Astronomy Chapters 20,21,22,23

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To get started, click on the blue Cepheids link on the main screen of the Interactive Figure. The Interactive Figure (in red) shows a graph of the Cepheid period-luminosity relation. This graph indicates that __________.

Cepheids with longer periods have higher luminosities

A Cepheid with a period of 30 days has an average luminosity that is about __________ times the luminosity of the Sun. (You will have to click on the blue Cepheids link on the main screen of the Interactive Figure to get back to the relevant plot.)


Click on the blue "Cepheid" label, then click the label on the bottom of the Interactive Figure that reads "Cepheids as Standard Candles" to bring up the next screen. Read the instructions that appear in the upper left hand corner and study the animation. What actually causes a Cepheid to vary in apparent brightness?

The Cepheid varies in radius, and its luminosity is greater when its radius is larger.

Click on the blue "Cepheid" label, then click the label on the bottom of the Interactive Figure that reads "Cepheids as Standard Candles" to bring up the next screen. Then click the "Next" button in the Interactive Figure to bring up the screen that shows a Cepheid light curve in the upper left. What is the approximate luminosity of the Cepheid whose light curve is shown in the graph?


Click on the blue "Cepheid" label, then click the label on the bottom of the Interactive Figure that reads "Cepheids as Standard Candles" to bring up the next screen. Then click the "Next" button in the Interactive Figure to bring up the screen that shows a Cepheid light curve in the upper left. Notice that the window with the light curve also has a box that tells you the apparent brightness of the Cepheid, which shows a value of 1.3×104. (The units are shown in the box, but you only need to focus on the numerical value.) Based on this apparent brightness and the luminosity you found in Part D, what is the approximate distance to this Cepheid?

2.5 million light years

Which of the following three-step processes correctly describes how we use Cepheids as a tool to make cosmic distance measurements?

Step 1: Measure the period of the Cepheid’s brightness variations. Step 2: Use the period-luminosity relation to determine the Cepheid’s luminosity. Step 3: Calculate the Cepheid’s distance from its luminosity and apparent brightness.

Radar, the first link in the cosmic distance chain, is used to establish the baseline distance necessary for the second link, parallax. What baseline distance must we know before we can measure parallax?

the Earth-Sun distance

Which cosmic distance measurement techniques are considered standard candle techniques?

Main-sequence fitting White dwarf supernovae Cepheids

Suppose that Cepheids did not exist and there were no other standard candle technique that worked at the same distances. Which statement would be true?

We would not be able to measure the distances of distant galaxies.

Why do we use Hubble’s law to estimate the distances of most distant galaxies, rather than using white dwarf supernovae in all cases?

We have not observed white dwarf supernovae in most galaxies

Imagine that radar had never been invented and that we instead had to rely on a less reliable method of measuring distances in our solar system. If that method led us to underestimate the Earth-Sun distance by 10%, how would it affect other measurements in the distance chain?

They would all be off by the same 10%

Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?


Hubble’s law expresses a relationship between __________.

the distance of a galaxy and the speed at which it is moving away from us

We can always determine the recession velocity of a galaxy (at least in principle) from its redshift. But before we can use Hubble’s law, we must first calibrate it by __________.

measuring the distances to many distant galaxies with a standard candle technique

Suppose that you measure a galaxy’s redshift, and from the redshift you determine that its recession velocity is 30,000 (3×104) kilometers per second. According to Hubble’s law, approximately how far away is the galaxy?

1.4 billion light-years

Based on what you have learned, which of the following best describes the meaning of Hubble’s constant (H0 )?

It describes the expansion rate of the universe, with higher values meaning more rapid expansion.

Following are a number of distinguishing characteristics of spiral and elliptical galaxies. Match each characteristic to the appropriate galaxy type.

Spiral Galaxies: contain abundant clouds of cool gass and dust, are rare in central regions of galaxy clusters, have significant, ongoing star formation, have flattened disk of stars, contain many bright hot stars. Elliptical Galaxies: Contain primarily old, low-mass stars, are more reddish in color.

The graph shows that galaxies with high speeds as measured from Earth are __________.

moving away from Earth and are farther from Earth than galaxies with lower speeds

By looking at the graph, what can we say about the galaxies that have the lowest speeds?

They are moving away from Earth and are closer to Earth than galaxies with high speeds.

