## Saturday, March 28, 2015 ... /////

### Dark matter: Science Friday with Weinberg, Hooper, Cooley

The background is temporarily "nearly white" today because I celebrate the Kilowatt Hour, also known as the Electricity Thanksgiving Day. Between 8:30 and 9:30 pm local time, turn all your electric appliances on and try to surpass one kilowatt. By this \$0.20 sacrifice, you will fight those who want to return us to the Middle Ages and who organize the so-called Earth Hour.

Ira Flatow's Science Friday belongs among the better or best science shows. Yesterday, he hosted some very interesting guests and the topic was interesting, too:
Understanding the Dark Side of Physics
The guests were Steven Weinberg, famous theorist and Nobel prize winner from Austin; Dan Hooper, a top Fermilab phenomenologist; and Judi Cooley, a senior experimental particle physicist from Dallas

And if you have 30 spare minutes, you should click the orange-white "play" button above and listen to this segment.

It doesn't just repeat some well-known old or medium-age things about dark matter. They start the whole conversation by discussing a very new story so that even listeners who are physicists may learn something new.

An observation was announced that imposes new upper limits on the self-interaction of dark matter. If it interacts with itself at all (it of course interacts gravitationally but if there is another contribution to its self-interaction), the strength of this force is smaller than an upper bound that is more constraining than those we knew before.

See e.g.
Hubble and Chandra discover dark matter is not as sticky as once thought
Dark matter does not slow down when colliding with each other, which means that it interacts with itself even less than previously thought.

The nongravitational interactions of dark matter in colliding galaxy clusters (by David Harvey+3, Science)
If you remember the "bullet cluster" that showed the existence of dark matter – and its separation from visible matter, they found 72 similar "clusters" and just like the 72 virgins waiting to rape a Muslim terrorist, all of them make the same suggestion: some dark matter is out there. They say that the certainty is now 7.6 sigma when these 72 observations are combined.

However, the dark matter location remains close enough to the associated visible stars which allows them to deduce, at 95% confidence level, that the cross section per unit mass isn't too high:$\frac{\sigma_{DM}}{m} \leq 0.47\,{\rm cm}^2 / {\rm g}$ The dark matter just doesn't seem more excited by itself than it is by the visible matter. Theories with "dark photons" are the first ones that are heavily constrained and many natural ones are killed. But maybe even some more conventional WIMP theories may be punished.

I think that if you have worked on my proposed far-fetched idea of holographic MOND, you have one more reason to increase your activity. And I guess that all axions are just fine with the new finding.

Weinberg clarifies the situation – why dark matter isn't understood too well (it's dark!) etc. – very nicely but many other things are said in the show, too. When the two other guests join, they also discuss other dark matter experiments, dark energy, gravitational waves, string theory etc.

Funnily enough, a layman listener wanted the guests to describe the cataclysms that would occur if the dark matter hit the Earth. The response is, of course, that dark matter hits our bodies all the time and nothing at all happens most of the time. I can't resist to ask: Why would a layperson assume that dark matter must be associated with a "cataclysm"?

People have simply liked to think about cataclysms from the very beginning of primitive religions, and the would-be modern era encourages people to unscientifically attribute cataclysms to many things – carbon dioxide was the most popular "culprit" in the recent decade (and of course, there are many retarded people around us who still believe that CO2 emissions are dangerous). People just can't get interested in something if it is not hyped by a talk about catastrophes.

At one moment, Weinberg (who also promoted his new book about the history of physics, To Explain the World) wisely says that dark matter is preferred because it's also supported by some precision measurements of the CMB – and because it's much easier and more conservative to introduce a new particle species than to rewrite the laws of gravity. Flatow is laughing but it is a serious matter. Flatow is a victim of the populist delusion that there are so many particles which must mean that they were introduced because they don't have any natural enemies. But particles are introduced when they are seen or at least glimpsed.

Lots of particles are used by theoretical physicists because they are being seen experimentally every day and even the new particles that are not sharply seen yet are being introduced because they explain some observations or patterns in them – in this sense, the particles are being seen fuzzily or indirectly (at least when the theorist behind them has any quality). And all theories involving new particles compete with other theories involving other new particles so it's no "unrestricted proliferation of new concepts without standards". Instead, it's the business-as-usual science.

