AdamWho

Is there any one out there who is troubled by the dark matter / energy theories out there? The fact that these seem to make great pop-sci books crowding chain book stores is enough to make me skeptical but the theories themselves just bug me.

The basic theories:
When we make measurements of stars on the edge of our (and other) galaxy they don't have the movement (faster velocity) that we expect, it seems that there is some mass pulling the starts, but when we count all the (visible) mass in the galaxy it isn't enough or properly distributed to cause this effect, hence dark matter.

So if Dark matter exists, then the universe would have significantly more mass than previously believed. This would change the topology of space (make it more curved) but all the measurements show the universe to be flat. To make the universe flat again after introducing all dark matter then need to introduce Dark Energy, which basically counteracts the global effects of the dark matter, thereby saving the flattens of the universe.

Problem:
Cosmological theories generally have wildly inaccurate experiments. So we are supposed to believe that they measured the velocity of the outer stars accurately enough, and then counted all the matter in the galaxy accurately enough to say there isn't enough matter. Then because of the assumption of dark matter and limited measurements of the topology of space we come up with dark energy, some weird anti-gravity energy that makes the universe flatter and causes it to expand.

Bottom line: This seems like adding epicycles.

lpetrich

The situation is not as horrible as AdamWho seems to think. In fact, a nice consequence of the analysis of all the data sent back by the Wilkinson Microwave Anisotropy Probe is that we get a reasonably coherent picture of the overall architecture of the Universe, even if nearly all of its mass resides in various unknown entities.

"Ordinary matter" is about 4% of the Universe
"Dark matter" is about 23% of the Universe
"Dark energy" is about 73% of the Universe

And the Universe's 3-space is essentially flat, though space-time as a whole is still curved.

Here is a nice article on that subject from 1995.

The first hint of dark matter came from observing galaxies. Their stars' orbital velocities ought to fall off much as the planets' orbital velocities do with increasing distance. However, they have nearly flat velocity curves, and this means that there is much more matter in their outer regions than can be accounted for by the stars.

v = sqrt(GM(r)/r)

flat v -> M(r) ~ r

Much the same conclusion comes from studying clusters of galaxies; these have even more matter than can be accounted for by their galaxies, at least as judged from how much matter is necessary to keep them from flying apart.

The Universe as a whole has even more mass than can be accounted for by its galaxy clusters; observations of distant supernovae indicate that the Universe is accelerating, which suggests that this extra matter has the odd property of a big negative pressure. This behavior is contrary to what one expects of swarms of particles, though some hypothetical elementary-particle fields can produce such negative pressure by being essentially flat over the size of the Universe. Thus, the name "dark energy".

And what are "dark matter" and "dark energy"?

The dark matter cannot be interstellar dust and gas, because otherwise it would cause much more obscuration than it is known to do. The main remaining "ordinary matter" possibility is Massive Compact Halo Objects (MACHO's), and there is evidence for their existence from what they do to light passing by them. However, there are not enough MACHO's to account for much of the dark matter.

This leaves Weakly Interacting Massive Particles, or WIMP's. An obvious one is neutrinos, but they are expected to be too light -- the Big Bang had left them with enough random kinetic energy to make them easily evaporate from early galaxy clusters. Much more massive WIMP's would not have that problem, and there are some elementary-particle theories like supersymmetry that offer some promising candidates. A good test would be lab detection of WIMP's, which may become feasible later this decade.

As to dark energy, it may be some odd leftover from very early in the Big Bang, but that's the most I'm willing to speculate about it.

Friar Bellows

Dark matter doesn't trouble me; it's just stuff we haven't directly detected yet. Dark energy is a bit unsettling, because of its seemingly odd properties, but only a bit.

Don't judge science by what appears on the pop-sci bookshelves of bookstore chains.

That's one way of inferring dark matter. There are many others. For example, the dynamics of stars within the local Galactic disk environment; the motion of galaxies within our Local Group; the extended x-ray emission from clusters of galaxies; gravitational lensing by clusters of galaxies of more distant galaxies; galaxy redshift surveys revealing large-scale streaming motions superimposed on the Hubble flow; all these observations imply some form of dark matter. There are more indirect studies: investigations into large-scale structure from peculiar motions and spatial distributions of distant galaxies coupled with N-body simulations; WMAP's observations of the angular power spectrum of the cosmic microwave background; inflation theories tend to support a flat a universe model. The best evidence for dark energy is in the observations of Type Ia supernovae (SNeIa).

