🧵 THREAD: Five Radical Alternatives to Dark Matter and Einstein's Gravity

What if we don't need dark matter? What if Einstein's equations need tweaking? These five theories challenge our most fundamental assumptions about gravity—and some have had surprising successes.

1/11 🌌 MOND (Modified Newtonian Dynamics)

In 1983, Mordehai Milgrom made a bold proposal: What if Newton's laws break down at extremely low accelerations? Below a critical threshold (a₀ ≈ 10⁻¹⁰ m/s²), gravity might behave differently than we think.

2/11 The motivation? Galaxy rotation curves. Stars at the edges of galaxies orbit way too fast—they should fly apart unless there's invisible dark matter holding them in. But MOND predicts these velocities WITHOUT dark matter by tweaking gravity itself at low accelerations.

3/11 The stunning part? MOND's single parameter (a₀) successfully predicts rotation curves for hundreds of galaxies with remarkable accuracy. It even predicted the velocity-luminosity relationship (Tully-Fisher) BEFORE observations confirmed it. That's not luck—that's something real.

4/11 But MOND has problems: How do you make it relativistic? How does it explain gravitational lensing and cosmic structure? TeVeS (Tensor-Vector-Scalar gravity) and other covariant versions try to solve this, but they're complicated and still struggle with some observations.

5/11 💡 VSL (Variable Speed of Light)

Here's a heretical idea: What if the speed of light ISN'T constant? What if it was much faster in the early universe and has been slowing down ever since?

6/11 VSL theories tackle cosmological puzzles like the horizon problem (why is the CMB so uniform?) without inflation. If light traveled faster early on, distant regions could communicate and equilibrate. As the universe aged and c decreased, we ended up with today's value.