# Start a new discussion

## Not signed in

Want to take part in these discussions? Sign in if you have an account, or apply for one below

## Discussion Tag Cloud

Vanilla 1.1.10 is a product of Lussumo. More Information: Documentation, Community Support.

• CommentRowNumber1.
• CommentAuthorUrs
• CommentTimeJan 10th 2013
• (edited Jan 10th 2013)

stub for dark matter

• CommentRowNumber2.
• CommentAuthorUrs
• CommentTimeJun 13th 2014

added two references (the original one and a review) on LSPs as CDM candidates

• CommentRowNumber3.
• CommentAuthorzskoda
• CommentTimeJun 15th 2014

In the case somebody is not aware, the dark energy listed under related concepts in fact (still) redirects to cosmological constant.

• CommentRowNumber4.
• CommentAuthorTobyBartels
• CommentTimeJun 16th 2014

There is written

See in particular at FRW model for the role of the cosmological constant in homogeneous and isotropic models as in the standard model of cosmology. In that context the cosmological constant is also called the dark energy (density), which makes up about 70% of the energy density of the observable universe (the rest being dark matter) and a comparatively little bit of baryonic matter.

Are we making a controversial claim here or merely reporting standard knowledge?

• CommentRowNumber5.
• CommentAuthorUrs
• CommentTimeJun 16th 2014

Sorry, which piece should be controversial? This is the standard model of cosmology. Of course it’s not a mathematical truth. Do you want more discussion of the justification of the standard model of cosmology?

• CommentRowNumber6.
• CommentAuthorTobyBartels
• CommentTimeJun 16th 2014

That dark energy is the cosmological constant, rather than something else.

• CommentRowNumber7.
• CommentAuthorUrs
• CommentTimeJun 16th 2014
• (edited Jun 16th 2014)

That’s just terminology. Please feel invited to expand and explain that an effective cosmological constant may be induced from all sorts of stuff.

Hm, I thought I did discuss this somewhere on the $n$Lab, where was it? Ah, at string theory FAQ:

The simple familiar example to compare this to is the cosmological constant in Einstein gravity: one can either consider it as an external parameter, a constant real number coefficient in front of the volume form summand of the Einstein-Hilbert Lagrangian, or else one can consider Einstein gravity coupled to a scalar field with some potential and consider those solutions to the equations of motion where this field is almost constant to good approximation. In such a case the field itself serves as an effective cosmological constant. (This is the mechanism behind the theory of cosmic inflation, see there for more details.) Hence the theory has one less external parameter (the “cosmological constant” is not fundamentally really a constant), which has instead been replaced by a field.

• CommentRowNumber8.
• CommentAuthorTobyBartels
• CommentTimeJun 16th 2014

I can't expand and explain, because I don't know. I just want to know if redirecting dark energy to cosmological constant and implying there that they are the same thing is a reasonable thing to do.

• CommentRowNumber9.
• CommentAuthorUrs
• CommentTimeJun 16th 2014
• (edited Jun 16th 2014)

On the one hand identifying the terms “dark energy” and “cosmological constant” is not an offense. Just consider that the official name of the standard model of cosmology is the $\Lambda$CDM-concordance model where “$\Lambda$” is the standard symbol for the cosmological constant and indicates the inclusion of “dark energy”, while “CDM” is for “cold dark matter”.

On the other hand, one may of course distinguish between the two terms, or at least have a discussion of their different history. The main technical distinction though is not actually reflected well by the difference in the two terms. The main technical fact to know is what is alluded to in the above blue box, that the Lagrangian of gravity on cosmological scales receives contributions to the term proportional to the volume form in two conceptually different ways: on the one hand there may be a (renormalized) constant in the theory, which just appears there, on the other hand the theory may contain fields whose Lagrangians effectively look like this constant, without techncially being equal to it.

