## Dark matter solution: not receiving all the light?

Hypothetically, if we weren't receiving all the light from a galaxy, is it possible that this would reconcile the gravitational shortfall we perceive?

Would it not also explain the gravitational lenses that we attribute to dark matter (since we are not receiving all the light to represent the appropriate amount of matter that causes the lens)?

Let me know if I'm not being clear enough I will try to elaborate. I look forward to your input.
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 Quote by euquila Hypothetically, if we weren't receiving all the light from a galaxy, is it possible that this would reconcile the gravitational shortfall we perceive?
No. I have no idea how you could come to that hypothesis

 Would it not also explain the gravitational lenses that we attribute to dark matter (since we are not receiving all the light to represent the appropriate amount of matter that causes the lens)?
No. Gravitational lensing is based on gravity. Having more or fewer photons is irrelevant.

 Let me know if I'm not being clear enough I will try to elaborate. I look forward to your input.
 But isn't light the only 'tool' that astronomers have to study objects outside our solar system? If we weren't getting all the light then our calculations would be off no? EDIT: If we weren't getting all the light from the galaxy, then our calculations of how much mass is present would be less. If this is less, then so would our calculations of the gravity required to hold to galaxy together.

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## Dark matter solution: not receiving all the light?

 Quote by euquila But isn't light the only 'tool' that astronomers have to study objects outside our solar system? If we weren't getting all the light then our calculations would be off no? EDIT: If we weren't getting all the light from the galaxy, then our calculations of how much mass is present would be less. If this is less, then so would our calculations of the gravity required to hold to galaxy together.
The rotational speed of the outer solar systems in the galaxy is utterly independent of how many photons reach Earth and it is that rotational speed that leads to conclusions about total mass.

You really should just study up on this stuff instead of apparently just making up a hypothesis without first understanding the fundamental facts.

 The rotational speed of the outer solar systems in the galaxy is utterly independent of how many photons reach Earth and it is that rotational speed that leads to conclusions about total mass.
I thought that the problem was missing mass, ie dark matter. Is this not correct?

And I thought that astronomers determined the mass by the luminosity of the galaxy. Perhaps this is where I err?

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 Quote by euquila I thought that the problem was missing mass, ie dark matter. Is this not correct? And I thought that astronomers determined the mass by the luminosity of the galaxy. Perhaps this is where I err?
Dark matter is inferred by the fact that the outer elements in galaxies rotate WAY differently than they would if the galaxies were made up only of the "visible" matter. This has NOTHING to do with how bright the galaxies are. I just don't see why you can't get that rotational speed and the number of photons reaching Earth have nothing to do with each other.
 Recognitions: Gold Member Science Advisor For discussion of brightness and orbital velocity methods of estimating galactic mass, see https://www.perimeterinstitute.ca/Pe...htness_Method/ a You will note in the case of the Triangulum galaxy, as discussed, the mass deficit is far beyond what could be explained by missing photons.

From https://www.perimeterinstitute.ca/Pe...htness_Method:
 In addition to the numerical discrepancy between the Orbital and Brightness methods, the overall pattern of the orbital speeds of the stars within galaxies (constant orbital speed with increasing distance) is fundamentally different from the expected pattern (orbital speed declining with increasing distance). Thus, even if the actual masses of stars in distant galaxies were higher than current estimates, this would only have the effect of moving the plot for expected orbital speed in Figure 6 (chapter 1) upwards. It would not alter the plot's overall pattern so that it matched the observed plot. Thus, the stars and gas alone cannot explain the observed speeds of stars, no matter how large their combined mass is.
I don't understand this statement. Wouldn't adding dark matter to the equation (in the stead of apparently absent luminous matter) have the same effect of pushing the expected orbital speed upwards as well? Or is it the way that the dark matter is supposedly distributed differ from that which the absent luminous matter would be distributed. If so -- ie if dark matter is distributed differently -- what is this unique pattern and how does it vary with r?

 Double posted by accident

 Quote by euquila Hypothetically, if we weren't receiving all the light from a galaxy, is it possible that this would reconcile the gravitational shortfall we perceive? Would it not also explain the gravitational lenses that we attribute to dark matter (since we are not receiving all the light to represent the appropriate amount of matter that causes the lens)? Let me know if I'm not being clear enough I will try to elaborate. I look forward to your input.

The gravitational shortfall is detected by observing the motion of stars in galaxies then doing some calculations. What matters is the motion, not how much light they give off.

With dark matter what matters is how much the light bends, not how much light there is.

 Quote by euquila From https://www.perimeterinstitute.ca/Pe...htness_Method: I don't understand this statement. Wouldn't adding dark matter to the equation (in the stead of apparently absent luminous matter) have the same effect of pushing the expected orbital speed upwards as well? Or is it the way that the dark matter is supposedly distributed differ from that which the absent luminous matter would be distributed. If so -- ie if dark matter is distributed differently -- what is this unique pattern and how does it vary with r? Thank you in advance for your careful consideration and explanation.
That's exactly right. The bright matter clumps up into a disk. The dark matter stays in a sphere with (I think, I'm not at all sure) possibly a larger radius than the disk. So the distribution is different.

They looked at the motion of the bright matter and it didn't make sense, so the dark matter with the needed distribution was hypothesized. They started to look for dark matter and found it IMO.
 Recognitions: Gold Member Science Advisor The distribution of dark matter is predominantly in a halo surrounding the galaxy, and can extend a considerable distance from the center of mass of the galaxy. This is logical if you consider dark matter is very weakly interactive with ... everything - including itself. Dark matter spends little time in the center of the galaxy where it travels at its maximum velocity. In the halo it is traveling at its lowest velocity, hence spends the majority of its time in this region. This extended region of relatively large amounts of gravitating matter is what causes the unusual pattern of orbital velocity of baryonic matter.

 Quote by euquila Hypothetically, if we weren't receiving all the light from a galaxy, is it possible that this would reconcile the gravitational shortfall we perceive?
No. If there were something blocking the light from galaxies it would cause the galaxies to blur and we don't see that. Also, something in between the galaxies would heat up and we'd see that.
 Are you people/"contributors" really this dense? He's asking if we are not actually seeing the entire non-dark matter galaxy i.e. is dark matter just regular matter that is obstructed from view. I really want to know why you guys love to be so damn condescending.