Can we study University level subjects without getting admitted into a university? I am currently a senior high school, and next year I will enroll for my undergraduate studies but I am afraid that my grades would not be good enough  to be admitted in good universities like MIT, Princeton etc.
So can I study advanced topics like general relativity, quantum mechanics etc., like at university but without getting into it?
This is a response that takes various comments of the OP into account rather than an explicit answer to the question asked - but it is too long for a comment, and I really think that the OP should be made aware of the following points.
@OP:
I do not intend to be rude, but I would strongly advise you to recalibrate your expectations and your perception of how research and academia work. For instance,

you say that you never managed to get good grades in highschool, but your question focusses on &quot;advanced topics like general relativity, quantum mechanics&quot;;

you are worried (in some of your comments) that you won't be admitted by any university, but your question focusses on places like MIT;

in another question you mention research of yours (which I find a bit odd, anyway, since you write you're a highschool student) and don't seem to be particularly convinced of its quality, but in a comment here you claim that you wish to &quot;interact with greatest minds of this world&quot;.


Now, it is important to note that none of your goals is completely unreasonable in itself: there are, of course, a lot of potential reasons why a smart student might have poor grades in highschool, and might anyway master theoretical physics later on; it is, as mentioned by others, also absolutely possible to do your undergraduate studies at a respected but not-at-all famous university, and then do your graduate studies at a very well-known place; it is also possible to start out with not particularly good research and to improve quickly and considerably, so that the &quot;greatest minds of this world&quot; would like to discuss your research with you.
However, the only way to achieve any of this is to keep a realistic perspective and to move forward in small and down-to-earth steps.
Currently, your contributions here merely focus on extremes (&quot;bad grades in highschool vs. studying advanced topics in theoretical physics&quot;; &quot;not admitted to any university vs. admitted to MIT&quot;; &quot;research of low quality, produced as a highschool student, vs. interacting with the greatest minds of this world.&quot;)
Here are a few suggestions of a more realistic approach:

If you had difficulties to earn good grades in highschool, make a thorough (and honest) analysis of the reasons, and try to figure out what you can do to improve this situation when you attend a university. In order to get any degree you will have to pass exams, too, and if you want to have any chance of your dreams coming true, you will have to do very well in most of them.
If you had poor grades in highschool, this might also indicate (though it is not sure, of course) that you don't know well some of the material from highschool. No matter whether this might not be your fault or what the precise reasons are - it might still pose a problem when you want to study topics in physics at university level. So please try to find out whether you have, for instance, serious gaps in your highschool knowledge of mathematics or physics (and maybe also some other subjects that could be relevant), and if you find some, try to close these gaps before you're heading towards more advanced topics.
(Please note that much more basic physical topics than general relativity and quantum mechanics - for instance, classical Newtonian mechanics - require mathematical knowledge which goes far beyond the mathematics taught in highschool.)

If you intend to become, in a few years, a graduate student in one of these &quot;top-tier&quot; universities, try to make choices right now which can help you get admitted then. For instance, make an effort to find out which kind of undergraduate experience (for instance, research experience, but also other things) are considered advantageous for the admission decision, and then try to choose a university for your undergraduate degree where you have good chances of acquiring this experience.

Getting involved with research early is certainly a good idea of you have ambitious plans - but the most reasonable way to do this is within the setting of a college or university and in collaboration with other people who already have experience in doing research.

Please try to inform yourself about some details of the academic system. For instance, you write in one comment that you are considering self-studying, then to write a few good research papers and to get a PhD in Cosmology. As mentioned by other users, this is a very unrealistic plan:
First, being admitted for graduate studies seems to be very unlikely if you do not have an undergraduate degree. So if you want to do a PhD, there is most likely no way around getting admitted for undergraduate studies first.
Second, producing good research and getting it published in reputable journals is very, very difficult if you do not have any formal education (= education at a university) in your subject and no senior colleagues that support and advise you. Producing really excellent research and getting it published in top journals (which would fit your, in some respects extremely ambitious, goals) is literally impossible without the aforementioned prerequesits.


Now, all these suggestions certainly come across as much less glorious and prestigious than getting admitted to MIT, or studying general relativity as soon as possible, or interacting with great minds. However, please be aware that even the greatest minds spend a considerable amount of their time with completely non-glorious, down-to-earth routine tasks (like advising students, marking exams, writing grant proposals, and so on).
Also, even if someone is born as a genius, a degree from a top university, a solid understanding of theoretical physics and the possibility to work with great people do not simply come to them out of the blue - even the smartest people have to work long and very hard for this, and they do it by taking many - maybe amibitious, but most often still small and realistic - steps. So precisely this is my advice for you.