Identification of matter and dark matter proportions

[Ranku]

Is it possible to identify a volume of space, observationally/statistically, that represents the 85:15 % ratio of dark matter and matter?

 

[Vanadium 50]

What is wrong with "the visible universe" as the answer to this question? It sounds like you have something in mind that remains unstated.

 

[anorlunda]

I'm not sure about that exact number, but the whole idea of dark matter came from observations of the rotation rates of galaxies and galaxy clusters.

From : https://en.wikipedia.org/wiki/Dark_matter

Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Dark matter can explain the 'flat' appearance of the velocity curve out to a large radius.

 

[Ranku]

What is wrong with "the visible universe" as the nswer to this question? It sounds like you have something in mind that remains unstated.

….well, I was wondering suppose dark matter is lighter than matter (mostly hydrogen), how might that show up gravitationally in a representative mix of dark matter and matter?

 

[Ranku]

Since this line of questioning is not eliciting a response, let me ask a related question. Can the proportion of dark matter vis-a-vis matter in the universe be determined from cosmological perturbations, and acoustic oscillations peaks profile?

 

[Vanadium 50]

I was wondering suppose dark matter is lighter than matter

A kilogram of dark matter is no lighter than a kilogram of hydrogen.

 

[Ranku]

A kilogram of dark matter is no lighter than a kilogram of hydrogen.

The way I am looking at it is this: Since we know the proportion of dark matter to matter in the universe, and if we were able to have a volume of space that is representative of the dark matter-matter ratio, then measuring the gravity of the proportion of matter we compare it with the gravity of the proportion of dark matter and see if the gravity of dark matter matches the gravity expected of the proportion of dark matter. If it is less, then that would mean that dark matter is lighter than matter.
This approach however requires that determining the proportion of dark matter in the universe is derived independently of its gravity. It seems one way to do this would be by subtracting the proportion of matter and dark energy from Ω = 1.

 

[collinsmark]

The way I am looking at it is this: Since we know the proportion of dark matter to matter in the universe, and if we were able to have a volume of space that is representative of the dark matter-matter ratio, then measuring the gravity of the proportion of matter we compare it with the gravity of the proportion of dark matter and see if the gravity of dark matter matches the gravity expected of the proportion of dark matter. If it is less, then that would mean that dark matter is lighter than matter.
This approach however requires that determining the proportion of dark matter in the universe is derived independently of its gravity. It seems one way to do this would be by subtracting the proportion of matter and dark energy from Ω = 1.

The argument seems a bit circular to me.

We estimate the proportion of (baryonic) matter to dark matter by their gravitational influence. Your statement, "since we know the proportion of dark matter to matter" is already based on gravitational influence.

 

[Ranku]

The argument seems a bit circular to me.

We estimate the proportion of (baryonic) matter to dark matter by their gravitational influence. Your statement, "since we know the proportion of dark matter to matter" is already based on gravitational influence.

Which is why I mentioned: This approach however requires that determining the proportion of dark matter in the universe is derived independently of its gravity. It seems one way to do this would be by subtracting the proportion of matter and dark energy from Ω = 1.

 

[Vanadium 50]

I don't understand what you are saying. But you can't say dark matter if lighter or heavier than ordinary matter. You can say you have more or less of it.

 

[Ranku]

I don't understand what you are saying. But you can't say dark matter if lighter or heavier than ordinary matter. You can say you have more or less of it.

Just as we have particle species which are heavier or lighter than other particle species, similarly we may wonder if dark matter is the lightest freely existing particle species. That’s all I’m saying.

 

[Vanadium 50]

That's not what your OP said. I think you need to work harder on clarifying exactly what you are asking.

 

[anorlunda]

Just as we have particle species which are heavier or lighter than other particle species,

Do you think that 85% of all particles are dark particles? When scientists say 85%, they are talking about mass.