Suppose that galaxy B is twice as far from Earth as galaxy A. Hubble’s law predicts that galaxy B will be moving away from Earth with approximately _____.

twice the velocity of galaxy A

Imagine that when we looked out into the universe we found that the light from all galaxies was blueshifted (rather than redshifted) and that the light from the most distant galaxies was blueshifted by the greatest amount. Which statement best describes what we would conclude about the motions of galaxies in this case?

All are moving toward Earth, with distant galaxies moving faster than nearby galaxies.

The age of the universe is related to the slope of the graph of Hubble’s law, and current data put the age of the universe at about 14 billion years. Suppose that future observations showed that the slope of Hubble’s law on the graph is actually steeper than that shown. In that case, the age of the universe would be _________ than 14 billion years because the universe is expanding ______ than current data suggest. (Each choice gives words to fill in the two blanks, separated by a slash.)

younger/more rapidly

Which of these galaxies would you most likely find at the center of a large cluster of galaxies?

a large elliptical galaxy

Which kind of object is the best standard candle for measuring distances to extremely distant galaxies?

A white dwarf supernovae

Which of these galaxies is most likely to be oldest?

A galaxy in the local group

About how many galaxies are there in a typical cluster of galaxies?

a few dozen

When the ultraviolet light from hot stars in very distant galaxies finally reaches us, it arrives at Earth in the form of

Visible Light

Why do virtually all the galaxies in the universe appear to be moving away from our own?

Observers in all galaxies observe a similar phenomenon because of the universe’s expansion.

If you observed the redshifts of galaxies at a given distance to be twice as large as they are now, then you would determine a value for Hubble’s constant that is

twice as large as its current value

In a photo like the Hubble Deep Field (Figure 20.1 in the textbook in your textbook), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?

the galaxies that are farthest away

Which of the following statements about types of galaxies is not true?

Elliptical galaxies are bluer and contain more dust than spiral galaxies

The most basic difference between elliptical galaxies and spiral galaxies is that ______.

elliptical galaxies lack anything resembling the disk of a spiral galaxy

Hubble’s galaxy classification diagram (the "tuning fork") ______.

relates galaxies according to their shapes, but not according to any evolutionary status.

Using the technique of main-sequence fitting to determine the distance to a star cluster requires that _____.

we identify enough main-sequence stars to make an H-R diagram for the cluster

Although it is highly unlikely to happen, 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?

It would mean the distance to the Andromeda Galaxy is also 10% greater than we thought.

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?

We can learn the distance to the galaxy.

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?

By observing individual Cepheid variable stars in Andromeda and applying the period–luminosity relation.

Assume that Hubble’s constant is 22 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.)

away from us at 2200 km/s

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

No, because galaxies in the Local Group are gravitationally bound together.

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

White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities.

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?

We could use a white dwarf supernova as a standard candle

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

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.

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

The faster the rate of expansion, the younger the age of the universe.

What does cosmological redshift do to light?

stretches its wavelength

The lookback time of the cosmological horizon is ______.

The age of the universe

Why can’t we see past the cosmological horizon?

Why can’t we see past the cosmological horizon?

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 _______.

28 billion years

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

The oldest galaxies we see at great distances are younger than the oldest galaxies we see nearby.

If you looked at the field of view seen in this Hubble Space telescope with your naked eye, about how big would it appear in the sky?

About the size of a grain of sand held at arm’s length against the sky.

What are we seeing in this photograph?

Almost every object in the photo is a galaxy, and these galaxies are at many different distances from us.

Is this a photo of a spiral galaxy or an elliptical galaxy, and how do you know?

It is a spiral galaxy, because we can see a dusty disk going across the center.

This photo shows the Large Magellanic Cloud, or LMC for short. Based on what you have learned in about the LMC from your textbook, which of the following statements about the LMC is not true?

Galaxies similar to the LMC are very rare.

What is this a picture of?

A cluster of galaxies.

Each dot on this graph represents an individual Cepheid variable star. Which of the following is true for a Cepheid that is 10,000 times as luminous as the Sun?

It varies gradually in brightness, with a peak in brightness about every 30 days.

The data points in this diagram represent the measured speeds and distances of various galaxies, and the solid line represents a best fit to these data. The trend indicated by the solid line is known as:

Hubble’s law

Suppose that Hubble’s constant were 11 km/s/million light years instead of 22 km/s/million light years. How would this graph look different?

