You are using the wrong verb. To raise= to lift something up.(among other meanings) to rise= to increase, to raise one's self up .
The population is rising very fast.
The population has risen very fast.
Both are ok. The meanings are slightly different.
The population is rising very fast. = You are stating a general trend going on right now that will probably continue.
The population has risen very fast. = You are noticing what has happened up to this moment. You are not saying anything the future.
You could also say "is increasing" or "has increased."