“All we know so far is what doesn’t work.” -Richard Feynman
The past month has seen a slew of papers out highlighting the tension between modified gravity and dark matter. Both recognize the same puzzles and problems with the Universe, and both ideas recognize that either one could be valid. In fact, if you look at the two greatest “crises” in gravity in the 19th century, it’s arguable that dark matter (Neptune) solved one, the Uranus problem, while modifying gravity (with Einstein’s general relativity) solved the other.
The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and dark matter. Image credit: X-ray: NASA/CXC/Ecole Polytechnique Federale de Lausanne, Switzerland/D.Harvey & NASA/CXC/Durham Univ/R.Massey; Optical & Lensing Map: NASA, ESA, D. Harvey (Ecole Polytechnique Federale de Lausanne, Switzerland) and R. Massey (Durham University, UK).
Now in the 21st century, we have a whole Universe to explain, and while dark matter is definitely the leading theory, the idea of modifying gravity isn’t crazy. Moreover, it has a success that dark matter can’t match: on galaxy-scales and below.
The details of the small-scale structure predicted by dark matter do not match with what we observe. The hope of the dark matter camp is that improved simulations and models will come to reproduce them accurately and robustly. Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).
In the end, it will take a big step forward for a true victor to emerge, but here’s where the science stands right now.