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420chan is Getting Overhauled - Changelog/Bug Report/Request Thread (Updated March 20)
Dark Matter and GR - struggling to understand by Bart Bok - Sun, 30 Sep 2018 09:15:14 EST ID:9RKOIT3O No.57458 Ignore Report Quick Reply
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Hey there fellow egghead stoners. Would you be able to guide me through this?

So, the whole concept of dark matter and energy. I find it a little dubious, I take it this hasn't been confirmed in any way - rather, like many theories in physics has been introduced as an attempted explanation of mismatch between empirical data and theoretical predictions.

But this is just an impression I've got from the superficial knowledge of modern physics I've got, so I'd rather first understand the basis for this hypothesis better before judging it dubious.

So the way I understand it is the concept was proposed in 30s to explain the observed movement of galaxies, in particular why some stay in clusters rather than be launched off due to the acquired speed.

So it is inferred that there must be enough gravity force to keep them together instead. This begs the first question: what is the expected mass calculation based on? As in, how'd they go about measuring that?

But all right, aside from that bit I'm missing, assume the mass predictions are sound. In Newtonian mechanics - so far so good. But now if we introduce GR we allow the concept of black holes which would escape our observations. Why does that not account for 'dark matter' effect? It fits the criteria of (1) great mass (2) inability to be observed

So the way this is taken into account, I understand, that all right - there may be black holes which we cannot observe, but we still have a rough prediction on their mass which we infer from gravitational lensing - and that is way not enough to account for 'dark matter', and hence the concept remains valid.

My questions to you anons:
1) Is the line of reasoning above more or less sound
2) Any resources on the subject to recommend which would explain in bit more detail how the calculations made? (seems like internet is full of pseudoscience articles but I can't seem to easily google a decent source)
3) Could it be possible that inferring
i) mass bends spacetime
ii) spacetime is observed to be bent
iii) therefore there must be mass
is wrong, because mass may not be the only reason the spacetime is bent. That is, could it be that the 'dark matter effect' is not due to some exotic type of matter, but rather an innate (or otherwise not yet understood by our science) geometrical feature of spacetime.
>>
Johann Encke - Mon, 01 Oct 2018 01:21:26 EST ID:457vC2+I No.57461 Ignore Report Quick Reply
>> what is the expected mass calculation based on?
Light from main sequence stars. We look at a galaxy, look at it's luminosity to estimate the number of main sequence stars, and estimate mass from that. Almost all galaxies thus have a totally insufficient amount of normal matter to account for their mass (although we have found some galaxies without dark matter, or with hardly anything but dark matter, but this is quite rare) or rather, what their mass would have to be in order for them to maintain coherency.

>> there may be black holes which we cannot observe, but we still have a rough prediction on their mass which we infer from gravitational lensing
You would think we could do this, but in practice we can only use gravitational lensing and other gravitational effects to detect black holes in this way if there is a significant amount of parallax on the black hole (otherwise, unless we are just lucky enough to have a sufficiently bright star in line with it and us along its event horizon, we will simply miss it) for this reason we can use this method to kind of hunt randomly for black holes, we can't use it to estimate how many black holes there actually are -- we have no idea as to the answer to that question, from an empirical standpoint.
>>2) Any resources on the subject to recommend which would explain in bit more detail how the calculations made?
https://ned.ipac.caltech.edu/level5/March05/Read/Read1.html
>>iii) therefore there must be mass
I think there are strong reasons to suspect this line of thinking might be flawed, especially since the only reason we describe dark matter as matter is simply because we can't think of anything else to describe it as. Relatively popular but unaccepted are theories that dark matter and energy represent the influence of alternate quantum realities upon our universe, or may otherwise be some sort of shadow of the m-brane. Unfortunately, a long running contender for non-DM/DE explanations, MOND, was recently disqualified due to new observations, although people are seeing if it can be saved with an update.

In short, absolutely it could be something else. DM/DE really is just a placeholder. But that begs the question: what else? We've been scratching our heads on that one for almost a hundred years, and we have uncovered startlingly little.
>>
Fred Hoyle - Sat, 06 Oct 2018 19:23:55 EST ID:8/fKg+Ea No.57462 Ignore Report Quick Reply
>>57461

Hopefully the James Webb space telescope will be launching under it's most recent date in 2021 and provide us some new insights.


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