Do we know what Dark Matter/Energy is?

[Martin]

canuck100 wrote:

Synopsis:
Dark Energy
Known?
70% of the universe is dark energy
it is responsible for the acceleration of the expansion of the universe.
Three kinds of theories attempt to explain it:

1. Einstein’s cosmological constant – i.e. vacuum energy, a property of space itself
2. a strange energy-fluid that fills space

~ fluid of virtual particles coming in/out of existence – this theory had serious problems
~ “quintescence” - a new kind of dynamical energy fluid/field
3. the theory of gravity needs to be modified - no gravitational theory modification has yet proven compelling

Yet gravity continues to complell us to modify it. Now that we know Newton's Theory of Gravity is ultimately, just a very good approximation.

[Chris Peterson]

Yet gravity continues to complell us to modify it. Now that we know Newton's Theory of Gravity is ultimately, just a very good approximation.

Speak for yourself. I find a modified theory of gravity the least compelling explanation for our observations. GR as it exists is just too good at explaining nearly everything; a modification of it seems unlikely.

[canuck100]

no gravitational theory modification has yet proven compelling

Chris is right about the huge success of general relativity. Newtonian gravity is an excellent approximation for v<<c and other situations. GR didn't overturn Newtonian gravitation -- it refined it and improved upon it. It had to be this way because Newtonian theory is so successful within it's limitations.

Similarly, any proposal dealing with 'dark energy' that proposes changes to gravitational theory will have to refine and/or build on GR. From what I've read, most of the proposals involving changes to GR simply suggest that gravitational effects either change over time or over cosmic distances. But they don't offer any evidence to support such claims and most of these proposals quickly run aground in many other respects - and are not particularly successful at dealing with dark energy either.

Such proposals are an important part of the debate, though, because it is important to keep checking the theories that we rely on to ensure that they still hold. However, if I had to bet, I would bet on the cosmological constant idea for dark energy instead. It's so similar to what Einstein originally proposed and he was right about so many other things that were at first counter-intuitive.

[Martin]

Interesting terms,.."refine and improve upon".

Please note: I do not propose that a complete modification is required or that any specific modification will completely explain all observations. On the other hand, I strongly suspect that we will continue to "refine and improve upon" gravity in the future as we have done in the past. Do you not understand what this implies?

[Chris Peterson]

Interesting terms,.."refine and improve upon".

Please note: I do not propose that a complete modification is required or that any specific modification will completely explain all observations. On the other hand, I strongly suspect that we will continue to "refine and improve upon" gravity in the future as we have done in the past. Do you not understand what this implies?

I don't think it is accurate to say we have refined our theory of gravity over time. Newtonian gravitational theory was empirically derived; a formula was found that fit observations, with little or no underlying theory. And it is wrong, plain and simple. Nobody noticed it is wrong because until recently we lacked the tools to test it outside a very narrow regime, where its error was too small for us to detect. We did not modify or refine Newtonian theory at all in coming up with GR. In fact, GR is much less empirical, and falls naturally out of a comprehensive model of the structure of the Universe. Given GR, it is possible to see why Newtonian theory provides a close approximation in some cases. But the two theories are not structurally related at all.

It is a logical fallacy to argue that because we went from Newtonian gravity to GR, we are in any way likely to go from GR to something else. We may, or we may not, but past history provides no reason to believe either is more likely.

Unlike Newtonian gravity, GR is deeply embedded in many other theories that have nothing to do with gravity at all (such as quantum mechanics and particle physics). If we assume that dark energy is explained by some theory of modified gravity, it means we need to find an entirely new theory, which reduces to GR in the realm in which we have tested it- and that realm is very wide. That would seem a much more complex solution than simply recognizing a new property of spacetime, especially as such a property is actually explainable on several fronts.

In the end, it comes down to the fact that modifying gravitational theory is a less elegant, less parsimonious solution. That doesn't mean it is wrong, but it does suggest a good possibility of that, which is why few cosmologists find it a very compelling argument.

[canuck100]

We did not modify or refine Newtonian theory at all in coming up with GR. In fact, GR is much less empirical, and falls naturally out of a comprehensive model of the structure of the Universe. Given GR, it is possible to see why Newtonian theory provides a close approximation in some cases. But the two theories are not structurally related at all.

True. My statement was valid for Special Relativity, but not General Relativity.

For me, any dark energy theory has to align with GR to hold water. Potentially improving a gravitational theory does not prima facie imply anything. It depends on the nature of such an improvement and how it integrates with existing models. Einstein's thowing away of his cosmological constant and its re-emergence as an explanation for dark energy both potentially constitute a refinement of GR that does not imply anything shattering as far as the rest of GR is concerned as the value of this cosmological constant does not affect the underlying structure of GR.

