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from the changing-our-understanding-of-the-universe dept.
Arthur T Knackerbracket has found the following story:
[...] Renowned astrophysicist and National Medal of Science awardee Vera Rubin passed away in Princeton N.J., the evening of December 25, 2016, at the age of 88. Rubin confirmed the existence of dark matter—the invisible material that makes up more than 90% of the mass of the universe. She was a retired staff astronomer at the Carnegie Institution's Department of Terrestrial Magnetism in Washington, D.C.
"Vera Rubin was a national treasure as an accomplished astronomer and a wonderful role model for young scientists," remarked Carnegie president Matthew Scott. "We are very saddened by this loss."
In the 1960s, Rubin's interest in how stars orbit their galactic centers led her and colleague Kent Ford to study the Andromeda galaxy, M31, a nearby spiral. The two scientists wanted to determine the distribution of mass in M31 by looking at the orbital speeds of stars and gas at varying distances from the galactic center. They expected the speeds to conform to Newtonian gravitational theory, whereby an object farther from its central mass orbits slower than those closer in. To their surprise, the scientists found that stars far from the center traveled as fast as those near the center.
After observing dozens more galaxies by the 1970s, Rubin and colleagues found that something other than the visible mass was responsible for the stars' motions. Each spiral galaxy is embedded in a "halo" of dark matter—material that does not emit light and extends beyond the optical galaxy. They found it contains 5 to 10 times as much mass as the luminous galaxy. As a result of Rubin's groundbreaking work, it has become apparent that more than 90% of the universe is composed of this invisible material. The first inkling that dark matter existed came in 1933 when Swiss astrophysicist Fritz Zwicky of Caltech proposed it. But it was not until Rubin's work that dark matter was confirmed.
Besides her remarkable scientific contributions, as noted by colleague Neta Bahcall of Princeton University: "Vera was an amazing scientist and an amazing human being. A pioneering astronomer, the 'mother' of flat rotation curves and dark-matter, a champion of women in science, a mentor and role model to generations of astronomers."
-- submitted from IRC
(Score: 3, Informative) by stormwyrm on Friday December 30 2016, @03:19AM
A large number of scientific theories start out that way, as fudge factors when things don’t quite work out. 19th century astronomers saw the observed ephemeris of Uranus did not conform to the computed ephemerides of the planet based on Newton’s laws. What did they do? They hypothesised the existence of another planet, and sure enough, they later found Neptune.
Similarly, Vera Rubin and others note that galaxies are rotating faster than they should be, Fritz Zwicky makes some gravitational estimates of the mass of the Coma Cluster and finds numbers that are way off judging from the visible matter, other folks observe galactic clusters colliding and gravitational lenses where there is no visible matter [wikipedia.org], people studying the cosmic microwave background see anomalous spikes that cannot be explained by the behaviour of normal matter, and so on. So no, it isn’t “no measurement required”. It has in fact been pinned down by various indirect measurements.
The other alternative is to modify the laws of gravity. However, for that to work you have to make a consistent modification to gravity that works at all scales, from terrestrial, to planetary, to stellar, to galactic, to galactic cluster, all the way to cosmological. This is why dark matter is so successful: it is too diffuse to affect things up to and including stellar scale, it is dense enough to affect cluster and cosmological scales in line with observation, and in the quantities required to support cluster and cosmological observations, there should be enough to affect galactic scales, though arguments still persist about the specifics. In contrast while you can construct a modified gravity that works at galactic scales while not screwing up the lower scales, all such attempts so far have failed to get to the larger scales without adding something that looks suspiciously like dark matter.
The only problem with dark matter is it’s proving fiendishly difficult to pin down its precise nature. The theory of dark matter says that such dark matter particles should be heavy and should interact only via gravity and possibly the weak interaction. The trouble is that gravity and the weak interaction are extremely weak forces. The neutrino is one example of such a particle that can only interact by these two forces and it has proved very challenging to detect (we still can’t detect very low energy neutrinos), but it is too light to be a good candidate for dark matter.
The prospect of dark matter is probably jarring in the way that the contemporaries of Copernicus and Galileo felt when faced with the prospect that the Earth (and hence they) were not the centre of the universe. It’s saying that not only are we not at the centre of the universe, furthermore we’re not even made up of the same kind of matter that the vast majority of the universe is made up of!
Numquam ponenda est pluralitas sine necessitate.
(Score: 2) by c0lo on Friday December 30 2016, @04:16AM
Just assume there's a "screening factor" to the gravity law, pretty much as the electric force damps in the presence of dielectrics. Compute then the "gravitational permittivity" and you have a testable theory potentially testable in the lab (assuming the instruments are sensitive enough).
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 2) by maxwell demon on Friday December 30 2016, @11:45AM
Dark matter is also a testable theory. Guess what people are searching for in accelerators? (Besides supersymmetric particles, of course).
Of course there is a chance that it ultimately turns out that it doesn't exist, but that's exactly what science is about. Neptun, the planet conjectured to explain the deviation s of Neptune's movement from the predictions, turned out to be real. Vulcan, the planet conjectured to explain the deviation of Mercury's movement from the predictions, turned out not to be real; rather the deviations were explained by a new theory of gravitation, Einstein's General Theory of Relativity.
Also note that Einstein did not find that theory by looking at the movement of Mercury, and would never have found it that way.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by maxwell demon on Friday December 30 2016, @12:06PM
This of course should have read:
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by requerdanos on Friday December 30 2016, @01:53PM
With respect, Neptune isn't invisible [space.com].
(Score: 3, Interesting) by maxwell demon on Friday December 30 2016, @02:57PM
It is, with the naked eye, as seen from earth.
But then, we know there exists matter which fulfils all properties of dark matter, with one exception: There's not enough of it around. Which is neutrinos.
Interestingly, Neutrinos also started out as something just introduced because the equations didn't add up. In that case, the conservation laws on beta decay. When they were introduced, people thought they would never be detected. Today, there is no doubt that neutrinos exist.
So strictly speaking, the question should not be whether dark matter exists (it does, in the form of neutrinos), but whether enough of it exists to explain the observations (the neutrinos alone are not sufficient, as their collective mass is too small).
The Tao of math: The numbers you can count are not the real numbers.