The data would follow a more shallow line, closer to horizontal than the current line.

The white arrow points to an object that lies within the disk of the galaxy shown. What is this object?

A supernova

This figure is a spacetime diagram, which means it shows space (distance in this case) on the horizontal axis and time on the vertical axis. Study it carefully to be sure you understand it. Now, look at the square around one of the galaxy images. What does the position of this image mean on this diagram?

It indicates that the galaxy was closer to the Milky Way when the supernova exploded 400 million years ago.

Based on the number of galaxies visible in the Hubble Deep Field (Figure 20.1 in the textbook in your textbook), the estimated number of galaxies in our observable universe is about ______.

100 billion

Which of the following is not one of the three major categories of galaxies?

Globular galaxies

Galaxies with disks but no evident spiral arms are called ______.

Lenticular galaxies

On a graph plotting galaxy luminosities against galaxy colors, the red sequence represents galaxies that are _________ than galaxies of the blue cloud.

larger and more elliptical

Which of the following best describes the status of the Milky Way in our Local Group of galaxies?

It is one of the two largest galaxies in the group.

A standard candle is _________.

a light source of known luminosity

What is a Cepheid variable?

A type of very luminous star that makes an excellent standard candle

What two observable properties of a Cepheid variable are directly related to one another?

the period between its peaks of brightness and its luminosity

What does Hubble’s law tell us?

The more distant a galaxy, the faster it is moving away from us.

Given that white dwarf supernovae are such good standard candles, why don’t we use them to measure the distance to all galaxies?

They are rare events, so we have observed them in only a tiny fraction of all galaxies.

Overall, what is our most accurate technique for measuring the distance to a nearby star?

Stellar Parallax

When we use an analogy that represents the expanding universe with the surface of an expanding balloon, what does the inside of the balloon represent?

The inside of the balloon does not represent any part of our universe.

If we say that a galaxy has a lookback time of 1 billion years, we mean that _________.

its light traveled through space for 1 billion years to reach us

Cosmological redshift is the result of ______.

the expansion of the universe

Although the entire universe may be much larger than our observable universe, we can see only within our observable universe. The "boundary" of our observable universe is called ______.

the cosmological horizon

Current estimates place the age of the universe at about ______.

14 billion years

You observe the peak brightnesses of two white dwarf supernovae. Supernova A is only ¼ as bright as Supernova B. What can you say about their relative distances?

Supernova A is twice as far away as Supernova B

The fact that the universe is expanding means that space itself is growing within ___________.

The observable universe

Spectral lines from Galaxy B are redshifted from their rest wavelengths twice as much as the spectral lines from Galaxy A. According to Hubble’s law, what can you say about their approximate relative distances?

Galaxy B is twice as far as Galaxy A.

Select the visible-light view of the galaxy M82. Based on its appearance, what type of galaxy is it?


Look again at the visible-light view of M82. What is the source of the white and blue light that dominates the image?


The infrared image of M82 shows a whitish-blue region running down the center, along with a much larger red region. What is emitting the light that is shown in red and what is emitting the light that is shown in whitish-blue?

The red is emission from warm dust grains; the whitish-blue is emission from stars.

Study all of the individual images and the Multi image that combines all of them. Which of the following statements provides the best interpretation of what we see in the Multi image?

Stars in the central region of this galaxy are exploding as supernovae, generating a galactic wind of hot gas and warm dust gains.

What can we conclude about this galaxy from the fact that there are so many supernovae occurring?

This galaxy contains an unusually large number of young stars.

As you learned in Part E, galaxy M82 has unusually active star formation. Why do astronomers think that this is a "burst" of star formation, rather than thinking that this galaxy always has such a high rate of star formation?

At its current rate of star formation, the galaxy would use up all its dust and gas in just a few hundred million years

Why do astronomers hypothesize that a massive black hole lies at the center of M 87?

The third image in the interactive photo (with the most detailed view of the galactic center) is labeled "gas disk." Which of the following best describes what we are seeing in this photo?

The third image in the interactive photo (with the most detailed view of the galactic center) is labeled "gas disk." Which of the following best describes what we are seeing in this photo?

The black hole is located deep within the bright central region, and around this region we see gas that is orbiting the central black hole.

