Why does dark matter pool in the CMB plasma?

[Dr. Strange]

I'm trying to understand the mechanisms of the anisotropies in the CMB. The general idea is that there are fluctuations in some field (e.g. inflation) and the baryonic and dark matter rush into the space compressing the fluid. The photon energy pushes back on just the baryonic matter while the dark matter pools in these wells.

What I don't understand is why does the dark matter pool? I assume it gathers some momentum on it's way into the gravity well, so why isn't that momentum conserved? The dark matter has no way of losing energy; it will just transfer it from potential to kinetic and back again, so why doesn't the dark matter oscillate around the fluctuations always returning to it's starting potential?

 

[marcus]

I'm trying to understand the mechanisms of the anisotropies in the CMB. The general idea is that there are fluctuations in some field (e.g. inflation) and the baryonic and dark matter rush into the space compressing the fluid. The photon energy pushes back on just the baryonic matter while the dark matter pools in these wells.

What I don't understand is why does the dark matter pool? I assume it gathers some momentum on it's way into the gravity well, so why isn't that momentum conserved? The dark matter has no way of losing energy; it will just transfer it from potential to kinetic and back again, so why doesn't the dark matter oscillate around the fluctuations always returning to it's starting potential?

Good thinking. Good question. Maybe the DM just rushes straight thru and takes an excursion on the other side, then falls back thru again etc.
I share your uncertainty about this.I'd like to hear what someone more knowledgeable has to say about it.

Wouldn't the initial terrain with its primordial fluctuations, consist largely of dark matter? In that case the DM would to a large extent be already "pooled" in patches of overdensity and underdensity.
And then FWIW a dynamic dance of DM would start---where outlying DM falls in towards a overdense DM region, and falls thru to the other side and then falls back thru. It may not matter to the analysis. Just my two cents.

 

[George Jones]

Off the top of my head, I don't know the answer, but thinking out loud ...

1) Can the small random velocities of dark matter particles in overdense regions result in virialized, gravitationally bound dark matter regions?

2) WIMPS (weakly interacting massive particles) often are taken as dark matter candidates, i..e, there is a small but non-zero interaction cross-section. Does this play a role?

 

[Dr. Strange]

Wouldn't the initial terrain with its primordial fluctuations, consist largely of dark matter? In that case the DM would to a large extent be already "pooled" in patches of overdensity and underdensity.
And then FWIW a dynamic dance of DM would start---where outlying DM falls in towards a overdense DM region, and falls thru to the other side and then falls back thru. It may not matter to the analysis. Just my two cents.

I am far from an expert, but if you're suggesting that dark matter pre-dates the inflation field then you're back at the original problem: inflation removed all inhomogeneities in whatever came before. I would assume that includes any dark matter overdensities.

 

[marcus]

If I understand correctly at the end of inflation the "inflaton" field decays producing matter (including DM) and whatever primordial fluctuations were in the inflation stage are transcribed into that decay-product matter (including DM)

So I would imagine---if we are going to take the inflation hypothesis seriously---that the post-inflation stage starts off with some DM overdensities. But you think not, which is fine, there are different possible views on inflation---and even scenarios that do without inflation. Like this:

http://arxiv.org/abs/1412.2914
A ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing
(Submitted on 9 Dec 2014)
We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Importantly, as this scenario predicts a positive running of the scalar index, observations can potentially differentiate between it and inflationary models. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
14 pages, 8 figures.

 

[marcus]

Off the top of my head, I don't know the answer, but thinking out loud ...

1) Can the small random velocities of dark matter particles in overdense regions result in virialized, gravitationally bound dark matter regions?
...

That's really interesting! You're the one to say, being more expert. But I'll put in a guess: I would say YES! Cloud of DM can eject excess gravitational energy by the SLINGSHOT interaction whereby some DM particles carry off excess energy and the remaining ones become gravitationally bound.

I guess this would be slow and inefficient compared with the clumping of ordinary matter. But in early universe with a high density of DM it might serve to make gravitationally bound clouds.

The ejected, sacrificed DM particles carrying away excess kinetic energy would eventually lose their momentum due to expansion and have a second chance to join into gravitationally bound clouds. The "redshift" of massive particles is a great slower-downer ; ^)

 

[Dr. Strange]

If I understand correctly at the end of inflation the "inflaton" field decays producing matter (including DM) and whatever primordial fluctuations were in the inflation stage are transcribed into that decay-product matter (including DM)

This scenario is disturbing to me. The general idea here is that a hypothetical field which creates negative energy densities is somehow able to decompose into a overdensities of a hypothetical particle that isn't part of the standard model. Let's assume we can continue from here without ever seeing the Feynman diagram of this interaction. Let's pick it up from there. We now have an overdensity of dark matter as a result of the spontaneous conversion of this field into matter and energy. I believe the original question still stands: how does this fluctuation grow? You have dark matter from the homogenous regions. That dark matter is pulled into the overdensities and flies back out the other side to the same potential that it started from (the only alternative to this is annihilation which would still be active to some degree but would produce energy which would add to the outward pressure, not more gravity). So how do these overdense regions of dark matter grow?

 

[Chalnoth]

I'm trying to understand the mechanisms of the anisotropies in the CMB. The general idea is that there are fluctuations in some field (e.g. inflation) and the baryonic and dark matter rush into the space compressing the fluid. The photon energy pushes back on just the baryonic matter while the dark matter pools in these wells.

