To estimate the expansion rate of the universe is a notoriously difficult problem because there is a lack of a single yardstick that all distances can be measured by.
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ATo estimate the expansion rate of the universe is a notoriously difficult problem because there is a lack of a single yardstick that all distances can be measured by
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BEstimating the expansion rate of the universe is a notoriously difficult problem because there is no single yardstick by which all distances can be measured
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CBecause there is a lack of a single yardstick to measure all distances by, estimating the expansion rate of the universe is a notoriously difficult problem
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DA notoriously difficult problem is to estimate the expansion rate of the universe because a single yardstick is lacking by which all distances can be measured
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EIt is a notoriously difficult problem to estimate the expansion rate of the universe because by no single yardstick can all distances be measured
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