Changing research interest in Math I am currently in my final semester of masters. I have some background in Algebraic Number Theory. I have one PhD offer from a US university. 

When I started getting into algebraic number theory, I did not have much trouble as most of the pre-requisites were from Galois Theory and Commutative Algebra. But through my study in the last two-three semesters, I have realized that Number Theory is a subject which requires knowledge of many different areas like Complex Analysis, Algebraic Geometry, Topology(subjects which I have not learned properly or not learned at all). This has put me under a quite a lot of stress: Am I good enough to study Number Theory or should I shift to something else, something more self-sufficient? 

Is it possible (and advisable) to change my research interest to something else? What should I do?

Edit- To add to my troubles, I don't know anything about Modular Forms or Automorphic Forms or Galois Representations.

Before voting to close this question down, I would like you to know this: I originally asked this question on Math SE. But people told me that it was more of an Academia SE question.
You haven't mentioned any alternatives to Number Theory.  Hmm.  Maybe it would be reassuring for you to have a small list in your hip pocket of possible areas.

If you don't have to decide right away, then I would suggest that you hold off on deciding, and meantime start remedying these gaps as soon as possible.  Without panic, but without any shilly-shallying.  Obviously, don't work on them all at once, but do start somewhere, and make a tentative map through this subject matter -- in what order, what books will you use, what topics could be skipped or skimmed over, which ones will you do in a class, which as self-study, and which in an independent study.

Even if you end up working on something other than Number Theory, this study will be helpful for you in the long run, either directly or indirectly.

If it feels daunting, think one step at a time, or Rome wasn't built in a day, or some similar phrase that helps you.

In the U.S., one is generally not expected to hit the ground already doing research, right from the first semester.  It's okay to take some time and get the solid foundation one needs.

Do you have an advisor, potential advisor, temporary advisor, or mentor of some sort?  If not, look around for one, and if necessary ask the department to assign you one temporarily.