You may not be a fan of dark matter, the hypothetical particle that makes up the bulk of the mass in the universe. And it’s true that the dark matter hypothesis has its flaws – and of course we haven’t found any dark matter particles yet. But the truth is that the alternatives are much worse.
The Universe Full of unexplained mysteries (which is what makes astronomers and astrophysicists happily work), many of those mysteries surround gravity. as we watch stars Orbiting around the centers of their galaxies, we find that they are moving very quickly due to the amount of visible matter that can keep them in those orbits with their gravity.
galaxies buzzing around galaxy clusters It also moves very quickly due to the amount of mass visible in the clusters. These same clusters bend the background light a lot. Even large structures in our universe arose very quickly without an additional source of mass.
The best hypothesis for scientists to explain all these disparate observations is that there is a new type of particle, known as dark matterwho inhabits the universe. This particle would be almost completely invisible (hence the name), and would rarely (if any) interact with ordinary matter. This idea is not as far-fetched as it seems; neutrinos They are particles that have exactly these properties. They don’t have enough mass to explain dark matter, but they do show that such particles can exist.
But the dark matter hypothesis is not perfect. Computer simulations of galactic growth suggest that dark matter-dominated galaxies must have incredibly high density at their centers. Observations of real galaxies show a higher density in their cores, but not nearly as sufficient as those simulations predicted. Also, simulations of dark matter evolving in the universe predict that each galaxy should contain hundreds of smaller satellites, while observations are always short.
Given that the dark matter hypothesis isn’t perfect—and that we don’t have direct evidence for any candidate particles—it’s worth exploring other options.
One of these options was introduced in the 1970s along with the original dark matter idea, when an astronomer Vera Robin He discovered for the first time the problem of stars moving very fast within galaxies. But rather than adding a new ingredient to the universe, the alternative changes the recipe by changing how gravity works on galactic scales. The original idea is called MOND, for “modified Newtonian dynamics”, but the name also applies to the general family of theories descended from this original concept.
With MOND, you get pretty much what’s on the label. at the level of the planets or the solar system, NewtonGravity works just fine (except, of course, when you need the more detailed gravity calculations it provides general relativity). But once you get old, the usual F = ma We are familiar with it does not quite apply, and the relationship between force and acceleration follows a different rule.
Under MOND, there is no need for an additional particle to explain the observations – only a slight modification of the force of gravity. And because the gravitational modulation under MOND is explicitly designed to explain the motions of stars within galaxies, it naturally does so very well. The theory also does not suffer from an overproduction of dark matter by satellites and very high galactic cores.
But MOND is far from perfect. Adjustments to gravity to explain stellar motions have difficulty explaining the motions of galaxies within clusters and the effect of background light on the lens. And MOND is not a complete theory of relativity (all modern theories in physics must conform to special relativity). An equivalent MOND update, called TeVeS, can compete head-to-head with general relativity – much less so. Models based on modified gravity have a major problem in explaining the growth of structure in the universe, and the properties of cosmic microwave background And more – all the places where dark matter works well.
There is no MOND-like theory that can explain every observation when it comes to dark matter. All of them failed at least one test. Although MOND may remain accurate when it comes to galactic rotation curves, there are enough observations to tell us that we resident Dark matter needs to exist in the universe.
No, the dark matter hypothesis is not perfect. But again, there is no scientific hypothesis. When evaluating competing hypotheses, scientists can’t just follow their lead, choosing one that looks cooler or looks simpler. We have to follow the guide wherever it leads. For nearly 50 years, no one has come up with a MOND-like theory that can explain the wealth of data we have about the universe. THIS DOESN’T MAKE MOND wrong – wrong – wrongBut it does make it a much weaker alternative to dark matter.
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