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> Dark matter halos aren't specially "designed" to solve a particular problem - they're an inevitable consequence of LCDM. You can't take one quote out of cont

> Dark matter halos aren't specially "designed" to solve a particular problem - they're an inevitable consequence of LCDM. You can't take one quote out of context as some meaningful criticism. The point they're making is that the particular distribution of dark matter and the initial conditions at the big bang have to be tuned to match observations because the natural predictions of CDM don't match observations. By contrast, in MOND many observations follow only from the amount of visible matter. > There are many different predictions like this, from very different areas of astrophysics, that are all explained by essentially one assumption - that there is some sort of cold matter that does not interact with light (i.e., it has no charge). That's the strength of the theory. This is the lie that's sold. There are numerous other parameters in LCDM (ie. assumptions) that must be tuned just right to match observations, and this paper goes over them and how MOND actually matches those observations without free parameters, or with some additional assumptions of its own. LCDM itself requires a number of assumptions to match observations, so the question is, given the evidence, how many of those observations are expected outcomes of the model without tuning, how many require tuning and/or specific initial conditions, and how many require additional assumptions? They do this for over 30+ observations and MOND comes out looking much better than you'd expect, so there's definitely something to it. It would just be a remarkable coincidence for MOND to make so many successful predictions based only on visible matter in cases where the LCDM requires visible matter + postulating invisible matter in a specific distribution. > 2. They are crafted to solve one or two particular problems in astrophysics, but they fail to solve all the other problems that cold dark matter solves. By contrast, most astrophysicists that are gung-ho on LCDM sweep under the rug the inconvenient tuning that's needed to actually match observations. If you actually read the paper they acknowledge that something like sterile neutrinos are needed to explain some observations under MOND, but this is considerably less dark matter than the predominant model suggests is needed. Nobody serious is claiming that MOND reproduces the full range of phenomena we see, but neither does LCDM, and LCDM without tuning doesn't look like our universe at all.

DiogenesKynikos 4y ago
> The point they're making is that the particular distribution of dark matter and the initial conditions at the big bang have to be tuned to match observations because the natural predictions of CDM don't match observations. This isn't true. The distribution of dark matter in LCDM matches the observed distribution remarkably well. There are still some uncertainties at the cores of galaxies, where baryonic physics is more complex (for example, star formation and black hole feedback are difficult to simulate accurately), but on larger scales, LCDM does very well without any tuning. > By contrast, in MOND many observations follow only from the amount of visible matter. MOND isn't an actual theory. It's an idea of how one might modify Newtonian mechanics, but any fully fleshed-out theory is relativistic. Which theory of modified general relativity are you referring to, specifically? > LCDM itself requires a number of assumptions to match observations It requires 6 parameters, as I said earlier, to explain everything in cosmology. > LCDM requires visible matter + postulating invisible matter in a specific distribution. You keep saying this, but it's simply wrong. LCDM only requires a Gaussian random field as an initial condition. It has one free parameter to specify that field's density distribution: a scalar spectral index. We actually know for certain that the universe was in such a state early on (from the Cosmic Microwave Background), so this is a very solid assumption. LCDM takes that initial state, applies the known laws of physics, and ends up with the matter density distribution we see today, without any tuning. > By contrast, most astrophysicists that are gung-ho on LCDM sweep under the rug the inconvenient tuning that's needed to actually match observations. Astrophysicists are not "gung-ho" about LCDM. They're constantly looking for ways in which it might break. It's actually quite depressing that LCDM does so incredibly well, because it would be exciting to find something new. Astrophysicists are actually very open-minded about modified gravity. The problem is that, contrary to what the paper you've found is claiming, there's no theory of modified gravity that explains away dark matter, while reproducing all the cosmological phenomena that are observed. Every theory of modified gravity messes up something big, like the halo mass function, the elemental abundances or the speed of gravitational waves. The only theories that don't mess things up are tiny modifications of general relativity, but those theories require dark matter.

DiogenesKynikos 4y ago

> The point they're making is that the particular distribution of dark matter and the initial conditions at the big bang have to be tuned to match observations because the natural predictions of CDM don't match observations. This isn't true. The distribution of dark matter in LCDM matches the observed distribution remarkably well. There are still some uncertainties at the cores of galaxies, where baryonic physics is more complex (for example, star formation and black hole feedback are difficult to simulate accurately), but on larger scales, LCDM does very well without any tuning. > By contrast, in MOND many observations follow only from the amount of visible matter. MOND isn't an actual theory. It's an idea of how one might modify Newtonian mechanics, but any fully fleshed-out theory is relativistic. Which theory of modified general relativity are you referring to, specifically? > LCDM itself requires a number of assumptions to match observations It requires 6 parameters, as I said earlier, to explain everything in cosmology. > LCDM requires visible matter + postulating invisible matter in a specific distribution. You keep saying this, but it's simply wrong. LCDM only requires a Gaussian random field as an initial condition. It has one free parameter to specify that field's density distribution: a scalar spectral index. We actually know for certain that the universe was in such a state early on (from the Cosmic Microwave Background), so this is a very solid assumption. LCDM takes that initial state, applies the known laws of physics, and ends up with the matter density distribution we see today, without any tuning. > By contrast, most astrophysicists that are gung-ho on LCDM sweep under the rug the inconvenient tuning that's needed to actually match observations. Astrophysicists are not "gung-ho" about LCDM. They're constantly looking for ways in which it might break. It's actually quite depressing that LCDM does so incredibly well, because it would be exciting to find something new. Astrophysicists are actually very open-minded about modified gravity. The problem is that, contrary to what the paper you've found is claiming, there's no theory of modified gravity that explains away dark matter, while reproducing all the cosmological phenomena that are observed. Every theory of modified gravity messes up something big, like the halo mass function, the elemental abundances or the speed of gravitational waves. The only theories that don't mess things up are tiny modifications of general relativity, but those theories require dark matter.