ASTRO HW 9, 10, 11, 12

The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they?

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

Neither Dark Matter nor Dark Energy

- "dark" because it emits only infrared light - blocks light from stars behind it

Today, most scientists accept the Big Bang theory because its predictions agree so well with observations. But a scientific theory can always be revised or discarded if future observations do not agree with its predictions. Consider the following hypothetical future observations. Which one(s) would be inconsistent with the Big Bang theory?

- Careful studies of quasar spectra show that, 12 billion years ago, the temperature of the cosmic microwave background was slightly lower than it is today. - Over the next 10 years, the temperature of the cosmic microwave background falls to 1 K. - Astronomers discover distant protogalactic clouds with a helium abundance below 20%.

Which cosmic distance measurement techniques are considered standard candle techniques?

- Cepheids - main-sequence fitting - white dwarf supernovae (distant standards)

Which of the following represent observations or measurements that have confirmed key predictions of the Big Bang model?

- Characteristics of the cosmic microwave background. - The abundance of helium in the universe.

Consider all of the observations shown in the video. Which of the following are reasonable conclusions?

- Gas orbits the radio source called Sgr A*. - Stars near the galactic center are much closer together than stars around our Sun. - There are strong magnetic fields in the central region of the galaxy.

Which of the following accurately describe some aspect of gravitational waves?

- Gravitational waves are predicted to travel through space at the speed of light. - The existence of gravitational waves is predicted by Einstein's general theory of relativity. - The first direct detection of gravitational waves came in 2015. - Gravitational waves carry energy away from their sources of emission.

Which of the following observations were made by Vera Rubin and Kent Ford?

- Many spiral galaxies have hydrogen gas clouds orbiting their centers at surprisingly high speeds. - Stars in the outskirts of the Andromeda galaxy orbit the galaxy's center at surprisingly high speeds.

Order from first to last to occur and also smallest to biggest (the era's) What is this backwards?

- Planck era: all 4 forces operated as one - GUT era: strong, electro weak forces unite as GUT force - Electroweak era:3 forces operated: gravity, strong, electroweak - Particle era: protons, neutrons both common - Era of nucleo-synthesis: fusion created helium nuclei - Era of nuclei: H, He nuclei and electrons existed but no neutral atoms _ - Era of atoms: neutral atoms existed, but not stars - Era of galaxies: stars and galaxies common coolest to hottest

Which of the following statements about gravitational waves are true?

- The first direct detection of gravitational waves, announced in 2016, came from the LIGO observatory. - Two orbiting neutron stars or black holes will gradually spiral toward each other as a result of energy being carried away by gravitational waves. - The emission of gravitational waves from merging black holes is predicted by Einstein's general theory of relativity.

Not true about Black Holes

- a 3-solar-mass black hole may be hidden between Jupiter and Saturn - a black hole will suck in any binary companion star - you would be squashed by gravity at the event horizon of any black hole - black holes emit x-ray light from within their event horizons

True about Black Holes

- a black hole can have the mass of a star in a space less than a few kilometers across - a black hole is an object smaller than its own Schwarzschild radius - two orbiting black holes can merge and emit gravitational waves - material from a binary companion can form an X-ray-emitting accretion disk around a black hole - a black hole can form during a supernova explosion

Largest mass to smallest mass

- a typical black hole(formed in a supernova) - a typical neutron star - a one-solar-mass white dwarf - main-sequence star of spectral M - Jupiter - the moon

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

Dark Energy(only)

- inferred to exist from the measured acceleration of the universe's expansion - the proposed source of a universal repulsive force

Dark Matter(only)

- likely to consist of tiny, weakly interacting particles - detected through its gravitational attraction - has mass - most common form of mass in the halo of the Milky Way Galaxy

Largest diameter to smallest diameter

- main-sequence star of spectral type A - Jupiter - a one-solar-mass white dwarf - the moon - a two-solar-mass neutron star - the event horizon of a two-solar-mass black hole

Oldest to youngest

- red giant in globular cluster M13, red main-sequence star in globular cluster M13 - the sun - hot, blue main-sequence star in disk

Highest density to lowest density

- the singularity of a black hole - a typical neutron star - a one-solar-mass white dwarf - a main-sequence star

Unknown about Black Holes

- the singularity of a black hole has infinite density - black holes make up 1% of the mass of the Milky Way Galaxy

Both Dark Matter and Dark Energy

- the universe contains more of this than it contains matter made from atoms - we do not know what it is made of

Which of the following provide evidence for dark matter in clusters of galaxies?

