Astronomy Quiz: Hubbles Law
20 questions · exam conditions
0:00
Hubbles LawQuestion 1 of 20

If a future, more precise measurement of the Hubble constant revealed its value to be 10% smaller than the current accepted value, what would be the primary implication for our understanding of the universe's age, assuming a constant rate of expansion?

The calculated age of the universe would be about 10% younger.
The calculated age would be unchanged, but the average density of the universe would be higher.
The calculated age would be unchanged, but the size of the observable universe would be smaller.
The calculated age of the universe would be about 10% older.
← Back to quizzes

Astronomy Quiz

Astronomy Quiz: Hubbles Law

Practice Hubbles Law in Astronomy with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Hubbles Law, giving you a quick way to practice the rules, question types, and explanations that matter most for Astronomy.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

If a future, more precise measurement of the Hubble constant revealed its value to be 10% smaller than the current accepted value, what would be the primary implication for our understanding of the universe's age, assuming a constant rate of expansion?

  1. The calculated age of the universe would be about 10% younger.
  2. The calculated age would be unchanged, but the average density of the universe would be higher.
  3. The calculated age would be unchanged, but the size of the observable universe would be smaller.
  4. The calculated age of the universe would be about 10% older. (correct answer)
Explanation: When you encounter questions about the Hubble constant and universe age, remember that these quantities have an inverse relationship. The Hubble constant (H₀) represents how fast the universe is expanding right now, and the age of the universe depends on how long it took to reach this current state. If we assume constant expansion, the universe's age is approximately t=1H0t = \frac{1}{H_0}. This means when the Hubble constant decreases, the calculated age increases proportionally. If H₀ becomes 10% smaller, you're dividing by a smaller number, which gives you a larger result—about 10% older. Here's why the other answers miss the mark: Answer A gets the relationship backwards, suggesting the universe would be younger when the Hubble constant decreases. This reflects a common misconception about the inverse relationship. Answer B incorrectly assumes the age calculation wouldn't change—but since age directly depends on H₀, any change in the Hubble constant necessarily affects the calculated age. The density might change too, but that's not the primary implication asked about. Answer C also wrongly claims the age would be unchanged and focuses on observable universe size, which isn't the main consequence of revising H₀. The correct answer is D: a 10% smaller Hubble constant means the universe has been expanding more slowly than we thought, so it took longer to reach its current size—making it about 10% older. Study tip: Remember the inverse relationship: smaller Hubble constant = older universe. Think of it as a slower expansion requiring more time to reach the same endpoint.

Question 2

A student correctly states that the space within our solar system is not expanding, nor is the space between stars within the Milky Way. Why does the cosmic expansion described by Hubble's law not apply on these smaller scales?

  1. Dark energy, which drives cosmic expansion, does not exist within galaxies.
  2. The Hubble constant is zero for distances less than one megaparsec and only becomes positive at larger distances.
  3. The expansion of the universe only began after galaxies had already formed and become stable.
  4. The force of gravity on local scales is vastly stronger than the 'stretching' effect of cosmic expansion. (correct answer)
Explanation: When you encounter questions about cosmic expansion, remember that Hubble's law describes the large-scale behavior of the universe, but local physics can override this global trend. The key insight is that cosmic expansion represents a very gentle "stretching" of space itself. This effect is incredibly weak compared to the fundamental forces that hold matter together. On local scales - within solar systems, around individual stars, and even within entire galaxies - gravity is enormously stronger than the expansion effect. Think of it like trying to stretch a rubber band that has several tight knots in it: the overall band might expand, but the knotted sections remain intact because the local binding forces are much stronger than the stretching force. Let's examine why the other options miss the mark. Choice A incorrectly suggests dark energy is absent from galaxies - dark energy exists everywhere but simply can't overcome local gravitational binding. Choice B misrepresents how the Hubble constant works; it's not zero at small distances but rather becomes irrelevant when other forces dominate. Choice C presents a false timeline - cosmic expansion began much earlier than galaxy formation and continues today. The correct answer is D because gravity's strength on local scales vastly exceeds the weak "stretching" effect of cosmic expansion. Stars remain bound in galaxies, planets stay in orbit, and even galaxy clusters can remain gravitationally bound despite the universe's overall expansion. Remember this hierarchy: local gravitational forces typically dominate over cosmic expansion until you reach the scale of galaxy clusters and beyond, where the cumulative expansion effect finally becomes significant.

