What is the Nature of Dark Matter and How Are Scientists Trying to Discover It?

[Fluxman]

It's extremely confusing to make comments like "dark matter is 100% pure gravitons," without putting this in context.

Firstly, I should remind everyone that idle speculation is not permitted on these forums. If in doubt, please check the guidelines in my signature. Secondly, I presume that obione is talking about some sort of braneworld scenario, whereby matter on an adjacent brane acts gravitationally with matter on our brane, thus looking like some sort of dark matter, however I would kindly request that obione both explains his thoughts in more detail and provides us links to published, peer-reviewed articles on the topic.

You are right about the mass of the graviton: it is a hypothetical particle that carries the gravitational force, and thus must be massless.

Is the amount of matter in the universe now equal or very close to equal the amount of matter at the beginning of the universe?

 

[Phy6explorer]

Matter is the substance of which physical objects are composed of. I find no meaning in your question!Matter cannot be physically measured, so the word"amount"is inappropriate.

 

[malawi_glenn]

Phy6explorer: Depends on how you define matter. On for example distinguish from the radiation domiated universe and matter dominated universe and so on.

 

[Phy6explorer]

You are quite right, but how else can we define matter?

 

[malawi_glenn]

You are quite right, but how else can we define matter?

leptons and baryons, is called baryonic matter.

 

[Phy6explorer]

But that is to be specific. But in general they are matter and matter is of what physical objects are composed of, isn't it? But I am saying this with reference to what Fluxman asked,if in general I am certainly wrong.

 

[malawi_glenn]

But that is to be specific. But in general they are matter and matter is of what physical objects are composed of, isn't it? But I am saying this with reference to what Fluxman asked,if in general I am certainly wrong.

In plain physics language, matter = baryonic matter.

Energy is constant in the universe, so it is a accepted question to ask if there was more matter bakc then with respect to today- since kinetic energy is not matter.

Photons are not regarded as matter in cosmology either, at least I have encountered thoses definitions in my studies, perhaps there are other conventions.

But most of the time, one knows what the term "matter" is referring to, at least cosmologists ;)

 

[Phy6explorer]

In plain physics language, matter = baryonic matter.

What about the exceptions of Non-baryonic matter such as neutrinos, that is, free electrons, black holes and dark matter?(For more info on dark matter go to "Dark matter-What is it" in the same forum.

 

[malawi_glenn]

that is why you introduce dark matter, you separate it from the ordinary matter, i.e the baryonic matter.

I know quite much about dark matter, and this IS the thread "Dark matter - what is it"..

I just wanted to stress that Fluxman's question is correct, since the total energy is the thing that is conserved, matter is not conserved. And if you want to call radiation matter or not is a 'matter' of taste. In cosmology, one usally distinguish from matter and radiation.

 

[Phy6explorer]

Oh,I got too involved and I forgot that this is the Dark matter thread.Now I have my doubt clarified.But why did people start using the term baryonic matter?

 

[Phy6explorer]

I meant no rudeness.I mentioned the place where we get info about DM b'cuz sometimes when I mention DM without any bacground info people get a bit disinterested.

 

[malawi_glenn]

Oh,I got too involved and I forgot that this is the Dark matter thread.Now I have my doubt clarified.But why did people start using the term baryonic matter?

recall that also electrons (in atoms and plasmas) are under the baryonic matter definition. It is a historical name which is not 100% logic (as many other things in astrophysics). I have no idea why and when one introduced the term.

But the definition today is that baryonic matter is particles that obeys a certain density relation with respect to time and shell factor. Photons and neutrinos does not fall under that definition.

 

[Phy6explorer]

Doesn't that mean we ar also baryonic matter?

 

[malawi_glenn]

Doesn't that mean we ar also baryonic matter?

well of course, has anyone said something else?

 

[Fluxman]

Matter is the substance of which physical objects are composed of. I find no meaning in your question!Matter cannot be physically measured, so the word"amount"is inappropriate.

Well, no disrespect intended, but I guess I am going with the idea that the quantity of a collection of matter can be measured by its mass.

 

[scupydog]

Well, no disrespect intended, but I guess I am going with the idea that the quantity of a collection of matter can be measured by its mass.

Perhaps DM should be called Dark Mass, since it is mass that is supposed to be missing from the critical density.

 

[Kurdt]

Protons, neutrons and electrons are considered baryons to a cosmologist. Neutrinos and radiation make up relativistic material (unless neutrinos have sufficient mass). Dark matter is non-baryonic, just to summarise what malawi glenn said.

 

[nikiforos]

What is Dark Matter?