- the way clusters bend light from galaxies behind them - the temperature of hot, X-ray emitting gas within clusters - orbital speeds of galaxies within clusters

In (Figure 1) at left, which of the four labeled supernovae is the most distant from us?

1

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

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.)

10,000

If you tried to fly into a ______________, you would be killed by tidal forces before you crossed the event horizon.

10-solar-mass black hole

Based on counting the number of galaxies in a small patch of the sky and multiplying by the number of such patches needed to cover the entire sky, the total number of galaxies in the observable universe is estimated to be approximately

100 billion

We have direct experimental evidence (from large particle accelerators) for the physical conditions in the universe back to about ________ after the Big Bang.

10^-10 seconds

Look again at the orbit of the star with the highlighted orbit. By comparing the orbit to the scale bar shown on the diagram, you can estimate that this orbit has a semimajor axis of about _____.

1150 AU

Based on current estimates of the value of Hubble's constant, about how old is the universe?

14 billion years old

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

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

To determine the mass of the central object, we must apply Newton's version of Kepler's third law, which requires knowing the orbital period and average orbital distance (semimajor axis) for at least one star. We could consider any of the stars shown in the figure, so let's consider the star with the highlighted orbit (chosen because its dots are relatively easy to distinguish). What is the approximate orbital period of this star?

20 yr

Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy?

200 million years

What is the temperature of the universe today?

3 K

What is the Schwarzschild radius of a 10 solar mass black hole?

30 km

Based on the measurements discussed in part D, the mass of the central black hole is calculated to be about __________ times that of the Sun.

4 million

The following equation, derived from Newton's version of Kepler's third law, allows us to calculate the mass (M) of a central object, in solar masses, from an orbiting object's period (p) in years and semimajor axis (a) in astronomical units: M=a3p2 Using this formula with the values you found in Parts C and D, what is the approximate mass of the central object?

4 million solar masses

Based on observations of the cosmic microwave background, the overall composition of the universe is approximately ________.

5% ordinary (baryonic) matter, 27% non-ordinary (nonbaryonic) dark matter, 68% dark energy

What is the approximate luminosity of the Cepheid whose light curve is shown in the graph?

9000 LSun

Which of the following best describes a black hole?

A place from which the escape velocity exceeds the speed of light.

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.

__________ predict(s) that the universe is expanding today.

All four models

The Big Bang theory is closely linked to Hubble's discovery that the universe is expanding, which seems to imply that there was a time in the past when the expansion first began. Nevertheless, the Big Bang theory did not gain widespread acceptance among scientists until the 1960s. Why wasn't expansion alone enough to convince scientists that the Big Bang really happened?

Although expansion seems to imply a Big Bang, no other specific predictions of the Big Bang theory were tested and confirmed until the 1960s.

Why might inflation have occurred at the end of the GUT era?

An enormous amount of energy was released when the strong and electroweak forces froze out from the GUT force.

Consider the statement from Part A reading "a 3-solar-mass black hole may be hidden between Jupiter and Saturn." How do we know this statement is not true?

An object of that mass would disrupt the orbits of the planets in our solar system.

What causes the radio pulses of a pulsar?

As the neutron star spins, beams of radio radiation sweep through space. If one of the beams crosses the Earth, we observe a pulse.

Which of the following does not accurately describe what we observe toward the Galactic center?

At optical wavelengths, we see a cluster of old, red stars.

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

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.

In principle, if we could see all the way back in time 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.

Why can't we see past the cosmological horizon?

