1. Galileo and the Tower of Pisa experiment
2. Einstein, A. (2001). Relativity: The Special and General Theory (Translated by Robert W. Lawson). M.Sc., University of Sheffield. https://www.f.waseda.jp/sidoli/Einstein_Relativity.pdf
3. Gravity and Acceleration Are the Same Thing (2016). https://www.pbslearningmedia.org/resource/nvem-sci-gravityacc/wgbh-nova-inside-einsteins-mind-gravity-is-acceleration/
4. Kolecki (2003). On the Expansion of the Universe. https://www.grc.nasa.gov/www/k-12/Numbers/Math/documents/ON_the_EXPANSION_of_the_UNIVERSE.pdf
5. Physics of the Universe (2024). General Relativity and Quantum Cosmology.https://www.physicsoftheuniverse.com/topics_relativity_gravity.html#:~:text=Einstein's ground-breaking realization,the “principle of equivalence”