Assuming that the bright core of M 87 is powered by a supermassive black hole, which of the following best describes the source of energy that makes the core appear so bright?

Gravitational potential energy is converted to thermal energy as matter from the surrounding gas disk spirals into the central black hole.

Which statement describes the trend among the galaxies shown on this graph?

More distant galaxies tend to have a larger infrared color ratio, but there are some exceptions.

You discover a new cluster of galaxies, and the brightest galaxy in this new cluster has an infrared color ratio of 0.65. Based on the data in the graph, what is the approximate distance to this new cluster?

1 billion light years

A follow-up study showed that the galaxies that are exceptions to the general trend on the original graph have unusually brighter centers; that is, these galaxies contain active galactic nuclei. Based on this follow-up study, could you improve your confidence in your distance estimate for the new cluster discussed in Part B, and, if so, how?

Yes, by photographing the new cluster with better angular resolution to see whether its brightest galaxy has an unusually bright center.

Consider both the original study (with data shown on the graph) and the follow-up study showing that the exceptions to the general trend are galaxies with unusually bright centers (as described in Part C). Which of the following conclusions are supported by this pair of studies?

Galaxies with unusually bright nuclei were more common when the universe was young than they are today. Galaxies with unusually bright nuclei are more common at greater distances. On average, more distant galaxies have larger infrared color ratios.

Which of these items is a key assumption in our most successful models for galaxy formation?

Some regions of the universe were slightly denser than others.

A collision and merger of two large elliptical galaxies will eventually produce

A large elliptical galaxy

The luminosity of a quasar is generated in a region the size of

The solar system

Supermassive black holes found at the centers of galaxies are related to the properties of those galaxies in which of the following ways?

The mass of the black hole is related to the mass of the galaxy’s bulge.

The primary source of a quasar’s energy is

Gravitational Potential Energy

We can study how galaxies evolve because ______.

The farther away we look, the further back in time we see.

Which of the following statements is not an assumption used in models of galaxy formation?

Gas contracted to form the disks of galaxies before any stars were born.

One possible explanation for a galaxy’s type invokes the angular momentum of the protogalactic cloud from which it formed. Suppose a galaxy forms from a protogalactic cloud with a lot of angular momentum. Assuming its type has not changed due to other interactions, we’d expect this galaxy to be ______.

a spiral galaxy

Two ways in which the starting conditions in a protogalactic cloud might cause it to become an elliptical (rather than spiral) galaxy are if the cloud begins with either:

relatively little angular momentum or relatively high density

Which of the following phenomena are not thought to be results of collisions or other interactions between galaxies?

The fact that spiral galaxies have both disk and halo components.

If the Andromeda Galaxy collided with the Milky Way, what would most likely happen to Earth?


Interactions among galaxies also are thought to influence a galaxy’s type in at least some cases. Which of the following does not support the idea that interactions can shape galaxies?

The fact that more distant galaxies have larger redshifts.

Observations indicated that over billions of years, galaxies in general tend to change from _________.

Smaller and bluer to larger and redder

Which characteristic is not generally true of a starburst galaxy?

The observed features of the starburst are thought to be caused by the presence of a supermassive black hole in the galaxy’s center.

Why should galaxy collisions have been more common in the past than they are today?

Galaxies were closer together in the past because the universe was smaller.

A quasar’s spectrum is hugely redshifted. What does this large redshift tells us about the quasar?

The distance to the quasar

Most active galactic nuclei are found at large distances from us, with relatively few nearby. What does this imply?

Active galactic nuclei exist tend to become less active as they age.

Suppose we observe a source of X rays that varies substantially in brightness over a period of a few days. What can we conclude?

The X-ray source is no more than a few light-days in diameter.

All of the following observations are real. Which one does not support the model in which active galactic nuclei are powered by accretion disks around massive black holes?

The most luminous active galactic nuclei have huge redshifts.

Central black holes can be very efficient for converting the mass-energy of infalling matter to thermal energy in the accretion disk. Roughly what percentage of the mass-energy can be converted to other forms of energy as matter falls into a black hole?


The observed relationship between the masses of central black holes and the bulge masses of galaxies implies that:

Galaxy formation and supermassive black hole formation must be related somehow.

Quasar spectra often show many absorption lines that all appear to be due to the same electron transition (such as level 1 to level 2 in hydrogen) but that fall at different wavelengths in the spectrum. Why do we think this is the case?

