Hi savvysatyam,

Good question! You are thinking along the correct lines about the sequencing of the verbs, but there is one important thing that you have missed. Yes the action of "began" happened before the action of "expanding". However, note that the author intends to say that the expansion of the universe is an action that is continuing in the present. And that is why this sequencing does not warrant use of past perfect and past tenses. From your doubt, I believe you have confusion between choices A and B. So let me present both choices and further clarify the confusion!

The widely accepted big-bang theory holds that the universe began in an explosive instant 10 to 20 billion years ago and has been expanding ever since.

A. that the universe began in an explosive instant 10 to 20 billion years ago andhas been expanding
B. that the universe had begun in an explosive instant 10 to 20 billion years ago and had been expanding

Choice A is the correct answer because it uses correct verb tenses. This sentence is talking about how universe was created. It is talking about a well-known action that took place in the past. Whenever we talk about universal truths or general information, we use simple past or present tense, depending upon the time of their happening. In this case, creation of universe took place in the past. Hence use of simple past tense "began" is absolutely correct. Also use of present perfect continuous tense "has been expanding" is correct because it is an action that started in the past and is continuing even today. There are no errors in choice A.

Now let's see why choice B is incorrect:

that the universe had begun in an explosive instant 10 to 20 billion years ago and had been expanding

Use of past perfect tense "had begun" is incorrect. Past perfect tense is used when we need to show that out of two events that took place in the past, which one took place earlier and which took place later. In this sentence, there is only one event that took place in the past. Hence, there is no later event in the past. This makes use of part perfect tense incorrect.

Also use of past perfect continuous tense "had been expanding" is incorrect because the event of expanding is not over in the past. It is still continuing. These verb errors make choice B incorrect.

Hope this helps.

Regards,

Payal