Alternative Theory of Gravity

Radio image of the neutral hydrogen gas in the galaxy AGC 114905. (Mancera Pina et al., MNRAS, 2021)

Out in the dark depths of space, our models of the Universe get messy. A new study looking at the ultra-diffuse dwarf galaxy AGC 114905 has revived a controversial theory (or more accurately a hypothesis) of gravity, and given us more questions than answers about what's making our galaxies tick.

It all starts with dark matter – or in this case, no dark matter. Although most cosmologists agree there's something out there called 'dark matter', causing spiral galaxies to rotate faster than they should, even dark matter doesn't answer all the questions we need it to.

So, it's not a bad idea to look at some alternative options. You know, just in case we are never able to find the stuff.

One alternative hypothesis to dark matter is called Modified Newtonian dynamics (MOND) or Milgromian dynamics framework. This hypothesis – first published in 1983 by physicist Mordehai Milgrom – suggests that we don't need dark matter to fill in the Universe's gravity gaps, if we calculate the gravitational forces experienced by stars in outer galactic regions in a different manner to how Newtonian laws suggest.

To test this idea, which involves working with proportionality to the star's radius or centripetal acceleration, we need to be looking at the speeds of galaxies – specifically weird ones like ultra-diffuse galaxies.

These very faint, ugly ducklings of the galaxy world have a habit of not acting like a galaxy should. For example, some ultra diffuse galaxies seem to be made almost entirely of dark matter, whilst others are almost completely dark matter-less.  READ MORE...