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Cosmology alert

Looks like there is now some hard evidence for dark matter. More commentary and explanation here. (Apparently it doesn't rule out MOND, but it does confirm that dark matter exists, which has been one of the most embarrassing questions in physics for a couple of decades now — given that visible matter accounts for only about 4% of the energy density of the universe, where is the rest hiding?

The two big question marks in our knowledge of what the universe is made of are dark matter (which doesn't interact with the type of matter we're familiar with, but which clumps gravitationally) and dark energy, which I can't get my head around (how the hell does something have negative pressure?). Dark matter appears to account for 22% of the universe, with dark energy making up the other 70-something percent (darn it, why is the universe seven-tenths made up of something I don't understand even the layman's definition of?). And today it looks like we're down one question mark.



Does this change our understanding of "He separated the light from the dark"


No: because the rest of that verse goes on to talk about the waters. The ancient hebraic cosmology is a trifle deficient when it comes to discussing Quasars ...


Negative Pressures and Cavitation in Liquid Helium, Physics Today, Humphrey Maris and Sebastien Balibar:

"... Before entering further into our subject, let us mention some everyday situations in which negative pressures occur. In a tree, water passes from the roots up to the leaves through the xylem and evaporates from the leaf surfaces (figure 3). Consider now the variation in the water pressure within the trunk of the tree. The pressure must decrease with height so as to balance the force of gravity acting on the water and drive the water through the xylem at the necessary rate. The pressure, which is 1 bar (10^5 pascals, or about 1 atmosphere) at ground level, must decrease by 1 bar for each 10 meters going up the tree. At the top of a California redwood tree—which may be 100 m above the ground—the pressure must therefore be about –9 bars. One might therefore think that a terrible accident would happen if a bird made a little hole in the trunk near the top: Air might rush into the hole and cause all the water to flow back to the ground, leaving a dried-out tree. In fact, the tree is protected against such a disaster, thanks to the presence of very small constrictions in the xylem channels, safety hatches that can hold water through capillarity...."


So we light-matter creatures are probably in the minority in this universe. Great. When can we expect the dark-matter death squads?


If I read that story right, the gas clouds were slowed down by colliding with each other, but the dark matter kept right on going. This seems to imply that not only does dark matter not interact with matter-as-we-know-it, it doesn't interact with itself, either.


I saw a Roger Penrose talk not long ago; he said that dark energy is exactly the same thing as the cosmological constant, the term /\ (big lambda) in general relativity that Einstein put there to stop the universe collapsing (or indeed expanding). He expressed surprise at the fact that an old thing got a new name, especially one that explains less than the old one.

So it's just a measure of the capacity of the vacuum to expand of its own accord.

I think that helped my understanding a bit, anyway!

Love the fiction, BTW!


The cosmological constant, as I understand it, is only one of the possible explanations for dark energy; see also quintessence.


I clump dark energy in with Dark Matter to get my head around it as you can't really see them both and I like to simplify everything in my head to particles..Of course that's probably not to great sjnce there is more of them then us. I'll check out that quintessence link. Cool stuff.

Steven Rogers:

...imply that not only does dark matter not interact with matter-as-we-know-it, it doesn't interact with itself, either.

Dark Matter is supposed to be a very tiny particle then what we are used of, so it goes around larger particles or through them. Through light speed particles so it should be faster with less friction.
I wonder how it relates to the religious spirit or anti-matter. Maybe it relates to the spiritual realm as some books describe just being in a 'software' state as spiritual, which doesn't necessarily have to be very obscure.

Supposedly a body like a planet creates gravity by somehow bending dark matter or Gravitons with it's more massive 'clumped' (?) dark matter creating a sink hole where we are sucked to the surface of the planet through negative forces. This is similar to a black hole but somehow the black wholes 'physical relating to us' planet isn't there anymore. Maybe it's just a Dark Planet. Anyway, I am real excited about all of this.

If completely understood Faster then Light Travel can become possible as there is proof that something is traveling faster then our perception of a constant speed of light. What we see. The theory is we basically have to create a scooping magnet or exotic particle machine to remove the larger matter creating a worm hole or dark matter road but is this archaic thinking? I think this is right but I would like them to put a speed on it. If it's traveling through things obviously there is less friction.

But the big question is are there complex 'intelligent' realms in this larger dimension. Anything more then just really tiny particles or a gravity stabilizer for us. If we could somehow physically see this Dark Matter with our own eyes what would we see?

Some people speculate, like me, that the universe is made up of very tiny stretched particles interweaving throughout everything so there really is no smallest particle or base substance as it's all dimensional. Yes I have has allot of fun with this subject.