No, you've got this wrong. The dark energy was (re)introduced to explain the accelerated expansion implied from the SNeIa observations. It has subsequently fit in nicely with (for instance) the WMAP observations which imply a flat universe.

By the way, "topology" is the wrong term to use. You could have a number of universes which have different topologies but have the same constant curvature. Best to stick to the term, "curvature".

Not true. We have the age of the universe down to about 2%, Hubble's constant to about 5%, the matter/energy density to about 2% and the redshift of decoupling to 0.1%. We are definitely in the era of high-precision cosmology. Of course, there are still large uncertainties in several key areas.

You don't have to believe: check out the papers and the data yourself. But I'll give you one example which shows we're on the right track. The dark matter implied from observations of nearby x-ray emitting clusters of galaxies has recently been identified and totally accounted for as hot intracluster gas. So at least one piece of the dark matter puzzle has been resolved. Hopefully more pieces will be resolved in the future (e.g. by direct detection of non-baryonic matter).

Seems, but is it? It's possible, sure, but can you point me to a theory which accounts for the data better than standard Big Bang + inflation? The day that happens, then maybe we can talk about epicycles.

Shadowy Man

Well, sort of. We currently have a standard cosmology theory. That theory has several input parameters to make the theoretical predictions fit the data. We have those parameters down to high precision now.

Friar Bellows

Thanks, Shadowy Man.

Shadowy Man

No problem.

I'm currently pretty skeptical about dark matter. It's pretty ad hoc and no one has a clue as to what it is. And not many people actually talk about that.

I commonly hear people invoke dark matter in conversation, but not much questioning about what it is. I think the dark matter problem is one of the biggest glaring problems in modern astrophysics.

I'm still pulling for MOND, but it's fun to root for an underdog.

lpetrich

To my mind, MOND is more ad hoc than dark matter, because MOND is difficult to make relativistic, while dark matter can come from several possible sources.

However, particle-detection experiments are approaching the sensitivity where they will be able to see certain sorts of WIMP's, if they are present in the expected abundances.

One interesting candidate is the Lightest Supersymmetric Particle (LSP). Its existence is expected from some supersymmetric extensions to the Standard Model of elementary particle physics, including the Minimal Supersymmetric Standard Model.

Although supersymmetry has the problem that we have yet to observe any particle that's the supersymmetry partner of some known particle, it has some theoretically attractive features, which explains the big bulk of supersymmetry literature.

The LSP is expected to be a neutral spin-1/2 particle ("neutralino") that is a mixture of the superpartners of the photon, the Z, and the neutral Higgs particles. This particle is expected to be very massive, with a mass of at least a few hundred GeV or so. This is just beyond the threshold of present-day accelerator technology, but upcoming accelerators like the LHC should be able to see it, if it exists.

This particle is expected to be left over from the Big Bang, of course; its low abundance would be the result of the Universe cooling below its rest mass before its annihilation mean free path passed the Hubble distance. Once it passes, then the particle will become unlikely to annihilate with itself over the lifetime of the Universe.

Answerer

Well, I thought dark energy is largely vaccum energy, wrong?

lpetrich

"Vacuum energy" is a common, but oversimplified description.

Judging from its density-pressure relationship, "dark energy" is likely some sort of scalar field that has an approximately constant value over all of the Universe, something like a universal "voltage value".

This is not theoretically unprecedented; there is something very similar in the favorite theory on why the weak interaction is weak. According to that theory, there is something called a "Higgs particle", a scalar field which has the same nonzero value everywhere. Other elementary particles interact with it, and the effect of this always-present nonzero value is to make them massive.

It should also be possible to produce excitations of this field, just as it is for other elementary-particle fields -- making "particles". And for the Higgs, this excitation mass is expected to be a few hundred GeV, which is just beyond current accelerator technology. Though such accelerators as the upcoming Large Hadron Collider ought to be able to produce Higgs particles, if they exist.

Shadowy Man

Well, I didn't say I thought MOND was correct, just that I liked rooting for the underdog.

Yes, MOND is ad hoc too. Both dark matter and MOND are ad hoc. But at least MOND fits the data with only one parameter. To fit galaxy rotation curves, you just pick the dark matter distribution that best fits the data.

Yes, the major problem is making MOND general relativistic. We will see. At least people are thinking about it.

It will be interesting to see what kinds of particles experiments like the LHC detect in the future.