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I Do we need Dark Matter or are we just time blind

  1. Feb 9, 2016 #1
    As humans, we intuitively tend to interpret the things we see in our local time frame. However, the rate at which time flows at any point (or time) in the Universe is affected by the strength of the gravitational field at that point (or time). Hence when we see distant events through our telescopes and interpret them in the lab in our own time frame (time rate), errors will occur.

    Nearly all the methods of measuring a star’s tangential velocity involve taking a number of measurements at known increments of time. (Red-shifting only measures radial velocity). The increment of time is measured in earth time units since it is difficult to remotely calculate the star’s time frame Hence we are always viewing the velocity of the star in the time frame of the earth. This will produce velocities which appear higher than the actual local velocities when viewing a star with mass greater than the earth. This is a type of time blindness.

    For example, the stars close to the Black Hole at the centre of the Milky Way are observed to be moving faster than predicted by classical theory To explain this, Dark matter has been “invented” to explain the higher than expected gravitational force necessary to produce these higher velocities.

    However, time in the extreme gravitational fields close to a black hole is known to slow down. So, are these stars actually moving at this unusual speed or is the difference in time frame making it appear to us that they are moving faster than they should be? If we can explain the observed anomalies in velocity by the difference in time frames between the star and the earth, do we still need Dark Matter?

    More worryingly, if time frame difference is not routinely corrected for, are we looking at a distorted picture of the Universe? The size of the error could potentially be very large. For example, the time rate close to a black hole is thought to approach zero and hence the corresponding velocity in our time frame would approach infinity. The data comparing actual speeds (measured in our time frame) of stars close to black holes with the speeds calculated from Newtonian theory could be used to calculate how much of the error is caused by changes in time frame.

    Time Rate Mapping
    A very useful tool would be if time frames could be mapped. The ratio of local time rate to GMT time rate could be used as a parameter. This could be done from gravitational lensing and by observing the speed of rotation of stars round black holes and galaxies round clumps of galaxies. This is similar to the methods used to map dark matter. This combined with the fact that dark matter appears to attract galaxies, which are areas of high gravity and slow time, could mean that a slow time map would be very similar to a dark matter map. It might even be possible to use existing dark matter maps to produce an approximate time correction when measuring the tangential velocity of stars.
     
    Last edited by a moderator: Feb 14, 2017
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  3. Feb 9, 2016 #2

    PeterDonis

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    No, actually it's the other way around. The stars close to the center are moving the way we would expect from the matter distribution we can see. The stars further from the center are the ones that are moving faster than we would expect from the matter distribution we can see. So your proposal, even if it were correct as regards the magnitude of the time dilation effect (which it isn't--see below), would not explain what we actually see.

    Have you actually done the math to see how close these stars would need to be to the central black hole for any such time dilation to be significant? (Hint: it's way closer to the hole than any of the stars actually are.)
     
  4. Feb 9, 2016 #3

    Orodruin

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    Yes we need dark matter. The effect of gravitation on time is miniscule in the relevant measurements and could anyway be easily corrected for if this was not the case.
     
  5. Feb 9, 2016 #4

    Chalnoth

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    To expand on this a little, gravitational time dilation is only significant very close to extremely dense objects (such as neutron stars or black holes). The fact that the "missing mass" is more of a problem further away from the centers of galaxies indicates that it can't be explained away through time dilation.

    Also, the existence of dark matter is supported by a wide body of evidence. Here's a blog post describing one particularly evocative piece of evidence:
    http://www.preposterousuniverse.com/blog/2006/08/21/dark-matter-exists/
     
  6. Feb 10, 2016 #5

    Chronos

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  7. Feb 17, 2016 #6
    Thanks everyone for your information its been really useful.
     
  8. Feb 14, 2017 #7
    How do we know what to expect, without knowing the depth our gravity well, relative to the depth of the gravity well of the galaxy we're observing? As a hypothetical example, suppose our closeness to the barycenter of our local group of galaxies makes clocks close to the center of the galaxy we're observing run at the same rate as ours as we measure.
     
  9. Feb 14, 2017 #8

    PeterDonis

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    The time dilation due to all of the gravity wells involved is way too small to affect any of these observations. The Milky Way's gravity well, for example, results in a time dilation of about one part in 100,000. Other galaxies are similar. The galaxy rotation curve anomalies that dark matter is postulated to explain are orders of magnitude larger than that.
     
  10. Feb 15, 2017 #9
    I see. Just entertaining ideas here, how can we know that for sure? Presumably to get that one part in 100,000 result you need the mass, and to get the mass we measure orbital velocity. But maybe the orbital velocity is slowed by significant gravitational time dilation, so we're fooled into thinking there's much less mass. What rules out that possibility?

    Edit: I see you gave an answer to this above already. I wasn't thinking about a black hole at the center of the galaxy, but rather a large quantity of densely packed stars (perhaps mixed in with black holes, I suppose) orbiting the center.
     
