ≈ 9.46 × 10¹⁵ m and scientific notation.

Concept Check

  1. In your own words, state Hubble's Law. What two quantities does it relate?
  2. A student says: "The Milky Way must be at the center of the universe because all galaxies are moving away from us." Explain the error in this reasoning.
  3. What is the difference between saying "galaxies are flying outward through space" and saying "space itself is expanding"? Why does the distinction matter?
  4. Why does looking at a more distant galaxy also mean looking further back in time?
  5. The light from a galaxy has its hydrogen absorption lines shifted to wavelengths 8 percent longer than in a laboratory. Is this a redshift or blueshift? What does it tell you about the galaxy's motion?

Practice Questions

Section AVocabulary in Context
  1. Define recession velocity and explain how it is measured from a galaxy's spectrum.
  2. What is the Hubble constant, and what are its units? Why do its units (km/s/Mpc) seem unusual?
  3. Explain in your own words what cosmological redshift is and how it differs from ordinary Doppler redshift.
  4. What is lookback time? If a galaxy is 500 million light-years away, what is the lookback time to that galaxy?
  5. Define the cosmological principle and explain why it implies there is no center to the expanding universe.
Section BCause-and-Effect Reasoning
  1. Why did the pattern of galaxy recession velocities suggest something different from random motion through space?
  2. Why does Hubble's Law predict that a galaxy twice as far away should be receding twice as fast?
  3. Galaxy A is at 200 Mpc with a recession velocity of 15,000 km/s. Galaxy B is at 400 Mpc with a recession velocity of 29,800 km/s. Is this consistent with Hubble's Law? Show your calculation.
  4. If the universe stopped expanding today, would we still observe redshifts from distant galaxies? Explain why or why not.
  5. Why does expansion not cause planets, stars, or galaxies to grow larger over time?
Section CScale and Calculation
  1. Using H₀ = 70 km/s/Mpc: (a) What is the recession velocity of a galaxy 300 Mpc away? (b) What is the recession velocity of a galaxy 1,500 Mpc away? (c) What is the distance to a galaxy receding at 21,000 km/s?
  2. A distant quasar has a recession velocity of 180,000 km/s (60 percent of the speed of light). Using H₀ = 70, estimate its distance in megaparsecs. Why should this answer be treated as an approximation?
  3. A galaxy at redshift z = 0.05 has its light stretched by 5 percent. If the rest wavelength of a hydrogen line is 656 nm, what is the observed wavelength in this galaxy's spectrum?
Section DEvidence and Reasoning
  1. List three independent lines of evidence that support the expanding universe model. For each, briefly explain how it supports expansion.
  2. Vesto Slipher found in the 1910s that most spiral nebulae were redshifted, but at the time, many astronomers were not sure whether these nebulae were inside the Milky Way or outside it. How did Hubble's discovery in the 1920s change the interpretation of Slipher's data?
  3. A student argues: "Redshift could be caused by something other than expansion — maybe light just gets tired and loses energy as it travels." What evidence would you use to evaluate this alternative? What observations might distinguish between tired light and genuine expansion?
  4. Hubble's original 1929 dataset contained significant errors in the distances he used. Yet his conclusion has held up remarkably well. What does this tell you about the nature of scientific discoveries built on imperfect data?

Graph and Data Questions

Data Set 22-D1
Interpreting a Hubble Diagram
  • SetupA Hubble diagram shows 24 galaxies. Horizontal axis: distance in megaparsecs (0 to 800). Vertical axis: recession velocity in km/s (0 to 60,000).
  1. Sketch the approximate best-fit line through the data.
  2. Using two points on your best-fit line, estimate the slope. What is the Hubble constant implied by this data?
  3. Two galaxies are plotted at similar distances but have noticeably different recession velocities. What might explain this scatter?
  4. Galaxy X is plotted at 600 Mpc with a recession velocity of 45,000 km/s. Is this consistent with Hubble's Law? Show your calculation.
Data Set 22-D2
Galaxy Spectra and Redshift
  • SetupThree spectra are shown. In the laboratory, a hydrogen absorption line appears at 486 nm. Spectrum A: line at 486 nm. Spectrum B: line at 510 nm. Spectrum C: line at 559 nm.
  1. Calculate the percent shift for Spectra B and C.
  2. Which galaxy is farther from us? How do you know?
  3. Which galaxy is receding faster? What is the approximate ratio of their recession velocities?
  4. Spectrum A shows no shift. What can you infer about this galaxy?

Error Analysis

Each statement contains an error. Identify it and write a corrected version.

Statement 1
"The universe is expanding from a central point where the Big Bang occurred."

What is the error? Write a correct description of what the Big Bang model says.

Statement 2
"Because galaxies are moving away from us, the Milky Way must be at the center of the universe."

What is the error? Use the raisin bread analogy to explain why this reasoning fails.

Statement 3
"Hubble's Law says that the closer a galaxy is, the faster it recedes."

What is the error? State Hubble's Law correctly.

Statement 4
"Since the universe is only 13.8 billion years old, the most distant objects we can observe are 13.8 billion light-years away."

What is the error? What is the actual radius of the observable universe and why is it larger?

Statement 5
"Cosmological redshift means galaxies are actually moving through space at high speed, just like a police siren moving away from you."

What is the distinction between cosmological redshift and Doppler redshift? Why does it matter?

Statement 6
"The expansion of the universe means that atoms, molecules, and planets are slowly growing larger over time."

Why is this incorrect? At what scale does expansion dominate, and why does it not affect bound objects?

