One of the features of science is that there is always a tension going on. We have standard paradigms that most scientists work within but on occasion a new result will turn up that seems to be violate the boundaries of that paradigm. What does one do then? Reject the paradigm and its associated underlying theory? Seasoned scientists know not to do that because throwing out a paradigm is not something to be undertaken lightly since good theories are hard to come by. What they do is treat the discrepant event as an anomaly meriting further study.
The community as a whole then falls into three camps: one group tries to see if the anomaly can be brought back into conformity with the paradigm by new experiments/observations and theoretical calculations. A second group takes more seriously the possibility that this might be a real effect that requires a new paradigm to accommodate and looks to see if they can find one that fits the bill. The third group, the largest by far, consists of those who are not working in that field and are thus not using that paradigm but are aware of the existence of the anomaly. They watch with interest from the sidelines to see how things fall out.
An example of this is what happened recently with the reported observation of a galaxy that did not seem to have any dark matter at all. Dark matter is the dominant paradigm in astrophysics at the moment, in which it is believed that all galaxies are immersed in a halo of this matter whose effects are seen by the gravitational forces exerted by it. The key problem is that there has been no direct detection of dark matter so its existence has not been confirmed. Hence the report of galaxy that is free of any dark matter opened up the possibility that the dark matter paradigm might be wrong.
But new analyses of data found that the anomaly can be explained away because the problematic galaxy is actually closer than had been previously thought and this removes the anomaly.
So an international team of researchers led by the Instituto de Astrofísica de Canarias (IAC) decided to take a closer look. And they found that all the anomalous measurements in the previous research that pointed to an absence of dark matter were reliant on the distance to the galaxy – 64 million light-years away.
This gave them something to work with. Using five separate methods, including photometry from the Hubble Space Telescope and the Gemini Observatory, they recalculated the distance to NGC1052-DF2.
Each method turned up the same result – NGC1052-DF2 is much closer than 64 million light-years away. According to the team’s multiple calculations, a more accurate distance would be around 42 million light-years.
Based on this new distance, the mass of the galaxy is about half of what it was thought to be previously – and the mass of the stars is only about a quarter of what previous analysis suggested.
So, not only does the galaxy itself have less mass, but the proportion of normal matter within that mass is smaller. This implies that the rest must be made up of – you guessed it – dark matter.
This does not end the story. Single experiments rarely do in science. Groups will investigate further to see if they can find further corroborating evidence for either position until finally everyone moves on to other things and the consensus view is filed under the ‘solved’ category.
I’m not a physicist. How do we know that “standard candle” calculations of astronomical distances are correct? If everything were really twice as far away as we had thought, would that make it possible for the dark matter concept to be unneeded? Thanks for any simple explanations or links that answer this.
Rob Grigjanis says
It’s not just about distance. Spiral galaxies have rotational curves which need explanation, and dark matter is the best we’ve come up with so far.
Mano Singham says
Measuring/calculating the distances to galaxies is very difficult task and taught with potential errors so you raise a very important question. Not being in the field of astrophysics, I cannot provide a confident answer to your question. What I do know is that before invoking an exotic new idea like dark matter, astrophysicists would likely have examined closely other factors that might have resolved the spiral galaxy rotational curves that Rob mentions, and distance effects would definitely have been investigated. So I suspect that the fact that distance played a role in this particular case is because of factors that are specific to it. Furthermore, going from distance to brightness to mass of galaxies is not straightforward either.
Most methods of measuring distance consist of ladder methods whereby we start with distances close by that can be measured directly by parallax methods and use those to set standard candles and then use those candles to measure greater distances and get new standard candles, and so on. Such methods are prone to systematic errors and people try to find different ways to do so and if the answers converge, that gives us some confidence that we are on the right track.
There are other paradigms such as the MOND (Modified Newtonian Dynamics) that do not invoke dark matter to explain the spiral galaxy data but that is a minority viewpoint at the moment.
Jenora Feuer says
Really, a lot of it comes down to ‘there are actually about five different ladder methods and they all seem to agree with each other on the results to within some fairly strict limits’.
My understanding (mostly from reading ‘Starts With a Bang!’) is that there IS some concern about this right now, because while the various ladder/standard candle methods all converge on the same answer, and while working forward from the Cosmic Microwave Background and quantum/particle physics approaches also all converge on an answer, those two groups of methods converge on slightly different answers, and there’s enough accuracy in both now that the error bars don’t overlap anymore. (There’s about a 8-9% difference between them, and only a 2-3% likely error on either.)
That said, the fact that two completely unrelated methods of measuring the Hubble Constant still return values less than 10% apart shows that no matter what the truth is in this argument, we’re not far off.
For any casual anyone who wants to learn more about this and astronomy in general, I wholeheartedly suggest the youtube channel “PBS Space Time”.
Don’t the alternatives to dark matter usually involve a modification of gravity? I would have thought that the discovery of a freak dark-matter-free galaxy would be regarded as proof that dark matter exists--it’s hard to be unusually deficient in something that doesn’t exist in the first place.
It would be unexpected for a random galaxy by itself to be free from dark matter. However, with certain unusual circumstances, the absence of dark matter could be confirmation of dark matter. For example, see the Bullet Cluster.
This particular result, the distance to the galaxy, has been the subject of a bitter arxiv war extending for almost a year now (there are several rebuttal papers to the one Mano cited). It is far from resolved, and the fact that this paper has finally been accepted by MNRAS (almost a year after being posted to arxiv) doesn’t mean it’s correct.