Astronomy Homework 11
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
What does this graph show?
It shows calculations indicating that the temperature of the universe began quite high but is now quite low.
Which cosmic distance measurement techniques are considered standard candle techniques?
-Main-sequence fitting -White dwarf supernovae -Cepheids
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
What is the current temperature of the universe?
A few K
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.
This figure shows a "slice of the universe" from the Sloan Digital Sky Survey. What is the Sloan Great Wall (indicated by the arrow)?
a huge collection of galaxies extending a billion light-years in length
Current estimates place the age of the universe at about ______.
14 billion years
Which of these galaxies is most likely to be oldest?
A galaxy in the local group
Estimate the luminosity of the Cepheid with a period of 8 days.
2000 solar luminosities
Based on inventoried matter in the universe, including dark matter known to exist in galaxies and clusters, the actual density of the universe is what fraction of the critical density?
26 percent
Approximately how long did the era of nucleosynthesis last?
5 minutes
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)?
5%
From here, 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?
8000Lsun
What is a Cepheid variable?
A type of very luminous star that makes an excellent standard candle
Prelecture Narrated Figure: Process of Science: Data to Distinguish among Expansion Models C. Which model predicts the largest average distance between galaxies 6 billion years ago?
Accelerating
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.
According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?
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 slight excess of matter particles.
What does the universe look like on very large scales?
Galaxies appear to be distributed in chains and sheets that surround great voids.
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.
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 do we think tiny quantum ripples should have been present in the very early universe?
Quantum mechanics requires that the energy fields at any point in space be continually fluctuating as a result of the uncertainty principle.
You observe the peak brightnesses of two white dwarf supernovae. Supernova A is only 1/4 as bright as Supernova B. What can you say about their relative distances?
Supernova A is twice as far away as Supernova B.
If the universe is accelerating, it will expand forever.
T
Evidence that the cosmic background radiation really is the remnant of a Big Bang comes from predicting characteristics of remnant radiation from the Big Bang and comparing these predictions with observations. Four of the five statements below are real. Which one is fictitious?
The cosmic background radiation is expected to contain spectral lines of hydrogen and helium, and it does.
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.
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.
Why do we believe 90 percent of the mass of the Milky Way is in the form of dark matter?
The orbital speeds of stars far from the galactic center are surprisingly high, suggesting that these stars are feeling gravitational effects from unseen matter in the halo.
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.
Why did the era of nuclei end when the universe was about 380,000 years old?
The universe had expanded and cooled to a temperature of about 3,000 K, cool enough for stable, neutral atoms to form.
Process of Science: Temperature of the Universe C.In basic terms, what does the graph show?
The universe is getting cooler with time.
Which of the following best explains why the night sky is dark?
The universe is not infinite in space.
This graph shows the rotation curves of four different spiral galaxies. Based on these curves, what do all four galaxies have in common?
Their most distant stars all orbit at about the same speed as stars located about 30,000 light-years from their centers.
Which of the following is not implied by this diagram?
There are six distinct forces known to operate in the universe, and a seventh force called the "super force" might also exist.
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.
In the distant past, the cosmic microwave background consisted primarily of infrared light.
True, because In the distant past, the cosmic radiation was waves smaller than gamma. quite penetrating, that caused many changes in genes leading to evolution.
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 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.
Olbers' 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?
About how many galaxies are there in a typical cluster of galaxies?
a few dozen
Process of Science: Dark Matter Evidence from the Bullet Cluster A. Examine the visible light image of the Bullet Cluster. What kind of cluster is it?
a galaxy cluster
Visual Activity: Rotation Curves and Evidence of Dark Matter A. 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
The most basic difference between elliptical galaxies and spiral galaxies is that ______.
elliptical galaxies lack anything resembling the disk of a spiral galaxy
Which of the following does not provide strong evidence for the Big Bang theory?
observations of the amount of hydrogen in the universe
Hubble's galaxy classification diagram (the "tuning fork") ______.
relates galaxies according to their shapes but not according to any evolutionary status
Hubble's "constant" is constant in
space
The critical density of the universe is the _______.
the total density of matter and energy needed to give the universe a "flat" geometry (in space time).