Beyond the cosmological horizon, we would be looking back to a time before the universe was born.

The _____________ is the name we give to the moment when the expansion of the universe began.

Big Bang

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

Why did the Big Bang not produce heavier elements?

By the time helium could survive, the temperature had become too low for heavier elements to form.

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.

What is the most accurate way to determine the distance to a nearby galaxy?

Cepheid variables

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

Cepheids

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

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

Dark energy does not exist and there is much more dark matter than we are aware of to date.

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.

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.

Which of the following statements about degeneracy pressure is not true?

Degeneracy pressure can arise only from interactions among electrons.

Notice that the "verdict" at the end of the Extraordinary Claims essay still leaves open the possibility that dark matter does not exist and that there is instead some problem with our theory of gravity. Which of the following hypothetical observations would give scientists substantially greater confidence in the existence of dark matter?

Direct detection of WIMPs in laboratories on Earth.

Each item below belongs either with the population of disk stars or the population of halo stars of the Milky Way Galaxy. Match each item to the appropriate population.

Disk Stars - youngest stars - the sun - stars that all orbit in nearly the same plane - high-mass stars Halo Stars - stars with the smallest abundance of heavy elements - oldest stars - stars whose orbits can be inclined at any angle - globular clusters

How does a black hole form from a massive star?

During a supernova, if the mass of the infalling core has enough gravity to overcome neutron degeneracy pressure, the core will collapse to a black hole.

According to the Big Bang theory, why do we live in a universe that is made of matter rather than antimatter?

During the rst 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 statements about electron degeneracy pressure and neutron degeneracy pressure is true?

Electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars.

Astronomers suspect that a galaxy's type can be determined either by initial conditions in a protogalactic cloud or by later interactions between galaxies. Each item below describes either an initial condition or a later interaction. Match these items to the galaxy type expected to form as a result.

Elliptical Galaxy - a galaxy collision strips away gas - stars form rapidly as the protogalactic cloud shrinks - protogalactic cloud has high density - protogalactic cloud rotates very slowly Spiral Galaxy - protogalactic cloud has high angular momentum - most protogalactic gas settles into a disk

Who first noticed the surprisingly high speeds of galaxies within clusters?

Fritz Zwicky

The _____________ refers to a time period in which the strong, weak, and electromagnetic forces were all unified.

GUT era

Which of the following best describes how galaxies are distributed on large scales in the universe?

Galaxies appear to be distributed in chains and sheets that surround great voids.

Consider the statement from Part A reading "the singularity of a black hole has infinite density." Why is this statement in the "unknown" bin?

General relativity and quantum mechanics give different answers about the nature of singularity.

What are the two possible explanations for not seeing something glowing brightly along every line-of-sight in the sky? I. The universe is infinite in size and infinitely old, but dust absorbs starlight from far away stars. II. The universe has a finite number of stars. III. The universe is changing in some way as to prevent us from seeing an infinite number of stars.

II and III

Which statement concerning black hole masses and Schwarzschild radii is not true?

In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star.

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, tiny quantum fluctuations in energy fields to grow into the density differences in the primordial gas from which galaxies formed.

Consider the portion of the video that starts with the all-sky view of the Milky Way and then zooms in to the galactic center. All of the images except the first two show radio, infrared, or X-ray light. Why don't these images show visible light?

Interstellar dust in the galactic disk prevents us from seeing the galactic center with visible light.

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.

Suppose there is a large amount of (weakly interacting) dark matter between us and a distant galaxy. How will this affect our view of the distant galaxy?

It does not affect our view; we can see right through the dark matter.

Why do we call dark matter "dark"?

It emits no radiation of any wavelength.

What is happening to the temperature of the universe?

It is becoming cooler over time. (today is the coolest)

What do we mean by the singularity of a black hole?

It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.

Which of the following statements about the cosmic background radiation is not true?

It is the result of a mixture of radiation from many independent sources, such as stars and galaxies that formed within the first billion years of the Big Bang.