We are seeing absorption lines from clouds of gas that lie between us and the quasar, and therefore each cloud has a different redshift.

Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in ______.

infrared light

Which of the following is an important starting assumption in models of galaxy formation?

Some regions in the universe start out denser than others.

According to observations, what type of galaxy was much more common when the universe was 2 billion years old than it is today?

Irregular galaxies

Collisions between galaxies typically unfold over a period of ______.

hundreds of millions of years

Why are collisions between galaxies more likely than collisions between stars within a galaxy?

Relative to their sizes, galaxies are closer together than stars.

Current understanding holds that a galaxy’s type (spiral, elliptical, or irregular) ______.

may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies

The distinguishing feature of a starburst galaxy is _________.

a rate of star formation that may be 100 or more times greater than that in the Milky Way

The unusually bright centers found in some galaxies are called ______.

active galactic nuclei

According to current understanding, what is a quasar?

an active galactic nucleus that is particularly bright

Which of the following phenomena is probably not related to the presence of a supermassive black hole?

the presence of globular clusters in the halos of galaxies.

The mass of a supermassive black hole thought to power a typical bright active galactic nucleus is roughly ______.

1 billion solar masses

According to the model in which active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of ______.

light emitted by hot gas in an accretion disk that swirls around the black hole

According to the model in which that active galactic nuclei are powered by supermassive black holes, the energy released as light comes from _____.

gravitational potential energy released by matter that is falling toward the black hole

Intergalactic hydrogen clouds are easiest to study by looking at ______.

absorption lines in quasar spectra

Hubble Space Telescope observations have shown that when the mass of the central black hole is very large, then ________

the mass of the bulge of the host galaxy is also very large

The best evidence for the existence of supermassive black holes is:

Very high orbital velocities in a very compact region.

During the history of the universe, what important event occurred about 0.001 seconds after the Big Bang?

Most matter in the early universe was annihilated by antimatter.

Which of the following important events occurred earliest in the history of the universe?

Spacetime rapidly expanded during a brief period of inflation.

During the history of the universe, what important event occurred about 380,000 years after the Big Bang?

Light began to travel freely through the universe.

Essentially all the hydrogen nuclei that will ever exist in our universe was created __________.

by the time the universe was about 3 minutes old

Compared to when the cosmic microwave background was first released, the radiation of the cosmic microwave background today is __________.

fainter and has most of its photons at longer wavelengths

In basic terms, what does the graph show?

The universe is getting cooler with time.

Approximately what is the current age of the universe in seconds?


Suppose you want to know what the temperature of the universe was 1 billion years ago. Where along the horizontal axis should you look?

almost all the way to the far right

What was the approximate temperature of the universe when the universe was just 1 second old?


From Part A, you know that in basic terms, the graph shows that the temperature of the universe decreases with time. In more specific terms, this graph shows that the temperature of the universe

fell very rapidly when the universe was young, but is changing very gradually today

What is the current temperature of the universe?

a few K

Which of the following does not provide strong evidence for the Big Bang theory?

observations of the amount of hydrogen in the universe

Which of the following does inflation help to explain?

the uniformity of the cosmic microwave background

Which of the following does inflation help to explain?

the origin of galaxies

The charge of an antiproton is


When a proton and an antiproton collide, they

convert into two photons.

Which of these pieces of evidence supports the idea that inflation really happened?

the apparently "flat" geometry of the universe

What is the earliest time from which we observe light in the universe?

a few hundred thousand years after the Big Bang

Which of the following best explains why the night sky is dark?

The universe is not infinite in space.

How do we determine the conditions that existed in the very early universe?

We work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size.

Why can’t current theories describe what happened during the Planck era?

We do not yet have a theory that links quantum mechanics and general relativity.

Which of the following statements best explains what we mean when we say that the electroweak and strong forces "froze out" at 10-38 second after the Big Bang?

These two forces first became distinct at this time

According to the Big Bang theory, how many forces – and which ones – operated in the universe during the GUT era?

2 forces: gravity and a single force that later became the strong, weak, and electromagnetic forces

Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies _________.

the electromagnetic and weak forces into the electroweak force

What was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?

The basic chemical composition of the universe had been determined.

The basic chemical composition of the universe had been determined.

During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.

