# Dark matter/virtual particles in standard model?

1. Sep 10, 2007

### jnorman

dark matter/virtual particles in standard model??

where do dark matter, dark energy, and virtual particles fit in the standard model? if those three mystifying concepts are necessary to explain the universe and its operation, why are they not included in the standard model?

IMHO, all three of those things are pretty much akin to stating flat out that we have NO IDEA what is actually going on... i mean, cant we just say that we do not yet understand something, instead of wholesale making up stuff like dark matter and energy that cant be detected, or creating some fantasy world of virtual particles that from what i can tell are nothing short of pure magic...

2. Sep 10, 2007

### malawi_glenn

Well, this is why there is research about supersymmetry etc, massive neutrinos does not fit into SM either (If I remember correct). The current standard model is not finished. And what the dark matter and dark energy is, i am not sure that no one knows yet, we just know that it must exist more matter than what is "visible" i.e baryonic matter.

What books/resource have you considered?

Virtual partciels, you mean those as "force carriers" in the fundemantal forces? i belive that that is experimentally proven that they exists...

3. Sep 16, 2007

### Timo

Let's start with a little recapitulation what the three terms actually mean and where they originate from - corrections and additions highly welcome:

Dark energy:
This is the one I know the least about, but I'll tell you how I think things are: General relativity tells us, whithin some limits, how spacetime behaves in reaction to a given particle content. The equations describing this are the Einstein equations. But: There is one catch in the Einstein equations: They allow for an a-priori unknown contant (cosmological constant). For experiments within our solar system (and most probably also within our galaxy), this constant is practically zero - as determined by experiment. But if it's not exactly zero, it can still have a tremendous impact on larger scales (like the whole universe). Experiments show that taking this value to be zero is not consitent with experimental observations of the universe. Now, having some constant which just came in mathematically (similarly to an integration constant) that has no phyical interpretation other than "setting it non-zero makes the theory consistant with observation" is physically unsatisfying. Hence, there is the idea that possibly this addend to the Einstein equations is caused by some exotic particles. If this was the case, you'd not have a purely mathematical term in your equations but have physical interpretations for all appearing mathematical objects.

Dark matter:
Observations on galactic scale show that rotational curves of the stars are not consistent with the matter the stars have. For general relativity to be correct, there has to be some other matter we do not see through our telescopes. Let's face it: Why should we see all matter that's out there? We can see (some) suns because they shine light, but why should we see some moon-size objects floating around at some distant point in our galaxy? We just had to redefine the definition of a planet because we recently found so many objects that size in our own solar system! Of course, the issue is not that easy. If you take e.g. the assumption "it's freely-floating planet-sized objects" as a scientific assumption, then you can test it. Afaik, simulations show that this assumption is not consistent with models on galaxy formation. Other candidates like black holes and MACHOS were indeed found (that's what I was told by some professor interested in astrophysics, at least) but considered not to contribute a sufficient amount of matter. Another assumption is that that invisible matter does not come in the form of compact object but is some evenly (it's not exactly evenly, for our galaxy I've seen predictions that their density peaks at some distance from the center of the galaxy) distributed particles. A problem with SM particles I'd see is that except for the neutrinos, they all interact with light which is contrary to the idea of dark matter (I don't know the experimental limit, but conceptually it's not too hard understanding that particles that interact with light are -at least in theory- visible). Bottom line: Dark matter is just some form of matter that we cannot experimentally see (yet).

Virtual particles:
The one I know most about and hence can make the story quite short: "Virtual particle" basically is the name for a mathematical factor appearing in calculations in particle physics when using a certain calculation technique. The name "virtual particle" is fitting e.g. because in a pictorial description (Feynman diagrams) of the appearing mathematical terms, the factor looks similar to a particle. Let's give an example: Take the function $$f(x) = x + \sin x$$. I could say it consists of the functions $$g(x) = x$$ and $$h(x) = \sin x$$ and call h a "virtual function". On the other hand, f is simply f and my g and h are just arbitrary inventions of mine and don't exist in f. Either way, there is certainly nothing mystical about the sine of x.