Notice the distorted galaxy images, such as the large arc-shaped structure, in this image of a galaxy cluster. What can astronomers learn by carefully measuring the distortions in this image?
the total mass of the cluster
Which of the following does inflation help to explain?
the uniformity of the cosmic microwave background
Process of Science: Interplay of Observations and Models in the Big Bang Theory C. One of the inferred statements from Part A is that "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
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
The Planck era refers to the time period
before the Planck time.
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 (Figure 20.1 in your text), 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
Key Concept: Dark Matter and Dark Energy B. 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.
If inflation really occurred, then our observable universe is only a tiny portion of the entire universe born in the Big Bang
T
The observed composition of ordinary matter in the universe-roughly 75 percent hydrogen and 25 percent helium-closely matches theoretical predictions based on the Big Bang model.
T
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.
Prelecture Video: Eras of the Universe D.Essentially all the hydrogen nuclei that will ever exist in our universe were created __________.
by the time the universe was about 3 minutes old
Suppose that the universe were infinite in both extent and age. In that case, we would expect the night sky to be ___________.
uniformly bright
Which of the following is an example of baryonic matter?
you
Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?
Cepheids
Prelecture Video: Eras of the Universe E.Drag words from the left to the blanks on the right.
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.
Which of the following statements about types of galaxies is not true?
Elliptical galaxies are bluer and contain more dust than spiral galaxies.
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.
Prelecture Video: Eras of the Universe C.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.
What is main-sequence fitting?
a method for determining the distance to a star cluster by assuming that its main sequence should line up with the main sequence on a standard H-R diagram
The white arrow points to an object that lies within the disk of the galaxy shown. What is this object?
a supernova
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 this period . viewed at arm's length against the sky
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 only as a result of Hubble's law.)
away from us at 2,200 km/s
Which of the following is not one of the three major categories of galaxies?
globular 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
Overall, what is our most accurate technique for measuring the distance to a nearby star?
stellar parallax
These diagrams represent four possible models for the universe. Which model presumes the existence of some type of dark energy in the universe?
accelerating universe
Process of Science: Testing the Expanding Universe C. Consider the observation "The Andromeda Galaxy, a member of our Local Group, is moving toward us." Why doesn't this observation contradict the idea that the universe is expanding?
Because the galaxies of the Local Group are gravitationally bound together.
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.
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.
Estimate the luminosity of the Cepheid with a period of 35 days.
10,000 solar luminosities
What is not a main source of evidence for the existence of dark matter?
Massive blue stars
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.
According to the Big Bang theory, how many forces, and which ones, operated in the universe during the GUT era?
Two forces: gravity and a single force that later became the strong, weak, and electromagnetic forces
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.
What is the earliest time from which we observe light in the universe?
a few hundred thousand years after the Big Bang
This figure shows an all-sky map of the cosmic microwave background recorded by the Planck telescope. How long did this light travel through space before it reached the telescope?
about 14 billion years
Gravitational lensing occurs when
massive objects bend light beams that are passing nearby.
Process of Science: Interplay of Observations and Models in the Big Bang Theory B.Consider this statement from Part A: "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
Process of Science: Dark Matter Evidence from the Bullet Cluster E. 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
Prelecture Narrated Figure: Process of Science: Data to Distinguish among Expansion Models A. 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
Process of Science: Temperature of the Universe A.The horizontal axis of the graph measures the time since the Big Bang in seconds. The extreme left of the horizontal axis, where it meets the vertical axis, is labeled 10−45 seconds (that is a decimal point followed by 44 zeroes and then a one). If we move along two tick marks to the right on the horizontal axis, it is labeled 10−35 seconds. By what factor does the time increase from one tick mark on the horizontal axis to the next tick mark toward the right?
1 x 10^(5)
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.
Why do virtually all the galaxies in the universe appear to be moving away from our own?
Expansion causes all galaxies to move away from nearly all others.
Although we don't know exactly when clusters, galaxies, or stars began forming, we do know that clusters came first, with galaxies and stars forming later.
F
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
Why do we call dark matter "dark"?
It emits no or very little radiation of any wavelength.
Why do we call dark matter "dark"?
It emits no radiation that we have been able to detect.
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.
This figure shows an all-sky map of the cosmic microwave background recorded by the Planck telescope. What do the dark and bright variations represent?
Regions of the sky with slightly different density at the time the radiation was emitted
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.
