[oshepherd]

> It is interesting that those who are looking experimentally for both dark matter and dark energy have come up empty in every experiment so far performed.

Given that <1% of the dark matter search space has been covered, this is an entirely unsurprising result.

The ΛCDM model doesn't come out of theory, it comes out of observation - the only thing we can make fit our observations of the universe is cold dark matter (and a cosmological constant, aka dark energy)

The thing a lot of people don't seem to get is that astronomers hate¹ dark energy and dark matter - they're really quite inelegant - but nothing else fits. Every other attempt (and lots and lots of time and energy have been spent on this) to explain our observations without hypothesising the existence of a lot of weakly interacting matter out there (e.g. MOND) has failed to match them

¹ In much the same way physicists dislike the standard model in all of its' ugliness - even though it works

[oldandtired]

The theory doesn't match the observations, so how do we tweak the theory to match the observations - oh wait - let's add dark matter to the theory. Partial fix - oh wait - we need something to add something else to he theory now - let's add ..... dark energy.

Oh yeah, this seems to fix the theory to now match observation.

The observations do not require dark energy or dark matter to fix things. It is the interpretation of those observations via the theory that requires these entities.

Observations are theory neutral. It should be fun to try an come up with a good model/theory and if new observations break the theory, then it should be more fun to see what is a better explanation.

For many, many years, I have watched very intelligent people get emotionally caught up in defending their theories and models, when it should be a case of just moving on. They just don't make it fun.

[oshepherd]

(And to put things another way: of course we've come back empty in all searches so far. If we hadn't, we'd stop looking!)

[raattgift]

Second thing first:

> ... much the same way physicists dislike the standard model in all of its' ugliness - even though it works

Personally I think the U(1)xSU(2)xSU(3) group theory of the Standard Model of Particle Physics is quite pretty. AFAIK the problems with the Standard Model are rarely put as issues of "ugliness" but rather that it's known to be incomplete -- in particular the gravitational sector is not dealt with at all -- and is probably slightly incorrect (depending, for example, on how you deal with neutrino masses). It has some practical problems too, namely that there are at least nineteen parameters that so far can only be determined empirically, and there are questions about whether the allowed symmetry breaking can resolve the hierarchy and strong CP problems.

However, the overall mathematical details are so attractive that perhaps most attempts to fix these problems introduce new internal symmetries (so far unsuccessfully) into the gauge QFT rather than take seriously the idea that the mathematical objects are inappropriate or unattractive.

So I think you have it very slightly backwards. We are probing the effective field theory limit of the SM and finding that likely the Standard Model doesn't work at accessible energies. (Unfortunately conclusive evidence of beyond-the-Standard-Model -- in the sense of an experimental result that the SM cannot explain -- hasn't been found yet). And unfortunately the most obvious and elegant approaches to extending the SM so far conflict with experimental results.

FWIW, Carroll at one point said that all physical cosmologists are gauge theorists. That's pretty hard to argue with these days. So I think you may have trouble enlisting many cosmologists into supporting your footnote.

> astronomers hate dark energy

I doubt many astronomers even care; it's a problem for cosmology since it literally only operates at extragalactic scales; there is no DE within galaxies. [4]

I'd bet that most astronomers mostly only care that astrophysics works as expected at high redshifts, i.e., that dark energy does not introduce Lorentz invariance violating terms into their calculations.

Cosmologists, being relativists, know that dark energy is reference-frame dependent, and in particular that DE is a feature of the cosmological frame. There are good reasons for using the cosmological frame when dealing with gigaparsec length scales, and so good reasons for calling it "dark energy" without clarifying that the very concept of energy in General Relativity (GR) is complicated. (To be fair, the concept of energy in Newtonian mechanics can be complicated too: how do you define potential energy in a rotating frame? The classical bead-on-a-rotating-hoop problem often shows up in graduate-level university exams.)

> astronomers hate ... dark matter

Do they? I mean it's frustrating that it's invisible, but so are neutrinos for all practical purposes, and astronomers don't hate those (at least not the astronomers studying solar neutrinos or the fine energy balance of supernovae, for example). Surely though it's more a puzzle which they also scratch their heads about when dealing with the motions of stars within galaxies and so forth.

> the only thing we can make fit our observations of the universe is cold dark matter

Firstly, I fully endorse the standard model of physical cosmology, \Lambda-CDM.

However, CDM is far from the only thing that can match our observations of galaxy-scale kinematics and dynamic stability. It is always possible to modify the LHS of G = T (pardon the dropping of indices and constant terms), it is generally possible to treat that modification as a (classical) field, and often possible to move that classical field over to the T side as a source term. So the space of solutions to the galaxy scale problems that CDM resolves is huge.

CDM's attraction is mainly in the hopes that beyond the Standard Model there lies a sterile neutrino or similar particle, but there are nearly endless ways the Standard Model could be extended into the dark matter sector. One line of thought behind this preference is that if such a particle is discovered, it's relativized and quantized "for free" within the BTSM QFT. MOND is hard to relativize [1] and may resist perturbative renormalization.

> (and a cosmological constant

Well, you've got me there. :-) Alternatives to the CC so far all have stability problems. They do exist though; for example one can find them in bimetric theories where the second metric decays away no later than the inflationary epoch. (There's some robust technical discussion at [2]).

> [dark energy] is inelegant

I have to disagree there. So would Schrödinger! [3] All you need is a representation of matter as a homogeneous and isotropic fluid with some density and pressure and a reasonable slicing of spacetime: if your spacelike slices expand and the matter stays homogeneous and isotropic on average then a negative pressure (or tension) with a value of ~ 1/3 of the density falls right out of practically any reasonable set of equations you might use.

In the FLRW model for the cosmological constant (CC) we just fix density_CC = const, pressure_CC = - density_CC -- the beauty of "dark energy" is that we can parametrize this pressure/density (it's the "w" parameter in the standard cosmology) and not have to worry about the microscopic details of the new component(s), other than that the microscopic behaviours of the component(s) and the bulk contribution to w must match. I think that's pretty elegant.

- --

[1] Famaey & McGaugh, https://arxiv.org/abs/1112.3960 ch. 7.

[2] https://physics.stackexchange.com/questions/123131/what-are-...

[3] Harvey, [physics.hist-ph] https://arxiv.org/abs/1211.6338v1

[4] DE is a set of non-metric-invariant quantities that arise in the Friedmann-Lemaître-Robertson-Walker (FLRW) metric. It is inappropriate to use the FLRW metric when matter is not (on average) homogeneous and isotropic. The matter in real galaxies is neither homogeneous nor isotropic, so one must use another metric to describe them, almost always a non-expanding one (e.g. Schwarzschild) which admits a boundary condition that one can use to patch that local non-expanding space into the expanding FLRW space. (I use "space" here rather than "spacetime", since the slicing of 4-d spacetime into 3+1 space + time is relevant.) If we do a lot of abuse we can pseudo-transport the DE into this other metric, wherein the best interpretation is that it contributes to the momentum tensor (but cf. Einstein's reply (his point 1) to that interpretation in [3] chapter 3, and also the Schwarzschild-de Sitter metric which we can with more work also stitch into FLRW).