The real question is whether a rather conservative theory with new particle species is more likely – and ultimately more true – than some totally new radical theory that denies that physics may be described in terms of particles and fields. Of course that a true paradigm shift may be needed. But the evidence that it is so – or the ability of the existing, radically new frameworks to convince that they are on the right track – isn't strong enough (yet?) which is why it seems OK to assume that the discrepancies may be fixed with some new particle species.

Also, Flatow is laughing when Weinberg calls the visible matter a contamination – because it's significantly smaller than dark matter which is still smaller than dark energy (by the magnitude of the energy density). Most laymen would find this laughable, too, and it's because the anthropocentrism continues to be believed by most laymen:

We are at the center of the Universe and everything we know from the everyday life must play an essential role in the most profound structure of the Universe. But as science has been showing for 500 years or so, this simply ain't so. If I ignore the fact that the Czechs are the ultimate average nation in the world, we the humans are a random update to one of many long branches of the evolution tree that arose from some rather random complex molecules revolving around an element that is not the most fundamental one, and the whole visible matter around us is a contamination and the clump of matter where we live is a mediocre rock orbiting a rank-and-file star in an unspectacular galaxy – and the Universe itself may be (but doesn't have to be!) a rather random and "not special" solution of string theory within the landscape.

Hooper mentions the 1960s and 1970s as the golden era of classical physics – and the recent years were slower.

At the end, Cooley and Weinberg discuss string theory – experimenters can't test it so the theory isn't useful for them but it's right that people work on it, and it has never been the case that all predictions of theories had to (or could) be tested. Weinberg wraps the discussion with some historical examples – especially one involving Newton – proving that the principle that all interesting predictions must be testable in the near future is misguided.

The short discussion on sciencefriday.com is full of crackpots irritated by the very concept of "dark matter" and the research of dark matter.

Off-topic: One of the good 2015 Czech songs, "[I Am Not a] Robotic Kid" (lyrics preaches against parents' planning their kids' lives and against conformism). Well, I should say "Czech-Japanese songs" because the leader of Mirai, the band, is Mirai Navrátil – as the name shows, a textbook example of a Czech-Japanese hybrid. He actually plans to sing in Japanese as well. It's their first song.

#### snail feedback (24) :

Thanks for this nice report Lumo ;-)

The remark involving calling visible matter a pollution could have made me chuckle in approvement too, because I would see it as another example of how theoretical physicists are able to state things in a fun cool and exactly to the point way, very often.

TRF articles are always full of such cool fun remarks and Lumo also knows how to properly use smileys ...;-P

reader Luboš Motl said...

Thanks, Dilaton, and yes, such explanations are cool! But not everyone wants to learn new cool surprising things; someone prefers to hear comments that he is important whether they are true or not! ;-)

Luboš, does the CDM theory provide some lower estimates for the DM cross section? AFAIK the CDM theory expect the DM is thermalized, so it has to interact with itself to get thermalized.

reader Luboš Motl said...

Yes, Pavle, a great point. CDM to work well has to be self-interacting. The mean free path in normal conditions should be between a kiloparsec and a megaparsec which excludes vanishing interactions.

reader NetDost - Visual Inspiration said...

sad story and a beautiful sculpture

reader Leo Vuyk said...

How do we know that these clusters have bounced already? that pink x-ray gas could also be based on an electric Birkeland Alfven current circuit between distant dark matter black holes sucking out the gas of the clusters. see:

http://vixra.org/pdf/1503.0186v2.pdf and below

Hmm, how is that that Sean Carroll, Dark Matter Expert extraordinaire, didn't insinuate himself into the program? :)

reader Uncle Al said...

Dark matter is a changing curve fit
with no apparatus detection. Milgrom acceleration is one universal number
arising from classical physics and confirmable in existing apparatus within 90
days. Look
(DOI: 10.5281/zenodo.15107). The rotor is 7 cm in diameter. Assay spacetime
geometry with test mass geometry (6.68×10^22 pairs of 9-atom enantiomorphic unit cells,
opposite sides of the rotor). Mercury's orbit was not anomalous, it was
diagnostic.

reader Luboš Motl said...