Question 3

Imagine you are an observer in Galaxy X. You observe that Galaxy Y, which is 50 Megaparsecs (Mpc) away, is receding from you at 3,500 km/s. In the opposite direction, you see Galaxy Z, also 50 Mpc away, receding at 3,500 km/s. What would an observer in Galaxy Y measure for the velocities of Galaxy X and Galaxy Z?

  1. Galaxy X recedes at 3,500 km/s, and Galaxy Z recedes at 3,500 km/s.
  2. Galaxy X recedes at 3,500 km/s, and Galaxy Z recedes at 7,000 km/s. (correct answer)
  3. Galaxy X approaches at 3,500 km/s, and Galaxy Z recedes at 3,500 km/s.
  4. Galaxy X is stationary, and Galaxy Z recedes at 7,000 km/s.
Explanation: This question addresses the cosmological principle that there is no center to the expansion. An observer in Galaxy Y would see Galaxy X receding at 3,500 km/s, consistent with the initial observation. Since Galaxy Z is in the opposite direction from X and is twice as far from Y (50 Mpc + 50 Mpc = 100 Mpc), its recessional velocity relative to Y would be twice that of X, or 7,000 km/s, according to Hubble's Law (v ∝ d).

Question 4

A hydrogen absorption line with a rest wavelength of 121.6 nm is observed in the spectrum of Galaxy A at 145.92 nm. The same line is observed in the spectrum of Galaxy B at 182.4 nm. What can be inferred about the relative recessional velocities of these galaxies, assuming they are low enough for v≈czv \approx cz to be accurate?

  1. The velocity of Galaxy B is approximately 2.5 times the velocity of Galaxy A. (correct answer)
  2. The velocity of Galaxy B is approximately 4 times the velocity of Galaxy A.
  3. The velocity of Galaxy B is approximately 1.25 times the velocity of Galaxy A.
  4. The velocity of Galaxy A and Galaxy B are nearly identical.
Explanation: First, calculate the redshift (z) for each galaxy using z=(λobs−λrest)/λrestz = (\lambda_{\text{obs}} - \lambda_{\text{rest}})/\lambda_{\text{rest}}. For Galaxy A: zA=(145.92−121.6)/121.6=24.32/121.6=0.2z_A = (145.92 - 121.6) / 121.6 = 24.32 / 121.6 = 0.2. For Galaxy B: zB=(182.4−121.6)/121.6=60.8/121.6=0.5z_B = (182.4 - 121.6) / 121.6 = 60.8 / 121.6 = 0.5. Since velocity is proportional to redshift (v≈czv \approx cz for low z), the ratio of their velocities is the ratio of their redshifts: vB/vA=zB/zA=0.5/0.2=2.5v_B / v_A = z_B / z_A = 0.5 / 0.2 = 2.5.

Question 5

An astronomer constructs a Hubble diagram (recessional velocity vs. distance) for a set of galaxies. If the universe's expansion has been accelerating over cosmic time (as is currently believed), how would the data points for the most distant galaxies deviate from the straight-line relationship defined by nearby galaxies?

  1. They would fall below the straight line. (correct answer)
  2. They would fall above the straight line.
  3. They would still fall perfectly on the straight line.
  4. They would form a random scatter with no clear trend.
Explanation: The straight line on a Hubble diagram represents the current expansion rate, H0H_0. For very distant galaxies, we are looking far back in time. If expansion is accelerating, it means the expansion rate in the past was slower than it is today. Thus, for a given distance (which corresponds to a certain lookback time), the recessional velocity would have been lower than predicted by the current, faster rate. This means the data points for distant objects fall below the line extrapolated from nearby objects.

Question 6

The fact that distant galaxies in every direction are moving away from us, with velocity proportional to distance, does not imply we are at the center of the universe. This observation can be explained by which fundamental concept in cosmology?