[harry]

G'day from the land of ozzzzzzz

Having a Holiday is great.

In simple English where do you find most of the Dark matter/energy?

Most papers that I have read tend to place most around the centre of a galaxies and clusters of stars and filaments that join such compact objects.

[bystander]

In simple English where do you find most of the Dark matter/energy?

Most papers that I have read tend to place most around the centre of a galaxies and clusters of stars and filaments that join such compact objects.

I've always believed that dark matter is found in galaxies and galaxy clusters, basically the same places you find visible matter, while dark energy was fairly evenly distributed through out space, but I've been wrong before.

[Chris Peterson]

In simple English where do you find most of the Dark matter/energy?

"Dark matter/energy" is a meaningless phrase, as your slash implies some sort of connection between the two.

Dark matter is concentrated in halos around galaxies and galaxy clusters- in general, it is found where ordinary matter is found.

Dark energy is presumed to be a property of space itself, and is therefore is everywhere. Asking where you find dark energy is like asking where you find space.

[mishkin]

No we do not know what Dark Matter Dark Energy is, or even if they are. However, it is fun to imagine what and if they are.

[apodman]

Asking where you find dark energy is like asking where you find space.

What, if any, are the pitfalls of saying dark energy is space? Or at least that dark energy is part and parcel of space in the same sense that the dimensions x, y, z, and t are? Is this kind of thinking at all similar to calling it a cosmological constant? I need a picture.

[Chris Peterson]

What, if any, are the pitfalls of saying dark energy is space? Or at least that dark energy is part and parcel of space in the same sense that the dimensions x, y, z, and t are? Is this kind of thinking at all similar to calling it a cosmological constant? I need a picture.

I think that if you polled cosmologists, most would consider your second statement close to the mark. Kind of like asking where in the atmosphere you find oxygen. It's just a fundamental component. (Don't stretch the analogy too far... you can take oxygen out of the atmosphere, but I doubt you can take dark energy out of space.)

[harry]

G'day from the land of ozzzzz

Chris your idea of dark energy found everywhere is an ad hoc idea.

That type of energy is theoretical.

I tend not to seperate, both matter and energy since they are one of the same.

If you think that dark energy as you want to define it is found everywhere. Please shows the evidence.

This keeps on reminding me of the King and the invisible robes.

[Chris Peterson]

Chris your idea of dark energy found everywhere is an ad hoc idea.

That type of energy is theoretical.

I have no idea what you mean by "an ad hoc idea". Of course dark energy is theoretical- so is much of what we observe. You use "theoretical" like it's a bad thing. If you think that, you shouldn't be hanging around on a science forum!

I tend not to seperate, both matter and energy since they are one of the same.

There is an equivalence between them, but that doesn't mean they are the same. They exhibit different behaviors, and so need to be treated separately in different situations. Ice and steam are the same, too, but you can't always treat them as such.

If you think that dark energy as you want to define it is found everywhere. Please shows the evidence.

The evidence is precisely what lead to the theory in the first place: the rate of universal expansion appears to increase with distance, an effect that is pervasive throughout all space and all the observable Universe. That's a powerful observation.

[The Code]

If you think that dark energy as you want to define it is found everywhere. Please shows the evidence.

First they must discover the correlation between 95% anti matter and 95% dark matter/energy... and find out what they missed when anti matter meets matter?

Mark

[Chris Peterson]

First they must discover the correlation between 95% anti matter and 95% dark matter/energy... and find out what they missed when anti matter meets matter?

What does that even mean?

[The Code]

What does that even mean?

http://map.gsfc.nasa.gov/universe/uni_matter.html

http://www.scientificamerican.com/artic ... -24&page=3

Have a read Chris.... Then consider what i just said.

[Chris Peterson]

http://map.gsfc.nasa.gov/universe/uni_matter.html
http://www.scientificamerican.com/artic ... -24&page=3

Have a read Chris.... Then consider what i just said.

Sorry, I see no connection. The first article simply describes the energy budget of the Universe, and possible candidates for both the dark matter and dark energy components. The second article explains what antimatter is, and the likely reason for the observed asymmetry between the amounts of matter and antimatter.

None of this remotely explains what you meant by "First they must discover the correlation between 95% anti matter and 95% dark matter/energy... and find out what they missed when anti matter meets matter?"

[The Code]

Sorry, I see no connection. The first article simply describes the energy budget of the Universe, and possible candidates for both the dark matter and dark energy components. The second article explains what antimatter is, and the likely reason for the observed asymmetry between the amounts of matter and antimatter.

