Magellanic Clouds in Motion
The two Magellanic Clouds are galaxies close to our Milky Way but their far Southern position inhibits viewing by Northern telescopes.
Announced in 2006, measurements with the Hubble Space Telescope suggest the Large and Small Magellanic Clouds may be moving too fast to be orbiting the Milky Way.
In 2014, measurements from the Hubble Space Telescope made it possible to determine that the LMC has a rotation period of 250 million years.
From a space.com story about LMC:
By pointing NASA's Hubble Space Telescope toward the two clouds, scientists began to catch a glimpse of the objects' histories. "Hubble's biggest contribution is enabling us to clock how fast the Magellanic clouds are moving," said Gurtina Besla, a researcher at the University of Arizona who studies dwarf galaxies. In 2007, Besla overturned conventional wisdom when she suggested that the LMC and SMC were making their first orbit of our galaxy.
"They're moving too fast to have been long-term companions of the Milky Way," Besla said.
She used data from the European Space Agency's Gaia spacecraft to clock smaller, satellite galaxies orbiting the LMC, as well. And, understanding how these galaxies move has helped researchers better calculate the mass of the LMC. Current estimates put the LMC at about 100 billion times as massive as Earth's sun, or a quarter the mass of the Milky Way. Besla said this size means the LMC is about 10 times heavier than previously calculated.
I am very curious how cosmologists will explain this unexpected lateral motion of the LMC and SMC.
While examining the Coma galaxy cluster in 1933, Zwicky was the first to use the virial theorem to infer the existence of unseen matter, which he referred to as dunkle Materie 'dark matter'. He calculated the gravitational mass of the galaxies within the cluster and obtained a value at least 400 times greater than expected from their luminosity, which means that most of the matter must be dark. The same calculation today shows a smaller factor, based on greater values for the mass of luminous material; but it is still clear that the great majority of matter was correctly inferred to be dark.
In 1933, I cannot imagine the accuracy of estimates for the mass of all the galaxies in that cluster.
Our current basis for the Milky Way:
M-S = Mass of Sun
Estimates of the mass of the Milky Way vary, depending upon the method and data used. The low end of the estimate range is 5.8×10^11 M-S, somewhat less than that of the Andromeda Galaxy. A recent mass estimate for the Milky Way is 1.29×10^12 M-S.
Our current estimates vary by a factor of 10. I expect in 1933 estimates would have been very crude.
We do not know for certain the luminosity of our galaxy. Because the Earth is within a spiral arm much of the galaxy is obscured so we do not know how many stars are present (between 100 - 400 billion).
Dark matter is proposed to explain unexpected galaxy motions in clusters.
Based on the precedence set in 1933 I expect these lateral motions of the Magellanic Clouds to be explained by dark matter.
A legitimate response could be 'we are working on it' simply because we must first improve our understanding for a real explanation.
The rest of this post is a personal opinion. If not interested, stop.
19th century astronomers set the bar high with a successful prediction of the planet Neptune!
However they were calculating orbits for only 7 planets, in conveniently predictable elliptical orbits. The orbit of Uranus takes 84 years.
20th centuy cosmologists have a high goal to match, with such a prediction, and it is not possible yet.
Beyond the solar system the universe is very complex.
Our Milky Way has over 200 billion stars and none of them move in an ellipse. Much of the galaxy is obscured so there are unknowns. The Sun's orbit is known to be 'disturbed' which should not be a surprise given it has a few billion stars that can affect it; the Sun's orbit takes > 220 million years.The Milky Way also has about 500 globular clusters, each with 10^5 stars. It also has over 50 satellite galaxies, with LMC and SMC the two largest.
At this time it is not possible to predict motions of individual stars in a galaxy with so many bodies in uncertain orbits. This scope is so far beyond that of the solar system. Cosmologists have developed models to simulate a spiral galaxy rotation, probably based on our limited knowledge of the Milky Way.
These models are being tested and get improved based on observation. However there are always outliers not conforming to the model's predictions. The deviation was greater further from the core.
At some point, perhaps after recognizing the model can never be 100% certain, or after recalling that bar set many years ago, this progress stopped.
Instead of admitting we are still learning about galaxies, an announcement was made: We understand and can predict the rotation of spiral galaxies! We have achieved the level of that bar previously set, making accurate predictions.
The announcement continued:
However the observations do not match for all stars and that deviation is caused by dark matter.
This result of 'however - dark matter' can be avoided with two changes in cosmology:
1) an uncertainty principle like one in quantum mechanics:
It is impossible to predict exact behaviors at the level of galaxies.
Maybe someday but not yet.
2) a tolerance for deviation, like one in manufacturing.
Even a very expensive high precision machine tool will have a tolerance for deviation, like repeatability and machined part deviations within 1 micron. It is impossible to be perfect every time especially with large complex machines subject to unpredictable temperature variations.
Cosmology needs a tolerance for deviation. Right now it is zero, so we got dark matter.
If the tolerance for a galaxy model were 10% deviations then the improvement in the models continues with further observations. Later it could be 1% or whatever the models can approach. Progress continues.
With dark matter, inadequate models are acceptable and the rotation deviation is explained with that excuse. This is not science - if the observation does not match the prediction the theory is wrong and must be fixed. I assume most know the famous Feynman quote.
This scenario for (1) applies to dark energy also. Distant galaxy clusters are millions or billions of light years away and can form structures spanning across a similar distance. It will take time to understand those behaviors from our remote observation point.
Until we understand how the nearby Magellanic Clouds move we have no basis for claiming we understand distant clusters where dark energy excuses what we cannot explain.
These two suggestions are a radical paradigm shift but interesting to consider.
If accepted, dark matter and dark energy must be reconsidered.
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