Where do the photons in the cosmic background radiation originate?
The end of the era of nuclei
How do we determine the amount of dark matter in elliptical galaxies?
We measure the speeds of stars at different radii from the galactic center and determine how much mass is interior to the orbit.
What is the approximate temperature of the universe (as a whole) today?
3K
What is the ultimate fate of an open universe?
All matter decays to a low-density sea of photons and subatomic particles
Why are Cepheid variables important?
Cepheids are pulsating variable stars, and their pulsation periods are directly related to their true luminosities. Hence, we can use Cepheids as "standard candles" for distance measurements.
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 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.
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.
Overall, what is our most accurate technique for measuring the distance to a nearby star?
Stellar Parallax
You observe the peak brightnesses of two white dwarf supernovae. Supernova A is only one-quarter as bright as Supernova B. What can you say about their relative distances?
Supernova A is twice as far away as Supernova B.
What happened to the quarks that existed freely during the particle era?
They combined in groups to make protons, neutrons, and their antiparticles.
What kinds of atomic nuclei formed during the era of nucleosynthesis?
hydrogen and helium and trace amounts of deuterium and lithium
Helium originates from
the Big Bang with a small contribution from stellar nucleosynthesis.
What is the most accurate way to determine the distance to a nearby galaxy?
using Cepheid variables
The Big Bang theory is widely accepted today because it successfully predicts and explains several key observed features of the universe. Two of its important predictions that have been verified are ___________.
(1) the existence and specific characteristics of the observed cosmic microwave background and (2) the observed overall chemical composition of the universe
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.
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
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.
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.
How did Edwin Hubble measure the distance to the Andromeda Galaxy?
He applied the period-luminosity relation to Cepheid variables.
Hubble's law expresses a relationship between __________.
the distance of a galaxy and the speed at which it is moving away from us
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.
Process of Science: Dark Matter Evidence from the Bullet Cluster F. 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.
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.
What two observable properties of a Cepheid variable are directly related to one another?
What two observable properties of a Cepheid variable are directly related to one another? the period between its peaks of brightness and its luminosity
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, whereas massive supernovae come in a wide range of peak luminosities
Based on current estimates of the value of Hubble's constant, how old is the universe?
between 12 and 16 billion years old
When a proton and an antiproton collide, they
convert into two photons
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
The fact that the universe is expanding means that space itself is growing within ___________.
the observable universe
A GUT (grand unified theory) refers to theories that
unify the strong force and the electromagnetic and weak forces
A grand unified theory (GUT) refers to theories that _________.
unify the strong force with the electromagnetic and weak forces
A "GUT" (grand unified theory) refers to theories that _________.
unify the strong force with the electromagnetic and weak nuclear forces
According to the Big Bang theory, the early universe had nearly equal amounts of matter and antimatter
False, because in order for photons to be able to travel freely, the opposite had to happen. Atoms that were ionized before became neutral.
Prelecture Narrated Figure: Process of Science: Data to Distinguish among Expansion Models D. Which model is most clearly ruled out by the data on the graph?
Recollapsing
Which model of the universe gives the youngest age for its present size?
a recollapsing universe
Which of the following particles are baryons?
protons
Cosmological redshift is the result of ______.
the expansion of the universe
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 the term dark matter?
matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
What is the significance of 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 to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times before 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.
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.
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.
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.
Which of the following best summarizes what we mean by the term dark energy?
It is a name given to whatever is causing the expansion of the universe to accelerate with time
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.
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.
This diagram represents what we call saddle-shaped geometry. In the context of this chapter, what is its significance?
It represents a two-dimensional analogy to one possible geometry for our universe.
The data points on this graph represent the measured spectrum of the cosmic microwave background, whereas the solid curve represents a theoretically calculated thermal radiation spectrum for a temperature of 2.73 K. What is the significance of the near-perfect match between the data and the theoretical spectrum?
It shows that the cosmic microwave background has a thermal radiation spectrum, just as the Big Bang theory predicts it should.
Prelecture Narrated Figure: Process of Science: Data to Distinguish among Expansion Models B. Which of the models predict that galaxies will eventually get closer together in the future?
Recollapsing
Visual Activity: Rotation Curves and Evidence of Dark Matter D. 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.
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.
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.
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.
