Why Is Dark Matter Essential for Understanding Gravity in the Universe?

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Discussion Overview

The discussion centers around the necessity of dark matter for understanding gravitational dynamics in the universe. Participants explore the relationship between energy, mass, and gravity, particularly in the context of general relativity and the implications of dark matter on galaxy formation and stability.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions the need for dark matter, suggesting that energy should also contribute to gravity based on the mass-energy equivalence principle (E=mc²).
  • Another participant asserts that while energy does have a gravitational effect, the total mass of ordinary matter and photons is insufficient to account for the gravitational forces needed to hold galaxies together.
  • There is a discussion about the misconceptions surrounding mass-energy equivalence, with one participant emphasizing that mass cannot simply be converted to energy and back without retaining some form of mass.
  • Some participants mention that all forms of energy, including electromagnetic radiation, are included in gravitational models, but the current energy density of radiation is very small compared to the total energy density.
  • Questions arise regarding the properties of dark matter, including its lack of electric charge and weak interactions with normal matter, as well as its inability to form large bodies due to these properties.
  • One participant introduces recent findings about dark galaxies, suggesting that they may contain dark matter halos that influence their gravitational dynamics.

Areas of Agreement / Disagreement

Participants express differing views on the role of energy in gravity and the necessity of dark matter. While some agree on the properties and implications of dark matter, there is no consensus on the fundamental questions regarding its necessity and the relationship between energy and gravity.

Contextual Notes

Participants note that the discussion is limited by the current understanding of dark matter and its properties, as well as the complexities involved in gravitational modeling. There is also mention of unresolved misconceptions regarding mass-energy equivalence.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in cosmology, gravitational theory, and the nature of dark matter and energy.

  • #31
Skolon said:
Very interesting discussion.

But it is something from my previous post that you don't discuss yet: "can we say that it (DM) is different by Dark Energy"?
After my post I found some strange theories based on this idea: "Dark fluid" models.

Are these models accepted by "official science"?
They're considered to be extremely speculative. Nearly all such speculative models turn out to be wrong. But I don't think they're considered ruled out just yet.
 
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  • #32
Yes, in the LCDM model, DM and DE behave very differently. DM clusters gravitationally, and indeed this clustering is responsible for the structure we see in the Universe, but DM is also pressure-less.

DE on the other hand does not cluster (it is very close to, or exactly homogeneously distributed) and has significantly negative pressure. This negative pressure is what cause the late time acceleration of the expansion of the Universe.

Now, thinking of alternatives to LCDM there are some attempts to construct single field models in which a single energy term has strange properties that allow it to both cluster on small scales but drive acceleration on large scales. These theories are not yet fully developed (at least as I understand it) in terms of having worked out all the details and implications that such a theory would have on the things we can observe.

There is no such thing as 'official science' but these aren't crackpot theories if that's what you mean. More that they are as yet under-developed possibilities that are being investigated. I don't know a lot about this to be honest, most of what I know comes from a conversation I had for several hours with one of the proponents of this idea. It seemed there were still a lot of details to be worked out before definite model predictions could be made in order to test this against data.

Probably a case of watch this space, though my personal view is that there doesn't seem to be a great motivator for this kind of approach, apart from a desire to simplify the theory by only needing one, rather than two, new unseen forms of energy. To me it seems that the single fluid would need to in fact act in a way that's much more complex and fine tuned that the two fluids we currently put in the model. In any case it's difficult to judge this yet in the absence of more details.
 

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