Astronomy: Dark Matter and Dark Energy Worksheet

Question 1

Explain the difference between dark matter and dark energy in terms of what each one appears to do in the universe.

Accepted Answer

Dark matter appears to add extra gravitational attraction, helping hold galaxies and galaxy clusters together. Dark energy appears to drive the accelerated expansion of the universe, causing distant galaxies to move away from one another faster over time.

Question 2

A spiral galaxy has stars far from its center moving at nearly the same speed as stars closer to the center. Explain why this observation is evidence for dark matter.

Accepted Answer

If most of the galaxy's mass were in its bright central region, stars farther out should orbit much more slowly. The nearly flat rotation curve suggests there is additional unseen mass spread around the galaxy, which astronomers call dark matter.

Question 3

A star orbits 30,000 light-years from the center of a galaxy at a speed of 220 km/s. Another star orbits 60,000 light-years from the center at 220 km/s. Based on these speeds, what can you infer about the mass distribution of the galaxy?

Accepted Answer

The similar speeds suggest that the galaxy's mass is not concentrated only near the center. There must be significant mass, including dark matter, extending far beyond the bright visible part of the galaxy.

Question 4

Define gravitational lensing and explain how it can reveal dark matter.

Accepted Answer

Gravitational lensing happens when mass bends the path of light from a more distant object. If the amount of bending is greater than visible matter can explain, astronomers infer that additional unseen mass, such as dark matter, is present.

Question 5

In a galaxy cluster, astronomers measure the total mass from gravitational lensing and compare it with the mass of visible stars and gas. The lensing mass is much larger. What conclusion should they draw?

Accepted Answer

They should conclude that the cluster contains a large amount of unseen mass. This unseen mass is evidence for dark matter because visible stars and gas cannot account for the total gravitational effect.

Question 6

The cosmic microwave background shows tiny temperature differences from one direction in space to another. Explain how these small variations are connected to dark matter.

Accepted Answer

The tiny temperature differences show where matter was slightly more or less dense in the early universe. Dark matter helped these denser regions grow through gravity, eventually forming galaxies and galaxy clusters.

Question 7

A Type Ia supernova in a distant galaxy appears dimmer than expected. Explain how observations like this led astronomers to infer dark energy.

Accepted Answer

Type Ia supernovae have predictable brightness, so astronomers can use them to estimate distance. When distant supernovae appeared dimmer than expected, it showed they were farther away than predicted, suggesting that the expansion of the universe has been accelerating due to dark energy.

Question 8

Describe the difference between expansion of space and motion through space. Use dark energy in your explanation.

Accepted Answer

Motion through space means an object travels from one place to another within space. Expansion of space means the distance between faraway galaxies increases because space itself stretches. Dark energy is the name given to the cause of the universe's accelerating expansion.

Question 9

The universe is often described as being about 5 percent ordinary matter, 27 percent dark matter, and 68 percent dark energy. If a model universe contains 10,000 total units of mass-energy, how many units are ordinary matter, dark matter, and dark energy?

Accepted Answer

Ordinary matter would be 500 units because 5 percent of 10,000 is 500. Dark matter would be 2,700 units because 27 percent of 10,000 is 2,700. Dark energy would be 6,800 units because 68 percent of 10,000 is 6,800.

Question 10

Explain why dark matter is called dark even though scientists think it has mass.

Accepted Answer

Dark matter is called dark because it does not emit, absorb, or reflect enough electromagnetic radiation for telescopes to detect directly. Scientists think it has mass because its gravity affects visible matter, light, and the structure of the universe.

Question 11

A student says, "Dark matter and black holes are the same thing because both are dark." Explain why this statement is incorrect.

Accepted Answer

The statement is incorrect because black holes are compact objects with gravity so strong that not even light can escape from inside the event horizon. Dark matter is thought to be a widespread form of matter that affects galaxies and clusters gravitationally, but it is not simply the same as black holes.

Question 12

Use the terms evidence, model, and uncertainty to explain why dark matter and dark energy are important scientific ideas even though they have not been directly observed.

Accepted Answer

Dark matter and dark energy are important because many lines of evidence support models that include them, such as galaxy rotation curves, gravitational lensing, the cosmic microwave background, and supernova distances. There is still uncertainty about what they physically are, so scientists continue testing and improving their models with new observations.

Astronomy: Dark Matter and Dark Energy

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