White-dwarf supernovae are good standard candles for distance measurements for all the following reasons except which?
White-dwarf supernovae occur only among young and extremely bright stars.
Process of Science: Dark Matter Evidence from the Bullet Cluster B. 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
Which region of the early universe was most likely to become a galaxy?
a region whose matter density was higher than average
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.
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).
Suppose that Hubble's constant were 11 km/s/million light-years instead of 22 km/s/million ligh-years. How would this graph look different?
The data would follow a more shallow line, closer to horizontal than the current line.
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.
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 (This is the essence of Hubble's law: More distant galaxies are moving away from us faster)
Process of Science: Testing the Expanding Universe B. Consider the hypothetical observation "Irregular galaxies outside the Local Group are moving toward us." From Part A, this observation would contradict the idea of an expanding universe. Why?
Because Hubble's law predicts that all galaxies outside our Local Group should be moving away from us.
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.
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.
Prelecture Narrated Figure: Acceleration of the Universe A. The video discusses four potential models of how the expansion of the universe changes with time. Drag the correct model description from the left-hand column to the appropriate blank in the sentences in the right-hand column.
-Only the coasting model assume(s) that the expansion rate of the universe always stays the same. -All four models predict(s) that the universe is expanding today. -Only the recollapsing model predict(s) that the universe will someday contract. -Only the critical model predict(s) that the average density of mass in universe is exactly the critical density. -Only the accelerating model predict(s) that the universe is expanding faster now than it was in the distant past.
Process of Science: Interplay of Observations and Models in the Big Bang Theory E.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?
-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%. -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.
Process of Science: Interplay of Observations and Models in the Big Bang Theory D.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 force froze out from the GUT force.
Key Concept: Dark Matter and Dark Energy A. Based on current evidence, decide whether each of the following statements apply to the concept of dark matter, dark energy, both, or neither. Drag each statement into the appropriate bin.
Dark Matter only - detected by its gravitational effects - has mass - distributed throughout the halo of the Milky Way Galaxy - likely to consist if subatomic particles Dark Energy only - inferred to exist from measurement of the universal expansion rate - acts as a force that can counteract gravity Dark Matter and Dark Energy - the universe contains more of this than it contains matter made from atoms - we infer it exists but do not know exactly what it is Neither Dark Energy or Dark Matter - blocks light from stars behind it - "dark" because it emits only infrared light
Prelecture Narrated Figure: Process of Science: Data to Distinguish among Expansion Models F. 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.
Prelecture Narrated Figure: Process of Science: Data to Distinguish among Expansion Models G. 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
Process of Science: Testing the Expanding Universe A. Consider the following hypothetical observations, some of which are real and some of which are fictional. For each observation, your job is to answer this question: If the observation were real, would it provide evidence for or against the idea that the universe is expanding? Sort each observation into the appropriate bin as follows:
Supports the expanding universe= -All galaxies in the Coma cluster of galaxies have redshifted spectra. -Galaxies 200 million light-years away move away from us twice as fast as galaxies 100 million light-years away. -The measured rate of expansion is the same in all directions. ----------- Contradicts the expanding universe= -Irregular galaxies outside the Local Group are moving toward us. -Galaxy speeds are faster in summer than in winter. -Spiral galaxies move away from us 10% faster than elliptical galaxies at the same distances. ----------- Neither supports nor contradicts= -The Andromeda Galaxy, a member of our Local Group, is moving toward us.
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.
Study this graph, focusing on the red curve and reddish horizontal swath. Which statement correctly interprets what the graph shows?
The measured abundance of deuterium agrees with the theoretically predicted abundance only if we make the prediction with a model of the universe in which ordinary matter makes up about 5% of the critical density.
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
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
Visual Activity: Rotation Curves and Evidence of Dark Matter B. 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
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
Which of the following methods used to determine the mass of a cluster does not depend on Newton's laws of gravity?
measuring the amount of distortion caused by a gravitational lens
Visual Activity: Rotation Curves and Evidence of Dark Matter F. 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
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
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 young
Which forces have physicists shown to be the same force under conditions of very high temperature or energy, as confirmed by experiments in particle accelerators?
the electromagnetic and weak forces
Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies _________.
the electromagnetic and weak forces into the electroweak force
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)
Visual Activity: Rotation Curves and Evidence of Dark Matter C. Why does the rotation curve for the solar system show speeds that become slower with increasing distance from the Sun?
