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> Lambda CDM makes very few assumptions. It only has 6 free parameters.[0] If you start with those 6 parameters, and then let the laws of physics work, you get

> Lambda CDM makes very few assumptions. It only has 6 free parameters.[0] If you start with those 6 parameters, and then let the laws of physics work, you get the universe we see today. Not including the DM distribution needed to match each galaxy's rotation curve, or external field effects, and dozens more observations: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm Contrast that with MOND's 1 parameter, visible mass.

DiogenesKynikos 4y ago
> Not including the DM distribution needed to match each galaxy's rotation curve Incorrect. The present-day distribution of dark matter is a consequence of the 6 parameters in Lambda CDM. If you take the Lambda CDM assumptions (with 6 free parameters) and apply the known laws of physics (gravity, hydrodynamics, nuclear fusion, radiative processes, etc.), you get something that looks like the universe that we observe. You get correct rotation curves, correct distributions of galaxy masses, and so on. If you can find a way in which Lambda CDM does not observe the known universe, then you will invalidate Lambda CDM (and maybe win a Nobel Prize). > Contrast that with MOND's 1 parameter, visible mass. There is currently no known theory of modified gravity that has the same predictive power as Lambda CDM. People have been trying very hard to find one, but so far without any luck. Lambda CDM works extremely well, as far as anyone can tell.
- naasking 4y ago
  > If you can find a way in which Lambda CDM does not observe the known universe, then you will invalidate Lambda CDM The paper I cited does exactly that, quantifying the extent to which CDM and MOND predict or must be adjusted to fit over 30 different observations via extra parameters.
  - DiogenesKynikos 4y ago
    The general consensus among astrophysicists is that MOND is not able to reproduce the full range of phenomena we observe, which LCDM does correctly reproduce. I'll just state that first, because the paper you're citing is representing an extremely fringe view in astrophysics. Looking at this paper, the first thing I notice is that it's not published in one of the standard astrophysics journals. There's probably a reason for that, but let's give it a chance. Next, I see that the introduction is making some very strange statements, like the following: > However, the MW retains a thin disc despite being much older than its dynamical time of a few hundred Myr [17]. For an early review of work on this problem, we refer the reader to [18]. A possible solution is that disc galaxies have a dominant pressure-supported spheroidal halo, even though such a halo is not observed [19]. The currently conventional solution to the above-mentioned issues is still to design invisible pressure-supported DM halos that surround, dominate, and stabilize galaxy discs [20,21]. Dark matter halos aren't specially "designed" to solve a particular problem - they're an inevitable consequence of LCDM. Not only that, but there's direct observational evidence for them: we can see their gravitational lensing signature. Here you see the same theory - cold dark matter - explaining two very different phenomena: galactic rotation curves and gravitational lensing. 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. There have been many modified gravity theories (modified Newtonian gravity, or "MOND," is not a serious theory, because it's not relativistic), but they all have the one or more of the following problems: 1. They mess something up that is very firmly established. For example, they might make gravitational waves propagate at less than the speed of light, when we know (since 2017) that they actually propagate at the speed of light. 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. 3. They don't actually predict anything new, beyond what vanilla general relativity predicts. They are essentially such slight modifications of relativity that they're experimentally indistinguishable. Such theories obviously can't solve the dark matter problem (because if they did, they'd be experimentally testable).
    - naasking 4y ago
      > 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

> Not including the DM distribution needed to match each galaxy's rotation curve Incorrect. The present-day distribution of dark matter is a consequence of the 6 parameters in Lambda CDM. If you take the Lambda CDM assumptions (with 6 free parameters) and apply the known laws of physics (gravity, hydrodynamics, nuclear fusion, radiative processes, etc.), you get something that looks like the universe that we observe. You get correct rotation curves, correct distributions of galaxy masses, and so on. If you can find a way in which Lambda CDM does not observe the known universe, then you will invalidate Lambda CDM (and maybe win a Nobel Prize). > Contrast that with MOND's 1 parameter, visible mass. There is currently no known theory of modified gravity that has the same predictive power as Lambda CDM. People have been trying very hard to find one, but so far without any luck. Lambda CDM works extremely well, as far as anyone can tell.

naasking 4y ago
> If you can find a way in which Lambda CDM does not observe the known universe, then you will invalidate Lambda CDM The paper I cited does exactly that, quantifying the extent to which CDM and MOND predict or must be adjusted to fit over 30 different observations via extra parameters.
- DiogenesKynikos 4y ago
  The general consensus among astrophysicists is that MOND is not able to reproduce the full range of phenomena we observe, which LCDM does correctly reproduce. I'll just state that first, because the paper you're citing is representing an extremely fringe view in astrophysics. Looking at this paper, the first thing I notice is that it's not published in one of the standard astrophysics journals. There's probably a reason for that, but let's give it a chance. Next, I see that the introduction is making some very strange statements, like the following: > However, the MW retains a thin disc despite being much older than its dynamical time of a few hundred Myr [17]. For an early review of work on this problem, we refer the reader to [18]. A possible solution is that disc galaxies have a dominant pressure-supported spheroidal halo, even though such a halo is not observed [19]. The currently conventional solution to the above-mentioned issues is still to design invisible pressure-supported DM halos that surround, dominate, and stabilize galaxy discs [20,21]. Dark matter halos aren't specially "designed" to solve a particular problem - they're an inevitable consequence of LCDM. Not only that, but there's direct observational evidence for them: we can see their gravitational lensing signature. Here you see the same theory - cold dark matter - explaining two very different phenomena: galactic rotation curves and gravitational lensing. 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. There have been many modified gravity theories (modified Newtonian gravity, or "MOND," is not a serious theory, because it's not relativistic), but they all have the one or more of the following problems: 1. They mess something up that is very firmly established. For example, they might make gravitational waves propagate at less than the speed of light, when we know (since 2017) that they actually propagate at the speed of light. 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. 3. They don't actually predict anything new, beyond what vanilla general relativity predicts. They are essentially such slight modifications of relativity that they're experimentally indistinguishable. Such theories obviously can't solve the dark matter problem (because if they did, they'd be experimentally testable).
  - naasking 4y ago
    > 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.