Which of the following is not an observed characteristic of the cosmic microwave background?

It contains prominent spectral lines of hydrogen, the primary chemical ingredient of the universe.

In principle, if we could see all the way to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times prior to about 380,000 years after the Big Bang. Why?

Before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely.

If observations had shown that the cosmic microwave background was perfectly smooth (rather than having very slight variations in temperature), then we would have no way to account for _________.

how galaxies came to exist

In stars, helium can sometimes be fused into carbon and heavier elements (in their final stages of life). Why didn’t the same fusion processes produce carbon and heavier elements in the early universe?

By the time stable helium nuclei had formed, the temperature and density had already dropped too low for helium fusion to occur.

How does the idea of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?

Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure.

Suppose that inflation did not occur. In that case, the fact that the cosmic microwave background has the same temperature in opposite directions of the sky would be considered ___________.

surprising, because those locations would never have been close enough to have any light or matter exchanged between them

Which analogy best explains why inflation predicts that the overall geometry of the observable universe should appear to be flat?

Earth is so big that it the part explored by an ant appears flat

Suppose that the universe were infinite in both extent and age. In that case, we would expect the night sky to be ___________.

uniformly bright

The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation below (these are not real observations) would call our current theory into question?

The discovery of a galaxy with a helium abundance of only 10% by mass.

Why do scientists assume that the early universe was much hotter and denser than the universe of today?

The fact that the universe is expanding implies that objects were closer together in the past, and compressing material makes it hotter and denser.

Experiments allow physicists today to reproduce (on very small scales) energy and temperature conditions thought to have prevailed in the early universe as far back in time as about _________.

one ten-billionth (10-10) of a second after the Big Bang

What is antimatter?

It consists of particles that are just like those of ordinary matter except with opposite properties, such as opposite charge.

What happens when a particle of matter meets its corresponding antiparticle of antimatter?

The combined mass of the two particles is completely transformed into energy (photons).

What is the significance of the Planck time?

Before it, conditions were so extreme that our current understanding of physics is insufficient to predict what might have occurred.

The four fundamental forces that operate in the universe today are _________.

strong force, weak force, electromagnetic force, gravit

A "GUT" (grand unified theory) refers to theories that _________.

unify the strong force with the electromagnetic and weak forces

In terms of the "eras" that scientists use to describe different times in the history of the universe, we live in the __________.

era of galaxies

What do we mean by inflation?

a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe’s first second of existence

Which of the following statements correctly summarizes the events in the early universe according to the Big Bang theory?

The universe began with the forces unified. During the first fraction of a second, the forces separated and there was a brief but important episode of inflation. Subatomic particles of both matter and antimatter then began to appear from the energy present in the universe. Most of the particles annihilated to make photons, but some became protons, neutrons, electrons, and neutrinos. The protons and neutrons underwent some fusion during the first three minutes, thereby determining the basic chemical composition of the universe.

Which statement about the cosmic microwave background is not true?

It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.

What is the approximate temperature of the universe (as a whole) today?


art M
The Big Bang theory is widely accepted today because it successfully predicts two key observed features of the universe. What are these two key successful predictions?

(1) the existence and specific characteristics of the observed cosmic microwave background; (2) the observed overall chemical composition of the universe.

Which of the following observations cannot be explained by the Big Bang theory unless we assume that an episode of inflation occurred?

the fact that the temperature of the cosmic microwave background is almost the same everywhere

The critical density of the universe is the _______.

the total density of matter and energy needed to give the universe a "flat" geometry (in spacetime).

Models of the Big Bang that include inflation predict that the overall geometry of the universe should be "flat" (in spacetime). This prediction _________.

agrees with data found by studying the cosmic microwave background

Which of the following statements cannot be tested by science today?

Prior to the Planck time, our universe sprouted from another universe

Olbers’s paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?

Why is the sky dark at night?

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

A galaxy cluster

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

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 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.

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

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.

Which of the models predict that galaxies should be getting farther apart now? (Keep in mind that now is located at at time=0years on the graph

accelerating coasting critical recollapsing

Which of the models predict that galaxies will eventually get closer together in the future?


Which model predicts the largest average distance between galaxies 6 billion years ago?


Which model is most clearly ruled out by the data on the graph?


Which model is most strongly supported by the data on the graph?