Real-World and Historical Applications

  1. Henrietta Swan Leavitt's discovery of the period-luminosity relationship was essential to Hubble's measurement of galaxy distances. Leavitt worked as a "human computer" at the Harvard College Observatory, a role often assigned to women who were paid less than the men who used their results. Research the Harvard Computers and write a paragraph connecting Leavitt's scientific contribution to the larger history of who does astronomy and who gets credit.
  2. Edwin Hubble used the Hooker Telescope at Mount Wilson Observatory. The site was chosen partly for its dark skies above Los Angeles. Today, light pollution from the metropolitan area makes Mount Wilson less productive for optical astronomy. What trade-offs do modern observatories face when choosing sites? What solutions have astronomers developed?
  3. The Hubble Space Telescope, launched in 1990, had a flawed primary mirror that required a servicing mission to correct. What does this episode illustrate about the relationship between scientific tools and scientific knowledge?
  4. The Hubble tension — the discrepancy between different measurements of H₀ — is one of the active frontiers of modern cosmology. Different teams using different methods get values that disagree slightly but persistently. What does this disagreement suggest about the current state of cosmological knowledge? Is a discrepancy between careful measurements always a problem, or can it sometimes be productive?

Mixed Review

The following questions draw on earlier chapters.

  1. (Chapter 7) A galaxy's calcium absorption line, normally at 393 nm, is observed at 412 nm. Is the galaxy approaching or receding? Calculate the recession velocity as a percentage of the speed of light.
  2. (Chapter 5) The observable universe has a radius of about 46 billion light-years. Express this in meters using scientific notation. (1 light-year ≈ 9.46 × 10¹⁵ m.)
  3. (Chapter 6) Why are radio telescopes useful for studying the most distant galaxies, even though those galaxies originally emitted much of their light in the ultraviolet? What happens to the wavelength of that light by the time it reaches us?
  4. (Chapter 16) Why do astronomers studying very high-redshift galaxies observe large numbers of hot, blue, massive stars that are rare in nearby galaxies? What does this suggest about the history of star formation in the universe?
  5. (Chapter 18) Quasars — active galactic nuclei powered by supermassive black holes — are found almost exclusively at high redshifts. What does this tell us about the history of quasar activity in the universe?

Chapter 22 Checkpoint

Chapter 22 Checkpoint
Complete without assistance — assesses all key skills and concepts from Chapter 22
Section 1 — Vocabulary and Concepts
  1. State Hubble's Law in words and in equation form. Define each variable.
  2. What is the cosmological principle, and what does it imply about the center of the universe?
  3. Explain the difference between cosmological redshift and ordinary Doppler redshift.
Section 2 — Calculation Skills
  1. Using H₀ = 70 km/s/Mpc, find the recession velocity of a galaxy 450 Mpc away.
  2. A galaxy has a recession velocity of 56,000 km/s. Using H₀ = 70, find its distance in megaparsecs.
  3. A hydrogen line at rest wavelength 486 nm is observed at 510 nm. Calculate the percent redshift. Is this galaxy approaching or receding?
Section 3 — Reasoning and Explanation
  1. Explain the raisin bread analogy. What does it show about the center of expansion?
  2. Why is the radius of the observable universe (~46 billion light-years) larger than the age of the universe in light-years (~13.8 billion)?
  3. A student says the universe is expanding because galaxies are flying outward through static space from a central explosion. Identify two errors in this statement and correct them.
Section 4 — Evidence
  1. Name five independent lines of evidence supporting the expanding universe. For each, write one sentence explaining what it shows.
  2. Why is it more convincing to have five independent lines of evidence than one very strong piece of evidence? What does convergence of evidence tell us about the model's reliability?
Section 5 — Error Analysis
  1. A student says: "The universe is expanding from a specific point in space where the Big Bang occurred." Identify the error and write a correct description.
  2. A student evaluates the size of the observable universe by multiplying the speed of light by the age of the universe (13.8 billion years) and concludes it is 13.8 billion light-years in radius. What is wrong with this calculation?
Looking Ahead
Chapter 23: The Big Bang, Cosmic History, and the Cosmic Microwave Background

Chapter 23 develops the full picture that expansion implies. If galaxies are receding and space is growing larger today, then in the past, galaxies were closer together and space was denser. Carried far enough back, the universe was once an almost unimaginably hot and dense state from which it has been expanding and cooling ever since.

Chapter 23 will explain what the Big Bang model actually says — and what it does not say. Students will learn about conditions in the early universe, the formation of the first atomic nuclei, the release of light in recombination, the discovery of the cosmic microwave background, the formation of the first stars and galaxies, and the cosmic timeline. The chapter will walk through this history with careful attention to evidence and appropriate scientific humility.

MGU Library Connection
  • Expanding Universe Guide — Recommended Companion Resources for this textbook
  • Chapter 7: Spectra, Color, Temperature, and Chemical Fingerprints — Doppler shift and redshift foundations
  • Chapter 21: Galaxy Clusters, Dark Matter, and Large-Scale Structure — large-scale structure of the universe
  • Chapter 23: The Big Bang, Cosmic History, and the Cosmic Microwave Background — follows directly from this chapter
  • Chapter 24: Dark Energy, the Fate of the Universe, and Open Questions — expansion continues to accelerate
  • Appendix B: Astronomy Units and Constants Reference — megaparsec, light-year, speed of light
  • MGU Dictionary entries: Hubble's Law, cosmological redshift, observable universe, lookback time, cosmological principle