Good that you mentioned that name. I mentioned "dark photons" as the textbook example of a concept that was mostly falsified by the new finding. A big part of Sean Carroll's writing about dark matter is about dark photons, of course. ;-)

http://www.preposterousuniverse.com/blog/2013/03/12/more-messy-dark-matter/

Lubos writes: "We are at the center of the Universe and everything we know from the everyday life must play an essential role in the most profound structure of the Universe. But as science has been showing for 500 years or so, this simply ain't so. If I ignore the fact that the Czechs are the ultimate average nation in the world, we the humans are a random update to one of many long branches of the evolution tree that arose from some rather random complex molecules revolving around an element that is not the most fundamental one, and the whole visible matter around us is a contamination and the clump of matter where we live is a mediocre rock orbiting a rank-and-file star in an unspectacular galaxy – and the Universe itself may be (but doesn't have to be!) a rather random and "not special" solution of string theory within the landscape."

Not trying to be perverse, but does that mean murder is no big deal? Of course, agree that from a merely scientific point of view, human beings are not very important in the grand scheme of things. But from a human/moral point of view about which all of us care a great deal, and about which science has nothing to say (David Hume's point about no ought from is), we are at the very center of creation. Hence to old rabbinic saying that when you murder another human being you destroy a world (or something like that, I forget the exact words).

I only bring this up because it annoys me when certain aggressive atheist types (and this does NOT include you) publically observe that humans are insignificant beings, as though this were some kind of ultimate moral judgment. Punch them in the mouth and they will feel differently. Not that I advocate that.

"Humans are insignificant in the whole universe" is a scientific fact following from current cosmological model ! It has nothing to do with whether you are theist or atheist. In fact this fact is quite important for eastern theists to argue that there could be a world beyond our little insignificant sensory perceptions!

reader Luboš Motl said...

Dear Luke, well, a murder, like a human, is insignificant from a cosmic or scientific viewpoint. On the other hand, science can in no way

LOL. Kudos for your "Electricity Thanksgiving Day" paragraph, I almost forget the ceremony :-D

Have to agree with you on the democracy thing. Before we went into Iraq and Afghanistan I couldn't imagine that people anywhere would not prefer to live in a free, democratic society with civil rights like we enjoy in the West. Truly the last few years have been a learning experience!

That's what you get when you don't travel and instead listen to CNN, Fox, etc.

reader Peter F. said...

I admit to feel a little bit put off or offended by the thought that I am a contaminant. %-{

:-)

reader Luboš Motl said...

Dear Peter, the claim that we are (and the visible matter is) a contaminant can't be interpreted in some universal moral, morally negative way.

It just means that without the visible matter, the composition of matter and density in the Universe would be simpler and in this sense "more pure", and the visible matter is a similarly small contribution as other things we call "contaminants".

It's incidentally very analogous to the situation of things like CO2. People also call it a contaminant although it's essential for life - a category of contaminants upon contaminants. Those are essential for us and things we love because we and things we love are "even more" contaminants in this counting of simplicity and majorities.

reader Leo Vuyk said...

Are you sure about no apparatus detection of dark matter?

http://vixra.org/pdf/1407.0001v2.pdf

reader Peter F. said...

Dear Lumo, it can be of importance to identify (be aware of) even weak knee-jerky responses within oneself. This was one such in me. I have also identified a current - probably to remain until I start rotting - primitive superstitious tendency in myself. It seems I enjoy sharing (possibly even provoking through doing so) some of my plain observations or odd ways of expressing them.
Actually, I most of the time am resisting letting fly of a frustration-fuelled angry attitude/trolling tendency! ;-)

If lubos characterizes something in a nasty way that makes you feel like you have to keep your mouth shut, then it's very likely an area he is being dishonest about. The attacks on laypeople now for anthropocentrism is almost certainly because Lubos doesn't want to think about the CMB anisotropy that has been around for over a decade that aligns the Solar System and ecliptic with the earliest and largest scale structures of the universe. It's not laypeoples fault this is the case. And it needs explaining as a matter of urgency.

The whole argument that we are not at the center of the Universe rests on the claim that Universe looks the same from any other location. But how do they know for sure it looks the same when they have never been at any other location? It's a conspiracy, I tell you.

reader Uncle Al said...

Leo, you have no empirical basis.