Because the Sun contains most of the mass of the solar system
Which of the following statements can not be tested by science today?
Before Planck time, our universe sprouted from another universe.
What does Hubble's law tell us?
The more distant a galaxy, the faster it is moving away from us.
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.
Strange as it may sound, most of both the mass and energy in the universe may take forms that we are unable to detect directly.
True, because the normal matter only counts for a small amount of the total mass of the universe. A lot more is in the forms of dark matter and dark energy.
We can estimate the total mass of a cluster of galaxies by studying the distorted images of galaxies whose light passes through the cluster.
True, because, people can get the mass of galaxy clusters through the effects of gravitational lensing.
Based on the observational evidence, is it possible that dark matter doesn't really exist?
Yes, but only if there is something wrong with our current understanding of how gravity should work on large scales.
What fraction of the mass needed to halt expansion is known to exist in the form of visible mass in the universe?
less than 1 percent
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
Process of Science: Dark Matter Evidence from the Bullet Cluster C. 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
The primary evidence that has led astronomers to conclude that the expansion of the universe is accelerating comes from __________.
observations of white dwarf supernovae
Dark energy has been hypothesized to exist in order to explain:
observations suggesting that the expansion of the universe is accelerating.
A large mass-to-light ratio for a galaxy indicates that
on average, each solar mass of matter in the galaxy emits less light than our Sun.
Experiments allow physicists today to reproduce (on 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
Measurements of how orbital speeds depend on distance from the center of our galaxy tell us that stars in the outskirts of the galaxy
orbit the galactic center just as fast as stars closer to the center
Which of the following sequences lists the methods for determining distance in the correct order from nearest to farthest?
parallax, Cepheid variables, Hubble's law
The four fundamental forces that operate in the universe today are _________.
strong force, weak force, electromagnetic force, gravity
What is postulated to have caused a sudden inflation of the early universe?
the "freezing out" of the strong force from the GUT force
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
The lookback time of the cosmological horizon is ______.
the age of the universe
On this graph, the time corresponding to the point at the far lower right of the diagonal line represents __________.
the age of the universe today, in seconds
Which of these pieces of evidence supports the idea that inflation really happened?
the apparently "flat" geometry of the universe
What are the two key observational facts that led to widespread acceptance of the Big Bang model?
the cosmic background radiation and the high helium content of the universe
Measuring the amount of deuterium in the universe allows us to set a limit on _________.
the density of ordinary (baryonic) matter the universe
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
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.
Process of Science: Testing the Expanding Universe D.We can in principle measure the expansion rate by studying galaxies in many different directions in space and at different times of year. If we compare such observations, we would find that the expansion rate is __________.
the same no matter when or in which direction we measure it
Process of Science: Dark Matter Evidence from the Bullet Cluster C. 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
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
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
How many forces operated in the universe during the GUT era?
two, gravity and the GUT force
What two quantities did Edwin Hubble plot against each other to discover the expansion of the Universe?
velocity and distance
The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation (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.
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
Process of Science: Temperature of the Universe E.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 is a standard candle?
an object for which we are likely to know the true luminosity
Do all three distance computations agree? Based on your results, estimate the uncertainty in the distance you have found.
20%
What does this diagram represent?
A particle and antiparticle colliding and converting all their mass into photons.
Prelecture Narrated Figure: Process of Science: Data to Distinguish among Expansion Models E. Which model is most strongly supported by the data on the graph?
Accelerating
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.
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.
Compute the distance to M 100 with data from each of the three Cepheids.
Cepheid 1: 6.1×10^7 ly Cepheid 2: 5.3×10^7ly Cepheid 3: 5.1×10^7 ly
How does the hypothesis 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.
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.
What is antimatter?
It consists of particles that are just like those of ordinary matter except with opposite properties, such as opposite charge.
This graph is made from measurements of the cosmic microwave background (dots) and a model based on the hypothesis of inflation. What does it show?
Measurements of the separation between regions of different temperature (in the cosmic microwave background) agree with predictions made by the hypothesis of inflation.
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.
Observations of the cosmic background radiation from the COBE satellite revealed tiny variations in its temperature from one place to another (corresponding to a few millionths of a degree Kelvin).
