[Ethereum] the “difficulty bomb” and what is the goal of it

From https://github.com/ethereum/EIPs/blob/master/EIPS/eip-2.md:

Mining difficulty is calculated from the time difference between blocks. The exact formula will change in Homestead. The symbol // in the following denotes integer division.

• Frontier:

block_diff = parent_diff + parent_diff // 2048 * (1 if block_timestamp - parent_timestamp < 13 else -1) + int(2**((block.number // 100000) - 2))

• Homestead:

block_diff = parent_diff + parent_diff // 2048 * max(1 - (block_timestamp - parent_timestamp) // 10, -99) + int(2**((block.number // 100000) - 2))

EDIT:

The problem with the frontier formula and the reason for the change was that the frontier version doesn't take into account how far off from 13 seconds the block time was. A block mined 1 second after the previous one has the same effect on the difficulty as one mined after 12 seconds. This causes block difficulty to adjust to a median block time rather than a mean.

Again, check out the EIP for more details.

The difficult bomb was actually added in a hard fork shortly after Ethereum's launch. It's not a necessary part of the protocol, and so if PoS isn't ready on time, it's most likely that another hard fork will delay or remove the bomb. In fact, if a portion of the Ethereum community desires to still use PoW, or does not agree with the final PoS protocol, there will likely be a hard fork on the remaining PoW network to remove the bomb.