Question 1

What does cosmological redshift do to light?

Accepted Answer

Cosmological redshift stretches the wavelengths of light, causing them to shift towards the red end of the spectrum. This is due to the expansion of the universe, which causes the space between galaxies to stretch, and the wavelength of light to stretch along with it.

Question 2

Which statement below correctly describes the relationship between expansion rate and age for the universe?

Accepted Answer

The faster the universe expands, the less time it would have taken to reach its current size, implying a younger age.

Question 3

Suppose an elliptical galaxy is so far away that we cannot see even its brightest stars individually.Which of the following techniques might allow us to measure its distance?

Accepted Answer

Cepheid variables and parallax require the ability to resolve individual stars, which is not possible in the scenario described. Radar ranging is not practical for distances beyond the Solar System. However, a white dwarf supernova, being extremely bright, can serve as a standard candle even for very distant galaxies, allowing for the measurement of distance without the need to resolve individual stars.

Question 4

Why can't we see past the cosmological horizon?

Accepted Answer

The cosmological horizon is the distance beyond which light has not had enough time to reach us since the universe began. Therefore, if we were to look beyond this horizon, we would be seeing events that occurred before the universe was born, which is impossible.

Answer: D
 The cosmological horizon defines the boundary of the observable universe, beyond which galaxies are moving away from us so rapidly that their light can never reach us. Therefore, every galaxy in the entire universe, including the ones that we cannot observe, exists within the cosmological horizon. Hence, there is nothing to see beyond it.

Note: B and C are incorrect as they do not explain the concept of the cosmological horizon.

Question 5

The lookback time of the cosmological horizon is

Accepted Answer

The lookback time of the cosmological horizon is the time it takes for light to travel from the farthest point of the observable universe to us. Since the age of the universe is the time elapsed since the Big Bang and light takes time to travel, the lookback time of the cosmological horizon is equal to the age of the universe. Therefore, the best choice is B, the age of the universe.

Question 6

In 1924,Edwin Hubble proved that the Andromeda Galaxy lay far beyond the bounds of the Milky Way,thus putting to rest the idea that it might have been a cloud within our own galaxy.How was he able to prove this?

Accepted Answer

Edwin Hubble was able to prove that the Andromeda Galaxy lay beyond the Milky Way by observing individual Cepheid variable stars in Andromeda and applying the period-luminosity relation. This allowed him to determine its distance from Earth and prove that it was not a cloud within our own galaxy.

Question 7

Suppose that Hubble's constant were 20 kilometers per second per million light-years.How fast would we expect a galaxy 100 million light-years away to be moving? (Assume the motion is due only to Hubble's law. )

Accepted Answer

According to Hubble's law, the recession velocity (or speed at which a galaxy is moving away from us) is proportional to its distance from us. The proportionality constant is Hubble's constant. So, using the given Hubble's constant value of 20 kilometers per second per million light-years, a galaxy 100 million light-years away would be expected to be moving away from us at a speed of (20 km/s) x 100 = 2000 km/s. Choice C is the closest option.

Question 8

Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?

Accepted Answer

White dwarf supernovae, also known as Type Ia supernovae, have a consistent peak luminosity because they result from a white dwarf in a binary system reaching a critical mass and exploding. This uniform brightness makes them excellent "standard candles" for measuring cosmic distances, unlike massive star supernovae, which vary widely in their peak luminosities.

Question 9

Suppose that we suddenly discovered that all these years we'd been wrong about the distance from Earth to the Sun,and it is actually 10% greater than we'd thought.How would that affect our estimate of the distance to the Andromeda Galaxy?

Accepted Answer

Our estimate of the distance to the Andromeda Galaxy is based on the measurement of its angular size and the assumption that the distance to the Sun is what we previously thought. If the distance to the Sun is actually 10% greater, then the distance to the Andromeda Galaxy would also have to be 10% greater to account for its observed angular size. Therefore, our estimate of the distance to the Andromeda Galaxy would need to be revised upward by 10%.

Question 10

What is the best way to determine a galaxy's redshift?

Accepted Answer

The most accurate way to determine a galaxy's redshift is to take a spectrum of the galaxy and measure the difference in wavelength of spectral lines from the wavelengths of those same lines as measured in the laboratory. This method is called spectroscopic redshift and is the most reliable way to determine a galaxy's redshift. Option A and D involve estimating the distance of the galaxy, which can introduce errors. Option B is not a reliable method as the color of a galaxy can be affected by various factors, such as dust and age, and does not necessarily correspond to its distance or redshift.

Question 11

If we went back in time 10 billion years,would we expect Hubble's constant to be identical to the present-day Hubble constant?

Accepted Answer

Hubble's constant is not actually a constant, but rather a measurement of the rate at which the universe is expanding. This rate changes as the universe ages, so if we went back 10 billion years, we would expect the Hubble constant to be different than it is today. Specifically, since Hubble's constant is roughly equal to 1 divided by the age of the universe, we would expect it to be about 1/10 billion years or 0.0000001 per year at that time.

Question 12

Does Hubble's law work well for galaxies in the Local Group? Why or why not?

Accepted Answer

Galaxies in the Local Group are gravitationally bound together, which means they do not obey Hubble's law. This is because Hubble's law only applies to galaxies moving away from us due to the expansion of the universe, and the motion of galaxies in the Local Group is mostly due to their mutual gravitational attraction.

Question 13

If we went back in time 10 billion years,how would we expect a plot of galaxy luminosities against colors to change?

Accepted Answer

10 billion years ago, the universe was much younger, and galaxies were in earlier stages of their evolution. This means there would be more young, hot, and therefore blue stars, making the blue cloud more heavily populated. At the same time, there would be fewer older, cooler, and therefore red stars, making the red cloud more sparse.

Question 14

Hubble's constant is about 22 km/s/million light-years,implying an age of about 14 billion years for the universe.If Hubble's constant were 11 km/s/million light-years,the age of the universe would be about

Accepted Answer

The age of the universe is inversely proportional to Hubble's constant. If Hubble's constant were half its current value (11 km/s/million light-years instead of 22 km/s/million light-years), the calculated age of the universe would double, making it about 28 billion years.

Question 15

Given that the universe is about 14 billion years old,which of the following statements is logically valid?

Accepted Answer

The answer of Given that the universe is about 14...

Question 16

Suppose we observe a Cepheid variable in a distant galaxy.The Cepheid brightens and dims with a regular period of about 10 days.What can we learn from this observation?

Accepted Answer

The regular period of brightening and dimming of a Cepheid variable is directly related to its luminosity. By measuring the period of a Cepheid variable, we can determine its true luminosity and compare it to its apparent brightness as seen from Earth. This allows us to calculate the distance to the galaxy in which the Cepheid variable is located. Therefore, option C is correct. Option A and B are incorrect as the rotation rate or the mass of the galaxy cannot be determined solely from the observation of a Cepheid variable. Option D is absurd and incorrect.

Quiz 20: Galaxies and the Foundations of Modern Cosmology