T
Process of Science: Dark Matter Evidence from the Bullet Cluster D. 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.
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.
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.
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 evidence suggests that the Milky Way contains dark matter?
We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at unexpectedly high speeds, higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.
How do we determine the conditions that existed in the 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.
What is this a picture of?
a cluster of galaxies
What is a rotation curve?
a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
A photograph of a cluster of galaxies shows distorted images of galaxies that lie behind it at greater distances. This is an example of what astronomers call:
a gravitational lens.
A standard candle is _________.
a light source of known luminosity
Consider a spinning disk of pizza dough, as shown here. What would the rotation curve for the spinning dough look like?
a perfectly slanted line straight up the middle--creates two triangles
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
According to current understanding, if the universe continues to expand forever, the last major source of light will come from _________.
evaporation of black holes
Notice the blue ovals (such as those indicated by the arrows) in this image of a galaxy cluster. The oval structures are not really located where they appear to be, but instead are multiple images of a single galaxy that lies directly behind the cluster. What do we call the process that creates these multiple images?
gravitational lensing
If observations had shown that the cosmic microwave background was perfectly smooth (rather than having slight variations in temperature), then we would have no way to account for _________.
how galaxies came to exist
Galaxies with disks but no evident spiral arms are called ______.
lenticular galaxies
A Cepheid with a period of 30 days has an average luminosity that is about __________ times the luminosity of the Sun.
10,000
Based on the number of galaxies visible in the Hubble extreme Deep Field (Figure 20.1 in your text), the estimated number of galaxies in our observable universe is about ______.
100 billion
Process of Science: Temperature of the Universe F.What was the approximate temperature of the universe when the universe was just 1 second old?
10^(10)K
Notice that the window with the light curve also has a box that tells you the average apparent brightness of the Cepheid, which shows a value of \(1.3 \times 10^4\). (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 million light years
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.
Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?
No, because galaxies in the Local Group are gravitationally bound together.
Prelecture Video: Eras of the Universe B. Which of the following important events occurred earliest in the history of the universe?
Space-time rapidly expanded during a brief period of inflation.
Which kind of object is the best standard candle for measuring distances to extremely distant galaxies?
a white dwarf supernova
We determine the distance of a Cepheid by
determining its luminosity from the period-luminosity relation and then applying the inverse square law for light.
Dark matter is inferred to exist because:
we can observe its gravitational influence on visible matter.
Prelecture Narrated Figure: Acceleration of the Universe B. In (Figure 1) at left, which of the four labeled supernovae is the most distant from us?
1
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.
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.
This image shows a colliding pair of galaxy clusters known together as the Bullet Cluster. The blue region represents a map of the cluster's dark matter. How was this blue map made?
The blue region was inferred from studies of how the cluster causes gravitational lensing of objects located behind it
This graph shows how the average distances between galaxies changes with time in the four models for the universe and also shows data points measured from white dwarf supernovae. Which of the following statements best describes what the data are telling us?
The data indicate that we live in an accelerating universe.
Study this diagram that summarizes the eras of the universe. Which of the following statements is not true?
The electroweak era was the longest lasting of all the eras in the history of the universe.
Process of Science: Dark Matter Evidence from the Bullet Cluster B. 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.
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.
Refer to this scenario for the following question: Lost in Spacetime. Just when you thought it was safe to take final exams . . . a vindictive multi-dimensional being reaches down (up? over? through?) to Earth and pulls you out of the universe. You are thrown back into the universe at a place of this being's choosing, and she permits you to leave only after you have identified your surroundings. You are subject to several tests. Through a scientifically unexplainable miracle, you are able to survive in every one of the places you are tested. (Lest you become too comfortable, however, you certainly are able to feel any associated pain due to high temperature, pressure, gravity, etc.) In each case described below, identify your surroundings. In some cases, the surroundings described may exist only during eras of the universe (past or future) other than our own time; in those cases, you should identify both the place and the time where you are located. You are once again in a hot, dense place. You are surrounded by protons and neutrons, some rapidly fusing into helium. You notice that your surroundings are cooling (good, because it's really hot!) and rapidly dropping in density. Within about 3 minutes, the fusion reactions stop. Where are you?
You are in the early universe during the era of nucleosynthesis.
