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I Dark matter/energy, why must it be there?

  1. Sep 9, 2017 #21

    Bandersnatch

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    This might sound a bit rude, so steel yourself. Keep in mind it's not an attack ad personam - I'm just trying to get you to reconsider your methodology of acquiring knowledge.
    What have you seen of the measurement methods used for either dark energy or dark matter to give weight to your assertion? You state is as a fact. Have you looked at error bars in spectroscopic measurements of rotation curves, and decided they report values beyond what the instrument is capable of? Have you read Adam Reiss' (et al.) research on dark energy, for which he received the Nobel prize, and found some errors?
     
  2. Sep 9, 2017 #22
    He does not state it as a fact. He said "I think". Apart from that, I agree: he does not seem to know how much work went into validating and cross-checking observational data.
     
  3. Sep 9, 2017 #23
    My understanding is MACHOs have been ruled out as a significant dark matter candidate by gravitational microlensing observations.

    https://en.wikipedia.org/wiki/Massive_compact_halo_object
    https://en.wikipedia.org/wiki/Gravitational_microlensing
    https://arxiv.org/abs/1607.06077
     
    Last edited: Sep 9, 2017
  4. Sep 9, 2017 #24

    Chronos

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    It might be safer to say we know more about what dark matter is not than what it is. First and foremost is it is not baryonic. The most damning evidence was provided by the cosmic background radiation data of the WMAP and Planck missions - the baryonic signature was conspicuously absent in both studies. The case for dark energy is less secure, but, has improved substantially since it was first detected as a consequence of supernova studies by Perlmutter and Riess - for which they, along with Schmidt, were awarded the Nobel prize in 2011, The Dark Energy Suvey [re: https://www.darkenergysurvey.org/the-des-project/overview/] is the most promising current attempt to confirm DE. The leading suspect is the cosmological constant proposed by Einstein.
     
  5. Sep 9, 2017 #25
    Hi @stoomart:
    Thanks for your response, particularly the third citation.

    I may well be mis-remembering something I read some time ago, but I think the conclusion that MACHOs cannot be dark matter is based on an assumption about their having a large size, perhaps like those that form from supernovas. Two of the Wikipedia articles you cited listed the candidates:
    brown dwarfs, red dwarfs, planets, white dwarfs, neutron stars, black holes.​
    but there is certainly no reason for them to be as large as these kinds of objects. These kind of objects could not be dark matter anyway because they would be normal baruonic matter (protons and neutrons) during PNP and that would prevent the survival of the observable amount of deuterium. However, primordial black holes (PNHs) do have to be large enough so that their evaporation by Hawking radiation would not be observed, as well as being small enough so that micro-lensing effects would not be noticeable.

    Wikipedia also mentioned specifically
    massive compact halo objects​
    which is what the acronym MACHO stands for.
    https://en.wikipedia.org/wiki/Massive_compact_halo_object
    A massive astrophysical compact halo object (MACHO) is any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter that emits little or no radiation and drifts through interstellar space unassociated with any planetary system.​

    The the third citation is more interesting. It says:
    The possibility that the dark matter is in intermediate-mass PBHs of 1 - 103M⊙ is of special interest in view of the recent detection of black-hole mergers by LIGO.​
    This seems to allow for the possibility that PBHs might be dark matter.

    Regards,
    Buzz
     
    Last edited: Sep 9, 2017
  6. Sep 9, 2017 #26
    The section "Lensing Constraints" on page 15 speaks directly about how little PBHs would contribute to the overall DM equation. My understanding is PBHs, if they existed, would be detected as a twinkling from microlensing, which is not observed.
     
  7. Sep 11, 2017 #27

    kimbyd

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    Bear in mind that there are two distinct classes of MACHOs:
    1. As normal matter collapses, some percentage of the collapsed matter becomes effectively invisible because it's too dim to detect.
    2. Some amount of matter started out as compact objects.

    Option (1) has been completely ruled out by the CMB data, which shows very clear evidence of dark matter long before any compact objects formed due to gravitational collapse. Option (2) is primordial black holes. Yes, they are a possibility. But they should still be considered an exotic possibility.
     
  8. Sep 11, 2017 #28

    kimbyd

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    My understanding is that those constraints still allow PBH's within a certain mass range, which is incidentally also the mass range of the observed black hole-black hole mergers that LIGO detected.
     
  9. Sep 12, 2017 #29
    Hi kimbyd:

    I agree that PBHs seem to be "an exotic possibility". However, at the present time, the other ideas previously discussed about what dark matter might be (e.g., neutrinos, WIMPs) seem to no longer be plausible candidates at all.

    What seems particularly odd about the PBH idea, based on the https://arxiv.org/abs/1607.06077 article, is that the only sizes for PBHs that have not yet been eliminated as a dark matter possibility are those that cannot with the present technology be detected. This would require a physical mechanism for PBH creation that by an odd coincidence would create only PBHs of a size that we cannot now detect.

    Regards,
    Buzz
     
  10. Sep 12, 2017 #30

    PeterDonis

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    Thread closed for moderation.

    Edit: The thread topic has been sufficiently addressed. The thread will remain closed.
     
    Last edited: Sep 12, 2017
  11. Sep 12, 2017 #31

    Drakkith

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    For clarification, this thread has been attracting some problematic posts that have now been deleted. Thread will remain locked since it is one of those topics that seems to inherently invite personal speculation.
     
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