Astronomy Hw/Practice Questions Unit 4
Dark energy has been hypothesized to exist in order to explain:
observations suggesting that the expansion of the universe is accelerating
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.
Is space expanding within clusters of galaxies? Why or why not?
No, because their gravity is strong enough to hold them together even while the universe as a whole expands.
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
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
A galaxy with all its mass concentrated at its center.
.
The current temperature of the universe as a whole is
a few K
Which of the following observations were made by Vera Rubin and Kent Ford?
-Stars in the outskirts of the Andromeda galaxy orbit the galaxy's center at surprisingly high speeds.-Many spiral galaxies have hydrogen gas clouds orbiting their centers at surprisingly high speeds.
Which of the following provide evidence for dark matter in clusters of galaxies? Select all that apply.
-The temperature of hot, X-ray emitting gas within clusters.-Orbital speeds of galaxies within clusters.-The way clusters bend light from galaxies behind them.
A galaxy with constant mass density within 20,000 light-years of its center, and beyond that an enclosed mass that increases proportionally to the distance from the center.
.
A galaxy with constant mass density within 20,000 light-years of its center, and zero density beyond that distance.
.
What was the peak wavelength of the background radiation at the time light left the most distant galaxies we can currently see? Assume those galaxies have a cosmological redshift of zz = 10. Express your answer to two significant figures and include the appropriate units.
.10 mm
In (Figure 1) at left, which of the four labeled supernovae is the most distant from us?
1
Match the words in the left-hand column to the appropriate blank in the sentences in the right-hand column. Use each word only once.
1. A grand unified theorypredicts that the strong, weak, and electromagnetic forces should become indistinguishable at high temperatures 2. The electroweak forceis a single force that unifies the electromagnetic and weak forces. 3. Inflationwas a dramatic expansion of the universe thought to have occurred when the universe was only a tiny fraction of a second old. 4. Olbers' paradoxforces us to think about why the sky is dark at night. 5. Observations of the cosmic microwave backgroundprovide a way to test our theory of the Big Bang. 6. When a particle of ordinary matter meets its precise opposite particle of antimatter, the result is annihilationwith complete conversion of mass into energy.
What is the mass-to-light ratio of a 30MSun30MSun supergiant star with luminosity 3×105 LSunLSun ? Express your answer as a multiple of mass-to-light ratio of solar system to two significant figures.
1.0*10^-4
What will the temperature of the cosmic microwave background be when the average distances between galaxies are twice as large as they are today?
1.365
What will the temperature of the cosmic microwave background be when the average distances between galaxies are twice as large as they are today? Express your answer in kelvins to three significant figures.
1.37 K
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
What was the approximate temperature of the universe when the universe was just 1 second old?
10^10 K
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 10−4510−45seconds (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 10−3510−35seconds. By what factor does the time increase from one tick mark on the horizontal axis to the next tick mark toward the right?
1x10^5
Compare the peak wavelength of the radiation in the universe at the end of the era of nucleosynthesis with the peak wavelength of the radiation in the universe currently. Assume the temperature at the end of the era of nucleosynthesis was 109K109K. Express your answer using three significant figures.
3.66x10^8
What is the temperature corresponding to that peak wavelength? Express your answer in kelvins to three significant figures.
30.0 K
What is the approximate temperature of the universe (as a whole) today?
3K
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?
4x10^17 s
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%
A cluster of galaxies has a radius of about 6.1 million light-years ( 5.8×1022m5.8×1022m ) and an intracluster medium with a temperature of 4.5×107K4.5×107K. Estimate the mass of the cluster. Express your answer in kilograms to two significant figures.
7.7x10^44
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.
Which of the following statements best 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.
Classify the given types of matter as either ordinary (baryonic) matter that contains protons and neutrons or as exotic (nonbaryonic) matter that consists of subatomic particles different from those that build atoms. Drag the appropriate items to their respective bins.
Baryonic matter (contains protons and neutrons)- matter in our bodies- matter in stars- matter in brown dwarfs- dark matter consisting of Jupiter-size objects in galactic halos Nonbaryonic matter- dark matter consisting of weakly interacting subatomic particles- matter that probably makes up the majority of dark matter
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.
Consider the portion of the video that shows two galaxy clusters that have collided, and includes a map showing the inferred distribution of dark matter among these clusters. How did astronomers infer the location of the dark matter?
By carefully observing how the light of galaxies beyond these clusters has been bent by the clusters.
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.
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.
Complete the following sentence correctly. 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.