But in cosmology it is quite common to subsume a huge amount of unknown territory in some simple constant. The “dark energy” could receive contributions from an infinitude of fields, but at the coarse-grained level of cosmology, it makes no difference. Even if one goes and specifies these fields a bit more, there is still immense no-knowledge: for instance in cosmic inflation the dark energy is taken to be the energy of a scalar field. But that scalar field in turn may very well be the effective result of a multitude of other scalar and non-scalar fields. Still, it is technically correct to speak of “the inflaton” where it is understood that this is an effective field of the model.

The same holds for “dark matter” itself. Dark matter could be lots of things and in particular could be compounds of plenty of different effects. Until one knows more, there is just one single parameter in the theory, the dark matter density.

(This is one of the mysteries of our world: that comparitively simplistic ordinary differential equations in a handful of such effective parameters provide such a astoundingly accurate model of the large scale structure of the observable universe. On the other hand, of course cosmic inflation itself is meant to be a partial explanation of that mystery…)

So in conclusion: as long as we don’t have more material, the redirect is perfectly fine. If on the other hand tomorrow there appears here a highly enegetic continutor who writes a bunch of paragraphs on cosmology and its more subtle aspects, then eventually it might be good to split into two entries that focus on different perspectives.

1. Edit to: dark matter by Urs Schreiber at 2018-03-31 00:26:11 UTC.

added new section “References – Evidence” with pointer to a new article on a new kind of evidence for dark matter

2. Edit to: dark matter by Urs Schreiber at 2018-03-31 00:42:24 UTC.

quote from van Dokkum et.al. slightly expanded

3. Edit to: dark matter by Urs Schreiber at 2018-03-31 01:03:14 UTC.

4. Edit to: dark matter by Urs Schreiber at 2018-03-31 01:31:31 UTC.

references expanded

5. Edit to: dark matter by Urs Schreiber at 2018-03-31 01:32:23 UTC.

reference expanded

• CommentRowNumber15.
• CommentAuthorUrs
• CommentTimeMay 7th 2018

• CommentRowNumber16.
• CommentAuthorUrs
• CommentTimeJul 25th 2018

which is pretty useful

• CommentRowNumber17.
• CommentAuthorUrs
• CommentTimeJan 5th 2019

added this pointer on further recent evidence for dark matter:

• CommentRowNumber18.
• CommentAuthorUrs
• CommentTimeJan 6th 2019

added a minimum paragraph to the section on Core-cusp problem and further pointers to the literature. Thanks to Justin Read for discussion.

• CommentRowNumber19.
• CommentAuthorUrs
• CommentTimeJan 31st 2019

added pointer to the recent followup

• Shany Danieli, Pieter van Dokkum, Charlie Conroy, Roberto Abraham, Aaron J. Romanowsky, Still Missing Dark Matter: KCWI High-Resolution Stellar Kinematics of NGC1052-DF2 (arXiv:1901.03711)

on the earlier

• CommentRowNumber20.
• CommentAuthorUrs
• CommentTimeFeb 16th 2019

started a section Galactic rotation curves and Tully-Fisher relation (here) with pointer to how the FIRE-2 simulation of $\Lambda CDM$ actually reproduces both of these well

• CommentRowNumber21.
• CommentAuthorUrs
• CommentTimeFeb 18th 2019

added also pointer to apparent resolution of the “missing satellite problem” by the FIRE-2 computer simulation

• Shea Garrison-Kimmel et al. Not so lumpy after all: modeling the depletion of dark matter subhalos by Milky Way-like galaxies (arXiv:1701.03792)

• Shea Garrison-Kimmel, Next-generation Galaxy Formation Simulations with FIRE, 2018 (video recording)

Exciting times.

• CommentRowNumber22.
• CommentAuthorUrs
• CommentTimeFeb 20th 2019

and today, more confirmation that the RAR comes out right with CDM:

• Aaron A. Dutton, Andrea V. Macciò, Aura Obreja, Tobias Buck, NIHAO XVIII: Origin of the MOND phenomenology of galactic rotation curves in a LCDM universe (arXiv:1902.06751)
• CommentRowNumber23.
• CommentAuthorUrs
• CommentTimeMar 11th 2019

changed the code for the images and the line breaks to the new format, then added this reference: