Dark Matter and Dark Energy

From this article

“Despite solid observational evidence for the existence of dark matter, its nature remains a mystery,” Derevianko, a professor in the College of Science at the University, said…

Modern physics and cosmology fail dramatically in that they can only explain 5 percent of mass and energy in the universe in the form of ordinary matter, but the rest is a mystery.”

I have a hard time with dark matter. To my knowledge, it’s only a theory to explain observations of universe expansion accelerating. Yet, we’ve designed many tests to evaluate, measure, and understand it with no results…at all. No direct observational evidence, despite what the article suggests. And nothing for dark energy, either, which is supposed to account for even more of the “missing” mass/energy of the universe.

What evidence to we have this stuff exists? It seems to me that many of our observational measurements of the universe don’t have a real benchmark - if we even minor errors in our measurements, constants, or assessment of the spectrum shift of moving objects or distance over the scale of the distances we are observing I would imagine the error could be significant. Are we really sure enough in these observations to theorize there is much more matter in the universe than we can observe?

Actually, that’s dark energy.

The observational evidence for dark matter is the rate at which spiral galaxies and galaxy clusters rotate - which requires far more gravitational mass than can otherwise be detected.

For dark matter, the galaxy rotation curves are the most intuitive evidence, but wikipedia provides a list of other more-or-less indirect observations. None of them are “here’s a lump of dark matter than I can put on my desk and use as a paperweight”, but altogether they are pretty conclusive.

In a way, dark matter is “obvious” in that human beings are made out of one kind of dark matter. But there are reasons to believe that most dark matter is non-baryonic dark matter, which is the interesting kind.

“Dark Energy” is a much more vague term, and is, pretty much, “the universe’s expansion is accelerating, and here is a reason why that might be”.

First off, the phrase “only a theory” is meaningless in a scientific context. When an idea is a theory, it means that we’re about as certain about it as it’s possible for us to be about anything. There is no level above “theory”.

Second, it’s correct to say that dark energy is really just a label for our ignorance. We don’t have any clue what it is, or how it has the properties it does. We know that the expansion of the Universe is accelerating, and logically there must be something causing that acceleration, and whatever that something is, we call it dark energy. Dark energy cannot be wrong, because the idea isn’t yet developed to the point where it could be wrong. You can’t just say “It’s not dark energy, it’s something else”, because in that case, the something else would be dark energy.

“Dark matter” merely describes all of the non-radiating matter. We’re (mostly) dark matter ourselves insofar as our minimal radiation (primarily infrared) cannot be seen at long distances. What is troubling that in order for general relativity to work with observations about the rotation of our galaxy and others, there must be a large amount of missing dark matter (MDM) that we can’t observe either directly (we can’t see it) or indirectly (it isn’t screening and absorbing electromagnetic radiation); hence, they hypothesis that MDM is primarily composed of some kind of matter that only weakly interacts with electromagnetic fields.

Some small amount of that material can be identified as “hot” (very fast, high mean free path) dark matter like neutrinos (which so weakly interact with normal matter and EM fields that of the billions from the Sun flying through every cubic centimeter every second we can only detect a few hundred a year using enormous apparatus like AMANDA) adds to the equation but can’t explain the overall behavior we observe in the galaxy, so there must be some kind of “cold” (slower, mean free path much shorter than the diameter of the galaxy) dark matter driving this. The leading hypothesis are called WIMPs (weakly interacting massive particles…yes, physicists need to work on better nomenclature) which are very massive but can’t be seen and due to their distribution don’t have a very pronounced effect on local structures and fields.

It should be noted that all astrophysicists and cosmologists regard the label (missing) dark matter and even proposed particles like WIMPs as merely being placeholders for a more rigorous explanation. (I’ve personally recommended [POST=6294452]renaming “dark matter” and “dark energy” to atramentous corporeity and crepuscular palpitation, respectively.[/POST]) Nobody thinks that slapping a label on the observation “solves” it in any material way, and there is certainly the possibility that the true explanation may revolutionize general relativity in the same way that the Michaelson-Morley experiment, seeking the “luminiferous aether”, gave way to the basis for special and then general relativity, though the threshold for overthrowing the current paradigm with its mass of confirming observations is certainly much higher today.

Stranger

I still miss phlogiston.

It’s not just that. We have a very good handle on how much baryonic matter (matter that gets most of its mass from protons and neutrons) there is in the Universe from the relative abundances of various low-mass isotopes (deuterium, helium-3, several lithium isotopes, etc.). These were all formed in the very early Universe, and how much of each there is depends on how common baryons are. But this amount falls well short of the mass we have that’s producing gravity, so most of the matter in the Universe (and hence most of the dark matter) must be non-baryonic.

