Dark energy and dark matter as curvature effects?

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Discussion Overview

The discussion revolves around the concepts of dark energy and dark matter, specifically exploring whether they can be understood as effects of curvature in spacetime rather than as separate entities. Participants examine various theoretical frameworks, including modified gravity theories like f(R)-gravity, and the implications of these ideas on cosmological observations and particle physics.

Discussion Character

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

Main Points Raised

  • Some participants propose that f(R)-gravity could explain the accelerated expansion of the Universe without invoking dark energy or dark matter, but acknowledge the lack of a comprehensive theory that fits all observations.
  • Others argue that modified gravity theories struggle to reconcile with observations such as the Bullet Cluster and Cosmic Microwave Background (CMB) without introducing some form of dark matter.
  • There is a suggestion that dark energy and dark matter might not exist and could instead be manifestations of curvature in spacetime or composed of undetectable weakly interacting particles like tachyons or axions.
  • Some participants highlight that the existence of dark matter is supported by gravitational interactions observable in phenomena like the Bullet Cluster and CMB anisotropies.
  • Concerns are raised about the implications of modifying gravity on both large and small scales, with some suggesting that such modifications could lead to detectable instabilities.
  • There is a discussion about the historical context of particle detection, comparing dark matter and dark energy to the delayed discovery of the Higgs Boson, suggesting that current unknowns do not negate their potential existence.
  • Some participants emphasize the need to focus on what is known about dark energy and dark matter, advocating for a discussion based on established observations and constraints rather than speculative models.
  • References to various papers are made, with some participants cautioning against the potential biases in counter-models and encouraging a deeper examination of their flaws.
  • There are mentions of recent developments in understanding negative pressure and repulsive gravitational forces, including concepts like quintessence fields.
  • Two main cases are proposed: one where dark matter and energy are simply manifestations of spacetime curvature, and another where they are composed of hypothetical particles.

Areas of Agreement / Disagreement

Participants express a range of views, with no clear consensus on the nature of dark energy and dark matter. Some support the idea of curvature effects, while others maintain that dark matter is necessary to explain certain observations. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants note limitations in current theories and observations, including the dependence on definitions and the unresolved nature of certain mathematical steps in proposed models. The discussion reflects ongoing uncertainties in the field.

petergreen
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The so called f (R)-gravity could be, in principle, able to explain the accelerated expansion of the Universe without adding unknown forms of dark energy/dark matter but, more simply, extending the General Relativity by generic functions of the Ricci scalar. However, a part several phenomenological models, there is no final f (R)-theory capable of fitting all the observations and addressing all the issues related to the presence of dark energy and dark matter. Astrophysical observations are pointing out huge amounts of ”dark matter” and ”dark energy” needed to explain the observed large scale structures and cosmic accelerating expansion. Up to now, no experimental evidence has been found, at fundamental level, to explain such mysterious components. The problem could be completely reversed considering dark matter and dark energy as ”shortcomings” of General Relativity.

sait.oat.ts.astro.it/MSAIt830312/PDF/2012MmSAI..83.1054C.pdf

[Publicated in Mem. S.A.It. Vol. 83, 1054]
 
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Sure - one can always just say that general relativity is wrong and space just happens to curve like that on the large scale but not on a small scale. So what?
 
This isn't very likely at all.

First, the Bullet Cluster and the CMB are impossible to reconcile with modified gravity without some form of dark matter (the TeVeS people claim to explain the Bullet cluster, but only by proposing a new, more massive species of neutrino, i.e. dark matter).

Second, it's rather difficult to actually modify gravity on large scales without also modifying it on small scales in a way that is detectable (sometimes causing matter to be unstable). For example:
http://www.hindawi.com/journals/ahep/2012/679156/

This doesn't mean it's impossible, it just means that there are strong constraints on the possibility. And as cosmological observations of structure formation come in, those constraints are going to get much, much stronger.
 
Chalnoth!

Since decades, we cannot solve the mystery of the dark matter and dark energy. Because perhaps dark energy and a dark matter does not exist (curvature), or composed from weakly interacting particles, which we cannot detect. Such as tachyons or axions.
 