 

[vanhees71]

I guess you refer to energy densities, expressed in terms of the $\Omega$-parameters in the $\Lambda$CDM concordance model of cosmology. It's about 5% "normal matter", 27% "dark matter", and 68% "dark energy".

https://en.wikipedia.org/wiki/Dark_energy

 

[Ranku]

That's not what your OP said. I think you need to work harder on clarifying exactly what you are asking.

The OP was about how to observationally determine the proportion of dark matter and matter. When you sensed something is ‘unstated’, I explained I was exploring how to observationally determine if dark matter might be lighter than matter.
I guess, on hindsight, if you simply flip the line of questioning, it becomes more logical. But then again that’s why it’s called hindsight

 

[Vanadium 50]

I explained I was exploring how to observationally determine if dark matter might be lighter than matter.

And it's not, in either sense of the word.

A kilogram is a kilogram
The total mass in dark matter is larger than the total mass of luminous matter.

 

[Ranku]

And it's not, in either sense of the word.

A kilogram is a kilogram
The total mass in dark matter is larger than the total mass of luminous matter.

As I clarified later in the thread, I meant lighter in the sense of dark matter being a lighter species of particles. Thus, even though the total mass of dark matter is more than matter, that mass can be constituted of a lighter species of particles.

 

[Vanadium 50]

OK, "Maybe". But your questions are moving all over the place.

 

[snorkack]

I'm not sure about that exact number, but the whole idea of dark matter came from observations of the rotation rates of galaxies and galaxy clusters.

From : https://en.wikipedia.org/wiki/Dark_matter

View attachment 313415
Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Dark matter can explain the 'flat' appearance of the velocity curve out to a large radius.

But note that if you do have such rotation curve, you might, in the case of spherical cow, simply convert this graph into graph of dark and light matter density, and their ratio!
The condition of orbiting is:
mv²/r=mMG/r²
from which simply
M=v²r/G
Only in case of spherical cow, or spherically symmetric mass distribution. Which spiral galaxies are not, and Gauβ Law does not apply to them
Going on with the said spherical cow, obviously ρ=δM/δr/4πr². So
ρ=δ(v²r)/δr/4πr²G
In case of spherical cow, deriving the light matter density, total density, dark matter density and their ratio should be trivial. In case of a disc galaxy, where Gauβ Law does not apply, it might be harder, but is it at all feasible? For me, explicit density profile of light and dark matter, rather than mere rotation curve, would be more illustrative.

 

[Ranku]

OK, "Maybe". But your questions are moving all over the place.

 

[ohwilleke]

The OP was about how to observationally determine the proportion of dark matter and matter. When you sensed something is ‘unstated’, I explained I was exploring how to observationally determine if dark matter might be lighter than matter.
I guess, on hindsight, if you simply flip the line of questioning, it becomes more logical. But then again that’s why it’s called hindsight

We have an observationally supported, although model dependent, estimate of the proportions of baryonic matter mass-energy, dark matter mass-energy, and dark energy mass-energy as a proportion of total mass-energy density in the universe.

We also have a good estimate (to one or two significant digits or so) of the total number of baryonic matter particles since we know the relative proportions of different kinds of baryonic mass particles (almost all protons and neutrons) and their respective masses.

We ordinarily don't think of dark energy as particles. We normally think about it as a field, and in the LambdaCDM model it is a fixed and universe energy density in every volume of space in the light-cone of the Big Bang. In some competing dark energy theories (that could explain the Hubble constant measurement disparity between CMB data and measurements based on observations of stars long after the Big Bang) the amount of dark energy per volume of space has varied at different times in the history of the universe.

We have a good estimate within the LambdaCDM model of the amount of dark matter that is out there by mass (which unlike dark energy, has remained more or less constant for the entire duration of the universe except immediately after the Big Bang).

But hypothetical masses for dark matter particles range from much less massive per particle than the average neutrino for axion-like particle (ALP) dark matter models, to asteroid mass clumps for primordial black hole dark matter candidates.