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:as mass.most common form of mass in the halo of the Milky Way Galaxy.detected through its gravitational attraction.likely to consist of tiny, weakly interacting particles. Dark Energy Only:inferred to exist from the measured acceleration of the universe's expansion.the proposed source of a universal repulsive force .Both Dark Matter and Dark Energy:the universe contains more of this than it contains matter made from atoms.we do not know what it is made of. Neither Dark Matter nor Dark Energyblocks light from stars behind it."dark" because it emits only infrared light.
Which of the following best sums up current scientific thinking about the nature of dark energy?
Dark energy probably exists, but we have little (if any) idea what it is.
Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?
Dark matter is the dominant form of mass in both clusters and in individual galaxies.
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.
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
Each phrase below describes some aspect of one of the four forces. Drag the correct force to go with each sentence. You may use the same force more than once.
Electromagnetism: governs virtually all chemistry and biology Electromagnetism: explains why two protons will repel each other Strong force: holds quarks together in protons and neutrons Strong force: holds protons and neutrons together in atomic nuclei Electromagnetism: explains the attraction between a proton and an electron Weak force: the only force besides gravity that affects neutrinos Gravity: dominates the universe at the largest scales Gravity: the weakest of the four forces on a per-particle basis
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.
Use the words at the left to complete the sentences at right correctly. You may use the same word(s) more than once.
Evidence suggests that about two-thirds of the total mass plus energy in the universe consists of dark energy. Large-scale structure in the universe has probably been shaped primarily by the gravity of dark matter. Astronomers suspect that dark matter consists of weakly interacting massive particles, or WIMPs. Matter that emits no light of any kind is called dark matter. Matter that emits X-rays but not visible light is considered a form of luminous matter. We say use the term dark energy as a name for whatever is causing the expansion of the universe to accelerate with time. Stars and gas clouds are considered to be forms of luminous matter. The bending of light by gravity is called gravitational lensing.
Shown following are the same eras shown in Parts A and B. Rank the eras from left to right based on the size of the observable universe during each one, from smallest to largest.
First: Planck era, GUT era, electroweak era, particle era, era of nucleosynthesis, era of nuclei, era of atoms, era of galaxies Last
Shown following are the same eras shown in Part A. Rank the eras from left to right based on the temperature of the universe during each one, from coolest to hottest.
First: era of galaxies, era of atoms, era of nuclei, era of nucleosynthesis, particle era, electroweak era, GUT era, Planck era Last
Shown following are the names of several eras that are used to describe the history of the universe. Rank the eras from left to right in the order in which they occurred, from first to last.
First: planck era, GUT era, electroweak era, particle era, era ofnucleosynthesis, era of nuclei, era of atoms, era of galaxies Last
What does a large-scale structure of the universe look like?
Galaxies are arranged in huge chains and sheets. Between the chains and sheets are giant voids.
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.
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.
Why do we call dark matter "dark"?
It emits no radiation that we have been able to detect.
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, 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.
During the history of the universe, what important event occurred about 0.001 seconds after the Big Bang?
Most matter was annihilated by antimatter.
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.
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.
Which of the following describes key evidence for dark matter in individual spiral galaxies like the Milky Way?
Orbital speeds of stars stay relatively high even at great distances from the galactic center.
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? Check all that apply.
Over the next 10 years, the temperature of the cosmic microwave background falls to 1 KK 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. Astronomers discover distant protogalactic clouds with a helium abundance below 20%.
Which of the following important events occurred earliest in the history of the universe?
Space-time rapidly expanded during a brief period of inflation.
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.
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.
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.
Observationally, how can we decide which of the four general expansion models best describes the present-day universe?
The best way of observing is the accurate measurements of distances between the galaxies. The most fitting standard candles for such measurements at such distances are white dwarf supernovae.
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
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.
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.
Why do we think this structure reflects the density of the early universe?
The large-scale structure is consistent with the density enhancements observed in the cosmic microwave background.
Decide whether the following statement makes sense (or is clearly true) or does not make sense (or is clearly false), and why. In the distant past, the cosmic microwave background consisted primarily of infrared light.
The statement is true. The cosmic background was released when the universe had the temperature of about 3000 KK. That is similar to the temperature of a red star, which emits mostly in infrared wavelengths. Then that wavelength was redshifted.
Which of the following best explains why the night sky is dark?
The universe has a finite age.
In basic terms, what does the graph show?
The universe is getting cooler with time.
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.
Why can't our current theories describe the conditions that existed in the universe during the Planck era?
