culmulative 5
Roughly what percentage of the mass-energy of in-falling matter can be converted to other forms of energy as the matter falls into a black hole?
10-40%
What is the approximate temperature of the universe (as a whole) today?
3K
What does this diagram represent?
A particle and antiparticle colliding and converting all their mass into photons.
Most active galactic nuclei are found at large distances from us, with relatively few nearby. What does this imply?
Active galactic nuclei exist tend to become less active as they age.
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.
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.
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 was annihilated except the slight excess of matter particles.
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.
Which of the following statements correctly explains why galaxy collisions should have been more common in the past than they are today?
Galaxies were closer together in the past, because the universe was smaller.
Which of the following statements is not an assumption used in models of galaxy formation?
Gas contracted to form the disks of galaxies before any stars were born.
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.
What is antimatter?
It consists of particles that are just like those of ordinary matter except with opposite properties, such as opposite charge.
Which of the following is not an observed characteristic of the cosmic microwave background?
It contains prominent spectral lines of hydrogen, the primary chemical ingredient of the universe.
In general, what happens to the density and temperature of a gas as it expands?
It gets less dense and cooler.
This visible-light photo shows a starburst galaxy. What is going on with the reddish gas that seems to be emerging on both sides of it?
It is gas being blown out in a galactic wind.
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.
What does this graph show?
It shows calculations indicating that the temperature of the universe began quite high but is now quite low.
The data points on this graph represent the measured spectrum of the cosmic microwave background, while 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.
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.
The photo shows a galaxy known as Arp 220, and the inset shows this galaxy's spectrum. What can you infer?
Most of the light from this galaxy is infrared, indicating active star formation.
If the Andromeda Galaxy collided with the Milky Way, what would most likely happen to Earth?
Nothing
Which of the following statements cannot be tested by science today?
Prior to the Planck time, our universe sprouted from another universe.
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.
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.
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).
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.
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.
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.
Interactions among galaxies also are thought to influence a galaxy's type in at least some cases. Which of the following does not support the idea that interactions can shape galaxies?
The fact that more distant galaxies have larger redshifts.
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.
All of the following observations are real. Which one does not support the model in which active galactic nuclei are powered by accretion disks around massive black holes?
The most luminous active galactic nuclei have huge redshifts.
This photo shows the central region of a cluster of galaxies. Based on what you have learned from your text, which of the following statements about this photo is not true?
The oldest stars in these galaxies are much older than the oldest stars in our own Milky Way.
Consider a spinning disk of pizza dough, as shown here. What would the rotation curve for the spinning dough look like?
The rotation curve is an upward sloping straight line, because the speed of rotation is zero at the center and increases with distance from the center.
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.
Suppose we observe a source of x-rays that varies substantially in brightness over a period of a few days. What can we conclude?
The x-ray source is no more than a few light-days in diameter.
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.
Which of the following is not a feature of central dominant galaxies?
They are spiral galaxies.
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
This diagram shows several stages in a computer simulation of a collision between two galaxies. What is being simulated?
Two spiral galaxies merge to become an elliptical galaxy surrounded by debris.
Quasar spectra often show many absorption lines that all appear to be as a result of the same electron transition (such as level 1 to level 2 in hydrogen) but that fall at different wavelengths in the spectrum. Why do we think this is the case?
We are seeing absorption lines from clouds of gas that lie between us and the quasar, and therefore each cloud has a different redshift.
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.
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.
Olbers's paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?
Why is the sky dark at night?
This Hubble Space Telescope photo shows the bright active galactic nucleus in the center of galaxy M87. What is the long bluish streak coming out of it?
a "jet" of material being shot out at high speed
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
The bright object in this photo is a quasar (named 3C 273). According to current understanding, about how big is the source of the bright light?
about the size of our solar system
Intergalactic hydrogen clouds are easiest to study by looking at __________.
absorption lines in quasar spectra
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
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
The unusually bright centers found in some galaxies are called __________.
active galactic nuclei
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
According to current understanding, what is a quasar?
an active galactic nucleus that is particularly luminous
The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they?