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
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 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 its 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
Visual Activity: Rotation Curves and Evidence of Dark Matter E. 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
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 primeval universe.
What makes white-dwarf supernovae good standard candles?
-They are very bright, so they can be used to determine the distances to galaxies billions of light-years away. -They should all have approximately the same luminosity. both A and B
Process of Science: Interplay of Observations and Models in the Big Bang Theory A.Classify each statement below as an observation or as an inference based on the current Big Bang model. (Note that the helium abundance is defined as the mass of helium relative to the mass of hydrogen.)
Observation: 3. Large-scale structures look about the same in all directions. 4. The temperature of the cosmic microwave background varies slightly with direction. 5. The cosmic microwave background temperature is 2.73 K. 6. The helium abundance is at least 25% in every galaxy studied so far. Inference: 1. Photons of the cosmic microwave background have traveled through space for almost 14 billion years. 2. The cosmic microwave background is radiation left over from the big bang. 7. Fusion during the universe's first five minutes produced 75% hydrogen and 25% helium. 8. Large-scale structure grew around density variations present in the early universe.
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.
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.
Refer to this scenario for the following question: Lost in Spacetime. Just when you thought it was safe to take final exams . . . a vindictive multi-dimensional being reaches down (up? over? through?) to Earth and pulls you out of the universe. You are thrown back into the universe at a place of this being's choosing, and she permits you to leave only after you have identified your surroundings. You are subject to several tests. Through a scientifically unexplainable miracle, you are able to survive in every one of the places you are tested. (Lest you become too comfortable, however, you certainly are able to feel any associated pain due to high temperature, pressure, gravity, etc.) In each case described below, identify your surroundings. In some cases, the surroundings described may exist only during eras of the universe (past or future) other than our own time; in those cases, you should identify both the place and the time where you are located. At last, someplace fairly comfortable. Very weak gravity is holding you to the surface of the small object on which you sit. Your object is apparently moving away from a star, perhaps one that it orbits with a period of thousands of years. Around you, geysers are spouting gas into space. Looking back along the object's orbit, you see particles of dust that the geysers apparently blew off the object when it was nearer to the star that it is now leaving behind. You conclude that the geysers were recently much more active but are now settling down into a quiescent state that may last for millennia. You also soon realize that you are closer to home than you have been in all your previous journeys. Perhaps if you can somehow find a small rocket, a heat shield, and a good parachute, you can escape and head home for your final exam. Where are you?
You are on comet Hale-Bopp, circa May 1997.
Process of Science: Dark Matter Evidence from the Bullet Cluster A.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.
What do we mean by the term 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
What do we mean by inflation?
a sudden expansion of the universe after the strong force froze out from the GUT force
These diagrams represent four possible models for the universe. Each model shows how the size of the observable universe changes with time. Of the four models, which one gives the universe the oldest age at present?
accelerating universe
How do we know that there is an insignificant amount of dark matter in the solar system?
we can measure the mass distribution in the solar system by analysis of the rotation curve and compare it with the visible mass distribution. analysis shows that there is no significant "hidden" mass in the solar system.
Process of Science: Temperature of the Universe B.The vertical axis of the graph measures the temperature of the universe in kelvin. If we move up one tick mark to the next, by what factor does the temperature increase?
100
Process of Science: Temperature of the Universe D.To get your bearings on the horizontal time axis of this plot, it is useful to calculate the age of the universe in seconds today. If you multiply the age of the universe in years by the number of seconds in a year, approximately what is the current age of the universe in seconds?
4 x 10^(17)s
This photo shows the Large Magellanic Cloud, or LMC for short. Based on what you have learned in about the LMC from your text, which of the following statements about the LMC is not true?
Galaxies similar to the LMC are extremely rare.
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.
Prelecture Video: Eras of the Universe A.During the history of the universe, what important event occurred about 0.001 seconds after the Big Bang?
Most matter was annihilated by antimatter.
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.
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
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%
Process of Science: Temperature of the Universe G.From Part C, 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
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
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
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
Based on this diagram, "red sequence" galaxies are __________ than "blue cloud" galaxies.
redder and more luminous
What is a Cepheid variable?
A type of luminous star that makes an excellent standard candle.
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.
What does cosmological redshift do to light?
stretches its wavelength