Stephen Baxter's "Xeelee Sequence" (various novels and short stories) deals with conflict between dark matter life and the Xeelee (amongst others) who are composed of regular, ordinary (baryonic?) matter. Or rather, as I recall, it deals with humans poking around in the rubble and generally trying to avoid getting squashed by the big guns. Entertaining stuff.

Um, I think my only other posts here have been about a Baxter story too. Ah... honestly, he's not paying me to promote his books on other peoples' blogs.


But, what is the actual proof? I keep getting lost in these articles. What observation was key? Forgive my obtuseness.


It's still probably more of an indirect comparison:

"The hot gas in this collision was slowed by a drag force, similar to air resistance. In contrast, the dark matter was not slowed by the impact, because it does not interact directly with itself or the gas except through gravity. This produced the separation of the dark and normal matter seen in the data. If hot gas was the most massive component in the clusters, as proposed by alternative gravity theories, such a separation would not have been seen. Instead, dark matter is required.

“because it does not interact directly with itself or the gas except through gravity.��?

How do they know this?

Also a correction for my post:
Basically they say that Dark Matter doesn't interact with itself (not logical to me because they can't observe Dark Matter directly only indirectly through gravity forces with stars, gas clouds, etc, but they are comparing to our dimension mainly I guess.) So the theory is that Dark Matter somehow is distorted near a planet's surface creating a vacuum effect of gravity. Maybe it just dissipates near other matter types. My question is how does it effect us if we don't effect it.


"When can we expect the dark-matter death squads?"

I mis-read that as "dark-matter death squids" ... there's gotta be a novel in that.




No, because the rest of that verse goes on to talk about the waters

True: Genesis 1 is a remarkably prescient anticipation of Talking Heads' Once in a Lifetime.


So now we know what dark matter is - plutonoid bodies sulking at being dropped from planet status.


This news may have interesting consequences for the quantum mechanics thread in the newsgroup.

(No, it wasn't me who started it.)


Mark : Some people speculate, like me, that the universe is made up of very tiny stretched particles interweaving throughout everything so there really is no smallest particle or base substance as it's all dimensional. Yes I have has allot of fun with this subject

Very tiny stretched particles...What else might that describe?


But the big question is are there complex 'intelligent' realms in this larger dimension. Anything more then just really tiny particles or a gravity stabilizer for us. If we could somehow physically see this Dark Matter with our own eyes what would we see?

Um, are there complex `intelligent' realms where you live, Mark? And are they all reading the same books about software spirituality, and somehow linking that with dark matter and dark energy?

Since I don't think English is your first language, Mark, I'm sure I probably misunderstood what you were trying to say. Did anyone else here have more luck?


I'm not quite sure what Mark's on about either. And the longer he posts here, the more unsure I get ...


Sorry, my main question was about dark matter's interaction with gravity.

The article:

“...because it(dark matter) does not interact directly with itself or the gas except through gravity.��?

How do they know this?


Observational Evidence for Self-Interacting Cold Dark Matter

Phys. Rev. Lett. 84, 3760–3763 (2000)
[Issue 17 – 24 April 2000 ]

David N. Spergel and Paul J. Steinhardt
Princeton University, Princeton, New Jersey 08544

Received 20 September 1999

Cosmological models with cold dark matter composed of weakly interacting particles predict overly dense cores in the centers of galaxies and clusters and an overly large number of halos within the Local Group compared to actual observations. We propose that the conflict can be resolved if the cold dark matter particles are self-interacting with a large scattering cross section but negligible annihilation or dissipation. In this scenario, astronomical observations may enable us to study dark matter properties that are inaccessible in the laboratory.


Mark, the whole point of dark matter is that it was inferred as an explanation for why the observed velocity of stars orbiting galactic nuclei was higher than it ought to be (when the orbital velocity of the galaxy was calculated from its mass, which in turn was calculated by counting the observable stars). It's stuff that clumps gravitationally, but which is invisible and doesn't appear to interact directly with anything we can observe ... except to make galaxies too massive.

If we could see it or detect it other than in this sort of indirect way, it wouldn't be "dark".


In the scientific method, theories are tested against observations, and if a theory fails the tests, it is thrown away. I don't remember the bit where they invent invisible stuff to make the equations balance (and more of it than the observed matter in the universe), or the bit where, if that doesn't work, they invent even more of another kind of invisible stuff.

It reminds me of crystal spheres and epicycles.

Being slightly less cynical, the negative-pressure business seems reminiscent of surface tension. It costs energy to make each square centimetre of air-water boundary, consequently the world seems reluctant to produce more boundary than is necessary and at small scales the boundary tends to be smoothish.

Observations seem to show that it costs energy to make each cubic centimetre of space. I guess this means that regions of high curvature will tend to be pulled smooth.


In this case, the theory that had to be thrown away (quite some time ago) was: "the whole universe is made of matter with interactions very like what we see around us normally."

