Dark matter science

Oct 31, 2011 By Steve Nerlich, Universe Today
Dark matter has been found to be (unexpectedly) evenly distributed across dwarf galaxies - rather than clumping together in the centre in the way that we had expected of 'cold' dark matter. Credit NASA.

Dark matter – there’s a growing feeling that we are getting closer to finding out the true nature of this elusive stuff. At least we are running a number of experiments that seem (on theoretical grounds) to have the capacity to identify it – and if they don’t... well, maybe it’s time for a rethink of the whole ball game.

There are two arguably quite separate requirements for dark matter to make sense of our current dataset and our theoretical schema for the universe. Firstly, the Standard Model of cosmology (Lambda-Cold Dark Matter) requires that 96% of the universe is composed of stuff of an unknown nature that cannot be directly observed.

About two thirds of this unknown stuff can’t possibly be matter since it apparently grows as the universe grows – so we call it dark energy. The remaining component we call dark matter since it represents a component of the dark side that is capable of generating gravity. But that’s about it. In this context, dark matter is invoked to balance the math – within a set of formulae which are already straining credibility by telling us that 96% of the universe is invisible and undetectable. So, if that was all there was to argue the case for dark matter, you would be justified if feeling a little skeptical.

But the second requirement for dark matter is much more grounded in sound observation and conventional physics. Galaxies – and the way in which galaxies cluster and dynamically interact – don’t make sense if they are composed of only the visible and other known types of matter that lie within them. The Milky Way itself is rotating in a manner that would result in much of it flying apart, if there wasn’t additional invisible matter generating additional gravitational attraction. So there are sound reasons to think that there really could be something else out there.

There’s been a recent kerfuffle about dark matter in dwarf galaxies – although this is largely about whether dark matter particles clump together at the centre or whether they are energetic particles whizzing about throughout the galaxy. Apparently the data better fit the latter scenario, which challenges the prevalent view that dark matter is ‘cold’ and prone to clumping.

Similar to the Bullet Cluster, MACS J0025.4-1222 represents the aftermath of the collision of two galaxy clusters. Most of the mass of each cluster remnant is in the cool blue regions - each having already moved beyond the collision point due to being only weakly interacting mass. The pink region represents strongly radiating and strongly interacting mass has been slowed up within the initial collision. Credit NASA.

A recent literature review on Arxiv provides a comprehensive coverage of the current status of dark matter science. Initial data from the PAMELA spacecraft, showing an anomalous cosmic ray flux, encouraged speculation that this might result from dark matter annihilating or decaying. This theory did not receive wide support, but such speculation was more recently revived with FERMI-LAT finding unexpected flows of positrons (i.e. antimatter) – followed by an announcement that FERMI-LAT and other telescopes will undertake a dedicated search for gamma ray lines arising from dark matter annihilation or decay. Here it is presumed – or at least hypothesised – that dark matter can be destroyed within the hot, dense and dynamic centres of galaxies, including our galaxy.

So space science could provide at least circumstantial evidence for one of the biggest mysteries in space science – although all findings to date are inconclusive at best.

Earth-based experiments are looking for more direct evidence of the particle nature of dark matter. For example, the Large Hadron Collider is looking for signs of supersymmetry particle signatures. The hypothesised neutralino would nicely fit the hypothesised characteristics of a dark matter particle (a particle that weakly interacts with other matter, has neutral charge, is stable over cosmic timescales and has no color charge), but there are no signs of the neutralino, or anything else clearly supersymmetrical, so far.

There are also experiments, like DAMA/LIBRA, deep down coal mines and the like, which are designed to directly identify weakly interactive massive particles – although again findings to date are all a bit inconclusive.

And ‘all a bit inconclusive’ is a statement that aptly represents the current state of science – we remain confident that there is something out there, but (obligatory play on words coming) we remain as much in the dark as ever about what exactly it is.

Explore further: How the physics of champagne bubbles may help address the world's future energy needs

More information: Capoziello et al The missing matter problem: From the dark matter search to alternative hypotheses.

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User comments : 25

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Temple
4.5 / 5 (4) Oct 31, 2011
The history of science is replete with somewhat unnecessarily exclusive and polarized schools of thoughts. Whether the wave/particle nature of light, the vulcanism/neptunism debate on geologic origins, etc, there have been a series of 'either or' theories proposed to explain physical phenomenon.

Study of the history of science has consistently revealed that these 'either or' dichotomies are somewhat unnecessary. Often it has shaken out that each school of thought had gotten part of the answer correct, but it was only realized when people decided to bridge the gaps between them.

