What is Dark matter?
Dark matter is a major component of Cosmology and Astrophysics. In the first case, the cosmological models of galactic evolution are based on this dark matter that does not emit or block light. Since it is assumed that it constitutes 85% of the total matter, the models are based on this matter and their aggregation produces lumps on which the ordinary matter accumulates, which finally give rise to the galaxies.
Let’s say that dark matter scaffolds ordinary matter to form galaxies and stars. The standard cosmological model that uses this idea is capable of reproducing in computational simulations the type of large-scale structure of the Universe that we see.
What evidence suggests that the milky way contains dark matter?
Every galaxy appears to have a halo of dark matter that is invisible to our instruments but can be inferred by certain measurements. The traditional way to infer the presence of dark matter is to see its gravitational effect. Spiral galaxies rotate on themselves. The stars that compose them orbit around the galactic centre with a speed that depends on the distance. This can be easily measured and it is verified that the speed of rotation does not correspond to what it would have if only the matter that was seen existed, but it would correspond if there were also a matter that we do not see.
Another way to infer the presence of dark matter is to observe gravitational lensing phenomena in distant galactic clusters. This also allows inferring a mass greater than the mass corresponding to the matter being seen.
The downside of this is that no direct irrefutable evidence has yet been found to indicate the existence of the particles that make it up.
It is under this situation that ESO astrophysicists work. They have studied the motion of 400 stars located up to a distance of 13,000 light years from us above the galactic plane and in a cone of 15 degrees. This has allowed them to calculate the gravitational influences to which they are subject. It is a volume four times greater than that studied so far in this type of research.
The study has enabled them to infer the presence of dark matter, since the models predict that there must be dark matter in particular area of the Milky Way. To get an idea of the distance, let’s mention that we are 27,000 light years from the galactic center.
In this study, the stars studied are considered to be “atoms” trapped in the gravitational well of the galaxy. By measuring its speeds in the three dimensions of space you can deduce the well shape and the amount of mass, both ordinary and dark. Subtracting the mass we see, we’d be dark.
It turns out that they have found no evidence of the existence of such matter. It’s not that it’s not known what it’s made of, it’s that it doesn’t seem to exist, at least in that part of the galaxy and according to this study. It is not easy to explain the presence of such matter in all those parts mentioned above and not in our galaxy.
The mass that infers from the movement of these stars can be explained with the presence of the ordinary matter we see.
But models of galactic formation and rotation indicate that our galaxy must have a halo of dark matter. You can even predict the shape of this halo and predict that there should be such matter in the region where the Solar System is located. Only highly unlikely halo shapes could explain the absence of dark matter in our neighbourhood. One possibility is that the dark matter of our galaxy will form a halo in the form of a pure cigar that passes through its centre and is not a sphere that envelops it.
But despite this result, the Milky Way spins faster than it would if there was no dark matter (like the other spiral galaxies). In fact, it is sufficient that only the movement of the Sun around the galactic centre is considered to check this point. So we have to find a solution to why dark matter is not detected in this study.
ESA’s future Gaia mission will allow further action to be taken and secured on this point.
Some scientists doubt the reliability of the method employed by these researchers, but if confirmed it would be a serious problem for the dark matter model.
Strictly, the result does not negate the existence of any such matter, but denies its presence in our galactic neighborhood.
One possible explanation is that dark matter is not composed of “cold” particles that move slowly but by light hot particles of rapid “movement” that would form a larger, more uniform halo. But this type of dark matter contradicts standard cosmological models.
Another explanation (as Pavel Kroupa of the University of Bonn suggests) is that dark matter formed lumps under the influence of its own gravity shortly after the Big Bang was given and that they grew within the galactic halos.
Perhaps the amount of ordinary matter is simply overestimated, or perhaps we miss something important about the Universe that we do not yet understand.
Many experts believe that dark matter is there and that future careful analyses will refute this latest study. The controversy is served. Critics point out that in order to achieve this result, certain hypotheses that do not necessarily have to be met and models whose applicability regime is not adequate are assumed.
For example, one of the equations used assumes several reasonable hypotheses for dark matter such as being in a steady-state, that the galactic rotation curve is flat, that the density of dark matter decays exponentially… In the end, it turns out that the theoretical model assumes that in the observed region dark matter has a spherical shape with constant density.
All of this is reasonable for the dark matter of the halo, but not necessarily for such a small region near the galactic disk in the vicinity of the Sun. In other galaxies, the distribution of dark matter has been inferred, noting that it may even be decoupled from ordinary matter and away from an ideal distribution.
Although there are other studies pointing in the same direction as this new result. Igor Karachentsev has calculated that there is much less dark matter than is assumed based on the gravitational field of a region of 163 million light-years in size from the Sun in any direction.
On the other hand, there will always be those who propose to abandon dark matter and modify the law of gravity. It’s what’s called MOND theories. According to these theories, gravity would work differently at cosmological scales. But they are very immature theories that do not convince the vast majority of physicists.
In short, dark matter remains as dark as ever, both literally and figuratively.
Recommended Book
Dark Matter and Dark Energy: The Hidden 95% of the Universe (Hot Science)
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