eliminating dark matter with an intuitive culprit
Few things are as reviled on popular science and physics comment sections as dark matter and dark energy because aside from indirect observations, we’ve never actually detected either. We can see that something is pushing galaxies apart from each other while another invisible force holds these galaxies together, but there have been many attempts to do away with both in a theoretical sense. From imagining universes filled with a low energy plasma, to trying to re-imagine the Big Crunch model, to systematic reviews of CMBR data, just about everything seems to have been thrown at the dark matter and dark energy, but no model can yet explain how galaxies are being held together in their current configurations and why they seem to be flying apart.
But there is a new idea out there that sounds like it may be on to something. Rather than mathematically rewiring the entire universe, it tries to eliminate the hidden forces by creating a new model of how the gravity of distant matter ripples throughout space, and concluding that their wake may explain the force behind dark matter not with exotic particles or quantum phenomena on cosmological spans, but with something far more familiar…
Generally, we don’t spend a whole lot of time thinking how distant objects would affect each other since, like a lot of other forces, gravity follows the inverse-square law, meaning that if you double the distance between two objects, their gravitational pull on each other would be reduced to a quarter of its strength. Basically, you could imagine gravity like a beam of light, diffusing with distance at an exponential pace, and hence, making the pull of distant stars and planets on each other a somewhat irrelevant concern. Yes, it registers when we get to the scale of galaxies spinning around huge central black holes, but when dealing with hundreds of thousands to millions of light years between two objects, even galactic scale entities shouldn’t matter, right?
Well, an Italian mathematician begs to differ and he’s come up with a metric which ties in gravitational wakes from sprawling webs of galaxies and casts these interactions as the enigmatic dark matter. In other words, he says, what we call dark matter is actually just gravity on an intergalactic level. And his paper even includes examples of real galaxies behaving right on track with his models, give or take an occasional nudge from a big dwarf galaxy or the occasional supermassive black hole belch. So mystery solved, right? Well, not quite yet…
One problem with calling the existing observations fitting the model a slam dunk is the fact that there are over 100 billion galaxies in various stages of development, growth, maturity, or turmoil out there, so odds are that you’ll be able to find galaxies that’ll match your predictions no matter that they are if you were to invest enough time in the search. While we haven’t mapped anywhere near even a tenth of them, we do have a huge galactic catalog spanning millions of entries, again enough to find virtually any behavior you’d want, and many caught rotating, forming, or colliding in ways we don’t even know are possible yet since we don’t have enough people to review all the data we currently have.
If you want to posit that new galaxies are birthed by the supermassive black holes of other galaxies, I’m sure you could find snapshots of early galaxies that look very much like they were being born from radioactive jets of quasars. Unless something like two thirds of all galaxies rotate just as this new model of dark matter predicts, we could say that we’re on to something. However, an effort to find out something as complex as this would take a long time and a lot of resources and when parts of the model don’t deal with such things as gravitational lensing or explain what look like random clumps of dark matter in intergalactic space, there may not be too many astronomers willing to devote a lot of time to testing it.
Similarly, when we’re dealing with a mathematical model, we always have to make sure that the numbers fit a particular set of observations not because the math has been retroactively set to fit them, but because they do simply by virtue of the equations’ results. In this case, an impressive test of this model would be pointing to a random region of space, using the model to calculate the rotation rate of a hypothetical galaxy, then seeing a new galaxy just like the one described in very similar circumstances doing the same thing. Though even then it could be argued that the model’s central metric is simply dark matter without being called dark matter since one of the key dilemmas with dark matter is its supposed preponderance.
With just about 4% of all universal contents being regular matter, there have to be somewhere in the neighborhood of five dark matter particles for every particle of plain old matter. Can rotating matter really have enough momentum to account for the pull and energy of something six times as abundant when averaged out across the entire cosmos? Seems rather unlikely, but then again, the universe keeps showing us that all sorts of intuitively unlikely things tend to be the norm rather than the exception we tend to think they are at first.
See: Carati A. (2011). Gravitational effects of faraway matter on the rotation of spiral galaxies arXiv: 1111.57…