Course Overview: https://youtu.be/xyF-MmGNxd0 The 20th century was known as the century of physics. In the past 120 years, concepts such as space, time, energy, entropy and particles were understood to much deeper levels. New paradigms of thinking such as relativity and quantum mechanics emerged. This course is the first course in the Understanding Modern Physics series, which covers an introduction to special relativity, general relativity and cosmology. We will find: (i) How space and time are relative to observers, and unified into a more fundamental construction of spacetime; (ii) Why the spacetime is not absolute, but rather curves in response to matter, and how gravity emerge as a result of such spacetime curvature; and (iii) What is the framework to understand the evolution of the whole universe, and how that is related to problems such as the origin of space and matter, and the fate of our universe. Note: the videos with a (*) are optional. They provide complementary information but not in the learning objective or assignment questions. Feel free to choose to watch them or not.