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Question, regarding 4:00 in that video. Dr. Becky states that GR, our best theory of gravity, at large scale, predicts dark matter. Is this correct? My understa

Question, regarding 4:00 in that video. Dr. Becky states that GR, our best theory of gravity, at large scale, predicts dark matter. Is this correct? My understanding is that the models that predict dark matter use Newtonian physics. And more, the problem with GR is that its calculations are complicated, in a partial-differential-equations sense.

antonioevans 3y ago
[flagged]
MattPalmer1086 3y ago
No GR does not predict dark matter. But when we look at galaxies and so on, things don't seem to add up if you only look at the visible matter. So, there could be a lot of dark matter we can't see. Or GR might not be correct and MOND (or a relativistic formulation like AQUAL) might be the right answer. It's true I guess that if GR is correct, then we need some kind of dark matter to explain observations. But it might not be correct.
- karmakaze 3y ago
  It's simpler to say Newtonian gravity and visible matter alone is sufficient to explain the need for dark matter. Adding GR to the picture doesn't take away the need for dark matter to account for observations.
  - MattPalmer1086 3y ago
    True, even without GR the same issue would exist with Newtonian gravity.
adrian_b 3y ago
She does not express this fact in the right way. Whenever the facts predicted by a theory do not match the experimental facts, one could say that the theory has predicted the existence of an unknown factor that has affected the experiments. Nevertheless, until there is an alternative way to determine the existence of that unknown factor, the right way is to simply say that there is a mismatch between predictions and observations and the reason for this mismatch must be determined in the future. For now, the current theory is not accurate enough. For instance, when some planetary movements did not coincide with the predictions, it was supposed that perhaps there exists an extra planet which explains the discrepancies between predictions and observations. This supposition was confirmed only when Neptune was also observed with a telescope. If Neptune had never been observed, perhaps it would have been discovered that the mathematical model of gravitation must be improved. For now, there are discrepancies between observations and the predictions of the current mathematical model of gravitation. Like in the cases of Neptune and Pluto, there is a supposition that perhaps there exists some kind of dark matter that would be the cause of the discrepancies. Until the moment when an alternative way to determine the existence of dark matter will be discovered, like the optical observations of Neptune and Pluto, the existence of dark matter remains just a hypothesis that cannot be used for any practical purpose, because it cannot predict anything. Dark matter can be added arbitrarily in any place and this can make any theory of gravity match the observations. Therefore now we have galaxies that are supposed to be rich in dark matter and galaxies that are supposed to be poor in dark matter, in order to fit the observations, but without any a priori rule that could be used to predict this.

antonioevans 3y ago

[flagged]

MattPalmer1086 3y ago

No GR does not predict dark matter. But when we look at galaxies and so on, things don't seem to add up if you only look at the visible matter. So, there could be a lot of dark matter we can't see. Or GR might not be correct and MOND (or a relativistic formulation like AQUAL) might be the right answer. It's true I guess that if GR is correct, then we need some kind of dark matter to explain observations. But it might not be correct.

karmakaze 3y ago
It's simpler to say Newtonian gravity and visible matter alone is sufficient to explain the need for dark matter. Adding GR to the picture doesn't take away the need for dark matter to account for observations.
- MattPalmer1086 3y ago
  True, even without GR the same issue would exist with Newtonian gravity.

adrian_b 3y ago

She does not express this fact in the right way. Whenever the facts predicted by a theory do not match the experimental facts, one could say that the theory has predicted the existence of an unknown factor that has affected the experiments. Nevertheless, until there is an alternative way to determine the existence of that unknown factor, the right way is to simply say that there is a mismatch between predictions and observations and the reason for this mismatch must be determined in the future. For now, the current theory is not accurate enough. For instance, when some planetary movements did not coincide with the predictions, it was supposed that perhaps there exists an extra planet which explains the discrepancies between predictions and observations. This supposition was confirmed only when Neptune was also observed with a telescope. If Neptune had never been observed, perhaps it would have been discovered that the mathematical model of gravitation must be improved. For now, there are discrepancies between observations and the predictions of the current mathematical model of gravitation. Like in the cases of Neptune and Pluto, there is a supposition that perhaps there exists some kind of dark matter that would be the cause of the discrepancies. Until the moment when an alternative way to determine the existence of dark matter will be discovered, like the optical observations of Neptune and Pluto, the existence of dark matter remains just a hypothesis that cannot be used for any practical purpose, because it cannot predict anything. Dark matter can be added arbitrarily in any place and this can make any theory of gravity match the observations. Therefore now we have galaxies that are supposed to be rich in dark matter and galaxies that are supposed to be poor in dark matter, in order to fit the observations, but without any a priori rule that could be used to predict this.