95% matter was annihilated by anti matter in the early universe,,, The universe is made up of 23% dark matter and 72% dark energy.... What other process could explain these percentages? other then something been missed when anti matter meets matter.

[The Code]

95% matter was annihilated by anti matter in the early universe,,, The universe is made up of 23% dark matter and 72% dark energy.... What other process could explain these percentages? other then something been missed when anti matter meets matter.

That go over your head chris?

[bystander]

95% matter was annihilated by anti matter in the early universe

Where did you get this figure??? The makeup of the early universe is completely unknown. 99.9999999% of ordinary matter could have been annihilated. We don't know. All it took was slightly more ordinary matter than anti-matter for matter to dominate.

The universe is made up of 23% dark matter and 72% dark energy

That's just a rough estimate, but probably close. What does that have to do with anti-matter?

What other process could explain these percentages? other then something been missed when anti matter meets matter.

What are you talking about??? What do you think you know???

Did that go over your head, Mark?

[The Code]

What are you talking about??? What do you think you know???

What 95% of matter got turned into half matter and half nothing? that can not be seen,, but can influence the universe? like anti matter?

[Chris Peterson]

95% matter was annihilated by anti matter in the early universe,,,

That's not likely. In fact, the initial imbalance needed to only be about one part in a billion in favor of ordinary matter. So virtually all of the matter that has antiparticles was annihilated in the early universe, producing photons (a well understood process) and eliminating antimatter from the Universe (except for a few stray particles, perhaps).

The universe is made up of 23% dark matter and 72% dark energy.... What other process could explain these percentages? other then something been missed when anti matter meets matter.

Why is a process required? These may simply be initial conditions. In any case, dark energy has no connection to either matter or antimatter, and dark matter probably doesn't have antiparticles. So it is likely that only the ordinary matter, now representing about 5% of the universal energy budget, is all that was involved. So the early annihilation may help explain the relatively small amount of ordinary matter compared with dark matter and dark energy, but it doesn't explain in any way the ratio between the latter two.

[canuck100]

These ideas of 95% of matter was destroyed in the early universe and so on get tossed around out of context. There is nothing magical about someone's loose quote about destruction of matter in the early universe and the fact that our present universe is made up of 4.7% baryonic matter.

The annihilations referred to at the end of various big bang epochs do not have specific % assigned to them. However, Scientists have a good handle on matter-antimatter reactions and none have yet violated conservation of energy or required dark energy or dark matter to explain. In fact, our understanding is developed enough that the above mentioned Scientific American article says that NASA is studying the feasibility of using such reactions as rocket fuel for interstellar exploration.

Not enough is known about the details of either of these dark entities to fit them in precisely with BBT. LambdaCDM says only that dark energy is dominant from the end of the inflation epoch (which means that matter-antimatter is not responsible for its creation BTW) and that dark matter is dominant from the beginning of the Matter Epoch so that it comes into play for subsequent structure formation.

The NASA article mentioned above comes down pretty firmly on the side of dark energy being regarded as a cosmological constant.

Neither of these articles support the speculative claims being made in this thread.

Furthermore, the properties of dark energy are such that it may not turn out to be energy in the E=mc^^2 sense. THis is why I wish that a different term had been chosen for this entity.

In any event, regarding dark matter and dark energy as somehow interchangeable or identical because of the equivalence of normal matter and photon energy is semantics arising from a poor choice of terminology -- it is not a scientific argument.

It's fun to speculate about the origins of the universe and about ideas and concepts in the forefront of research into the as yet unexplained. Folks are entitled to their beliefs that may arise from such speculations. Aspects of such speculations may indeed turn out to be a part of some future scientific theory. However we need to be clear that such speculations are not science.

[Chris Peterson]

Understanding phase transition in Nuclear matter gives us a bit of understanding of matter/energy understanding.

and yes I have read this paper. This will give you some idea, although limited.

http://arxiv.org/abs/0907.4065
Quark Phase Transition Parameters and $\delta$-Meson Field in RMF Theory

This paper has absolutely nothing to do with dark matter or dark energy. Nuclear matter phase transitions have nothing to do with dark matter or dark energy.

FYI, the paper is about phase changes that might occur in hadrons (particles made of quarks) under the extreme pressure found in compact objects like neutron stars. Hadrons are not candidates for dark matter, and dark energy is unrelated to any discussions of matter.

It is not sufficient simply to read the papers that you list; some understanding of their content is also desirable, otherwise you contaminate the discussion with non-topical material.