What I don't understand is why does the dark matter pool? I assume it gathers some momentum on it's way into the gravity well, so why isn't that momentum conserved? The dark matter has no way of losing energy; it will just transfer it from potential to kinetic and back again, so why doesn't the dark matter oscillate around the fluctuations always returning to it's starting potential?

In an expanding universe, the expansion itself acts as a kind of friction: any particle that is moving with respect to the background catches up to parts of the universe that are expanding away, becoming slower with respect to the background.

If I'm remembering my structure formation correctly, if you look at structure formation from a non-expanding frame of reference, to a first approximation structures don't grow (much): overdense regions simply stop expanding when the expansion slows sufficiently. There's definitely some collapse that occurs because self-gravitating clouds are unstable (periodically gravitational interactions between dark matter particles will shuffle kinetic energy between them, and the particles with highest kinetic energy will be expelled, reducing the energy remaining within the cloud), but this effect is quite slow.

 

[marcus]

Hi Chalnoth, I expect you are familiar with this (a side aspect) but it is so interesting I want to mention it. Leonard Parker's 1966 Harvard PhD thesis explaining how rapid expansion can create particles. There is quite a lot of follow-up literature on this. I think the mechanism (studied using QFT in curved space time) can be imagined as analogous to how we sometimes intuitively picture Hawking radiation arising at a BH horizon.
Parker and Ivan Agullo have an essay about this written for non-specialists that won first prize in the 2011 Gravity Foundation essay contest. It has references to earlier papers.
http://arxiv.org/abs/1106.4240
Stimulated creation of quanta during inflation and the observable universe
Ivan Agullo, Leonard Parker
(Submitted on 21 Jun 2011)
Inflation provides a natural mechanism to account for the origin of cosmic structures. The generation of primordial inhomogeneities during inflation can be understood via the spontaneous creation of quanta from the vacuum. We show that when the corresponding stimulated creation of quanta is considered, the characteristics of the state of the universe at the onset of inflation are not diluted by the inflationary expansion and can be imprinted in the spectrum of primordial inhomogeneities. The non-gaussianities (particularly in the so-called squeezed configuration) in the cosmic microwave background and galaxy distribution can then tell us about the state of the universe that existed at the time when quantum field theory in curved spacetime first emerged as a plausible effective theory.
9 pages. Awarded with the First Prize in the Gravity Research Foundation Essay Competition 2011
================
For some reason we don't seem to talk much about the Parker effect. There's a recent retrospective by Parker recounting how he discovered the idea.
http://arxiv.org/abs/1503.00359
Creation of quantized particles, gravitons and scalar perturbations by the expanding universe
Leonard Parker
(Submitted on 1 Mar 2015)
Quantum creation processes during the very rapid early expansion of the universe are believed to give rise to temperature anisotropies and polarization patterns in the CMB radiation. These have been observed by satellites such as COBE, WMAP, and PLANCK, and by bolometric instruments placed near the South Pole by the BICEP collaborations. The expected temperature anisotropies are well-confirmed. The B-mode polarization patterns in the CMB are currently under measurement jointly by the PLANCK and BICEP groups to determine the extent to which the B-modes can be attributed to gravitational waves from the creation of gravitons in the earliest universe. It was during 1962 that I proved that quanta of the minimally-coupled scalar field were created by the general expanding FLRW universe. This was relevant also to the creation of quantized perturbations of the gravitational field, since these perturbations satisfied linear field equations that could be quantized in the same way as the minimally-coupled scalar field equation. In fact, in 1946, E.M. Lifshitz had considered the classical Einstein gravitational field in FLRW expanding universes and had shown that the classical linearized Einstein field equations reduced, in what is now known as the Lifshitz gauge, to two separate classical minimally-coupled massless scalar field equations. These field equations of Lifshitz, when quantized, correspond to the field equations for massless gravitons, one equation for each of the two independent polarization components of the spin-2 massless graviton. I will discuss this further in this article.
14 pages. Plenary Lecture given September 2, 2014 at the ERE2014 Conference in Valencia, Spain To appear in the Proceedings of the ERE2014 Conference

 

[nikkkom]

What I don't understand is why does the dark matter pool?

Because DM particles do interact. Gravitational interaction is an interaction too. They exchange some energy and momentum via it. And as a result, some of them are faster than average (and they escape the potential well) and some are slower - and these are trapped in the well.

It's similar to "evaporation" of globular clusters. Stars exchange momentum via gravity. Some acquire enough momentum to escape. Which, as a whole, drains globular cluster of kinetic energy, and it gradually shrinks.

 

[Chalnoth]

Because DM particles do interact. Gravitational interaction is an interaction too. They exchange some energy and momentum via it. And as a result, some of them are faster than average (and they escape the potential well) and some are slower - and these are trapped in the well.

Gravity is, however, mostly a conservative force: unless the interaction emits gravitational waves, then no energy is lost. And gravity waves are usually only significant for things orbiting very close to very dense objects (e.g. black holes, neutron stars).

It's similar to "evaporation" of globular clusters. Stars exchange momentum via gravity. Some acquire enough momentum to escape. Which, as a whole, drains globular cluster of kinetic energy, and it gradually shrinks.

Yes, but this is a very slow process. I don't think it makes a significant difference for the early formation of structure.

 

[nikkkom]

You don't need to lose energy. You only need to have it redistributed.