by Maybe you’re not a fan of dark matter, the hypothetical particle that makes up most of the mass in the universe. And it’s true that the dark matter hypothesis has its shortcomings — and, of course, we haven’t discovered any dark matter particles yet. But the truth is that the alternatives are much worse.
The IS globe full of unexplained mysteries (that’s what keeps astronomers and astrophysicists happily employed), and many of those mysteries around gravity. While we watch stars In the middle orbits of their galaxies, we see that they are moving far too fast because of the amount of visible matter that can hold them in those orbits with their gravity.
Galaxies buzzing around galaxy clusters also moves much too quickly given the amount of visible mass in the clusters. Those same clusters bend background light too much. Even the large structures in our Universe evolved far too quickly without an additional source of mass.
Related: Should we be so sure that dark matter exists?
The best hypothesis scientists have to explain all these disparate observations is that there is a new type of particle, called dark dark, who lives in the cosmos. This particle would be almost entirely invisible (hence the name), rarely (if ever) interacting with normal matter. This idea is not as far-fetched as it seems; neutrino they are particles that have exactly these properties. They do not have enough mass to explain dark matter, but they show that such particles can exist.
But the dark matter hypothesis is not perfect. Computer simulations of galaxy growth suggest that galaxies with dark matter in their centers should have extremely high densities. Observations of real stars show higher densities in their cores, but not nearly as much as those simulations predicted. Also, models of dark matter arising in the universe predict that each galaxy should have hundreds of smaller satellites, and that observations are consistently short.
The case for MOND
Since the dark matter hypothesis isn’t perfect — and we have no direct evidence that there are any candidate particles — it’s worth exploring other options.
One such option was introduced back in the 1970s alongside the original dark matter idea, when astronomer Vera Rubin he discovered for the first time the problem of stars moving too fast within galaxies. But instead of adding a new ingredient to the universe, the alternative changes the recipe by changing how gravity works at galactic scales. The original idea is called MOND, for “newly modified Newtonian dynamics”, but the name also applies to the general family of theories that arose from that original concept.
With MOND, you get pretty much what’s on the label. On planetary or solar system scales, Newtongravity works fine (except, of course, when you need the more detailed gravity calculations provided by general relativity). But when you go big, it’s normal F = ma We do not fully apply it, and the relationship between force and acceleration follows a different rule.
Under MOND, no extra particle is needed to explain the observations — just a slight change in the force of gravity. And because the variation of gravity under MOND is expressly designed to explain the motions of stars within galaxies, it naturally does that very well. The theory also does not suffer from the overproduction of dark matter satellites and galactic cores.
The flawed champion
But MOND is far from perfect. The modifications made to gravity to explain stellar motion have difficulty explaining the motions of galaxies within clusters and the lensing of the background light. And MOND is not a fully relativistic theory (all modern theories in physics must be compatible with it special relativity). An equivalent update to MOND, called TeVeS, can compete head-to-head with general relativity — and it’s not much shorter. Models based on modified gravity have significant trouble explaining the growth of the universe’s structure, aspects of the cosmic microwave background and more – everywhere dark matter works quite well.
There is no theory like MOND that can account for every single observation when it comes to dark matter; each fails at least one test. Although MOND may still be accurate about the rotation curves of galaxies, there are enough observations to tell us that we would yet dark matter must exist in the universe.
No, the dark matter hypothesis is not perfect. But then again, there is no scientific hypothesis. When evaluating competing hypotheses, scientists cannot simply go easy, or choose one that is colder or simpler. We must follow the evidence, wherever it leads. For nearly 50 years, no one has come up with a theory like MOND that can explain the wealth of data we have about the universe. That doesn’t make MOND wrongbut it is a much weaker alternative to dark matter.
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