  1. The universe is isotropic but not homogeneous.
  2. The expansion of space is uniform, so every observer would see the same Hubble law. (correct answer)
  3. The anthropic principle, which suggests the universe must be as we observe it.
  4. The accelerating expansion driven by dark energy, which pushes all galaxies apart equally.
Explanation: The Cosmological Principle states that the universe is both homogeneous (the same everywhere on large scales) and isotropic (the same in all directions). A consequence of a uniform expansion in a homogeneous universe is that every observer, regardless of their location, will see all other distant galaxies receding from them. This is often explained with the raisin bread analogy, where every raisin sees every other raisin moving away as the dough expands.

Question 7

Hubble's observations showed a linear relationship between a galaxy's distance and its recessional velocity. This empirical law provides strong evidence for which conclusion about the past state of the universe?

  1. The universe was once much hotter and denser than it is today. (correct answer)
  2. The universe has always been expanding at a constant rate.
  3. The universe was static and unchanging for billions of years before expansion began.
  4. The laws of physics were fundamentally different in the distant past.
Explanation: If all distant galaxies are moving away from us, and the farther they are, the faster they are moving, then it implies that in the past, everything must have been much closer together. Extrapolating this expansion backward in time leads to a state of extremely high density and temperature, which is the fundamental concept of the Big Bang model. While the expansion rate isn't perfectly constant (distractor B), the overall trend supports a denser past.

Question 8

An observer measures the redshift of a distant quasar to be z=4z = 4. What does this imply about the wavelength of a specific spectral line (e.g., Lyman-alpha) from this quasar as observed on Earth?

  1. The observed wavelength is four times its rest wavelength.
  2. The universe was one-fourth of its current size when the light was emitted.
  3. The quasar is receding at four times the speed of light.
  4. The observed wavelength is five times its rest wavelength. (correct answer)
Explanation: When you encounter redshift problems in astronomy, you're dealing with how light from distant objects gets stretched as it travels through expanding space. The key relationship to remember is that redshift zz directly tells you how much longer the observed wavelength is compared to the original. The redshift formula is z=λobserved−λrestλrestz = \frac{\lambda_{observed} - \lambda_{rest}}{\lambda_{rest}}, which can be rearranged to λobserved=λrest(1+z)\lambda_{observed} = \lambda_{rest}(1 + z). With z=4z = 4, the observed wavelength equals λrest(1+4)=5λrest\lambda_{rest}(1 + 4) = 5\lambda_{rest}. So the Lyman-alpha line appears five times longer than its rest wavelength, making D correct. Let's examine why the other answers miss the mark. Choice A claims the wavelength is four times longer, but this ignores the "+1" in the formula—a common error where students confuse the redshift value with the wavelength ratio. Choice B discusses the universe's size when light was emitted, which relates to the scale factor relationship a=11+za = \frac{1}{1+z}, giving one-fifth current size, not one-fourth. Choice C suggests the quasar moves at four times light speed, but this misapplies the Doppler formula and violates relativity—cosmological redshift isn't simple Doppler motion anyway. Remember this pattern: for cosmological redshift, the observed wavelength is always (1+z)(1 + z) times the rest wavelength. Don't forget that crucial "+1"—it's the difference between getting redshift problems right or falling into the most common trap on astronomy exams.

Question 9

The 'tired light' hypothesis, now discredited, proposed that the redshift of distant galaxies was caused by photons losing energy on their long journey, not by cosmic expansion. If this hypothesis were true, what would be a key difference in our observations of the distant universe?

  1. Distant galaxies would appear blueshifted instead of redshifted.
  2. The cosmic microwave background radiation would not exist.
  3. Time dilation effects, such as the observed stretching of supernova light curves, would not be present. (correct answer)
  4. Hubble's law would still be observed, but the Hubble constant would be negative.
Explanation: In an expanding universe, distant events appear to be slowed down, a phenomenon known as cosmological time dilation. For example, the light curves of Type Ia supernovae at high redshift are observed to be stretched over a longer period. This is a direct consequence of cosmic expansion stretching not just the wavelength of light, but the duration between photon emissions. A 'tired light' model, where only energy is lost, would not predict this time dilation effect. This observation is strong evidence for expansion.

