In the dark depths of space, our models of the Universe are getting messy. A new study of the ultra-diffuse dwarf galaxy AGC 114905 has revived a controversial theory (or more precisely a hypothesis) of gravity, and has given us more questions than answers about what makes our galaxies work.
It all starts with black matter – or in this case, no dark matter. Although most cosmologists agree that there is something called “dark matter”, which causes spiral galaxies spin faster than they shouldeven dark matter does not answer all the questions we need.
So, it’s not a bad idea to look at some alternative options. You know, In case we are never able to find things.
An alternative hypothesis to dark matter is called Modified Newtonian Dynamics (MOND) or the framework of Milgromian dynamics. This hypothesis – first published in 1983 by physicist Mordehai Milgrom – suggests that we do not need dark matter to fill the gravity gaps in the Universe, if we calculate the gravitational forces experienced by stars in the outer galactic regions in a way different from how Newtonian laws suggest.
To test this idea, which is to work with proportionality to star radius or centripetal acceleration, we need to look at the velocities of galaxies – especially weird ones like ultra-diffuse galaxies.
These very weak ugly ducklings of the galactic world have a habit of not acting like a galaxy should. For example, some ultra diffuse galaxies appear to be made almost entirely dark matterwhile others are almost completely devoid of dark matter.
But this team discovered that the galaxy’s rotation was extremely slow – slow enough that not only did they not need dark matter to confirm the models, but the galaxy’s rotation curve also cast huge doubt on the WORLD frame. This does not correspond to either of the two hypotheses.
“The reported very low rotational velocity of this galaxy is inconsistent with both MOND and the standard approach with dark matter,” says University of St Andrews physicist and one of the researchers of the new paper, Hongsheng Zhao.
“But only MOND is able to circumvent this apparent contradiction.”
The new paper has “debunked” the 2021 discovery, suggesting that the problem is not with MOND, but rather with the tilt of the galaxy itself.
When we look at distant galaxies in the depths of space, it can sometimes be difficult to confirm what angle we are seeing. The original team found that AGC 114905 looked elliptical, suggesting we’re looking at the galaxy from one angle.
But using simulations, researchers now suggest the galaxy could appear elliptical even when facing us. A change in the galaxy’s angle relative to us would also change the galaxy’s rotational speed, making all MOND calculations add up after all.
“Our simulations show that the tilt of AGC 114905 could be significantly less than reported, which would mean that the galaxy is actually rotating much faster than people think, in line with MOND’s expectations,” says lead author of new paper, physicist Indranil Banikalso from the University of St Andrews.
Now, that’s still an open question. We don’t know if this new paper, or the 2021 paper is going to be crowned victorious – or at least the most correct.
In the meantime, if this new discovery holds, it looks like the MOND framework might live another day. As wild as MOND may be, with dark matter still elusive and many other questions still unanswered, we need every possible option.
The research was published in the Royal Astronomical Society Monthly Notices.
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