And really, even aside from the multiple lines of direct evidence, it’d be quite surprising if there weren’t a significant amount of non-baryonic dark matter. Different particles are subject to different forces; why shouldn’t there be a few out there that aren’t subject to the weak force? Heck, we already know that there are some, the neutrinos: Why shouldn’t there be more?

I very much like the explanation that Pedro G. Ferreira, a professor of astrophysics, gives in The Perfect Theory: A Century of Geniuses and the Battle over General Relativity.

Chapter 3, “Correct Mathematics, Abominable Physics,” starting on page 28, outlines the cosmological constant. Although only the first six pages are given online, they cover the reasoning behind it and why others quickly realized what the equations really implied.

I think you mean electromagnetic. Neutrinos are subject to the weak force.

Yeah, but “dark matter” and “dark energy” sound way more likely than “atramentous corporeity” and “crepuscular palpitation” to somehow wind up giving you superpowers if you study them.

And how has that worked out so far? One giant green rage monster, a guy who spends his time in a run down gym literally beating the stuffing out of punching bags because he can’t cope with pop culture, a manic-depressive self-aggrandizing “billionaire, genius, playboy, philanthropist”, and a couple of alien invasions from another dimension that first destroyed a small town in New Mexico and then a few tens of billions of dollars of real estate on the island of Manhattan later, and I think we can all agree that superheroism is not exactly all it is cracked up to be.

Or were we talking about Batman? 'Cause, he’s just kind of a dick, and he didn’t even need to have superpowers bestowed on him to get there; just the tutelage of one Liam Neeson and a shitton of money that he inherited and squandered on cool toys rather than follow in his father’s footsteps and continue good works for the people of Gotham.

I’m sorry, what were we talking about? Oh yeah, cosmology. Also not all it is cracked up to be. Basically something unexpected is going on and we don’t know exactly what but we can make a couple of placeholders that seem to fit the bill for no especially good reason. Which is how science works; not with a “bang”, but with the whimper of a researcher going, “That’s not how the data was supposed to come out! Five years of research, my PhD thesis, and any hope of getting that post-doc position at UH down the toilet. Well, fuck. Let’s bowl.”

Stranger

I’ve said before that “dark matter”–and especially “dark energy”–reminds me a bit of the “epicycles” in the old heliocentric model of the Solar System. (And I’m quite sure the astronomers and cosmologists would dearly love to have some wonderfully elegant new model of…something; gravity, the Big Bang, whatever…that would neatly and in some sense simply account for all our observations of reality in some way that would make everyone–or at least all the people with a good grasp of physics and advanced mathematics–go :smack: and say “Oh, of course the expansion of the Universe is accelerating, because [???]”.)

Oops, yeah, that’s what I meant.

How seriously would a hypothesis be taken that allowed a bunch (most) of an object’s mass to be unobservable due to it being in a dimension beyond our perception? Using Flatland as a model, it’s fairly easy to believe only a tiny fraction of, say a 4-dimensional, object’s mass is available to us. What we call dark matter would simply be “the rest” of matter that we can’t get at. And dark energy would not be nergy, so much as normal motion explained by physical attachment to stuff we can’t perceive.

If extradimensional presumptions are typically rejected, why is that?

But we can measure the gravitational effects of normal baryonic matter, such as stars and planets, and we have a pretty good idea of how much gravity we’d expect if we dumped X petagrams of protons or neutrons or whatever in a pile.

So it’s not like stars or other everyday objects have a lot more gravity than we expect. Or rather, if stars had a lot more gravity that we expect, we’d expect them to have the amount of gravity we would measure them to have.

So whatever “dark matter” is, it isn’t stars or planets or dust or lone hydrogen atoms in interstellar space.

If dark matter was extra gravity generated from normal matter that we can’t normally experience, why would it affect the galaxy? If it’s off in some other dimension where it can’t be accessed, why doesn’t it stay there?

How about extradimensional matter that DOESN’T protrude energy or mass into our realm? Instead, it warps spacetime in such a way that galaxy-sized structures form wherever it isn’t, much like irrgularities in the ground create puddles? In a puddle, water comes together because it is effectively pushed together by outside conditions, not because of any particular attractive force between the molecules. Maybe gravity doesn’t HAVE to explain galactic behavior?

It certainly is possible that the explanation for “dark matter” isn’t that galaxies have lots of weakly interacting massive particles–particles that interact gravitationally but not electromagnetically. Instead there’s some other force that smooshes galaxies into shape. But why does such a force act exactly the same as extra gravity? Hey, maybe it’s because intergalactic space is filled with negative-gravity particles and reverse-negative gravity is acting exactly the same way as regular gravity, just like light is really the blinding absence of darkness.

But that doesn’t seem likely. The simplest explanation for why galaxies seem to have extra mass is that they do in fact have extra mass. The only problem with this explanation is that we can’t directly detect this mass, since it only interacts gravitationally and gravity is so much weaker than electromagnetic interactions that they are almost impossible to detect.