PrepTest 134, Section 3, Question 13
Astronomers have found new evidence that the number(51 more words not shown)
Which one of the following,(12 more words not shown)
- AThe mass of(15 more words not shown)
- BThe overwhelming majority(15 more words not shown)
- CThe number of(18 more words not shown)
- DTheories about how(13 more words not shown)
- EThere is no(13 more words not shown)
Reading the argument
Five times as many galaxies, but almost no change in the estimated mass? Something about our expectation needs another look.
One possibility is that astronomers now think the average galaxy has less mass. More galaxies could then fit within roughly the same total. Another possibility is that galaxies account for only a tiny part of the universe's mass. If most of the mass comes from something else, a big increase in the galaxy count might barely move the overall estimate.
What we’re looking for
The question asks us to explain the unchanged estimate, not to explain why astronomers found more galaxies. We should accept both reported developments and look for a way they can fit together. Either of those possibilities would be worth checking in the answers.
Working the answers
- ACorrect
This supports the second possibility: galaxies contribute only a tiny share of the total mass. A large change in that small component can make very little difference to the whole.
Imagine estimating the combined weight of a house and everything inside it. You thought it contained one can of soda; now you discover five. There are five times as many cans, but four additional cans barely affect that enormous total. The number of cans changed dramatically while the estimate of the whole stayed virtually the same. That's the relationship A supplies for galaxies and the universe's mass.
- B
Rough estimates can still change when you discover many more objects to count. If you roughly estimate each galaxy's mass and then learn there are five times as many galaxies, why wouldn't you revise the total? B establishes uncertainty but doesn't explain why this new finding has so little effect.
- C
Better instruments can explain the increase in the estimated number of galaxies. That answers a different question. We already accept that astronomers now think more galaxies exist; we want to know why their estimate of total mass hasn't changed accordingly.
- D
This concerns whether mass estimates affect theories of galaxy formation. We're asking why a changed galaxy count has not substantially affected mass estimates. D doesn't connect the new count to the stable total; it discusses a different influence in the other direction.
- E
Disagreement about methods doesn't mean astronomers ignore new evidence. Scientists using different procedures might all revise their own estimates when they learn there are more galaxies. To make E explain the result, we'd have to add that their procedures were insensitive to galaxy counts. The answer never says that. A directly explains why the count can change so much while total mass changes so little.
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