Calculating Dark Matter's Acoustic Peak Resonance: A Homework Assignment

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Discussion Overview

The discussion revolves around the challenges and theories related to the detection of dark matter, particularly in the context of cosmic background radiation (CBR) and its implications for understanding dark matter's properties and interactions. Participants explore various hypotheses, observational evidence, and the implications of different models, including the bullet cluster and the nature of dark matter particles.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • Some participants question why dark matter has not been directly observed in the cosmic background radiation, despite its gravitational influence being evident.
  • Others argue that while the dark matter hypothesis is compelling, alternative theories like MOND do not adequately explain certain observations, such as the bullet cluster.
  • One participant suggests that low-mass free particles must exist in the CBR, proposing that dark matter cannot be composed of heavy mass particles due to the lack of hard scattering observed in the bullet cluster.
  • Another participant challenges the reasoning that dark matter must hard scatter, stating that massive, electrically neutral particles would not hard scatter regardless of their mass.
  • Questions are raised about the interaction cross-section and coupling strength between photons and non-relativistic neutrinos, with some asserting that neutrinos do not couple directly to photons.
  • A homework assignment is proposed, asking participants to plot the mass distribution of dark matter and gas at the crossover point for the acoustic peak and calculate the peak resonance.

Areas of Agreement / Disagreement

Participants express differing views on the nature of dark matter, its interactions, and the implications of observational evidence. There is no consensus on the validity of the various models or the reasoning presented.

Contextual Notes

Discussions include unresolved questions about the interaction properties of dark matter candidates and the implications of different theoretical frameworks. The limitations of current observational techniques and definitions are also acknowledged.

Who May Find This Useful

Readers interested in dark matter research, cosmology, and theoretical physics may find the discussions and proposed homework relevant to their studies.

Orion1
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No, not really, however my question is why has not dark matter been discovered, observed directly in the cosmic background radiation?
[/Color]
Reference:
http://en.wikipedia.org/wiki/Dark_matter
 
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Orion1 said:
No, not really, however my question is why has not dark matter been discovered, observed directly in the cosmic background radiation?

When people talk about "direct detections" of dark matter, they generally mean detection of individual particles in a collider, cosmic ray detector, or some other ground-based experiment. We already know (in part because of the CBR) that something is out there exerting a great deal of gravitational influence, but we haven't been able to determine much beyond its average mass distribution.
 
The DM hypothesis is fairly compelling. Lensing studies and virial theory, for example, offer powerful observational evidence there is a huge amount of gravitating, undetected mass in the universe at large. MOND is a seductve alternative, but fails to explain things like the bullet cluster.
 

The wiki describes dark matter in the 'bullet cluster' as 'collisionless' dark matter.

My opinions are these:

Any low-mass free particles that the universe is composed of MUST exist in the CBR as background radiation.

The collision cross-section for dark matter particles is smaller than the electron and magnitudes weaker, of which there is only one candidate in the CBR, the neutrino.

Even if dark matter were non-baryonic, it would still hard-scatter by collisions as the baryonic matter does in the 'bullet cluster'.

The dark matter in the bullet cluster does not hard-scatter, therefore it cannot be composed of heavy mass particles, at least nothing more massive than a neutrino.

Also, even non-baryonic CBR dark matter 'particles' would have been detected by hard-scattering in collider and cosmic ray detectors as background radiation, therefore, particled non-baryonic dark matter CBR does not exist.

wiki said:
In particular, measurements of the cosmic microwave background anisotropies correspond to a cosmology where much of the matter interacts with photons more weakly than the known forces that couple light interactions to baryonic matter.

CBR photonic anisotropies suggests to me that dark matter does not hard-scatter but rather can soft-scatter photons and is 'polarized' to its focus, such as a lens.

wiki said:
Anisotropy (pronun. with the stress on the third syllable, is the property of being directionally dependent, as opposed to isotropy, which means homogeneity in all directions. It can be defined as a difference in a physical property (absorbance, refractive index, density, etc.) for some material when measured along different axes. An example is the light coming through a polarizing lens.
What is the interaction cross-section between a photon and a non-relativistic neutrino?

What is the interaction coupling strength between a photon and a non-relativistic neutrino?

Is the non-relativistic photon-neutrino interaction coupling strength less than the known forces that couple photon interactions to baryonic matter?

Do photons soft-scatter through a massively large 'cluster cloud' of non-relativistic neutrinos?

Could a massively large 'cluster cloud' of non-relativistic neutrinos anisotropicly polarize cosmic microwave background radiation?
[/Color]
Reference:
http://en.wikipedia.org/wiki/Dark_matter
http://en.wikipedia.org/wiki/Bullet_cluster
http://en.wikipedia.org/wiki/Anisotropy
http://upload.wikimedia.org/wikipedia/commons/e/ea/Bullet_cluster.jpg
 
Last edited:
Orion1 said:
Any low-mass free particles that the universe is composed of MUST exist in the CBR as background radiation.

You could call dark matter particles a "background" of sorts, but CBR refers to the cosmic microwave background. By definition, dark matter can't be a part of it.


The collision cross-section for dark matter particles is smaller than the electron and magnitudes weaker, of which there is only one candidate in the CBR, the neutrino.

The cross section is smaller than for electrons... I don't know what it means for the cross section to be "weaker". There is a cosmological background of neutrinos, but not enough to make up the dark matter.


The dark matter in the bullet cluster does not hard scatter, therefore it cannot be composed of heavy mass particles, at least nothing more massive than a neutrino.

Your reasoning falls apart here -- a particle can be massive and still have a low interaction cross section. There are quite a few candidates in theories of supersymmetry.
 
Orion1 said:
Even if dark matter were non-baryonic, it would still hard-scatter by collisions as the baryonic matter does in the 'bullet cluster'.

Hard scattering is an electromagnetic interaction. Massive, electrically neutral particles will not hard scatter, no matter how large their masses.

The dark matter in the bullet cluster does not hard-scatter, therefore it cannot be composed of heavy mass particles, at least nothing more massive than a neutrino.

If dark matter were composed of neutrinos, there would be a very large lower limit on the sizes of dark matter halos, due to the free-streaming of the neutrinos. Neutrinos have long since been ruled out as DM candidates because this limit is significantly broken in the sizes of observed structures.

What is the interaction cross-section between a photon and a non-relativistic neutrino?

Extraordinarily small. The lowest order interaction is a 1-loop diagram with a way off-shell W or Z.

What is the interaction coupling strength between a photon and a non-relativistic neutrino?

Zero. Neutrinos don't couple directly to the photon. Any [tex]\gamma / \nu[/tex] interactions will either involve a lepton/antilepton loop in the photon exchanging a Z with the neutrino, or the photon being absorbed and reemitted by the charged lepton in a lepton/W loop in the neutrino.
 
homework101


ok, Chronos, SpaceTiger and Parlyne, I have a simple independent homework assignment for you all.

1. plot the 'mass distribution' Dark Matter-Gas curve at the crossover point for the acoustic peak animation sequence listed in reference.
2. calculate the peak resonance for this 'mass distribution' curve.
3. publish your results on this thread.
[/Color]
Reference:
http://cmb.as.arizona.edu/~eisenste/acousticpeak/acoustic_physics.html
http://cmb.as.arizona.edu/~eisenste/acousticpeak/acoustic_anim.html
http://cmb.as.arizona.edu/~eisenste/acousticpeak/acoustic_anim_dens.html
 

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