Question 10

Two separate model universes, Universe Alpha and Universe Beta, are assumed to have been expanding at a constant rate since their beginning. Cosmologists determine that the Hubble constant in Universe Alpha (HαH_{\alpha}) is double the value of the Hubble constant in Universe Beta (HβH_{\beta}). What is the most direct physical implication of this finding?

  1. Universe Alpha is approximately half as old as Universe Beta. (correct answer)
  2. Universe Alpha is approximately twice as old as Universe Beta.
  3. Universe Alpha must be significantly denser than Universe Beta.
  4. The observable universe is smaller in Universe Alpha than in Universe Beta.
Explanation: The age of the universe, in a simple model with a constant expansion rate, is inversely proportional to the Hubble constant (T≈1/H0T \approx 1/H_0). A larger Hubble constant implies a faster expansion rate, meaning it would have taken less time to reach the current state from the initial singularity. Therefore, if Hα=2HβH_{\alpha} = 2H_{\beta}, then Tα≈1/(2Hβ)≈Tβ/2T_{\alpha} \approx 1/(2H_{\beta}) \approx T_{\beta}/2. Universe Alpha is younger.

Question 11

An astronomer observes Galaxy P and Galaxy Q. The light from Galaxy P has a redshift of z=0.5z = 0.5, while the light from Galaxy Q has a redshift of z=1.0z = 1.0. Which statement is a necessary consequence of these observations based on the standard cosmological model?

  1. Galaxy Q is exactly twice as far away from us as Galaxy P.
  2. The universe was younger when the light from Galaxy Q was emitted than when the light from Galaxy P was emitted. (correct answer)
  3. Galaxy Q's recessional velocity is exactly twice the recessional velocity of Galaxy P.
  4. The observed spectrum of Galaxy Q is twice as bright as the observed spectrum of Galaxy P.
Explanation: Redshift is directly related to the expansion of the universe and therefore to lookback time. A higher redshift means the light has been traveling for longer, and was therefore emitted earlier in the universe's history when the universe was younger and smaller. Distractors A and C are incorrect because the relationship between redshift, distance, and velocity is not linear at high z values. Distractor D is incorrect because brightness depends on luminosity and distance, not just redshift.

Question 12

Galaxy A is at a distance dd and has a redshift zz. Galaxy B is observed to have a redshift of 4z4z. Assuming zz is small enough for the linear Hubble relation (v≈czv \approx cz) to hold, and that both galaxies lie along the same line of sight from Earth, what is the approximate distance from Galaxy A to Galaxy B from our perspective?

  1. 2d2d
  2. 5d5d
  3. 4d4d
  4. 3d3d (correct answer)
Explanation: When you encounter redshift problems involving multiple galaxies, remember that Hubble's Law connects redshift to distance through recession velocity. The key insight is understanding how redshifts translate to relative positions in an expanding universe. Using Hubble's Law v=H0dv = H_0 d, and the given approximation v≈czv \approx cz for small redshifts, we can establish that redshift is directly proportional to distance from Earth. Galaxy A has redshift zz and distance dd, while Galaxy B has redshift 4z4z. Since redshift scales linearly with distance, Galaxy B must be at distance 4d4d from Earth. Since both galaxies lie along the same line of sight and Galaxy B has the larger redshift, it's farther from us than Galaxy A. The distance between the galaxies is simply the difference in their distances from Earth: 4d−d=3d4d - d = 3d. Let's examine why the other answers miss the mark. Choice (A) 2d2d incorrectly assumes some kind of doubling relationship that doesn't follow from Hubble's Law. Choice (B) 5d5d mistakenly adds the distances rather than finding their difference (4d+d=5d4d + d = 5d). Choice (C) 4d4d confuses the distance from Galaxy B to Earth with the distance between the two galaxies. The correct answer is (D) 3d3d. Remember this pattern: when galaxies are aligned along your line of sight, their separation equals the difference in their distances from you. Always convert redshift ratios to distance ratios first, then apply basic geometry.

Question 13

Imagine you are an observer in Galaxy X. You observe that Galaxy Y, which is 50 Megaparsecs (Mpc) away, is receding from you at 3,500 km/s. In the opposite direction, you see Galaxy Z, also 50 Mpc away, receding at 3,500 km/s. What would an observer in Galaxy Y measure for the velocities of Galaxy X and Galaxy Z?

