Thursday, June 27, 2013

Passing through cosmic walls

Foam. Image source: DoITPoMS
Axions are hypothetical particles that are presently being searched for as possible dark matter candidates. The axion is a particle associated with the spontaneous breaking of a symmetry in the early universe. Unlike the case for the Higgs field, there can be a large number of ground states for the axion field. These states all have the same energy, but different values of the field. Since the ground states all have the same energy, they can coexist with each other, filling the universe with patches of different values of the axion fields, separated by boundaries called 'domain walls'.

The best visualization that came to my mind is a foam-like structure that fills the universe, though you shouldn't take this comparison too seriously.

At the domain walls, the axion field has to change values in order to interpolate between the different domains. This position-dependence of the field however creates a contribution to the energy density. Since the energy density of the domain walls decays slower with the expansion of the universe than the energy density of ordinary matter, this can become problematic, in the sense that it's in conflict with observation.

There are various ways to adjust these models or to pick the parameter ranges so that the domain walls do not appear to begin with, decay quickly, or are unlikely to be present in our observable universe. These are the most commonly used strategies for those interested in the axion as a particle. But in the recent years there has also been an increasing interest in using the domain walls themselves as gravitational sources, and so it has been suggested that they might play the role of dark energy or make contributions to dark matter.

In an interesting paper that appeared recently in PRL, Pospelov et al lay out how we could measure if planet Earth passed through such a domain wall
    How do you know if you ran through a wall?
    M. Pospelov, S. Pustelny, M. P. Ledbetter, D. F. Jackson Kimball, W. Gawlik, D. Budker
    Phys. Rev. Lett. 110, 021803 (2013)
    arXiv:1205.6260 [hep-ph]
(Apparently the arXiv-title did not survive peer review.)

The idea is to use the coupling of the gradient of the axion field, which is non-zero at the domain walls, to the spin of particles of the standard model. A passing through the domain wall would oh-so-slightly change the orientation of spins and align them into one direction.

This could be measured with devices normally used for very sensitive measurements of magnetic fields, optical magnetometers. Optical magnetometers consist basically of a bunch of atoms in gaseous form, typically alkali metals with one electron in the outer shell. These atoms are pumped with light into a polarized state of higher angular momentum, and then their polarization is measured again with light. This measurement is very sensitive to any change to the atomic spin's orientation, which may be caused by magnetic fields - or domain walls.

In the paper, and in a more recent follow-up paper, they estimate that that presently existing technology can test interesting parameter ranges of the model when other known constraints (mostly astrophysical) on the coupling of the axion have been taken into account. It should be mentioned though that they consider not a pure QCD axion, but a general axion-like field, in which case the relation between the mass of the particle and its coupling is not fixed.

The sensitivity to the event of a domain wall passing can be increased by not only reading out one particular magnetometer, but by monitoring many of them at the same time. Then one can look for correlations between them. This way one is not only able to better pick out a signal from the noise, but from the correlation time one could also determine the velocity of the passing through the domain wall.

I think this is an interesting experiment that nicely complements existing searches for dark matter. I also like it for its generality. Maybe while searching for axion domain walls, we'll find something else that we're moving through and that happens to couple very weakly to spins.


Uncle Al said...

Macroscopic domain wall magnetic effects, given years of observation, seem unlikely. Microscopic effects are not optimistic.

Fantastical dark matter theories discredit unremarkable Milgrom acceleration re the Tully-Fisher relation. Fermionic matter plus photon vacuum mirror symmetry suffers parity violations then manually patched with unending symmetry breakings. Noether's theorems couple vacuum isotropy and angular momentum conservation. Trace vacuum chiral anisotropy selective toward matter leaks that conservation as MOND's 1.2×10^(-10) m/s^2 Milgrom acceleration. Dark matter is flapdoodle.

