Could a large group of miners theoretically lower the network hashrate to manipulate difficulty?

Question

I am relatively new to understanding the fundamentals of the Bitcoin network, so I am still trying to learn how the actions of miners can influence the direction of the blockchain.

Back to my main question, since the difficulty adjusts at a fixed rate of every 2016 blocks (~2 weeks), what would prevent a large hashrate miner or pool from entirely shutting down their mining power minutes before the 2016th block was found? The incentive here would be that the next difficulty would become significantly lower until it is readjusted in 2016 more blocks. This process of course, could be repeated.

Is there some sort of method in place which averages transaction times preventing this from happening? If so, would such an action still have any affect?

Thanks :)

Answer 1

The difficulty adjusts every 2015 blocks (due to an unintentional "off-by-one" error), taking into account how much time elapsed for those 2015 blocks to be mined. If it took less than two weeks, the difficulty goes up. If it took longer than two weeks, the difficulty goes down.

If a large amount of mining (hashrate) was shutdown for the entire 2015 block period, it would take longer to mine the blocks and surely it would bring the difficulty down; but it would cost those miners substantially (in lost opportunity, blocks they could have mined), and further, after the difficulty adjusted down, it would not be long for it to once again adjust up, so really not much point in doing that.

Answer 2

The difficulty adjusts every 2016 blocks based on the average block time for all of those 2016 blocks. So just making the 2016th block take longer to mine has little effect on adjustment. A miner would have to manipulate block times for the entire 2016 block period to get any significant difficulty change.