Dark matter particle mass is very important for direct dark matter searchers and gravitational lensing based searches for very high mass dark matter particles/clumps, but in efforts to determine the properties of dark matter from things like galaxy and galaxy cluster dynamics, dark matter particle mass doesn't matter much.

Instead, the property of dark matter that drives galaxy and galaxy cluster dynamics is mean dark matter particle velocity. High velocity dark matter particles are called "hot dark matter" and has been ruled out as an important component of dark matter. "Cold dark matter" particles are dark matter particles with a much lower velocity. "Warm dark matter" particles are much closer to cold dark matter particles than to hot dark matter but a little higher in velocity (and has a smaller mass that gives it quantum mechanical interactions not found in more massive dark matter particles).

The LambdaCDM Model of Cosmology doesn't really make a distinction between Warm Dark Matter and Cold Dark Matter. Both fit that model's definition of Cold Dark Matter.

There are lots of ways that dark matter could have hypothetically been created. One of the most popular theories is called "thermal freeze out" which hypothesizes that there is a statistical relationship between the mass of a dark matter particle and the temperature at which it "freezes out" as the average temperature of the universe falls after the Big Bang as the universe expands. In a thermal freeze out scenario particles with a mass that causes them to freeze out at a particular temperature acquire their mean velocity at the mean velocity of particles at that temperature.

In a thermal freeze out scenario, cold dark matter has particle masses of ca. 10 GeV to 1000 GeV, warm dark matter has particle masses of ca. 1 keV to 10 keV, and hot dark matter has a mass on the same order as neutrinos or less.

Dark matter candidates with particle masses of less than warm dark matter that nonetheless have warm dark matter or cold dark matter mean velocities (like axion-like particles) are hypothesized to come into being by means other than thermal freeze out, often with these particles being continually created and destroyed in a dynamic equilibrium.

Unlike dark energy, dark matter is assumed to gather into clumps, although not as tightly as baryonic matter does. In the dark matter hypothesis, galaxies, galaxy clusters, and cosmic scale strings of baryonic matter between galaxies arise from baryonic matter being drawn into halos of dark matter.

The inferred proportion of baryonic matter mass to dark matter mass that is observed, however, is not uniform even setting aside the large "voids" in space with very little ordinary baryonic matter or dark matter of any kind.

There is a tight relationship between the distribution and mass of ordinary matter in a system, and the properties that the system has which are attributed to its inferred dark matter, implying in the dark matter paradigm, a tight relationship between the ordinary matter distribution in a system and the dark matter distribution in that system. See, e.g., Pavel E. Mancera Piña, et al., "A tight angular-momentum plane for disc galaxies" arXiv 2107:02809 (July 6, 2021) (accepted for publication in A&A Letters). While a correlation between ordinary matter distributions and dark matter distributions is expected, it isn't entirely clear why the relationship between the two such a tight one.

Generally speaking, the dark matter proportion by mass of the total mass of the system is lowest in elliptical galaxies (with the least inferred dark matter in the most spherical elliptical galaxies and the most in the least spherical ones), is higher in typical spiral galaxies, and is higher still in low surface brightness dwarf galaxies and in galaxy clusters, although there are also a small number of dwarf galaxies for which one can infer from their dynamics that they have little or no dark matter.

These patterns of dark matter proportions by galaxy type were not predicted in advance by dark matter theorists, and are still not well understood in the dark matter paradigm.

 

[snorkack]

in efforts to determine the properties of dark matter from things like galaxy and galaxy cluster dynamics, dark matter particle mass doesn't matter much.

Instead, the property of dark matter that drives galaxy and galaxy cluster dynamics is mean dark matter particle velocity. High velocity dark matter particles are called "hot dark matter" and has been ruled out as an important component of dark matter. "Cold dark matter" particles are dark matter particles with a much lower velocity. "Warm dark matter" particles are much closer to cold dark matter particles than to hot dark matter but a little higher in velocity (and has a smaller mass that gives it quantum mechanical interactions not found in more massive dark matter particles).

