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stub for dark matter
added two references (the original one and a review) on LSPs as CDM candidates
In the case somebody is not aware, the dark energy listed under related concepts in fact (still) redirects to cosmological constant.
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?
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?
That dark energy is the cosmological constant, rather than something else.
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has one less external parameter (the “cosmological constant” is not fundamentally really a constant), which has instead been replaced by a field.
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.
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.
Edit to: dark matter by Urs Schreiber at 2018-03-31 00:26:11 UTC.
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added new section “References – Evidence” with pointer to a new article on a new kind of evidence for dark matter
Edit to: dark matter by Urs Schreiber at 2018-03-31 00:42:24 UTC.
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quote from van Dokkum et.al. slightly expanded
Edit to: dark matter by Urs Schreiber at 2018-03-31 01:03:14 UTC.
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added cross-link with “bullet cluster”
Edit to: dark matter by Urs Schreiber at 2018-03-31 01:31:31 UTC.
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references expanded
Edit to: dark matter by Urs Schreiber at 2018-03-31 01:32:23 UTC.
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reference expanded
added pointer to
which is pretty useful
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