In the 1970s the astronomer Vera Rubin and her colleague Kent Ford measured how fast stars orbit the centres of spiral galaxies. The expectation was straightforward. Most of a galaxy's visible mass is concentrated near its core, so, as with planets around the Sun, stars far from the centre should move more slowly than those close in. They did not. Across galaxy after galaxy, orbital speeds stayed roughly constant out to the visible edge and beyond. Either the law of gravity was wrong on galactic scales, or galaxies contained a great deal of mass that emits no light.
Most physicists chose the second option, and the evidence since has been cumulative and varied. Clusters of galaxies bend the light of more distant objects far more strongly than their visible matter can account for. The pattern of tiny temperature fluctuations in the cosmic microwave background, the afterglow of the early universe, fits calculations only if ordinary matter is outweighed roughly five to one by something else. And computer simulations of structure formation reproduce the observed web of galaxies only if that something clumps under gravity without interacting with light. The case for dark matter rests not on any single observation but on the convergence of independent lines of evidence on a single number.
What that matter actually is remains unknown, and here the story becomes less comfortable. For three decades the favoured candidate was a weakly interacting massive particle, a hypothetical entity predicted by extensions of the standard model of particle physics that happened to yield about the right cosmic abundance. Enormous detectors, shielded deep underground from cosmic rays, were built to catch the rare collision of such a particle with an atomic nucleus. They have become steadily more sensitive and have found nothing. Particle accelerators, which might have produced such particles directly, have likewise come up empty.
Some researchers read these null results as a reason to widen the search: dark matter might consist of far lighter particles such as axions, or of primordial black holes, or of a whole 'dark sector' with its own forces. Others regard them as a warning that the original inference was flawed. The proponents of modified Newtonian dynamics, or MOND, argue that a small change to the law of gravity at very low accelerations explains galactic rotation curves more economically than invisible mass does—and, strikingly, predicts in advance relationships between a galaxy's visible matter and its rotation that dark matter models must be tuned to reproduce. MOND's weakness is on larger scales, where it struggles with galaxy clusters and the cosmic background.
The debate has a philosophical edge. Dark matter's defenders point out that postulating unseen entities to rescue a successful theory has a distinguished pedigree: the planet Neptune was discovered because Uranus wobbled. Their critics reply that the same move once produced Vulcan, a planet invented to explain Mercury's anomalous orbit, which turned out not to exist; the anomaly was explained instead by general relativity. The history of science, in other words, offers precedents for both strategies, and it does not tell us in advance which one we are living through.
Most physicists chose the second option, and the evidence since has been cumulative and varied. Clusters of galaxies bend the light of more distant objects far more strongly than their visible matter can account for. The pattern of tiny temperature fluctuations in the cosmic microwave background, the afterglow of the early universe, fits calculations only if ordinary matter is outweighed roughly five to one by something else. And computer simulations of structure formation reproduce the observed web of galaxies only if that something clumps under gravity without interacting with light. The case for dark matter rests not on any single observation but on the convergence of independent lines of evidence on a single number.
What that matter actually is remains unknown, and here the story becomes less comfortable. For three decades the favoured candidate was a weakly interacting massive particle, a hypothetical entity predicted by extensions of the standard model of particle physics that happened to yield about the right cosmic abundance. Enormous detectors, shielded deep underground from cosmic rays, were built to catch the rare collision of such a particle with an atomic nucleus. They have become steadily more sensitive and have found nothing. Particle accelerators, which might have produced such particles directly, have likewise come up empty.
Some researchers read these null results as a reason to widen the search: dark matter might consist of far lighter particles such as axions, or of primordial black holes, or of a whole 'dark sector' with its own forces. Others regard them as a warning that the original inference was flawed. The proponents of modified Newtonian dynamics, or MOND, argue that a small change to the law of gravity at very low accelerations explains galactic rotation curves more economically than invisible mass does—and, strikingly, predicts in advance relationships between a galaxy's visible matter and its rotation that dark matter models must be tuned to reproduce. MOND's weakness is on larger scales, where it struggles with galaxy clusters and the cosmic background.
The debate has a philosophical edge. Dark matter's defenders point out that postulating unseen entities to rescue a successful theory has a distinguished pedigree: the planet Neptune was discovered because Uranus wobbled. Their critics reply that the same move once produced Vulcan, a planet invented to explain Mercury's anomalous orbit, which turned out not to exist; the anomaly was explained instead by general relativity. The history of science, in other words, offers precedents for both strategies, and it does not tell us in advance which one we are living through.
Question 1
Single correct option
Rubin and Ford's observations were surprising because:
- A. stars far from galactic centres moved faster than stars close to the centre
- B. galaxies emitted more light than their estimated mass could explain
- C. distant stars orbited at roughly the same speed as inner ones, contrary to what the visible mass predicted
- D. planets in the solar system were found not to obey Newtonian gravity
Show answer and solution
Answer: (C) distant stars orbited at roughly the same speed as inner ones, contrary to what the visible mass predicted
Paragraph 1: since visible mass is concentrated near the core, 'stars far from the centre should move more slowly… They did not… orbital speeds stayed roughly constant out to the visible edge and beyond.' That is option 3.
- Option 1 reverses the finding; the surprise was that outer stars did *not* slow down, not that they moved faster than inner ones.
- Option 2 is not stated; the issue was mass that emits *no* light.
- Option 4: the solar system is only an analogy for the expected pattern. Nothing suggests planets break Newtonian gravity.
- Option 1 reverses the finding; the surprise was that outer stars did *not* slow down, not that they moved faster than inner ones.
- Option 2 is not stated; the issue was mass that emits *no* light.
- Option 4: the solar system is only an analogy for the expected pattern. Nothing suggests planets break Newtonian gravity.
Question 2
Single correct option
According to the passage, the case for dark matter is strong chiefly because:
- A. underground detectors have directly recorded dark-matter particles
- B. several independent kinds of evidence point to the same conclusion
- C. MOND has failed to explain the rotation of individual galaxies
- D. primordial black holes have been shown to exist in sufficient numbers
Show answer and solution
Answer: (B) several independent kinds of evidence point to the same conclusion
The last sentence of paragraph 2 states it directly: 'The case for dark matter rests not on any single observation but on the convergence of independent lines of evidence on a single number.' Those lines are gravitational lensing, the cosmic microwave background and structure simulations, so option 2 is correct.
- Option 1 contradicts paragraph 3: the detectors 'have found nothing'.
- Option 3 is inaccurate: the passage says MOND explains rotation curves 'more economically'; its weakness is on *larger* scales.
- Option 4: primordial black holes are only one speculative candidate.
- Option 1 contradicts paragraph 3: the detectors 'have found nothing'.
- Option 3 is inaccurate: the passage says MOND explains rotation curves 'more economically'; its weakness is on *larger* scales.
- Option 4: primordial black holes are only one speculative candidate.
Question 3
Single correct option
The author refers to Neptune and Vulcan in order to:
- A. show that dark matter, like Neptune, will eventually be detected
- B. demonstrate that general relativity has already made dark matter unnecessary
- C. argue that astronomers should stop inferring the existence of unseen objects
- D. show that positing unseen entities to explain anomalies has both succeeded and failed before, so history cannot settle the present debate
Show answer and solution
Answer: (D) show that positing unseen entities to explain anomalies has both succeeded and failed before, so history cannot settle the present debate
Neptune shows the 'unseen entity' strategy succeeding: it was postulated to explain Uranus's wobble and then found. Vulcan shows it failing: the planet did not exist, and a change in theory (general relativity) resolved the anomaly. The author concludes that history 'offers precedents for both strategies, and it does not tell us in advance which one we are living through'. That is option 4.
- Option 1 picks only the Neptune half.
- Option 2 misapplies the Vulcan case: general relativity explained Mercury, not dark matter.
- Option 3 goes too far; the author does not recommend abandoning the strategy.
- Option 1 picks only the Neptune half.
- Option 2 misapplies the Vulcan case: general relativity explained Mercury, not dark matter.
- Option 3 goes too far; the author does not recommend abandoning the strategy.
Question 4
Single correct option
Which of the following, if true, would most seriously WEAKEN the position of MOND's proponents as described in the passage?
- A. A newly observed galaxy's rotation matches the curve MOND predicted from its visible matter alone.
- B. Underground detectors, after a further upgrade, still record no collisions with dark-matter particles.
- C. In a collision between two galaxy clusters, the mass inferred from gravitational lensing is found clearly separated from the clusters' visible gas.
- D. Axions are shown to be too light to account for the rotation of galaxies.
Show answer and solution
Answer: (C) In a collision between two galaxy clusters, the mass inferred from gravitational lensing is found clearly separated from the clusters' visible gas.
MOND says the anomaly comes from modified gravity acting on *visible* matter, not from unseen mass. The passage notes its weakness 'on larger scales, where it struggles with galaxy clusters'.
Option 3 describes a cluster collision in which the gravitational (lensing) mass sits apart from the visible gas. If gravity simply tracked visible matter, as MOND implies, this separation should not happen. Unseen mass explains it naturally, so this hurts MOND the most.
- Option 1 *supports* MOND: it confirms a prediction made in advance.
- Option 2 also tends to support MOND's critics of dark matter, since detectors keep finding nothing.
- Option 4 rules out one dark-matter candidate, which if anything helps MOND.
Option 3 describes a cluster collision in which the gravitational (lensing) mass sits apart from the visible gas. If gravity simply tracked visible matter, as MOND implies, this separation should not happen. Unseen mass explains it naturally, so this hurts MOND the most.
- Option 1 *supports* MOND: it confirms a prediction made in advance.
- Option 2 also tends to support MOND's critics of dark matter, since detectors keep finding nothing.
- Option 4 rules out one dark-matter candidate, which if anything helps MOND.
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