UA is my favorite proof of Flat Earth. There are different kinds of experiments which suggest that the surface of the Earth is physically accelerating upwards. It is considered to be an absurd phenomenon which inspired the modification of the nature of space and time at the beginning of the 20th century to account for the effects under a Round Earth. The geometry of space of such a nature that the earth is physically accelerating up to objects at a physical level, but there is an unseen nature of the universe called "space-time" or "curved space" which creates this illusion.
See the information in these links:
https://wiki.tfes.org/Equivalence_Principlehttps://wiki.tfes.org/Evidence_for_Universal_AccelerationThe objection to UA mainly comes from imagining the Earth as a disk that accelerates through the universe that can hit objects in the way, or that the disk can exceed the speed of light.
Firstly, it doesn't need to be a disk accelerating through a Cartesian universe. The traditional expanding universe model postulates that the universe can expand and accelerate away from itself and create new space without mass destruction. It could also be something akin to a universe accelerating through a larger meta-universe and we feel the inertial effects.
Per speed of light limits, it's not a limit in Special Relativity and things can continuously accelerate without reaching the speed of light as a function of the laws through dilation effects. But Special Relativity was created to explain the
Michelson-Morley Experiment which suggests that the Earth is horizontally motionless. So while continuous acceleration isn't a problem in SR, I have some doubts about those laws myself.
Much of the major theoretical science in Astronomy is really about why the FE seems to be true but is "really" not. It is continuously fascinating. The "but but how about this" just doesn't compare to the bulk of this evidence.