Question 1

What produces the arcs of light that appear to be centered on several clusters of galaxies,such as those shown in Figure 13-72 of Comins,Discovering the Essential Universe,6th ed.?

Accepted Answer

The arcs of light are produced by gravitational lensing, where the light from a single distant source is bent by the gravitational force of both the visible material and the dark matter of the intervening cluster of galaxies. This bending causes the image of the source to appear distorted and stretched out into arcs.

Question 2

You are examining quasars and you happen upon two quasars in the same part of the sky that have identical spectra.You cannot see any massive object in the region between the two quasars.What is a reasonable explanation for this discovery?

Accepted Answer

The identical spectra suggest that the two quasars are likely images of the same quasar, created by gravitational lensing. The lensing object could be composed mostly of dark matter, which would not be visible.

Question 3

In observations of a double-quasar image produced by gravitational lensing,fluctuations on the intensity of one image appear to be delayed compared with equivalent variations on the other image.Explain.

Accepted Answer

The gravitational field of the lensing object curves the path of light passing through it, resulting in one beam taking a longer optical path than the other beam. This causes fluctuations in intensity to appear delayed in one image compared with the other image.

Question 4

For gravitational lensing of a distant quasar to occur,the galaxy producing the lensing must:

Accepted Answer

In order for gravitational lensing of a distant quasar to occur, the galaxy producing the lensing must be almost perfectly placed on a line between Earth and the quasar. This alignment causes the light from the quasar to be bent and magnified as it passes through the gravitational field of the intervening galaxy. The rotation of the galaxy or the presence of a supermassive black hole at its center is not necessary for gravitational lensing to occur.  The size of the galaxy does not determine whether or not the quasar's light can pass through it and reach Earth.

Question 5

An active galaxy is measured to be 100 Mpc away.How many million years ago did the light we see now leave this galaxy?

Accepted Answer

We can use the equation t=d/v, where t is the time taken for light to travel a distance d at a speed v. 
Here, d = 100 Mpc = 100 million parsecs and we know that the speed of light, v = 3 x 10^8 m/s. 
However, we need to convert Mpc to meters by using the conversion factor 1 Mpc = 3.086 x 10^22 m. 
So, d = 100 x 3.086 x 10^22 m = 3.086 x 10^24 m. 
Using the equation now, t = d/v = (3.086 x 10^24 m) / (3 x 10^8 m/s) = 10.287 x 10^15 s. 
We can convert this to million years by dividing by the number of seconds in a year and then by 1 million. 
So, t = 10.287 x 10^15 s / (3.154 x 10^7 s/yr) / 1 million = 326 million years. 
Therefore, the answer is C: 326.

Quiz 13: The Galaxies

What produces the arcs of light that appear to be centered on several clusters of galaxies,such as those shown in Figure 13-72 of Comins,Discovering the Essential Universe,6th ed.?

You are examining quasars and you happen upon two quasars in the same part of the sky that have identical spectra.You cannot see any massive object in the region between the two quasars.What is a reasonable explanation for this discovery?

In observations of a double-quasar image produced by gravitational lensing,fluctuations on the intensity of one image appear to be delayed compared with equivalent variations on the other image.Explain.

For gravitational lensing of a distant quasar to occur,the galaxy producing the lensing must:

An active galaxy is measured to be 100 Mpc away.How many million years ago did the light we see now leave this galaxy?