We have strong indications that gravity works on a very broad scale, as we can see signs of gravitational interactions between galaxies, but both matter within the galaxies and galaxies themselves are interacting as if the galaxies weigh more than they should based on the matter we can observe through other means. This particular galactic cluster collision serves to show that there is mass there which is neither the stars nor the gas. All the attempts to patch up our theories of cosmology with, say, weird adjustments to gravity or to Newton's laws which should apply only on certain scales haven't worked, and have been jettisoned, just as you noted they should be.

This isn't really much different than, say, Rutherford's inference that atoms must be made of really tiny nuclei plus a lot of empty space, by considering the way particles bounce back from a thin film when it's bombarded with electrons or beta particles. That sure did draw a lot of outraged "common sense" back at the turn of the last century, though; how dare Science tell us that matter isn't solid, when all the evidence of our senses and logic says the reverse!

I think the scientific jury is still out on "dark energy" and a lot of physicists are still looking for a better answer and explanation on that one. The problematic observations, though, are showing that the expansion of the universe is accelerating when the standard theories predict it should be slowing due to gravity. Make your "energy to make each cubic centimetre" a real mathematical expression or law which fits all observed data well, instead of a quick handwave, and you can probably win a Nobel. If you can't match the observed data, though...


This could easily be wrong, but my impression is that the argument for non-self-interacting dark matter is as follows: if it self-interacted, at least in the frictional way baryonic matter does, it would collapse down into rotating disks, just like galaxies and solar systems do. But in order to account for observed galactic rotation velocities without changing Newtonian gravity, a visible galaxy has to be embedded in a sphere of matter. A sphere is what conservational of angular momentum and no frictional collisions gives you, hence non-self-interacting dark matter.

The space opera webcomic Schlock Mercenary has also had storylines involving conflict with dark matter entities. Recommended.


Dark Matter and "Hobbit Galaxies"
[by the way, Dr. Thomas McDonough had a fine novel, The Missing Matter, Spectra, 1991, with a fascinatingly different hypoethsis of the phenomenon conventionally taken to indicate dark matter. His is multidimensional, and not string theory, either. It was the last book "discovered by" Isaac Asimov for his imprint].


Sky survey nabs four new Milky Way satellites

* 09:00 28 August 2006
* news service
* David Shiga

One of the four new dwarf galaxies is seen here in an illustration generated from sky survey catalogue data. Foreground stars have been removed (Image: V Belokurov/IoA Cambridge/SDSS)
Enlarge image

One of the four new dwarf galaxies is seen here in an illustration generated from sky survey catalogue data. Foreground stars have been removed (Image: V Belokurov/IoA Cambridge/SDSS)

Four new satellite galaxies of the Milky Way have been discovered, bringing the total known to about 20. The pace of new discoveries suggests that many more such satellites remain unknown, which would present a serious challenge to models of dark matter as "warm", fast-moving particles.

The satellites are dwarf galaxies a few hundred to a few thousand light years across. The tiny galaxies are thought to be the building blocks of large galaxies, such as our own Milky Way – which is about 100,000 light years across.

So astronomers try to study them to understand more about how this merger process took place. Their exact numbers could also tell astronomers how easily dark matter forms clumps, which might offer clues about the nature of this mysterious substance, whose presence is detected only by its gravitational effect on ordinary matter and light (see Cosmic smash-up provides proof of dark matter).

But the small galaxies are faint and difficult to observe – they tend to get lost behind the throngs of foreground stars from the Milky Way. Just two years ago, only 10 satellite galaxies were known, with the exact classification of some objects in dispute.
"Like hobbits"

Now, thanks to new observing techniques and detailed sky maps such as the Sloan Digital Sky Survey (SDSS), that number has doubled. Astronomers use SDSS to look for the particular types of stars expected to lie in dwarf galaxies, then detect the dwarfs as slight "overdensities" in these types of stars – patches of the sky where there are more of the stars than in surrounding areas.

The four new discoveries were made by a team led by Vasily Belokurov of the University of Cambridge, UK. Named after the constellations in which they were found – Coma Berenices, Canes Venatici II, Hercules, and Leo IV, all of them lie between roughly 100,000 and 500,000 light years from Earth.

The largest and smallest are Hercules and Coma Berenices, which are about 1000 and 200 light years across, respectively. Like most of the other dwarfs discovered by SDSS, the new finds are much smaller and fainter than the 10 dwarfs that were known previously, Belokurov says. "They should not really be called dwarfs – they are more like hobbits," he told New Scientist.

Based on the number of dwarf galaxies SDSS has found so far, the researchers estimate that there may be a total of about 50 small satellites around the Milky Way.
Diverging models

If that number is correct, it could provide clues as to the nature of dark matter, says James Bullock of the University of California, Irvine, US, who was not part of the team.