Theory panders. A rigorously derived axiomatic system cannot be internally falsified, even if incomplete (Euclid) or empirically defective (Newton, Dirac equation). Vacuum being trace chiral toward fermionic matter is observable external to theory. Look.

Zephir said...

In my theory the current wave of global warming is caused with passing of solar system through dark matter cloud. It links the global warming which occurs at many places of solar system t (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11.. ) with shift of geomagnetic poles, the increasing geovolcanism, changes of physical constants or even with changes of kilogram and meter iridium prototypes and many other effects (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,..)

Uncle Al said...

@Zephir: Dunning-Kruger effect (2000 Ig Nobel Prize): ignorance more frequently begets confidence than does knowledge

1) Incompetent individuals tend to overestimate their own level of skill; 2) Incompetent individuals fail to recognize genuine skill in others; 3) Incompetent individuals fail to recognize the extremity of their inadequacy.

If you know you are an idiot, and we know you are an idiot, and each knows the other knows you are an idiot, and even other idiots know you are an idiot - and you are an empirically boring idiot - why do you further pursue the point?

Robert L. Oldershaw said...

Thinking of the previous thread: "Science Should Be More Like Religion", one might offer the following comment.

In the complete absence of any empirical evidence for axions, axion fields or domain walls, are we dealing with faith-based science here?

In some ways, perhaps, science should be less like religion. Perhaps radically distinct.

Nemo said...

Thanks for this nice and very interesting article :-)

I hope it will attract some non-trolling comments too ... ;-)

Sabine Hossenfelder said...

Thanks... Blogger would really benefit from a possibility to uprate and downrate comments...

Zephir said...

/* Blogger would really benefit from a possibility to uprate and downrate comments..*/

I dunno about blogger, but the scientific thinking requires a reasoning - which is just what the upvoting/downvoting of comments is lacking. Science is not a democracy - a logical reasoning should always get a priority before subjective stance.

/* Incompetent individuals tend to overestimate their own level of skill*/

You do behave like if you would want to disprove some of my claim - but your level of skill is not sufficient for it..

Zephir said...

BTW Hypothetical axions are probably equivalent to hypothetical anapoles

Chett Mitchell said...

Hate to be so ignorant, but are there any a priori ways to estimate the dimensions of these domain walls? If they are light years thick, It could take a really long time to get a signal. We'd have to be really lucky to be watching exactly when a crossing took place.

I've been rooting for Axion dark matter for a long time, but have learned to live with disappointment (I'm pretty sold on LSP these days). I agree that it would be a very worthwhile search for whatever other fields might be hiding out there.

Robert L. Oldershaw said...

Some of us are dismayed by what passes for science these days.

For example,

Multiverse - not testable in any definitive manner; cannot make definitive predictions.

String theory - no predictions.

Sparticles and extra-dimensions - just over the next energy hump; take our word.

Quarks - (hidden inside black boxes, but hey, they are as real as the trinity, take our word)

WIMPS, axions, sterile neutrinos, etc. - we WILL find them no matter if it takes an infinite amount of time. Failure is not an option.

Planck Scale - unobservable, but again take our word and use it heavily in your reasoning.

Anthropic arguments - dog chases tail.

Boltzmann brains - EGAD!

So I ask you: Are you not worried that this, ahhh, stuff is getting more than a bit unscientific?

Is being worried not both reasonable and scientific?

Robert L. Oldershaw

Zephir said...

/*..are there any a priori ways to estimate the dimensions of these domain walls?..*/

They're scale invariant in wide extent.

Zephir said...

/*stuff is getting more than a bit unscientific?*/

Yes, until you call "scientific" the low-dimensional formal stuff based on deterministic rigor. But the current state of technology enables us to observe more distant scales, than this rigor can effectively describe. I compared this situation to perspective at the water surface many times here: at proximity the surface ripples are spreading deterministically in circles and they don't interact with (reference frame of) underwater way too much. But at sufficiently small or large distances they get scattered with density fluctuations of (compacted dimensions) of underwater and the motion of such a waves is impossible to describe with deterministic math anymore. This is simply how our Universe appears and the physicists must adopt to this situation, or they will extinct like any other rigid species.

