Evidence that Dark Matter is real stuff

[Martin]

a NASA press conference is scheduled for Monday 21 August (in about 4 days) to announce the first observations confirming that Dark Matter is real substance and not just a modified gravity effect

 

Here are some pretty nice pictures. It is PDF of a talk Maxim Markevitch gave, scroll down to see the pictures.

 

http://cxc.harvard.edu/symposium_2005/proceedings/files/markevitch_maxim.pdf

 

the pictures show the two clusters of galaxies colliding, and the very hot ball of gas (ordinary matter)

and then they show the LENSING BACKGROUND mapping the levelcurves of the dark matter density

to show that the dark matter has passed through----and so has been spatially separated from the gas.

so it is not just a modified gravitational effect of the gas, becauase that would be located around the gas.

 

Maxim's team is part of the CHANDRA Xray astronomy program. The first clear observation of Dark Matter as such, not just as one possible hypothetical explanation, is likely to be a big deal. So the press conference will have NASA people and CHANDRA project leaders as well as Maxim's group that actually did the research.

 

Here are some announcements and discussion. I got the news from John Baez, but it has been all over blog-land.

 

NASA announcement http://www.nasa.gov/home/hqnews/2006/aug/HQ_M06128_dark_matter.html

Baez TWF 238 http://math.ucr.edu/home/baez/week238.html

 

Conference presentation abstract

http://www.cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf

Dark matter and the bullet cluster

M. Markevitch (1), S. Randall (1), D. Clowe (2), A. Gonzalez (3), M. Bradac (4) (1) Harvard-Smithsonian Center for Astrophysics, (2) University of Arizona, (3) University of Florida, (4) KIPAC, Stanford University

 

"1E0657-56, the "bullet cluster", is a merger with a uniquely simple geometry. From the long Chandra X-ray observation which revealed a classic bow shock in front of a small subcluster, we can derive the velocity of the subcluster and its direction of motion. Recent accurate weak and strong lensing total mass maps clearly show two merging subclusters, including the host of the gas bullet seen in X-rays. This cluster provided the first direct, model-independent proof of the dark matter existence (as opposed to any modified gravity theory) and a direct constraint on the self-interaction cross-section of the dark matter particles. I will review these and other related results."

------------------------

 

I've met and talked to Maxim a couple of years ago. At that time he was already doing Xray astronomy as his specialization. He is a young Russian scientist working the US, who likes the outdoors and is a nice quiet modest person. Easy to be around. He takes very fine photographs

http://hea-www.harvard.edu/~maxim/personal/

I especially like the underwater coral reef pictures from Bonaire, and the art pictures from Greece.

He has a good quiet sense of humor, one can see him in this picture from Santa Fe VLA telescope site

he is the one in blue

http://hea-www.harvard.edu/~maxim/personal/SFE04/2761.html

=====================

[Royston]

Good stuff, I'm guessing the title Dark Matter will change, now it's lost it's anonymity. The link I found doesn't appear to have any images embedded, were you referring to the recent galaxy clusters ?

 

http://chandra.harvard.edu/photo/2006/1e0657/

[Martin]

Snail it is strange you dont get pictures from that PDF link. Already there is a stunning one on page 2

... scroll down to see the pictures.

 

http://cxc.harvard.edu/symposium_2005/proceedings/files/markevitch_maxim.pdf

 

the pictures show the two clusters of galaxies colliding' date=' and the very hot ball of gas (ordinary matter)

[/quote']

 

I get several pictures from that link, illustrating what they are talking about. Maybe try again?

[Royston]

Thanks Martin, I didn't see a link on your original post...so I tried looking for one myself, obviously the wrong one.

 

Wow, I like the eureka moment showing the offset between gas and mass peaks (1E 0657-56.) Thanks for the links.

[[Tycho?]]

Neato.

 

Now that we know it exists: what the hell is it?

[Dr. Dalek]

']Neato.

 

Now that we know it exists: what the hell is it?

Yeah, is it a chemical or an exotic form of matter, plasma, solid, condensate,

[Martin]

it has to be outside the standard version of particle physics

because it does not interact

 

it does not absorb or radiate light

it doesnt have collisions---a whole bunch of it just past right thru a whole bunch of ordinary matter as if it wasnt there and we saw this----technically it has an unprecedentedly low (perhaps zero) cross-section

 

"chemicals" and "condensates" are made of usual baryonic matter----electrons, quarks combined into protons etc.----DM is not that

 

even neutrinos have been ruled out----that was one idea they had earlier

 

"plasma" wouldn't explain anything because then a plasma of WHAT?

 

This has to be a totally different KIND of matter, made of a DIFFERENT FUNDAMENTAL PARTICLE. One that is not included in the Standard Model scheme of fundamental particles. It can't just be an "exotic form" of the kind we know.

 

Another thing to remember is that the dark matter that has been inferred to make up some 23 percent of the U is COLD. It is not like a plasma, which is where the particles are moving randomly around very fast.