Dark Matter must exist because too many parameters (some very fundamental) and many observations depend on its existence. Having said that: What is it?
There is a cosmology hypothesis (very extensive and several decades old “CCA”) from which one can calculate a plethora of parameters and including the determination of how much DM is there in the universe and a possibility for what DM is. {For example, by the CCA hypothesis, the DM states are, unlike any up to now hypothesized state, a stable member of a family of “archaic” particles (identified and well defined, all bosons) that are “totally blind” to “all but” gravitational field (perhaps also to a scalar). They are not of leptons or quarks but actually in the evolution of the universe, are “progenitors” of both leptons and quarks. These DM stable states currently have a number density ½ that of baryonic matter. If this is valid, then their “equivalent rest mass”, mo, from the above DM cosmic value and Nucleosynthesis and WMAP inferred age can be found from the amount of DM (Mdm) in the “accessible” universe which is:

Mdm = 6.41x10^55 g. and its “equivalent rest mass”, mo ~ (Mdm)/(3.5x10^78) = 1.82x10^-23 g. This is about 11 times more massive than the proton.
(From this, CMB details can also come about and can be calculated).

Why not discovered? Among other peculiarities, not “easily produced”. Very weak interacting (if these statements so far have not shocked everyone, the next statement, no doubt, will) “photon-blind antimatter”; no annihilation, but a “destructive” interaction shows photons up to energies of neutral kaon and neutral D-meson decays. They also, have cosmic behavior that causes non-linear oscillations (fast, across strong gravitational field, “hot” regions) to form (slow moving) concentrations at the most, weak field, “cold” regions of large structures. This means small number densities within visible matter concentrations}
Just to show that this is not, ad hoc and how the rest of the components of the universe fit with this, the densities D (for the total critical, the Dark Energy (vacuum energy) Dark Matter and Baryonic Matter respectively) are calculated and given to be compared with observations.
Dc = Dde + Ddm + Dbm
9.60 x10^-30 = 7.0 x10^-30 +2.16 x 10^-30 + 4.16 x10^-31 g-cm^-3
For the total critical value of omega = 1, or (100%)
100% ~ 73.6% + (22.2% + 4.3%)
and can be compared to the published values.
How does this fit with GR? The Ricci Tensor cannot vanish (no such thing as “empty space”). A different metric (non-Minkowski, and with past-future asymmetry). The Equivalence Principle needs modification. Not a “pure” Cosmological “Constant”. Otherwise, GR is ok.

 

[gravity guru]

turbo-1, i like the way you think. here"s something to consider that may pertain to DM it"s only conceptual in nature and might only be enteresting and nothing more. if out in space away from objects a static charge was made to spin where would the magnetic field that it produces come from? might that be DM that is being gathered by the spinning charge and formed into a magnetic field around the charge? since DM lacks energy of it"s own perhaps space is filled with it in varying degrees of densities and is manifested only when acted upon by moving charge. just a thought. gravity guru.

 

[Phy6explorer]

I don't think DM needs to gather gravitational field.DM itself has gravitational properties!

 

[Poop-Loops]

I'm working on a Dark Matter project for my professor currently. Here is the website I made for the project:

http://www.phys.washington.edu/groups/admx/home.html

This is a candidate for DM that would also solve some other stuff. Personally, based on what I've found out about the project, there is a very slim chance of actually detecting one of these Axions because of how little range we can scan and how big of a range these particles can theoretically occupy.

The sad part is that I was at a lunch thing where someone who works on WIMPs was going to give a colloquium later in the day and he said the same thing about his project and pretty much all DM projects: that the chance of detecting something is very slim because pretty much everybody is just taking a shot in the dark as to where to scan.

EDIT: I should note that the website isn't very theory heavy because we wanted laypersons to be able to understand it without getting scared off. The bulk of the theory is in the publications section. I myself had to do quite a bit of research before I fully understood what an Axion was and why it was important, and even now I only understand it qualitatively.

 

[Orion1]

nikiforos, I request citations and references for all the variable numerical values you stated on post #39.

I presume that your numerical value of '3.5x10^78' from your 'equivalent rest mass' equation is equivalent to the number of protons in the 'observable' Universe?

I deduce that your parameters were derived from a observable steady-state Universe model and originated from the following equations:

Dark matter mass:
M_{dm} = \Omega_{dm} \cdot M_u
M_u - Universe mass

Universe composition: 0 \leq \Omega \leq 1
\Omega_{de} - dark energy
\Omega_{dm} - dark mass
\Omega_{bm} - baryonic mass

\rho_c = \Omega_{de} \rho_c + \Omega_{dm} \rho_c + \Omega_{bm} \rho_c
\boxed{\rho_c = \rho_c (\Omega_{de} + \Omega_{dm} + \Omega_{bm})}
\rho_c - Universe critical density
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