I suspect when the dust settles, we will discover that the MOND/LCDM (and their variants) debate will follow a similar pattern, with each revealing truths about the universe.
hemitite
not rated yet Oct 31, 2011
In those heavy ion collisions that are said to produce quark-gluon plasma of the sort that made up the "cosmic soup" of the early universe, wouldn't some of those dark mater particles be produced too?
theon
1.3 / 5 (3) Oct 31, 2011
How ardently is it overlooked that galactic dark matter cannot be WIMPS (a nightmare for the Galaxy), but must be MACHOs (a blessing). And then there is not any reason to exclude neutrinos as the non-baryonic dark matter. Good new comes from KATRIN next year: this case will be tested with a definite result.
Vendicar_Decarian
0.2 / 5 (36) Oct 31, 2011
@hematite

Yes, but since they haven't been observed, the theorists have had to propose theoretical particles that are larger than the energies currently attainable.

Of course, those particles are just theoretical... Right?
Vendicar_Decarian
0.3 / 5 (36) Oct 31, 2011
@theon

MACHOs haven't been observed either. And there have been scintillation studies done to look for them.

TimESimmons
1.6 / 5 (7) Nov 01, 2011
No such thing as dark matter. The Bullet cluster is part of the evidence. http://www.presto...ndex.htm
Vendicar_Decarian
0.9 / 5 (41) Nov 01, 2011
If anti-gravity matter does not interact with photons as TimE says on his web page, then conversion of mass to energy and back again would allow matter to be created - pushed away from the anti-gravity matter thus gaining energy - transformed into photons, beamed back to the anti-gravity matter source amplified by the gain in energy. The cycle could be repeated continually thereby allowing the generation of energy from nothing.

TimE wants his energy from nothing and his chicks for free.

MarkyMark
5 / 5 (2) Nov 01, 2011


TimE wants his energy from nothing and his chicks for free.


What kind of red blooded male are you? I would love to get my chicks for free, and i would need lots of free energy otherwise i may quickly get exausted from the efforts of, "getting the most from my free chicks".

SteveL
not rated yet Nov 01, 2011
Nothing is ever free. Ever.
Ethelred
3 / 5 (2) Nov 01, 2011
Steve, I can assure that it is possible to borrow The Moon Is a Harsh Mistress from many libraries for free.

Ethelred
Shelgeyr
2.3 / 5 (3) Nov 02, 2011
Dark matter science


Actually more like "Dark matter wild postulating".

When someone can post the details and parameters of a Dark Matter experiment (and don't leave out the control group!) then perhaps it might rank the label "science". As is, the "S" in "SWAG" itself is being too gracious.
SteveL
1 / 5 (1) Nov 03, 2011
Steve, I can assure that it is possible to borrow The Moon Is a Harsh Mistress from many libraries for free.

Ethelred

And who pays the help and the light bill? Who pays for the carpet cleaning and dumping the trash? Did the book appear out of nowhere? If so Mr. Heinlein somehow got the credit.
Ethelred
3 / 5 (2) Nov 03, 2011
And you post is free.

Humor free.

Ethelred
theon
2 / 5 (3) Nov 04, 2011
@theon

MACHOs haven't been observed either. And there have been scintillation studies done to look for them.

There is an observation claim in quasar microlensing by Schild 1996. Sadly, this has never been discussed by the search teams who failed to observe MACHOs in front of the Magellanic Clouds. There are papers from the MOA collaboration (Sumi et al) who find more and more new MACHOs. In other words, the old no-finds may have lacked the claimed accuracy. For now, the case is still open.


Vendicar_Decarian
0.3 / 5 (36) Nov 05, 2011
I know of no observation that points to macho densities that are great enough to make up more than 4 times the observed matter in the universe.

Three are three possibilities if you are looking for missing matter.

1. Large globs of the stuff in the form of Macho's
2. Intermediate particles of charged hydrogen dust, a few billion atoms in size.

3. Some new subatomic particle that hasn't yet been discovered.

Option 2 seems the most probable to me.
Twin
2 / 5 (3) Nov 05, 2011
Discussions of Dark Matter always ignore the obvious.

Dyson Spheres! Billions of them. Ok, taking off the foil hat now.
yyz
5 / 5 (2) Nov 06, 2011
"Dyson Spheres! Billions of them."

