MadAtom said:
this won't be a disadvantage while applying to an university? The admission committee won't consider me as a not focused student and consequently lose interest in me?
You must prove your interest regardless of major.
There's some areas, such as physical chemistry, condensed matter physics, materials science, biomedical engineering, biochemistry, computational biology, etc. that are much more welcome to outsiders (major switches) than others like theoretical math or high energy astro. Its simply because some areas are quite interdisciplinary and a great variety of people can contribute their skills, and others are very specialized and only a few people can.
However, if you truly are unfocused, say, going from physical chemistry all the way into high energy astro, then yes, they will junk your application outright unless you could prove (with publications and/or research experience) that you can succeed.
When I applied to grad school several schools junked my paper outright because I expressed interest in a research area that I didn't have background for (nonlinear optics). On the other hand I was readily accepted by programs when I expressed interest in their condensed matter research, because I already had extensive lab and research experience in materials science.
I don't sit on an admissions committee so take my thoughts as reference only.