Ok, and now for your actual question:
- Dark energy has some exotic properties which almost guarantee that it's not described in the Standard Model. Seen from the narrowed-down perspective of particle physics: If the SM describes all currently-observed particle-physical observations, why should we include some strange terms to fit observations on gravitational interaction which the SM doesn't describe, anyways? In the end, it could really just be a constant without any particle-physics interpretation as a particle or field.
- I don't know to what extent particles predicted by the SM are ruled out to be dark matter. Some prominent extensions of the standard model predict (independently from the reasons leading to introduce dark matter into astrophysics) new particles that would well fit the requirement of dark matter. These extensions are not part of the SM because there is no particle-physical experimental evidence for them - finding those evidences (or proofs) are among the most important goals of future particle physics experiments.
- The question to what extent virtual particles exist or not depends on your understanding of the term "existance". Theoretical calculations and experimental results are absolutely clear on this matter and consistent with the SM (meaning that if they exist, every particle of the SM can be the base for a virtual particle).

I disagree. At least in the case of dark matter we actually have quite a few ideas of what might be going on. A very interesting thing is that completely different approaches from different fields (supersymmetry in particle physics) seem to give possible explanations of what this invisible matter could be. It is true that we don't know what is actually going on, but we do have ideas - most probably dozens of them. Note: By "we have ideas" I am not talking about metaphilosophical crackpot ideas like "everything consists of photons" but of actual mathematical models that are principially testable and consistent with past experiments.

Introducing dark matter and especially dark energy imho exactly is saying "we do not yet understand it". We know that for experiment to be consistent with theory, there must be something (on the most abstract level some mathematical terms) which must have certain properties. You should not misunderstand dark matter and dark energy to be necessarily assumed something mystical (ok, dark energy has some strange properties). Of course, there still is the simple alternative that the theory is wrong - which then comes with the not-so-simple problem to find a different theory that explains the experiments needing dark matter/energy and the whole lot of experiments that work fine without dark matter/energy in the old theory.

If dark matter is some for of particles predicted by BSM models (=models that extend the SM), then it might be detected on future collider experiments.

I hope that I cleared up your misconception (or total lack of understanding) of the term "virtual particle" a bit. It's pure mathematics, not pure magic.

In short:
- Virtual particles are a well-understood concept completely in agreement with experiment.
- To my understanding, dark matter and dark energy should not be understood as the claim that when we add some hocus pocus then everything is fine but rather as the admission that there is something we have not understood, yet. They are placeholders that hopefully will be filled at some time.

@Malawi_glenn: Finding explanations for astrophysical/cosmological problems is one (admittedly very prominent and inspiring) reason to look for physics beyond the SM. It's not the only one, though. There are a few (mostly aesthetical) reasons withing the SM to look for extensions/alternatives, already - the most obvious being that it does not seem to describe gravity.

Last edited: Sep 17, 2007
4. Sep 17, 2007

### arivero

In short:

- Virtual particles are the layman explanation of Quantum Field Theory, as they address both the force exchange and the vacuum field.
- The vacuum field from QFT is said to be one of the big failures of QFT in cosmology, as it does not predict the right density of energy, aka cosmological constant.

So it is very good point to ask what the virtual particles and the cosmological constant have to do. Although, to invoke the rest mass of the virtual particles is not a big start.

5. Sep 17, 2007

### arivero

Uuups! As this ending remark shows, I was answering another thread where the first posted asked if virtual particles can be argued as the cause of the dark matter. A partial answer is that they can be argued for the cosmological constant, and then for dark energy, and the prediction fails a lot of powers of ten.

6. Sep 18, 2007

### arivero

To be precise. I was answering to this entry:

https://www.physicsforums.com/blogs/bobdeloyd-89659/do-virtual-particles-have-mass-1086/

7. Sep 19, 2007

### George Jones

Staff Emeritus
Also, without dark matter, galaxies would have "boiled off" from clusters of galaxies.

Big bang nucleosynthesis gives further strong evidence that dark matter is not made from ordinary stuff like neutrons and protons, including black holes that have formed from the collapse of objects made of protons and neutrons.

After the universe cooled enough for neutrons and protons to freeze out individually, nucleosynthesis via nuclear fusion started. Because of the expansion of the universe, this stopped after a few minutes. The relative abundances of the elements produced during these few minutes is dependent on the absolute density of baryons at the onset of big bang nucleosynthesis. For matter that hasn't been too polluted by stuff like stellar nucleosynthesis, measuring the relative abundances of light elements now gives relative abundances then and thus absolute density of baryons then. Finally, running the expansion of the universe forward from the time of big bang nucleosynthesis to now gives the absolute density of baryons now. This density is too low to explain the motions of stars in galaxies, the motions of galaxies in clusters, and the fact that cosmological observations seem to show that universe is (almost) spatially flat.

My view on dark energy.