(1) the existence and specific characteristics of the observed cosmic microwave background(2) the observed overall chemical composition of the universe
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
The observed relationship between the masses of central black holes and the bulge masses of galaxies implies that __________.
galaxy formation and supermassive black hole formation must be related
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
According to the model in which that active galactic nuclei are powered by supermassive black holes, the energy released as light comes from __________.
gravitational potential energy released by matter that is falling toward the black hole
In the distant past, the temperature of the universe was __________.
hotter than it is today
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
Collisions between galaxies typically unfold over a period of __________.
hundreds of millions of years
Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in __________.
infrared light
According to observations, which type of galaxy was much more common when the universe was 2 billion years old than it is today?
irregular galaxies
According to the model in which active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of __________.
light emitted by hot gas in an accretion disk that swirls around the black hole
Based on this graph, if you can measure the mass of a galaxy's central bulge, then you will also know the approximate __________.
mass of the galaxy's supermassive black hole
Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) __________.
may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies
Experiments allow physicists today to reproduce (on very small scales) energy and temperature conditions thought to have prevailed in the early universe as far back in time as about __________.
one ten-billionth (10-10) of a second after the Big Bang
This figure shows an all-sky map of the cosmic microwave background recorded by the Planck mission. What do the dark and bright variations represent?
regions of the sky with very slightly different density at the time the radiation was emitted
Two ways in which the starting conditions in a protogalactic cloud might cause it to become an elliptical (rather than spiral) galaxy are if the cloud begins with either _________________.
relatively little angular momentum or relatively high density
Observations indicate that over billions of years, galaxies in general tend to change from _________.
smaller and bluer to larger and redder
The four fundamental forces that operate in the universe today are __________.
strong force, weak force, electromagnetic force, and gravity
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
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
A quasar's spectrum is hugely redshifted. What does this large redshift tells us about the quasar?
the distance to the quasar
Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies _________.
the electromagnetic and weak forces into the electroweak force
Which of the following phenomena is not thought to be a result of collisions or other interactions between galaxies?
the fact that spiral galaxies have both disk and halo components
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
We can study how galaxies evolve because ______.
the farther away we look, the further back in time we see
Which of the following observations provides direct evidence that galaxies containing active galactic nuclei have central, supermassive black holes?
the high orbital velocities of stars and gas clouds at the very centers of these galaxies
Which of the following observational phenomena is probably not a direct consequence of the presence of a supermassive black hole?
the large numbers of stars found in globular clusters
Observations have shown that when the mass of the central black hole is large, then __________.
the mass of the bulge of the host galaxy is also large
This image combines a visible-light view of the galaxy Hercules A with an image showing radio wave emission (the reddish colors) in its vicinity. The likely explanation for the two huge clumps of radio emission far to either side of the visible galaxy is __________.
the radio emission comes from gas shot out into space by jets powered by a supermassive black hole in the galactic center
A grand unified theory (GUT) refers to a type of theory intended to unify __________.
the strong force with the electromagnetic and weak forces
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
The critical density of the universe is the __________.
total density of matter and energy needed to give the universe a "flat" geometry (in spacetime)
Suppose that the universe were infinite in both extent and age. In that case, we would expect the night sky to be ___________.
uniformly bright
The mass of a supermassive black hole thought to power a quasar is typically __________.
1 billion solar masses
The three cubes represent three stages in a computer simulation of the formation of protogalactic clouds; they represent a region about 500 million light-years across. Which of the following statements about this model is true?
Although the gas looks uniform in the first frame, the model assumes there were small regions with slightly higher densities than other regions.
This large fuzzy blob in the center of this image is the central dominant galaxy of the cluster that surrounds it. The arrow points to several large clumps of stars near the center of the galaxy. What do those clumps probably represent?
Each clump was probably once the center of an individual galaxy.
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.
Why are collisions between galaxies more likely than collisions of stars within galaxies?
Relative to their sizes, galaxies are much closer together than stars.
Which of the following is an important starting assumption in models of galaxy formation?
Some regions in the universe start out denser than others.
Which is not a general characteristic of starburst galaxies?
The "starburst" is thought to be caused by the presence of a supermassive black hole in the galaxy's center.
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
The distinguishing feature of a starburst galaxy is __________.
a rate of star formation that may be 100 or more times greater than that in the Milky Way
One possible explanation for a galaxy's type invokes the angular momentum of the protogalactic cloud from which it formed. Suppose a galaxy forms from a protogalactic cloud with a lot of angular momentum. Assuming its type has not changed as a result of other interactions, we'd expect this galaxy to be ______.
a spiral galaxy
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
