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.)
answer
accelerating
coasting
critical
recollapsing
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Which of the models predict that galaxies will eventually get closer together in the future?
answer
recollapsing
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Which model predicts the largest average distance between galaxies 6 billion years ago?
answer
accelerating
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Which model is most clearly ruled out by the data on the graph?
answer
recollapsiong
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Which model is most strongly supported by the data on the graph?
answer
accelerating
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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 __________.
answer
farthest away from us
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What additional data would be most valuable in helping scientists evaluate whether the accelerating model really is the best of the four models?
answer
More observations of very distant white dwarf supernovae
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A _________ predicts that the strong, weak, and electromagnetic forces should become indistinguishable at high temperatures
answer
grand unified theory
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The _________ is a single force that unifies the electromagnetic and weak forces.
answer
electroweak force
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___________ was a dramatic expansion of the universe thought to have occurred when the universe was only a fraction of a second old.
answer
Inflation
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__________ forces us to think about why the sky is dark at night.
answer
Olbers' paradox
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Observations of the ________ provide a way to test our theory of the Big Bang.
answer
cosmic microwave background
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When a particle of ordinary mater meets its precise opposite particle of antimatter, the result is __________ with complete conversion of mass into energy.
answer
annihilation
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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.)
answer
Observation:
-Large-scale structures look about the same in all directions.
-The temperature of the cosmic microwave background varies slightly with direction.
-The cosmic microwave background temperature is 2.73 K.
-The helium abundance is at least 25% in every galaxy studied so far.
Inference:
-Photons of the cosmic microwave background have traveled through space for almost 14 billion years.
-The cosmic microwave background is radiation left over from the big bang.
-Fusion during the universe's first five minutes produced 75% hydrogen and 25% helium.
-Large-scale structure grew around density variations present in the early universe.
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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 __________.
answer
the universe began very hot and dense and has been cooling as it expands
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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 __________.
answer
tiny temperature variations in different directions
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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?
answer
Although expansion seems to imply a Big Bang, no other specific predictions of the Big Bang theory were tested and confirmed until the 1960s.
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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?
answer
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.
question
Which of the following best summarizes what we mean by dark matter?
answer
matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
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Which of the following best summarizes what we mean by dark energy?
answer
It is a name given to whatever is causing the expansion of the universe to accelerate with time.
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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 _________.
answer
dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way
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What is a rotation curve?
answer
a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
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What is the primary way in which we determine the mass distribution of a spiral galaxy?
answer
We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.
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What do we mean when we say that the rotation curve for the Milky Way galaxy is "flat"?
answer
Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
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Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?
answer
Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.
question
Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters?
answer
measuring the temperatures of stars in the halos of the galaxies
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When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?
answer
It bends or distorts the light coming from galaxies located behind it.
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Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?
answer
Dark matter is the dominant form of mass in both clusters and in individual galaxies.
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What is the distinguishing characteristic of what we call ordinary (or baryonic) matter?
answer
It consists of atoms or ions with nuclei made from protons and neutrons.
question
Measuring the amount of deuterium in the universe allows us to set a limit on _________.
answer
the density of ordinary (baryonic) matter the universe
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What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
answer
They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.
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Which of the following best sums up current scientific thinking about the nature of dark matter?
answer
Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.
question
When we speak of the large-scale structure of the universe, we mean _________.
answer
the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe
question
The primary evidence that has led astronomers to conclude that the expansion of the universe is accelerating comes from __________.
answer
observations of white dwarf supernovae
question
Which of the following best sums up current scientific thinking about the nature of dark energy?
answer
Dark energy probably exists, but we have little (if any) idea what it is.
question
Based on evidence both 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)?
answer
5%
question
According to current understanding, if the universe continues to expand forever, the last major source of light will come from _________.
answer
evaporation of black holes
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A massive object can distort the light of more distant objects behind it through the phenomenon that we call
answer
gravitational lensing
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are defined as subatomic particles that have more mass than neutrinos but do not interact with light.
answer
WIMPs
question
of spiral galaxies provide strong evidence for the existence of dark matter.
answer
rotation curves
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Matter made from atoms, with nuclei consisting of protons and neutrons, represents what we call
answer
ordinary (or baryonic)
question
of the universe is better-explained when we include the effects of dark matter along with the effects of luminous matter.
answer
large-scale structure
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Matter consisting of particles that differ from those found in atoms is generally referred to as
answer
exotic (or nonbaryonic)
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