The LambdaCDM Model of Cosmology doesn't really make a distinction between Warm Dark Matter and Cold Dark Matter. Both fit that model's definition of Cold Dark Matter.

There are lots of ways that dark matter could have hypothetically been created. One of the most popular theories is called "thermal freeze out" which hypothesizes that there is a statistical relationship between the mass of a dark matter particle and the temperature at which it "freezes out" as the average temperature of the universe falls after the Big Bang as the universe expands. In a thermal freeze out scenario particles with a mass that causes them to freeze out at a particular temperature acquire their mean velocity at the mean velocity of particles at that temperature.

In a thermal freeze out scenario, cold dark matter has particle masses of ca. 10 GeV to 1000 GeV, warm dark matter has particle masses of ca. 1 keV to 10 keV, and hot dark matter has a mass on the same order as neutrinos or less.

Dark matter candidates with particle masses of less than warm dark matter that nonetheless have warm dark matter or cold dark matter mean velocities (like axion-like particles) are hypothesized to come into being by means other than thermal freeze out, often with these particles being continually created and destroyed in a dynamic equilibrium.

Unlike dark energy, dark matter is assumed to gather into clumps, although not as tightly as baryonic matter does. In the dark matter hypothesis, galaxies, galaxy clusters, and cosmic scale strings of baryonic matter between galaxies arise from baryonic matter being drawn into halos of dark matter.

The inferred proportion of baryonic matter mass to dark matter mass that is observed, however, is not uniform even setting aside the large "voids" in space with very little ordinary baryonic matter or dark matter of any kind.

Generally speaking, the dark matter proportion by mass of the total mass of the system is lowest in elliptical galaxies (with the least inferred dark matter in the most spherical elliptical galaxies and the most in the least spherical ones), is higher in typical spiral galaxies, and is higher still in low surface brightness dwarf galaxies and in galaxy clusters, although there are also a small number of dwarf galaxies for which one can infer from their dynamics that they have little or no dark matter.

Is dark matter "assumed to gather into clumps" or "observed to gather into clumps"?
Once you have converted the rotation curve into actual density distribution, how about density as a function of local gravitational potential?
If dark matter were ideal gas of constant temperature and molecular mass - that is, if the mean dark matter particle velocity were a constant - the density of dark matter should have exponential dependency on the local gravitational potential.
The rotation curve does give you both the local total density (and thus local dark matter density after subtracting the light matter density) and the local potential (not relative to infinity, because that depends on the unobserved mass outside the curve, but relative to anywhere else inside the curve). When you plot the dependence of local dark matter density on local gravitational potential, does the mean dark matter velocity stay constant inside same galaxy and between galaxies, or does it vary?

 

[Ranku]

If dark matter were ideal gas of constant temperature and molecular mass - that is, if the mean dark matter particle velocity were a constant - the density of dark matter should have exponential dependency on the local gravitational potential.

Do we see observational evidence of this exponential dependency?

 

[ohwilleke]

Is dark matter "assumed to gather into clumps" or "observed to gather into clumps"?
Once you have converted the rotation curve into actual density distribution, how about density as a function of local gravitational potential?
If dark matter were ideal gas of constant temperature and molecular mass - that is, if the mean dark matter particle velocity were a constant - the density of dark matter should have exponential dependency on the local gravitational potential.
The rotation curve does give you both the local total density (and thus local dark matter density after subtracting the light matter density) and the local potential (not relative to infinity, because that depends on the unobserved mass outside the curve, but relative to anywhere else inside the curve). When you plot the dependence of local dark matter density on local gravitational potential, does the mean dark matter velocity stay constant inside same galaxy and between galaxies, or does it vary?

The inferred distribution of dark matter from observations, unlike dark energy, is clumpy as opposed to uniform across all volumes of space.

