" Shouldnt the tenses in a single sentenece remain the same? No, there is no such requirement. Otherwise in what situation we can change the tenses in a single sentence?
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beatgmat"The widely accepted big bang theory holds that the universe began in an explosive instant ten to twenty billion years ago and has been expanding ever since."This is a variant of a very typical pattern with since, in which a process has been continuing since an event occurred in the past.