Robert L. Oldershaw said...

I am hoping that Dr. Hossenfelder will consider a review of Jim Baggott's new book Farewell To Reality.

I believe the author was trained as a physicist and worked in physics before turning to writing.

He appears to have a very good understanding of the relevant physics, various theoretical models and the scientific method.

I cannot think of a more important set of issues that need to be objectively confronted and debated at the present time.

Sabine Hossenfelder said...

Hi Chett,

The width of the domain walls is a function of the mass of the (excitation of) the field, which is subject to constraints from observables which leads to a typical width that one could expect, and that would be possible to measure. The numbers they quote in the paper are a relative velocity of 10^-3 c and a signal duration of 1ms. Now you can convert that into a width if you want. Best,


Sabine Hossenfelder said...


As I've said previously, you are greatly distorting reality. Besides this, poking in the dark is an entirely healthy scientific process that's essential to progress. I share your worry in some regards, but as the German saying goes you're trying to turn a mouse into an elephant. Best,


Phillip Helbig said...

The equivalent English expression is "making a mountain out of a molehill" (mole is "Maulwurf").

Robert L. Oldershaw said...

It is a verifiable fact that I am not alone in this concern about the drift of theoretical physics into the realm of untestable pseudo-science.

Like Jim Baggott, I expressly endorse speculation so long as it is not hyped as tested physics, or in some cases as "the only game in town".

Some highly educated people with strong backgrounds in physics have called attention to this problem.

But perhaps I am choosing an inappropriate venue for expressing my concerns.

Robert L. Oldershaw
Discrete Scale Relativity/Fractal Cosmology

Don Foster said...

“…spontaneous breaking of a symmetry in the early universe.”

I am wondering if these early symmetry breaks are of known progression, anticipated by theory and well established.

They would seem no small matter, these first embryonic foldings: not simply a matter of mathematical subtly, but rather manifesting the fundamental tensioning of the universe.

In some movie version their appearance would surely be worthy of potent sound effects, perhaps something like the sweep of Luke Skywalker’s light saber. Or is this making too much of them, giving them unwarranted dramatic treatment?

Eric said...

Robert, the existance of quarks are proved science. Just because they don't fit into other tenets of your DSR theory does not mean you can simply discount them, just as you can't discount GR. To lump quarks in with other items you mentioned just tells everyone to not bother with DSR because it's coming from someone with an agenda.

Robert L. Oldershaw said...

If a "quark" is ever actually observed in a reasonably direct manner, instead of indirectly inferred (as is currently fashionable), then your arguments would carry some weight.

If Discrete Scale Relativity is a useful new paradigm, nature will verify that and the opinions of pseudo-scientists will count for noting.

Don Foster said...

Dr. Bee,

So the domain walls of the axion fields mark a detectible boundary of distinction between their differing field values. The axions themselves arise with the breaking of a hypothesized symmetry within the nascent universe.

This is necessarily naive, but I wonder if we can postulate that for any given distinction between things there is perforce some attribute of similarity across any boundary. That is, the universe is of a piece and therefore symmetry “breaks” are more in the nature of folds in its entirety.

If we begin with the “ultimate symmetry” of a singularity, it would be a simple comfort to recognize the instance in which one becomes two, to understand the nature of some first fold, the turning of one attribute to lay cross-grained with respect to itself.

It seems a paradox. How do you make this first distinction in an untroubled symmetry? What do you have to work with? There is no detail of time or distance, no inside or outside. Is there any attribute with an axis to be twisted?

Yet, I would like a story in which one becomes two without parting, to understand the nature of this first distinction, how it might come to be enfolded in all that follows, endlessly marbled but never quite meeting with it’s counterpoise, the two legs bounding the universe’s many angles of repose.