How does the interstellar medium affect our view of most of the galaxy?

It prevents us from seeing most of the visible and ultraviolet light from the galactic disk.

Consider a binary system of two neutron stars. How should the emission of gravitational waves affect this system?

It should cause the orbits of the two objects to decay with time.

Which of the following offers at least some evidence that inflation really occurred in the early universe?

Measurements of brightness variations in the cosmic microwave background.

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

If the electromagnetic force is stronger than gravity, then why is it overpowered by gravity on large scales?

Most objects are electrically neutral.

Why can't the dark matter in galaxies be made of neutrinos?

Neutrinos travel at extremely high speeds and can escape a galaxy's gravitational pull.

Why is the Big Bang theory the only theory of the universe you learn about in a typical astronomy class?

No competing theory has survived the test of evidence.

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.

On average, is the distance between galaxies increasing within clusters of galaxies?

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

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.

________________ predict(s) that the universe is expanding faster now than it was in the distant past.

Only the accelerating model

__________ assume(s) that the expansion rate of the universe always stays the same.

Only the coasting model

_______________ predict(s) that the average density of mass in universe is exactly the critical density.

Only the critical model

______________ predict(s) that the universe will someday contract.

Only the recollapsing model

Which of the following describes key evidence for dark matter in individual spiral galaxies like the Milky Way?

Orbital speeds of stars stay relatively high even at great distances from the galactic center.

What direct evidence do we have that the weak and electromagnetic forces were once unified as a single force?

Particle accelerators on Earth can reach energies equivalent to the high temperatures of the Electroweak era and have produced particles predicted by the unified theory.

The ___________ refers to a time period in which all four forces are thought to have been unified.

Planck era

How does the theory of inflation explain the near-uniformity of the cosmic microwave background?

Prior to rapid inflation, all regions of space were close enough to bounce radiation back-and-forth and reach the same temperature.

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

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

If you wanted to observe the center of our galaxy, you would need to point a telescope in the direction of the constellation __________.

Sagittarius

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

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

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.

Which of the following three-step processes correctly describes how we use Cepheids as a tool to make cosmic distance measurements? In all cases, assume that the Cepheid's apparent brightness has been carefully measured through observations.

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.

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.

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.

If all the "dark matter" in our universe were to be instantaneously removed, which of the following would not happen?

The Solar System would fly apart.

How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?

The accretion disk around a neutron star is much hotter and emits higher-energy radiation.

Why do astronomers think the expansion rate of the universe is accelerating, rather than decelerating (as previously thought)?

The average distance between galaxies is greater than expected when looking at very distant galaxies.

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.

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).

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.

Notice that some of the stars on the diagram are represented by a series of dots that are very close together, while others have their dots farther apart. Keeping in mind that all the stellar positions were measured at approximately one-year intervals, which stars are moving the fastest in their orbits during the time period indicated by the dots?

The fastest stars are the ones with: the dots farthest apart

Why isn't the space within our solar system or the Milky Way expanding according to Hubble's Law?

The gravity exerted by the solar system and the Milky Way is strong enough to hold them together against the expansion of the universe.

Which of the following is a consequence of Hubble's Law?

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

What evidence supports the existence of a very massive black hole at the center of our galaxy?

The motions of the gas and stars at the center indicate that it contains 4 million solar masses within a region no larger than our solar system.

What is it about the observations of Zwicky, Rubin, and Ford that suggests the existence of large quantities of dark matter?

The observed orbital speeds are too high to be explained by the gravity of the visible stars and gas clouds.

Based on observations, which of the following statements about stars in the Milky Way is generally true?

The older the star, the lower its abundance of heavy elements.

Why do we expect the cosmic background radiation to be almost, but not quite, the same in all directions?

The overall structure of the universe is very uniform, but the universe must have contained some regions of higher density in order for galaxies to form.

Why do we think tiny quantum ripples should have been present in the very early universe?

The principles of quantum mechanics require that the energy fields at any point in space be continually fluctuating.

How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?

The stars that formed first could orbit the center of the galaxy in any direction at any inclination.