Cosmology simulations with dark matter also see it clumping although the fit of the simulations to the observational data usually have one or more discernible, statistically significant, shortcomings. See, e.g., Zhixing Li, Hong Guo, Yi Mao, "Theoretical Models of the Atomic Hydrogen Content in Dark Matter Halos" arXiv:2207.10414 (July 21, 2022) and Moritz Haslbauer, Indranil Banik, Pavel Kroupa, Nils Wittenburg, Behnam Javanmardi, "The High Fraction of Thin Disk Galaxies Continues to Challenge ΛCDM Cosmology" arXiv:2202.01221 (February 2, 2022) (published at 925 ApJ, 183 (2022) DOI: 10.3847/1538-4357/ac46ac and Aidan Zentner, Siddharth Dandavate, Oren Slone, Mariangela Lisanti, "A Critical Assessment of Solutions to the Galaxy Diversity Problem" arXiv:2202.00012 (January 31, 2022) and Saeed Tavasoli, "Void Galaxy Distribution: A Challenge for ΛCDM" arXiv:2109.10369 (September 21, 2021) (Accepted in ApJ Letter) DOI: 10.3847/2041-8213/ac1357.

The assumption is not that all dark matter has precisely the same velocity, but that it has order of magnitude similarity.

For example, in the strict LambdaCDM hypothesis, CDM is assumed to have velocities << c that tend to be on the order of hundreds of km/s, but there is not one exact speed that all CDM particles are expected to have for all time.

Dark matter is collisionless or indistinguishable from being collisionless in the LambdaCDM model, so it experiences only gravitational interactions and those interactions are, generally speaking in the astronomy context, effectively Newtonian gravitational interactions with relativistic adjustment assumed to be negligible.

So, the idea is that at thermal freeze out, in a thermal freeze out version of CDM, DM particle velocities were closely clustered in a normal distribution with not much variance around a certain velocity related to its mass, and that since then, only well understood Newtonian gravitational forces in the course of galaxy and structure assembly have tweaked those values (i.e. that significant deviations from the original velocity distributions should have identifiable causes that we are capable of understanding).

The details of observed dark matter density distribution within a given system like a galaxy, the extent to which it is a single halo or made up of subhalos, and the details of halo shape and velocity, are a matter of ongoing investigation and are open questions.

Generically, the dark matter distribution inferred from observations of the dynamics of a galaxy or galaxy cluster and gravitational lensing of that system is rarely unique. It is a matter of finding some distribution from one of several leading predetermined hypothetical possibilities that are mathematically feasible to work with that are consistent with the data within the range of observational uncertainties (which in astronomy tend to be quite large relative to other parts of physics), rather than finding from the observations that only one distribution of dark matter mass and velocity is possible.

We can directly measure stellar velocities, and there are empirically supported relationships between the stellar and halo virial velocities, but, of course, we can't directly measure dark matter particle velocities. See, e.g., Lorenzo Posti, S. Michael Fall "Dynamical evidence for a morphology-dependent relation between the stellar and halo masses of galaxies" arXiv:2102.11282 (February 22, 2021) (Accepted for publication in A&A).

An analytically calculated halo model called NFW (for the initials of the people to first describe it in journal articles, Navarro, Frenk, and White) is what you would expect of CDM, all other things being equal, but other halos models which don't obviously flow from theory (e.g. the Burkert profile seem to be better descriptions of what is inferred from observations for reasons that aren't well understood. See also, e.g., Mariia Khelashvili, Anton Rudakovskyi, Sabine Hossenfelder, "Dark matter profiles of SPARC galaxies: a challenge to fuzzy dark matter" arXiv:2207.14165 (July 28, 2022).

A Snowmass 2021 paper on "Astrophysical and Cosmological Probes of Dark Matter" explores within the dark matter paradigm, the open questions, and possibilities for deviating from the strict collisionless Cold Dark Matter paradigm of LamdaCDM to reconcile observation and the model. For example, it notes that:

Evidence of small-scale power suppression could, for example, suggest that dark matter is warmer (i.e., not nonrelativistic) during structure formation, is not collision-less, is wavelike rather than particle-like, or underwent non-trivial phase transitions in the early Universe.