Different models of dark matter make different predictions about how easily it clumps together. The particles in so called "warm" dark matter models move too quickly to be easily pulled into clumps by gravity, while slower-moving "cold" dark matter particles congeal into clumps easily.

Studying the size distribution of galaxies can help distinguish between the two models because galaxies are thought to form in the middle of concentrations of dark matter. So finding many small galaxies suggests dark matter clumps together easily – without the need for a lot of mass to hold itself together.

"If there really are 40 to 50 clumps out there, it really starts to look bad for warm dark matter models," Bullock told New Scientist.

That contradicts other recent observations of the movement of 12 of the satellite galaxies, which suggested that dark matter is made of warm particles (see 'Tepid' temperature of dark matter revealed).


The optical/xray/dark matter picture has been my desktop background for the last week. I worked on the 2nd xray satellite observatory (OSO-7) 72-74, mostly looking at clusters of galaxies, unbelievable now to see a picture of a cluster collision with an xray shock wave.

Re dark energy, I also have had a hard time getting my brain wrapped around it. I grepped my blog, here's what was there:

January 24, 2004: Just finished Feb Scientific American. Four cosmology articles. Dark energy is just too weird. Had one thought, re the big bang was decelerating until 5 billion years ago, but is now accelerating. Maybe the expansion has a 1st harmonic vibration, the expansion alternating between accelerating and decelerating. Basically, the echo of the inflationary period. Interesting concept, tho, that the big bang is accelerating because gravity is leaking away to other dimensions or branes. Seems like that would violate conservation of something tho.

May 12, 2003: The latest Sky & Telescope has an article on new cosmologies, talking about parallel universes on membranes (branes). These could correspond to the 6 extra tiny dimensions of string threory. And the dark energy, which has seemed like a total kludge to me, is gravity from adjacent branes -- gravity is the only force that crosses branes, which is why it's so weak compared to the other forces (electomagnetic, weak, stong).

I think my conclusion, the last time I thought about it, was that dark energy represented in some way the interaction of our universe with the multiverse. Still seems like a kludgy cop-out to me to have to invoke other universes because we can't get our own to balance.


Chris Heinz is correct that Lisa Randall et al predict energy (as gravitons) leaking off our 4-D brane in 5-space or above, and that gravitons can wander in from other branes. Further, this (not technically a multiverse) would appear as a violation of conservation of energy and conservation of momentum (linear and angular) in a laboratory in our brane.

I have published refereed papers that the large Hadron Collider might, as soon as 2007, in searching for the Higgs boson, also show apparent violations of conservation of momentum when particle-antiparticle pairs are generated with complex (non-real) mass in comving reference frame. This would also happen for a while after the Big Bang, thus cooling the cosmos faster than otherwise allowed, and could happen in other high-energy events at supernovae, black holes, and the like.

I write this kind of thing in Physics papers, and hope to later show them in fiction, with characters, narrative, and avoidance of expository blocks.

Lisa Randall was in the very same class at Stuyvesant High School (my alma mater) as Brian Greene -- what are the odds of this Dickensian coincidence?


Astrophysics, abstract

From: Garry Angus
Date (v1): Wed, 6 Sep 2006 17:50:00 GMT (32kb)
Date (revised v2): Thu, 7 Sep 2006 16:46:57 GMT (32kb)

On the Law of Gravity, the Mass of Neutrinos and the Proof of Dark Matter
Authors: Garry W. Angus (1), HuanYuan Shan (2,1), HongSheng Zhao (1,2), Benoit Famaey (3) ((1) University of St. Andrews, (2) NAOC, Beijing, (3) Universite Libre de Bruxelles)

Comments: 5 pages, 1 figure, submitted to ApJL

We fit the weak lensing map of the bullet merging galaxy cluster 1E0657-56 in a class of gravity theories interpolating between GR and MOND (General Relativity and Modified Newtonian Dynamics), so to infer the nature and amount of non-baryonic particles with less dependence on the validity of GR on cluster scales. In agreement with Clowe et al. (2006), we show that the bullet cluster is dominated by a non-baryonic component - in GR as well as in MOND. This result adds to the number of known generic pathologies for a purely baryonic MOND, namely the inabilities in explaining dynamics of other X-ray emitting clusters, and in fitting the Cosmic Microwave Background (CMB). A plausible resolution of all these issues is the "marriage" of MOND with ordinary neutrinos of maximum mass 2eV, which the Karlsruhe Tritium Neutrino (KATRIN) experiment can falsify by 2009. Some issues of consistency with earlier analysis of the bullet cluster are also raised.



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This page contains a single entry by Charlie Stross published on August 22, 2006 9:50 PM.

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