Why did the era of nuclei end when the universe was about 380,000 years old?

The universe had expanded and cooled enough for stable, neutral atoms to form.

Which of the following is not consistent with observations of the cosmic microwave background?

The universe is at least 20 billion years old.

Why do spiral galaxies appear blue in color?

Their light output is dominated by hot, massive blue stars.

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.

You've now found that the central object has a mass of about 4 million solar masses but is no more than about 70 AU in diameter—which means it cannot be much larger than the size of our planetary system. Why do these facts lead astronomers to conclude that the central object is a black hole?

There is no known way to pack so much mass into such a small volume without it collapsing into a black hole.

What is happening to the wavelength of the universe?

They are becoming longer. (500,000 years after the Big Bang has a shorter wavelength than today)

By looking at the graph of Hubble's law, 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.

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.

What happened to all of the quarks that existed freely during the particle era?

They combined in groups to make protons, neutrons, and their antiparticles.

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.

What makes white-dwarf supernovae very good standard candles for distance measurements?

They should all have approximately the same luminosity.

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.

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

This force first became distinct at this time.

Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?

Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.

How many forces operated in the universe during the GUT era?

Two: gravity and the GUT force

Which of the following is the most likely candidate for what makes up the majority of dark matter?

WIMPs

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.

Consider the statement from Part A reading "black holes make up 1% of the mass of the Milky Way Galaxy." Why is this statement in the "unknown" bin?

We cannot detect all black holes and therefore don't know the percentage of the galaxy's mass they make up.

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 from the X-rays it produces.

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 describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?

We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.

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.

What evidence suggests that the Milky Way contains dark matter?

We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.

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 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.

Which of the following is not a good reason why white-dwarf supernovae are good standard candles for distance measurements?

White-dwarf supernovae occur only among young and extremely bright stars.

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

Why is the sky dark at night?

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

How do X-ray measurements help us measure the amount of dark matter in galaxy clusters?

X-rays are emitted by hot gas, and the intracluster gas is heated by the gravitational effects of dark matter. More dark matter leads to greater heating, and hence stronger X-ray emission.

Suppose you were unfortunate enough to fall into a black hole in a binary system where the black hole was accreting matter from its companion star. Which of the following is most likely to kill you first?

X-rays from the accretion disk

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

a galaxy cluster

A standard candle is ________.

a light source of known luminosity

Which model of the universe gives the youngest age for its present size and expansion rate

a re-collapsing universe

What do we mean by inflation in the context of the Big Bang theory?

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

What do we mean by inflation?

a sudden expansion of the universe driven by the energy released when the strong and electroweak forces froze out from the GUT force

This figure shows how the luminosity of supernovae change over time. How long does it take a white dwarf supernova to decrease in luminosity by a factor of 100 from its peak?

about 200 days

The actual matter density of the universe, accounting for all of the luminous matter and all of the dark matter known to exist in galaxies and clusters, is what fraction of the critical density?

about 30 percent

The cosmic microwave background peaks at a wavelength of about 1 mm, and the universe has a temperature of about 3 K. If the microwave background peaked at a wavelength of 10 micrometers, what would its temperature be? (Hint: see Cosmic Calculations 5.1 and 17.1 in the textbook)

about 300 K

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

accelerating

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

accelerating

Which model(s) predict(s) that galaxies are getting farther apart NOW?

accelerating coasting critical recollapsing (all of them)

A(n) _____________ can form around a white dwarf, neutron star, or black hole in a binary system.

accretion disk

From an observational standpoint, what is a pulsar?

an object that emits flashes of light several times per second (or even faster), with near perfect regularity

What is the basic definition of a black hole?

an object with gravity so strong that not even light can escape

The distribution of the dark matter in a spiral galaxy is

approximately spherical and much larger than the galaxy's visible halo.