Theory that links quantum mechanics and general relativity is required to describe the universe during the Planck era.
Scientists do not know what dark energy is, yet they 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: (1) measurements of the accelerating expansion and (2) the universal geometry inferred from the cosmic microwave background.
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.
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.
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
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.
Although we can divide the history of the universe into many distinct "eras," sometimes it's useful just to have a broad perspective on what events occurred in the very early universe and what events occurred later. Listed following are events that occurred either before or after the first five minutes in the history of the universe. Match these events to the appropriate time period.
Within the first five minutes*antielectrons (positrons) as common as electrons*temperature fell to 10^15K*strong force and electroweak force first became distinct*inflation occuredSometime after the first five minutes*galaxies formed*photos of cosmic microwave*stars formed*carbon nuclei formed by fusion
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
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 is the earliest time from which we observe light in the universe?
a few hundred thousand years after the big bang
Examine the visible light image of the Bullet Cluster. What kind of cluster is it?
a galaxy cluster
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
Which region of the early universe was most likely to become a galaxy?
a region whose matter density was higher than average
The major evidence for the idea that the expansion of the universe is accelerating comes from observations of:
a white draw supernova
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
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
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
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
Given that the electromagnetic force is far stronger than gravity on a per-particle basis, why doesn't the electromagnetic force dominate the interactions of large objects like planets, stars, and galaxies?
because there are both positive and negative electric charges
Essentially all the hydrogen nuclei that will ever exist in our universe were created __________.
by the time the universe was about 3 minutes old
Strong evidence for the existence of dark matter comes from observations of:
cluster of galaxies
Listed following are three possible models for the long-term expansion (and possible contraction) of the universe in the absence of dark energy. Rank each model from left to right based on the ratio of its actual mass density to the critical density, from smallest ratio (mass density much smaller than critical density) to largest ratio (mass density much greater than critical density).
coasting universe, critical universe, recollapsing universe
When a proton and an antiproton collide, they
convert into two photons
Which of the following unifications as been directly tested and confirmed through experiments in particle accelerators?
electroweak unification
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
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
Consider a spinning disk of pizza dough, as shown here. What would the rotation curve for the spinning dough look like?
line is going straight from bottom left to upper right.
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 does not provide strong evidence for the Big Bang theory?
observations of the amount of hydrogen in the universe
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.)
observations: Large-scale structure looks 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. inferences from a model: 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 (by mass). Large-scale structure grew around density variations present in the early universe.
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 possible types of universe would not expand forever?
recollapsing universe
Listed following are four models for the long-term expansion (and possible contraction) of the universe. Rank the models from left to right based on their predictions for the average distance between galaxies five billion years from now, from smallest to largest.
recollapsing universe, critical universe, coasting universe, accelerating universe
Listed following are four models for the long-term expansion (and possible contraction) of the universe. Rank each model from left to right based on its prediction for the current age of the universe, from youngest to oldest
recollapsing universe, critical universe, coasting universe, accelerating univese
According to current understanding of physics, which four of the following are the four fundamental forces in nature?
strong force gravity weak force electromagnetism
Which of the four forces that exist today would be merged together at high energies in what scientists call the "GUT force"?
strong force weak force electromagnetism
Based on current evidence, which of the following is considered a likely candidate for the majority of the dark matter in galaxies?
subatomic particles that we have not yet detected in particle physics experiments
Suppose that inflation did not occur. In that case, the fact that the cosmic microwave background has the same temperature in opposite directions of the sky would be considered ___________.
surprising, because those locations would never have been close enough to have any light or matter exchanged between them
Which of these pieces of evidence supports the idea that inflation really happened?
the apparently "flat" geometry of the universe
Based on current evidence, a supercluster is most likely to have formed in regions of space where _________.
the density of dark matter was slightly higher than average when the universe was very young
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)
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
Which of the following does inflation help to explain?
the origin of galaxies
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
A grand unified theory (GUT) refers to a type of theory intended to unify __________.
the strong force with the electromagnetic and weak forces
Which of the following does inflation help to explain?
the uniformity of the cosmic microwave background
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
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
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
Shown following are several times in the history of the universe. Rank these times from left to right based on the average temperature of the universe at each time, from coolest to hottest.
today, 1.5 billion years after the Big Bang, 100 million years after the Big Bang, 1 million years after the Big Bang, 500,000 years after the Big Bang
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
When the universe was 380,000 years old, its thermal radiation spectrum consisted mostly of
visible and infrared photons.
Dark matter is inferred to exist because:
we can observe its gravitational influence on visible matter