In December 2014 I had asked a question about the nature of the big bang and presented the following analogies: “So in the former of the two beliefs, an analogy would be a tiny collection of matter in a giant space called the universe, in which the matter is distributed. The latter case would be analogous to the universe itself (and its matter (and everything else, including light and time)) being a small “rubbery”, stretchy ball which explosively starts to expand.”
In the ensuing discussion I learned that the latter case, above, was correct except that the big bang is not a literal explosion.
My next question is: In simplified terms, is dark matter believed to be the rubbery, stretchy ball (for analogy purposes only) material within which all perceptible matter resides or floats or is suspended? In other words is it the actual material of “empty space” that we have not yet found or observed?
No dark matter does not have a uniform distribution but is clumped around galaxies so it is not the same thing as space. The first evidence for dark matter was that galaxies must have more mass than is indicated by their visible stars – hence dark matter.
By some definitions, you are sitting on top of a big ball of dark matter right now. If it’s not glowing ridiculously hot, regular matter is dark matter, in the sense that you wouldn’t be able to see its presence directly in another galaxy.
There are reasons why we think that there is a lot more dark matter than can be accounted for as “ordinary matter that isn’t glowing white hot right now”. People talk about non-baryonic dark matter, which comprises hypothesized unknown particles that interact gravitationally, but not electromagnetically. These particles would not be any more exotic than neutrinos are (but probably more massive).
Long story short, hypotheses of what dark matter is are usually “more of the same stuff we already see, just a little different” rather than “fundamentally new type of thing”.
Having two negative answers to my question, I now have another question. Once again assuming that everything we see, and don’t see, in the universe is all “space-time stuff” that has been expanding since the big bang. Is the “non-obvious” matter, or empty space, or vacuum (I apologize for the terminology. I’m clearly a layman (though very curious and interested) wrt this.) considered by physicists to be “something”?
It is difficult to understand what you’re getting at as I feel there might be a fundamental misunderstand going on here. Obviously whether the vacuum is “something” is too vague to have a proper answer. However in general relativity, due to spacetime curvature the vacuum does have properties of its own. Compare to Newtonian physics where the vacuum is merely a featureless background for matter to reside/fields to be define on. To explain in a little more depth:
In general relativity the Einstein field equations relates spacetime curvature to the stress-energy. The stress-energy is a tensor field (just think of a tensor as an extension of the concept of a vector)that covers the whole of spacetime, the value of its components at any given point gives you a lot of information about matter at the point: i.e, its energy density, momentum density, pressure and shear stress. It is easy to identify vacuum regions of a spacetime as components of the stress-energy tensor will be zero at all points in a vacuum.
So any point in space containing normal matter will have a non-zero stress-energy tensor at that point, similarly any point in space containing dark matter will have a non-zero stress-energy tensor. Any point that is in a vacuum will have a zero stress-energy tensor.
In addition to the curvature related to the stress-energy we can give everywhere a natural curvature by introducing a cosmological constant. Dark energy is usually modeled as a cosmological constant. However we can also translate the contribution of the cosmological constant as being a contribution from the stress-energy tensor describing a perfect fluid covering the whole of spacetime with a non-zero energy and negative pressure.
(Ignoring the cosmological constant) The lack of stress energy would suggest that the vacuum is flat a featureless in GR which is not the case. We know in GR that gravity is spacetime curvature and we also know that gravity works even in a vacuum. So how can gravity work if there is no curvature in a vacuum? The answer is that the Einstein field equations don’t tell the whole story in regards to curvature. There is a type of curvature not described by the Einstein field equations and in a vacuum how this curvature manifests itself will be dependent on the boundary of the vacuum (e.g. the boundary with a gravitational source or the boundary at infinity)This means also means that the vacuum can have its own dynamic properties such as gravitational waves.
In cosmological solutions though the whole of space is assumed to be full of matter of some sort, so we don’t encounter vacuums. However the spatial part of the metric in these solutions is not just a featureless background either, it has curvature, it is dynamic.
Dark matter isn’t expected to have negative mass, because it acts gravitationally as if it has regular old mass. Dark matter in our galaxy acts to attract matter in our galaxy as if it were regular matter, and if it had negative mass it would repel matter.
While negative mass is theoretically possible, in the sense that we can imagine it, there’s no indication that any particles with negative mass exist in our universe. There are no known physical phenomena that could be explained if we postulated matter with negative mass, rather the opposite. There seems to be too much mass in the universe that acts gravitationally than we can explain by the normal stuff we see, like stars and planets and dust clouds. And so we imagine that a possible explanation is that there is matter out there that we can’t see, that could be present in our solar system only because it does not interact electromagnetically we can’t detect it.
I heard of the possibility of negative mass before negative energy and negative mass began to be widely considered. The proponent of the idea suggested the possitive mass fraction of the universe may represent a mere layer of foam floating on an ocean of negative mass stuff.
If the main observable characteristic of negative matter is its gravitational interraction, then it has nothing to do with the concept of negative mass.
But to answer the second question (regarding the apparent vacuum that most of the universe is made of), could we deem the entire “empty space” as endowed with potential energy, on which lies afloat a thin sheet of visible stuff held together by the guessestimated rubbery dark energy?
Dark matter isn’t too mysterious. We think we know what sort of thing it is; we just don’t know the details: As leahcim says, dark matter particles probably aren’t any more exotic than neutrinos, we just haven’t found any directly yet.
Dark energy, however, is. We can enumerate some of the properties it must have to account for our observations, but we’re worse than clueless as to just what sort of substance of phenomenon would have those properties. I say “worse than clueless” because we do actually have one clue: Most models of particle physics predict that the vacuum itself should have an energy associated with it which would have the right qualitative properties. But all estimates from particle physics of the strength of this energy put it at over a hundred orders of magnitude stronger than what we actually see in the Universe. Now, these are only estimates, and it wouldn’t surprise anyone too much if there turned out to be both positive and negative contributions which exactly canceled out… but for them to almost cancel out, to over a hundred decimal places, but not quite entirely is nearly inconceivable.
“Dark matter” is a single term, as is “dark energy”. Don’t try to interpret them by breaking those terms into their components, or by relating the word “dark” in both of them. We could as easily have called them “phlogiston” and “ether” (and in fact, one near-synonym for “dark energy”, “quintessence”, is in fact an old word for ether).
Thanks for all the responses and I apologize for not acknowledging earlier. And if I should start a new thread, mods (or anyone else) let me know.
So if dark matter isn’t all of the other stuff that we don’t really see, does that stuff (the material on/in which matter, light, particles etc (or the stuff we generally call “space”, or even “ether”) resides/floats/sits) have a specific, scientifically accepted name. If you think of the rubbery membrane that people use as an analogy for the space/time continuum, that represents the material I’m asking about.