  1. Galaxy X recedes at 3,500 km/s, and Galaxy Z recedes at 3,500 km/s.
  2. Galaxy X recedes at 3,500 km/s, and Galaxy Z recedes at 7,000 km/s. (correct answer)
  3. Galaxy X approaches at 3,500 km/s, and Galaxy Z recedes at 3,500 km/s.
  4. Galaxy X is stationary, and Galaxy Z recedes at 7,000 km/s.
Explanation: This question addresses the cosmological principle that there is no center to the expansion. An observer in Galaxy Y would see Galaxy X receding at 3,500 km/s, consistent with the initial observation. Since Galaxy Z is in the opposite direction from X and is twice as far from Y (50 Mpc + 50 Mpc = 100 Mpc), its recessional velocity relative to Y would be twice that of X, or 7,000 km/s, according to Hubble's Law (v ∝ d).

Question 14

An astronomer observes Galaxy P and Galaxy Q. The light from Galaxy P has a redshift of z=0.5z = 0.5, while the light from Galaxy Q has a redshift of z=1.0z = 1.0. Which statement is a necessary consequence of these observations based on the standard cosmological model?

  1. Galaxy Q is exactly twice as far away from us as Galaxy P.
  2. The universe was younger when the light from Galaxy Q was emitted than when the light from Galaxy P was emitted. (correct answer)
  3. Galaxy Q's recessional velocity is exactly twice the recessional velocity of Galaxy P.
  4. The observed spectrum of Galaxy Q is twice as bright as the observed spectrum of Galaxy P.
Explanation: Redshift is directly related to the expansion of the universe and therefore to lookback time. A higher redshift means the light has been traveling for longer, and was therefore emitted earlier in the universe's history when the universe was younger and smaller. Distractors A and C are incorrect because the relationship between redshift, distance, and velocity is not linear at high z values. Distractor D is incorrect because brightness depends on luminosity and distance, not just redshift.

Question 15

The cosmological redshift zz is related to the universe's scale factor a(t)a(t), which represents the relative size of the universe at a given time. If light is emitted at time tet_e and observed at the present time tot_o, what is the correct relationship?

  1. 1+z=a(to)/a(te)1 + z = a(t_o) / a(t_e) (correct answer)
  2. z=a(te)/a(to)z = a(t_e) / a(t_o)
  3. 1+z=a(te)/a(to)1 + z = a(t_e) / a(t_o)
  4. z=(a(to)−a(te))2z = (a(t_o) - a(t_e))^2
Explanation: Cosmological redshift is a direct measure of how much the universe has expanded since the light was emitted. The ratio of the observed wavelength to the emitted wavelength is equal to the ratio of the scale factor at the time of observation to the scale factor at the time of emission: λo/λe=a(to)/a(te)\lambda_o / \lambda_e = a(t_o) / a(t_e). Since 1+z=λo/λe1+z = \lambda_o / \lambda_e, it follows that 1+z=a(to)/a(te)1+z = a(t_o) / a(t_e).

Question 16

The 'tired light' hypothesis, now discredited, proposed that the redshift of distant galaxies was caused by photons losing energy on their long journey, not by cosmic expansion. If this hypothesis were true, what would be a key difference in our observations of the distant universe?

  1. Distant galaxies would appear blueshifted instead of redshifted.
  2. The cosmic microwave background radiation would not exist.
  3. Time dilation effects, such as the observed stretching of supernova light curves, would not be present. (correct answer)
  4. Hubble's law would still be observed, but the Hubble constant would be negative.
Explanation: In an expanding universe, distant events appear to be slowed down, a phenomenon known as cosmological time dilation. For example, the light curves of Type Ia supernovae at high redshift are observed to be stretched over a longer period. This is a direct consequence of cosmic expansion stretching not just the wavelength of light, but the duration between photon emissions. A 'tired light' model, where only energy is lost, would not predict this time dilation effect. This observation is strong evidence for expansion.

Question 17

A hydrogen absorption line with a rest wavelength of 121.6 nm is observed in the spectrum of Galaxy A at 145.92 nm. The same line is observed in the spectrum of Galaxy B at 182.4 nm. What can be inferred about the relative recessional velocities of these galaxies, assuming they are low enough for v≈czv \approx cz to be accurate?