If we could view our galaxy from a distance of several hundred thousand light-years, it would appear ________.

as a flattened disk with a central bulge and spiral arms

The critical density of the universe is the ________.

average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist

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.)

away from us at 2,000 km/s

Why is the sky dark at night?

because the universe is not infinitely old, so the distance we can see is limited

Given that the electromagnetic force is far stronger than gravity on a per-particle basis, why doesn't the electromagnetic force dominate the interactions of large objects like planets, stars, and galaxies?

because there are both positive and negative electric charges

The Planck era refers to the time period

before the Planck time.

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

If you tried to visit a _____________, you would probably be killed by radiation well before you reached the black hole itself.

black hole in an X-ray binary system

LIGO detects gravitational waves because the lengths of its arms change as gravitational waves pass by. About how much are these lengths expected to change when LIGO detects gravitational waves from the merger of two neutron stars or two black holes?

by an amount smaller than the diameter of a proton

How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?

by applying Newton's version of Kepler's third law (or the equivalent orbital velocity law) to the Sun's orbit around the center of the Galaxy

Consider the portion of the video that shows two galaxy clusters that have collided, and includes a map showing the inferred distribution of dark matter among these clusters. How did astronomers infer the location of the dark matter?

by carefully observing how the light of galaxies beyond these clusters has been bent by the clusters

The _________________ is our name for the photons we see that were first released when the universe was about 380,000 years old.

cosmic microwave background

Evidence suggests that about two-thirds of the total mass plus energy in the universe consists of ____________.

dark energy

We say use the term ________________ as a name for whatever is causing the expansion of the universe to accelerate with time.

dark energy

Astronomers suspect that ____________ consists of weakly interacting massive particles, or WIMPs.

dark matter

Large-scale structure in the universe has probably been shaped primarily by the gravity of ___________.

dark matter

Matter that emits no light of any kind is called ______________.

dark matter

Given such small length changes (as noted in Part D), what can give scientists confidence that they have really detected a gravitational wave signal?

detecting the same changes at more than one location

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

After a massive-star supernova, what is left behind?

either a neutron star or a black hole

__________ explains why two protons will repel each other

electromagnetism

___________ governs virtually all chemistry and biology

electromagnetism

____________ explains the attraction between a proton and an electron

electromagnetism

A white dwarf can remain stable in size because of _____________.

electron degeneracy pressure

Which of the following unifications as been directly tested and confirmed through experiments in particle accelerators?

electroweak unification

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

elliptical galaxies lack anything resembling the disk of a spiral galaxy

The most massive galaxies in the universe are

elliptical.

Which of the following types of galaxies appear reddest in color?

ellipticals

Most of the helium in the universe formed during the _____________.

era of nucleosynthesis

The ____________ marks the boundary between the inside and outside of a black hole.

event horizon

The boundary from within which light cannot escape from a black hole is called the black hole's __________.

event horizon

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 makes up the interstellar medium?

gas and dust

The bending of light by gravity is called ________________.

gravitational lensing

__________ dominates the universe at the largest scales

gravity

____________ the weakest of the four forces on a per-particle basis

gravity

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

What kinds of atomic nuclei formed during the nucleosynthesis era?

hydrogen, helium and trace amounts of lithium, beryllium, and boron

To calculate the dashed orbits from the stellar positions, astronomers had to assume that __________.

if they observed for many more years, the dots would trace out ellipses

The Sun's location in the Milky Way Galaxy is ________.

in the galactic disk, slightly over halfway out from the center

A very short period of extremely rapid expansion, called ___________, is thought to have occurred when the universe was about 10-38 second old.

inflation

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

Matter that emits X-rays but not visible light is considered a form of _________.

luminous matter

Stars and gas clouds are considered to be forms of ____________.

luminous matter

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

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

The graph of Hubble's law 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

A neutron star can remain stable in size because of _______________.

neutron degeneracy pressure

A 10 solar mass main sequence star will produce which of the following remnants?

neutron star

From Part E you know the mass of the central object. Now consider its size. Based on what you can see in the diagram, you can conclude that the diameter of the central mass is __________.

no more than about 70 AU

A(n) ________ occurs when fusion ignites on the surface of a white dwarf.

