Yes, it is much harder to get into a program when listing interests in cosmology or string theory, even more so for internationals. You have fewer groups around the country involved in this, much less funding than that of other specialties, and a host of students interested in it which creates exceedingly high competition. However, listing theory in general knocks your application standards up, this just does even more so. With all that being said you need to answer honestly about what you're interested in. They understand people can drift in their interests to a degree, but if you for instance list experimental condensed matter and all of a sudden you're looking for a string theory group to get into they're not going to take that well, and you might find yourself being blackballed by certain groups because of that action and your relationship with your advisor strained. These are all people you're going to need when you graduate, they're not people you want to be on bad terms with. So be honest, because you may get into the school and find you can't get into that research, what are you going to do then? Switching schools is very rare, so you're kind of stuck in a situation you're taking a big gamble on. It's better to get a no beforehand and try your luck elsewhere.