Fill in the gaps with the verbs could, may, must, have to, should and the appropriate infinitive 1)Einstein’s theory predicted that the universe was not static, but (be) either expending or contracting.
2)Because the universe is expanding it (cool), which means that it (be) much hotter when it was young.
3)Therefore, a significant fraction of the universe (consist) of non-baryonic matter.
4)There were no longer free electrons to absorb and scatter light, and photons (travel) freely through the universe.
5)These fluctuations (cause) by something that happened even earlier.
6)We , however, (be) careful about taking these models too seriously; they all (be) wrong.

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