nova

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

What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?

observations of white dwarf supernovae

Based on our current understanding of physics, we can understand the conditions that prevailed in the early universe as far back in time as about ________.

one ten-billionth of a second after the Big Bang

With current technology, we expect to be able to detect (directly) gravitational waves from a binary system of two neutron stars or two black holes __________.

only from the instant when the two objects merge into one

A(n) _________ is rapidly rotating neutron star.

pulsar

Astronomers are seeking to obtain an image of the region around the black hole's event horizon with a project called the Event Horizon Telescope. What type of light does this project seek to observe?

radio waves

Considering the graph of expansion models, which model(s) predict(s) that galaxies will eventually get closer together?

recollapsing

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

recollapsing

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

recollapsing

The ____________ is the place to which all of a black hole's mass is in principle located within the black hole.

singularity

Which adjective does not necessarily describe a known feature of the early universe? (Be sure to consider the universe as a whole, not just the observable universe.)

small

Where is most of the dark matter in our galaxy thought to exist?

spread throughout the halo of the galaxy

What is the most accurate way to determine the distance to a nearby star?

stellar parallax

_________ holds quarks together in protons and neutrons

strong force

____________ holds protons and neutrons together in atomic nuclei

strong force

Which of the four forces that exist today would be merged together at high energies in what scientists call the "GUT force"?

strong force electromagnetism weak force

According to current understanding of physics, which four of the following are the four fundamental forces in nature?

strong force weak force gravity electromagnetism

Degenearcy pressure arises when ________.

subatomic particles are packed as tightly as the laws of quantum mechanics allow

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

Ignoring any radiation, you could in principle survive the journey across the event horizon of a ___________.

supermassive black hole

Which of the following observatories is most likely to discover a black hole in a binary system?

the Chandra X-Ray Observatory

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

What caused the rapid spin of a neutron star that we see as a pulsar?

the conservation of angular momentum during the collapse of the original star's core

What are two key observational facts that led to widespread acceptance of the Big Bang model?

the cosmic background radiation and the helium content 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

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

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

Which of the following observations (if made) would disprove an important prediction of the Big Bang theory?

the discovery of a galaxy with 10% helium abundance

Hubble's law expresses a relationship between __________.

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

From what cosmic epoch do the photons in the cosmic background radiation originate?

the end of the era of nuclei

What is postulated to have caused a sudden inflation of the early universe?

the energy released from the "freezing out" of the strong force from the GUT force

Cosmological redshift is the result of ________.

the expansion 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

In a photo like the Hubble eXtreme Deep Field (the figure 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

What is the main idea of the Big Bang theory?

the idea that all matter and energy in the universe began in an unimaginably dense state, and then space itself began expanding in all directions

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

Gravitational waves were first detected directly in 2015. According to models, the source of these gravitational waves was __________.

the merger of two black holes

"Photons of the cosmic microwave background have traveled through space for almost 14 billion years." This statement follows from our model of the Big Bang, because the Big Bang model is based on the idea that __________.

the universe began very hot and dense and has been cooling as it expands

What characteristics of the orbiting stars do we need to measure in order to calculate the mass of the central object, Sgr A*?

their orbital periods and average orbital distances

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

Hubble's "constant" is not constant throughout

time

"Large-scale structure grew around density variations present in the early universe." Observational evidence that such density variations really existed comes from the fact that the cosmic microwave background exhibits __________.

tiny temperature variations in different directions

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

The first gravitational waves were detected in 2015 by the LIGO observatories in Washington and Louisiana. What event was thought to cause these gravitational waves?

two black holes merging

What are the Magellanic Clouds?

two small galaxies that orbit the Milky Way Galaxy

What two quantities did Edwin Hubble compare for a sample of galaxies to discover the expansion of the universe?

velocity and distance

Most stars in the Milky Way's halo are ________.

very old

_____________ the only force besides gravity that affects neutrinos

weak force

What is the most accurate way to determine the distance to a very distant galaxy?

white dwarf supernova

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