We also could not detect until recently the Higgs Boson. There are types of Neutrinos that are still elusive. Our standard model of particles is by no means complete. That does not entail that the particles do not exist. It took a long time of counter arguments against dark energy and dark matter to become validated as the most plausible possibility.
 
petergreen said:
Chalnoth!

Since decades, we cannot solve the mystery of the dark matter and dark energy. Because perhaps dark energy and a dark matter does not exist (curvature), or composed from weakly interacting particles, which we cannot detect. Such as tachyons or axions.

Or perhaps dark energy and dark matter are fairly close to what we have already predicted and we just haven't been able to gather enough evidence yet.
My point is that simply saying "it's been a while and we haven't figured this out" isn't really a reasonable argument. For example, the Higgs Boson was predicted something like 40 years ago. It simply took so long to find because we had to build a multi-billion dollar collider to find it.
 
...nor is it true that dark matter is undetectable. Dark matter, at the very least, interacts gravitationally which gives rise to observable phenomena like the Bullet cluster and can be constrained by measurements of CMB anisotropies, as Chalnoth mentioned. If dark matter is additionally weakly interacting, various direct detection efforts have a hope of measuring certain properites of the dark matter particles. And, while axions remain a dark matter candidate, tachyons likely have nothing to do with it. petergreen -- where'd you hear this?

EDIT: And please, dark matter and dark energy have nothing necessarily to do with each other, aside from the fact that both possesses the word "dark" in their names. It makes little sense, from a physics standpoint, to discuss both in the same thread as if they are equally mysterious or physically related in some way. They're not.
 
bapowell said:
...nor is it true that dark matter is undetectable. Dark matter, at the very least, interacts gravitationally which gives rise to observable phenomena like the Bullet cluster and can be constrained by measurements of CMB anisotropies, as Chalnoth mentioned. If dark matter is additionally weakly interacting, various direct detection efforts have a hope of measuring certain properites of the dark matter particles. And, while axions remain a dark matter candidate, tachyons likely have nothing to do with it. petergreen -- where'd you hear this?

EDIT: And please, dark matter and dark energy have nothing necessarily to do with each other, aside from the fact that both possesses the word "dark" in their names. It makes little sense, from a physics standpoint, to discuss both in the same thread as if they are equally mysterious or physically related in some way. They're not.

Dark matter... OK! Can be detected, but we can not what it is! Axions? The dark energy in turn may be a tachyon field!

http://arxiv.org/pdf/1105.0325v2.pdf

DOI: 10.1007/s10509-012-0977-x
 
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Petergreen, you seem to want to just post whatever happens to agree with your train of thought on Dark Energy/Dark matter. Instead of posting things it might be, it would probably be infinitely more helpful to ask what we DO know about both topics and why we have come to those conclusions. This allows FAR more concrete answers to be given, such as constraints placed on different theories based on observations.
 
  • #10
Http://arxiv.org/abs/1301.6832

this paper shows the problems with the ghost dark matter model

The last paper you posted derives from that model. Counter models are well and good but when randomly browsing them can be VERY misleading. Every paper tries to convice you. This is the way it is.

However in many cases of these counter models if you dig deep enough have flaws which prevent them from becoming popular throughout the science community.

Edit One side note if you wish to propose counter arguments use the papers as suportive to your view point but take the time to share your reasoning in posting them. Incuding your personal arguments trust me we will all learn more from that than a bunch of random papers.
 
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  • #11
... Any latest updates/reading materials on repulsive gravitational force, matters on negative pressure/false vacuum? Last time I've heard is that they add a new component, negative equation of state e.g (Quintessence fields minimally-coupled scalar field evolves in a potential V in which energy and pressure combined resulted in negative equation of state) and the geometrical modification of gravity besides several hints from QG (modification of gravitational low energy lagrangian).
 
  • #12
Guys! Two cases are possible...

A: Dark matter and energy are simply space-time curvature or torsion.
B: Weakly interacting real and/or imaginary (hypothetical) particles.
 

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