    Last edited: Feb 15, 2017
  11. Feb 15, 2017 #10

    Chalnoth

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    Gravitational time dilation is an incredibly tiny effect in nearly all cases. Basically, if you're not talking about the area immediately surrounding a black hole, you can pretty safely ignore gravitational time dilation in most contexts.
     
  12. Feb 15, 2017 #11
    I'd still like to know how we're sure it's a tiny effect in this case. What if for every star in the galaxy's central bulge that we see, there are 1000 black holes, or highly compact / highly redshifted stars, all orbiting the galaxy's center? I suppose we could rule that out because we don't see evidence of gravitational lensing. Is that how it's ruled out? Yes the gravitational time dilation might be significant only in the area immediately surrounding the bulge, but it seems that's all it would take to measure a slower (than locally measured) orbital velocity for a star in the area, and hence greatly underestimate the mass within the bulge.
     
  13. Feb 15, 2017 #12

    Orodruin

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    So you would remove the need for dark matter by introducing copious amounts of matter that cannot be seen? Do you see the irony in this?

    To be perfectly frank, your argumentation is based on suppositions and word play. What people are telling you is generally supported by the mathematical predictions of general relativity which has been shown to be in very good agreement with observations. If you want to dispute this, do the math. (But note that PF is not the place for personal theories or original research. The proper place for that is in a reputable peer reviewed journal (if accepted).
     
  14. Feb 15, 2017 #13

    Chalnoth

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    In order for time dilation around a black hole to reach 1%, you'd need to be about 100 times the Schwarzschild radius away. That may sound large, but bear in mind that black holes are tiny. A black hole with the mass of the Earth would be about the size of a golf ball. A black hole the mass of the Sun would only have a radius of about 3000 meters. That means that you'd have to be within 300km of a black hole the mass of the Sun to have time dilation greater than 1%. Contrast this with typical distances between astronomical objects which are often measured in the trillions of kilometers (1 light year = 9.4 trillion kilometers).
     
  15. Feb 15, 2017 #14
    The current thinking is that the flat rotation curves are accurate. I'm indirectly asking about the possibility that maybe the curves aren't flat if they were completely locally measured. The copious amounts of matter I'm talking about wouldn't be distributed the same as is currently thought it must be.

    I'm not proposing a personal theory. I'm asking how some possibilities are ruled out.
     
  16. Feb 15, 2017 #15

    Orodruin

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    This does not change the fact that what you are describing just cannot be a viable solution if you actually do the maths - as has been explained by many posters in this thread. GR just does not behave the way you would need it to.
     
  17. Feb 15, 2017 #16
    Okay, so I guess if the central bulge was much more dense, say distances between astronomical objects averaging 0.05 light years, with 10000 dark stars (maybe black holes) for every visible star, we'd definitely notice this, right? We'd notice gravitational lensing, visible stars being dimmed or blocked as dark stars pass in front of them, or other obvious things?

    Yes, a black hole the mass of the Sun would only have a radius of about 3000 meters, but I'm thinking about black holes the size of the Sun, with 10000 of them for every visible star in the central bulge. Just to entertain the idea and see how we rule out that possibility.
     
  18. Feb 15, 2017 #17
    I can do the math in my head, to know that if enough dark mass is packed into the galaxy's central bulge, it's possible to inaccurately measure the orbital velocity of a star to any degree. That's why I'm asking how this possibility is ruled out. I'm not the OP. My questions are within the context of GR.
     
  19. Feb 15, 2017 #18
    Like anything else that enjoys scientific acceptance, it's not due to one measurement. Fred Zwicky hypothesized about the existence of dark matter because of the velocity of entire galaxies within a cluster, not the stars themselves. Vera Wang then proposed the same hypothesis when noticing that the stars don't orbit the way they were expected to. Gravitational lending has also allowed us to map where the anomalies are and they don't always match up to where the matter is. The bullet cluster contains galaxies that smashed through each other. The stars of course didn't collide so they passed right through each other. The loose gas and dust did, so it got ripped away from the galaxies and stayed in between. The dark matter stayed with the stars, if it were regular matter, it should have had friction with itself and stayed with the gas.
     
  20. Feb 15, 2017 #19
    Can you elaborate on this? What is the basis of the assumption that the dark matter can't be dark stars or black holes, for the Bullet Cluster? In which case it seems we could expect them to stay with the stars and not the gas.
     
  21. Feb 15, 2017 #20

    Chalnoth

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    Can't work. Just adding a bunch of extra mass to the center of the galaxy won't flatten the rotation curves. You need matter distributed smoothly throughout the galaxy.

    Also, your scenario still has typical separations between "dark stars" at 20 billion kilometers, which is much too far to experience substantial time dilation for anything but a supermassive black hole.
     
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