14.1

Why does the rotation curve for the solar system show speeds that become slower with increasing distance from the Sun?

Because the Sun contains most of the mass of the solar system

Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?

Dark matter is the dominant form of mass in both clusters and in individual galaxies.

When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?

It bends or distorts the light coming from galaxies located behind it.

As noted in the Introduction, some scientists have proposed that dark matter does not really exist. According to this view, all matter is ordinary (baryonic), but at large distances from matter, gravity does not precisely obey either Newton's or Einstein's theories of gravity. Is this alternative view of gravity consistent with what we observe in the Bullet Cluster? Why or why not?

No. If all matter was ordinary, then the blue region representing the location of most of the matter would line up with the red region representing the hot gas.

Open the Rotation Curve of a Spiral Galaxy animation and click at various distances from the galactic center to create a graph of the galaxy's rotation curve; be sure to start with points very close to the galactic center and continue to well beyond the visible part of the galaxy. Which of the following statements best describes the pattern of the graph?

Starting from the center, speeds at first rise rapidly, and then become nearly constant with increasing distance.

From Parts A though C, you should understand the three major components shown in the composite Bullet Cluster image. Notice that the hot, X-ray-emitting gas (red) is not in the same place as most of the visible galaxies and most of the matter (blue). How do we think the hot gas came to be in a different place from the galaxies and most of the matter?

The Bullet cluster is actually two separate galaxy clusters that have collided, stripping out hot gas in the process.

Which of the following best explains why a higher-mass cluster of galaxies causes light from a distant galaxy to bend more than a lower-mass cluster of galaxies?

The stronger gravity of a larger cluster curves space itself by a greater amount, and light follows the curvature of space.

Study the composite image of the Bullet Cluster, as well as the animations. What do the two large, red-colored regions in the composite image represent?

X-ray emission from hot gas

Based on the observational evidence, is it possible that dark matter doesn't really exist?

Yes, but only if there is something wrong with our current understanding of how gravity should work on large scales.

Examine the visible light image of the Bullet Cluster. What kind of cluster is it?

a galaxy cluster

A photograph of a cluster of galaxies shows distorted images of galaxies that lie behind it at greater distances. This is an example of what astronomers call

a gravitational lens

Consider a distant galaxy located directly behind a cluster of galaxies, as shown in this interactive figure. As seen from Earth, the gravitationally lensed images of the distant galaxy will appear more widely separated if the intervening cluster of galaxies has __________.

a relatively high mass and is located relatively near Earth.

Now, let's turn our attention to rotation curves for orbiting masses which are not rigidly held together like the points on a merry-go-round. The Rotation Curve of the Solar System animation shows the orbits of Mercury, Venus, Earth, and Mars. Click on each of the orbits, and watch the corresponding points appear on the graph below it. If a planet (or asteroid) orbited the Sun at three times the distance of Mars, its orbital speed would be __________.

between one-third and two-thirds the orbital speed of Mars

All the mass of the galaxy is concentrated in the center of the galaxy.

close to "orbital speed" (x) and "

Strong evidence for the existence of dark matter comes from observations of

clusters of galaxies.

The Rotation Curve for a Merry-Go-Round animation shows a rotating merry-go-round. Click several points on the merry-go-round, and watch the corresponding points appear on the graph below it. This graph is what we call a rotation curve, because it plots the speed at which any point rotates around the center (the orbital speed) against the distance of that point from the center. Based on the graph, you can conclude that doubling the distance of a point from the center of the merry-go-round would __________.

double its orbital speed

Which of the following is NOT one of the three main strategies used to measure the mass of galaxy clusters?

measuring the temperatures of stars in the halos of the galaxies

There are also two large blue-colored regions in the composite image. These blue regions are labeled as "dark matter" based on __________.

observations of gravitational lensing by the cluster

From Part D, you know that the galaxy's rotation curve is nearly flat, even well beyond the point at which we see any stars in the galaxy. According to the law of gravity as we understand it today, the only explanation for these this flatness is that __________.

substantial amounts of mass must reside at great distances from the galactic center

To understand why the galaxy rotation curve in Part D is flat, we must first understand how gravity determines the orbital speeds of stars around the galactic center. Consider any star on a circular orbit around the center of the galaxy. The speed of the star's orbit depends on the star's distance from the galactic center and __________.

the mass of the galaxy that resides within the star's orbit

Consider a distant galaxy located directly behind a cluster of galaxies as shown in this interactive figure. Knowing the distance to the cluster of galaxies and the angular separation of the lensed images of the distant galaxy, astronomers can estimate:

the total amount of matter in the cluster of galaxies, including both dark matter and matter in stars

Careful measurements show that the hot, X-ray-emitting gas in the Bullet Cluster contains about 7 times as much total mass as all the stars in the cluster's galaxies combined. Therefore, the fact that the most of the gravity is in the blue regions that surrounds the visible galaxies, rather than in the regions with the hot gas, indicates that __________.

there is even more matter surrounding the galaxies than there is in the hot gas

Dark matter is inferred to exist because

we can observe its gravitational influence on visible matter.

Measurements of how orbital speeds depend on distance from the center of our galaxy tell us that stars in the outskirts of the galaxy

orbit the galactic center just as fast as stars closer to the center.