Each data point represents a single white dwarf supernova that has been observed. The data points that appear farthest to the left on the graph represent the white dwarf supernovae that are the __________.

Farthest away from us

What additional data would be most valuable in helping scientists evaluate whether the accelerating model really is the best of the four models?

More observations of very distant white dwarf supernovae

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.

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.

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

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

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

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

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.

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

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

What is the mass-to-light ratio of the solar system?


Why do we call dark matter "dark"?

It emits no radiation that we have been able to detect.

Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it?

There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.

Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did not contain dark matter. How would their rotation curves be different?

The orbital speeds would fall off sharply with increasing distance from the galactic center.

The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?

Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.

It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?

Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.

How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?

We detect this gas with X-ray telescopes.

Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?

The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.

How does gravitational lensing tell us about the mass of a galaxy cluster?

Using Einstein’s general theory of relativity, we can calculate the cluster’s mass from the precise way in which it distorts the light of galaxies behind it.

If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?

They travel at speeds close to the speed of light.

Is space expanding within clusters of galaxies?

No, because their gravity is strong enough to hold them together even while the universe as a whole expands.

Which of the following statements about large-scale structure is probably not true?

Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.

Based on current evidence, a supercluster is most likely to have formed in regions of space where _________.

the density of dark matter was slightly higher than average when the universe was very young

Imagine that it turns out that dark matter (not dark energy) is made up of an unstable form of matter and that all of it suddenly decays tomorrow into photons or other forms of energy. Based on current understanding, which of the following would begin to occur?

The galaxies in clusters would begin to fly apart.

Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?

The actual average density of matter, even with dark matter included, is only about a quarter of the critical density.

Hubble’s constant is a "constant" in that its value __________.

is the same across all of space and does not change on human time scales

Hubble’s constant is related to the age of the universe, but the precise relationship depends on the way in which the expansion rate changes with time. For a given value of Hubble’s constant (such as 22 km/s/Mly), the age of the universe is oldest if __________.

the expansion rate has been increasing with time (an accelerating universe)

Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. Based on current understanding, which of the following would have to be true for this to be the case?

Dark energy does not exist and there is much more matter than current evidence suggests.

Scientists do not know what dark energy is, yet they still claim that it is the dominant constituent of the mass-energy of the universe. What (if any) evidence supports this claim?

There are two lines of evidence: measurements of the accelerating expansion, and the universal geometry inferred from the cosmic microwave background.

Which of the following statements correctly describes current understanding of the eventual fate of the universe?

Although the accelerating expansion suggests that the universe will continue to expand forever, it is possible that future discoveries will indicate a different fate.

Which of the following best summarizes what we mean by dark matter?

matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light

Which of the following best summarizes what we mean by dark energy?

It is a name given to whatever is causing the expansion of the universe to accelerate with time.

The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that _________.

dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way

What is a rotation curve?

a graph showing how orbital velocity depends on distance from the center for a spiral galaxy

What is the primary way in which we determine the mass distribution of a spiral galaxy?

We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy’s center.

What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?

Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward

Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?

Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.

In general, when we compare the mass of a galaxy or cluster of galaxies to the amount of light it emits (that is, when we look at it mass-to-light ratio), we expect that ______.

the higher the amount of mass relative to light (higher mass-to-light ratio), the greater the proportion of dark matter

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

measuring the temperatures of stars in the halos of the 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

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.

What is the distinguishing characteristic of what we call ordinary (or baryonic) matter?

It consists of atoms or ions with nuclei made from protons and neutrons.

Measuring the amount of deuterium in the universe allows us to set a limit on _________.

the density of ordinary (baryonic) matter the universe

What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?

They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.

Which of the following best sums up current scientific thinking about the nature of dark matter?

Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.

When we speak of the large-scale structure of the universe, we mean _________.

the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe

The primary evidence that has led astronomers to conclude that the expansion of the universe is accelerating comes from __________.

observations of white dwarf supernovae

Which of the following best sums up current scientific thinking about the nature of dark energy?

Dark energy probably exists, but we have little (if any) idea what it is

Based on evidence from measurements of the acceleration of the expansion rate and from careful study of the cosmic microwave background, about what percentage of the universe’s total mass and energy takes the form of ordinary atomic matter (protons, neutrons, and electrons)?


According to current understanding, if the universe continues to expand forever, the last major source of light will come from _________.

evaporation of black holes

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