  1. The velocity of Galaxy B is approximately 2.5 times the velocity of Galaxy A. (correct answer)
  2. The velocity of Galaxy B is approximately 4 times the velocity of Galaxy A.
  3. The velocity of Galaxy B is approximately 1.25 times the velocity of Galaxy A.
  4. The velocity of Galaxy A and Galaxy B are nearly identical.
Explanation: First, calculate the redshift (z) for each galaxy using z=(λobs−λrest)/λrestz = (\lambda_{\text{obs}} - \lambda_{\text{rest}})/\lambda_{\text{rest}}. For Galaxy A: zA=(145.92−121.6)/121.6=24.32/121.6=0.2z_A = (145.92 - 121.6) / 121.6 = 24.32 / 121.6 = 0.2. For Galaxy B: zB=(182.4−121.6)/121.6=60.8/121.6=0.5z_B = (182.4 - 121.6) / 121.6 = 60.8 / 121.6 = 0.5. Since velocity is proportional to redshift (v≈czv \approx cz for low z), the ratio of their velocities is the ratio of their redshifts: vB/vA=zB/zA=0.5/0.2=2.5v_B / v_A = z_B / z_A = 0.5 / 0.2 = 2.5.

Question 18

For galaxies with redshifts z>1z > 1, the recessional velocity calculated with the simple formula v=czv = cz can exceed the speed of light. Why does this not violate the theory of special relativity?

  1. Special relativity does not apply to objects as massive as galaxies, only to small particles.
  2. The 'recessional velocity' is due to the expansion of space itself, not motion through space, which is what relativity limits. (correct answer)
  3. The speed of light was significantly higher in the early universe, allowing for faster-than-light recession.
  4. These galaxies are thought to be powered by black holes, which can eject material at speeds exceeding that of light.
Explanation: Special relativity states that no object can travel through space faster than the speed of light. However, cosmological redshift is not caused by motion through space. It is caused by the expansion of space itself. Two distant points in space can be moving apart from each other at a rate greater than the speed of light because the space between them is being created or stretched. This does not violate relativity.

Question 19

An astronomer constructs a Hubble diagram (recessional velocity vs. distance) for a set of galaxies. If the universe's expansion has been accelerating over cosmic time (as is currently believed), how would the data points for the most distant galaxies deviate from the straight-line relationship defined by nearby galaxies?

  1. They would fall below the straight line. (correct answer)
  2. They would fall above the straight line.
  3. They would still fall perfectly on the straight line.
  4. They would form a random scatter with no clear trend.
Explanation: The straight line on a Hubble diagram represents the current expansion rate, H0H_0. For very distant galaxies, we are looking far back in time. If expansion is accelerating, it means the expansion rate in the past was slower than it is today. Thus, for a given distance (which corresponds to a certain lookback time), the recessional velocity would have been lower than predicted by the current, faster rate. This means the data points for distant objects fall below the line extrapolated from nearby objects.

Question 20

Two separate model universes, Universe Alpha and Universe Beta, are assumed to have been expanding at a constant rate since their beginning. Cosmologists determine that the Hubble constant in Universe Alpha (HαH_{\alpha}) is double the value of the Hubble constant in Universe Beta (HβH_{\beta}). What is the most direct physical implication of this finding?

  1. Universe Alpha is approximately half as old as Universe Beta. (correct answer)
  2. Universe Alpha is approximately twice as old as Universe Beta.
  3. Universe Alpha must be significantly denser than Universe Beta.
  4. The observable universe is smaller in Universe Alpha than in Universe Beta.
Explanation: The age of the universe, in a simple model with a constant expansion rate, is inversely proportional to the Hubble constant (T≈1/H0T \approx 1/H_0). A larger Hubble constant implies a faster expansion rate, meaning it would have taken less time to reach the current state from the initial singularity. Therefore, if Hα=2HβH_{\alpha} = 2H_{\beta}, then Tα≈1/(2Hβ)≈Tβ/2T_{\alpha} \approx 1/(2H_{\beta}) \approx T_{\beta}/2. Universe Alpha is younger.