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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 th

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. There are a huge number of different types of phenomena that the theory has to correctly predict (e.g., the abundance of different elements, the numbers of galaxies with different masses, the rotation curves of galaxies), and it does so extremely well. 0. https://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters https://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters

naasking 4y ago
> 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

> 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).