Sometime I do want to know who has changed which file in the last few days.

How? I think 'history' is maybe one way. But there should be other better ways.

Take a look at the "find" command.
example:
finds all files in the current directory (and below) that were modified less than 1 minute ago.

"man find" for details

This just find out the changed file under current dir. How about find out all the files?

Look at the man page!!!

The full syntax is:
find [options] [path] [expression]

"." (meaning current directory) is only one example of a pathname.

If I read your question correctly, you also want to know WHO changed a file. I don't believe there is any data on the OS level that would be of much help; you should look at the application level for this kind of info.
That said:
ls -altr
shows in reverse order of data/time when the files have changed, the most recent being at the bottom of the list.
ls -altru
shows in reverse order of data/time when the files have been accessed. Very useful during upgrades for weeding out junk that is never accessed. Under Windows this information is corrupted by anti-virus scanning. Under linux this information is usually corrupted by backup software unless explicitly told to retain the last time accessed.