All-sky Dyson Sphere searches have already been conducted using data from the InfRared Astronomical Satellite(IRAS). After mimicking and erroneous sources have been eliminated, a *very* few ambiguous candidates remain. IRAS' Low Resolution Spectrometer was able to sample nearly 10^6 solar-type stars out to a distance of ~300pc, while detecting no unambiguous evidence for a single DS:

http://home.fnal....arch.htm
http://iopscience...2075.pdf

Perhaps deeper, more sensitive surveys by Spitzer and Akari may find otherwise, but the prospects look dim.
antialias_physorg
not rated yet Nov 06, 2011
In those heavy ion collisions that are said to produce quark-gluon plasma of the sort that made up the "cosmic soup" of the early universe, wouldn't some of those dark mater particles be produced too?

Very good question.
Graeme
2.3 / 5 (3) Nov 06, 2011
If the hypothetical DM particle has a very low cross section with normal matter, then it likely has a very low cross section with hot normal matter as in the quark-gluon plasma. Some may be formed, but it may take millions of years to come to equilibrium. The big bang did not have that time.

The same kind of story with neutrinos that the cross section for a cool neutrino is so small that it will be invisible. But at least the ultrahigh energy neutrino has a greater chance of interacting. So we may find that if we could accelerate protons or the like to similar energies there could be some interaction with what is otherwise cool neutrinos in order to get a measurement of the density of them around the earth. We already know for solar system gravity that there are not invisible accumulations of dark matter around the sun or earth.

And hydrogen ion clusters of billions of atoms are not likely to be stable as they would evaporate in the vacuum and heat of space.
Graeme
2.3 / 5 (3) Nov 06, 2011
If hydrogen ion clusters make up a large proportion of matter then the element ratios with helium in the universe need serious reconsideration, and you may well ask how come the sun does not have more hydrogen? The gigaclusters of hydrogen should be detectable in the solar system if they exist. A simple experiment with a charged plate on a microphone in space should get some loud pings and pops as it attracts these ions (if they existed).
Vendicar_Decarian
0.2 / 5 (36) Nov 07, 2011
"If hydrogen ion clusters make up a large proportion of matter then the element ratios with helium in the universe need serious reconsideration." - Graeme

Ya. Very good point. Perhaps the initial helium density of the universe was 1/4 of what is observed in the youngest objects.

Is it even possible to measure the helium content of the extended halo of matter surrounding galaxies?

"you may well ask how come the sun does not have more hydrogen?" - Graeme

Or less Helium at the outset.

"A simple experiment with a charged plate on a microphone in space should get some loud pings and pops as it attracts these ions (if they existed)." - Graeme

Also true. Has anyone been listening?
Ethelred
3.7 / 5 (3) Nov 07, 2011
OK I was reading anyway. I didn't see anything that made sense regarding 'hydrogen ion clusters' which is a bit of an oxymoron.

If its ionized how the hell does it form a cluster? Ionized hydrogen is a proton. Protons are all positively charged. They CANNOT cluster except under influence of a LOT of gravity. So what is causing this purely hypothetical, no evidence of it existing, cluster and why do you even think they might exist?

So with that said the parts about the proportions of H and He in the Sun don't make any sense either since they were dependent of the bizarre concept of hydrogen ion clusters.

Ethelred
antialias_physorg
not rated yet Nov 07, 2011
"A simple experiment with a charged plate on a microphone in space should get some loud pings and pops as it attracts these ions (if they existed)."

Have you heard of solar wind? You'd hear nothing but that. Please, for the love of all that is holy: why doesn't anyone use google or wikipedia before posting crap?
Graeme
not rated yet Nov 08, 2011
Hydrogen ion clusters contain one proton and a whole bunch of hydrogen molecules, or they can also have an even number of atoms. They have been shown to exist with over 100 atoms. A particularly stable cluster is H6 The can be made by expanding compressed hydrogen gas through a tiny hole into a vacuum, and then using an electric field to attract the ionized component. Radiation on solid hydrogen also makes these cluster ions HD is preferentially turned into the positive ion. This may be a hidden store of deuterium in space.
Graeme
not rated yet Nov 08, 2011
>love of all that is holy

cluster ions with a mass of several billion would behave differently to the solar wind. They would be very little affected by magnetic fields, and would impact with a dull thud on the hypothetical cathode, unlike the protons from the solar wind. You could make a simple mass spectrometer arrangement to separate ions of different mass. My point is that these clusters probably do not exist, and if they did they should have been detected already.

I don't really believe that hydrogen ion clusters exist in such massive gigaclusters, and that if the clusters abound they are more likely to be condensed on dust grains.

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