 

the cloud of DM that, for example, is accompanying our own galaxy is stuff that is not moving around very much relative to us. It is drifting, not whizzing. That is what "cold" means when they speak of COLD DARK MATTER (CDM)

 

What makes this fascinating to me is that it means the list of particles, the standard model scheme, has to be enlarged somehow to include one or more that DO NOT INTERACT with our kind EXCEPT BY GRAVITY.

 

they are immune to any of the other forces, they are not affected by electric or magnetic attraction or by the forces that hold the nuclei of atoms together and govern decay.

 

Every other kind of particle we know DOES INTERACT with other particles in some way besides mere gravity.

 

this doesnt (if it really exists). all it can do is gradually collect in big clouds around galaxies made of our kind----drawn in by our gravity.

 

it acts like it is "anaesthetized" so that all it can feel is gravity---it can't feel any other forces and it cant have collisions with ordinary stuff.

 

I still remain skeptical that DM is a new kind of substance. It might still be an artifact of some modification of the law of gravity. But this has become less likely, so I am prepared to be convinced. For me, the key thing that has been demonstrated is that it has MOMENTUM so it can coast thru a galaxy and come out the other side and that it apparently can occupy a radi cally SEPARATE LOCATION from the ordinary matter that it was originally surrounding.

[CPL.Luke]

what prevents this stuff from being very very cold clouds of hydrogen and or iron/other elements?

[Martin]

what prevents this stuff from being very very cold clouds of hydrogen and or iron/other elements?

 

there are ways of "seeing" cold clouds of gas. the atoms and molecules interact with light. radio astronomy has been mapping neutral hydrogen for some decades.

 

you saw the pictures, the two clusters of galaxies collide, their gas clouds collide and light up from the collision energy, but the DM just PASSED CLEAN THROUGH

 

you have more questions than I can answer CPL

I enjoy it but I have to do some stuff.

I hope swansont or other folks will step in on this

 

btw Severian in real life is an expert on particles which might account for DM------if "supersymmetry" is real then their might be really peculiar particles that nobody has seen yet which could act like this stuff. maybe.

 

the next 3 or 4 years in physics/astronomy are going to be very exciting.

Tell your friends they should all take a physics and an astronomy class if they can so as to get a ringside seat:-)

[Dr. Dalek]

What makes this fascinating to me is that it means the list of particles, the standard model scheme, has to be enlarged somehow to include one or more that DO NOT INTERACT with our kind EXCEPT BY GRAVITY.

So I assume that means that Dark Matter can interact with other Dark Matter through means appart from gravity?

 

Or is that just an uncertain guess?

[Martin]

So I assume that means that Dark Matter can interact with other Dark Matter through means appart from gravity?

 

Or is that just an uncertain guess?

 

I don't know that it is certain. I think it is an interesting question.

 

In the past couple of days, reading background to the upcoming Monday press conference, I came across an abstract where one of the issues was

ESTIMATING THE SELF-INTERACTION CROSS SECTION

 

I personally don't know (and can't guess) how that would be done.

 

for me, it is too early to guess. I entertain all reasonable possibilities and just pay attention to what questions people are asking.

I suppose it is possible for a DM particle to have ZERO interaction cross-section with itself! To not interact at all.

 

Also I suppose it would be possible for every DM particle to repel every other aside from gravity attraction which is very weak.

 

I don't have any conjectures----it seems to me that many things are possible because so little is known at this point.

 

Maybe someone else has more definite ideas.

===================

 

your question provokes an idea. this is an amusing possibility:-)

suppose it turns out from these pictures that the cloud of DM,

which the collision has now disconnected from the Baryonic matter cloud that it originally surrounded, and which is now ADRIFT, shows signs of DISPERSING. you know, as if it needed the extra baryonic matter providing extra mass to help it collect-----and deprived of a core of baryonic matter it drifts apart. then maybe one could try to calculate what other forces affect DM (if any) besides gravity.

 

in any case it will be fascinating to get their report on Monday!

[Martin]

anybody interested in the DM issue should check this out

 

http://cosmicvariance.com/2006/08/21/dark-matter-exists/

 

and especially watch this movie of two clusters of galaxies colliding

http://chandra.harvard.edu/photo/2006/1e0657/media/

scroll down to number 4

where it says "4. Animation of Cluster Collision MPEG | Quicktime"

 

(it shows what they think really happened to bring about the situation that the Chandra team has been observing)

[Bettina]

anybody interested in the DM issue should check this out

 

http://cosmicvariance.com/2006/08/21/dark-matter-exists/

 

and especially watch this movie of two clusters of galaxies colliding

http://chandra.harvard.edu/photo/2006/1e0657/media/

scroll down to number 4

where it says "4. Animation of Cluster Collision MPEG | Quicktime"

 

(it shows what they think really happened to bring about the situation that the Chandra team has been observing)

 

"Einstein continues to have the last laugh"

 

I liked that quote. Thanks Martin this is way cool.

 

I can't view the images because I get this..This Account Has Exceeded Its CPU Quota

 

I'll try tommorrow.

 

Bee

[Martin]

 

I can't view the images because I get this..This Account Has Exceeded Its CPU Quota

 

probably you can get it tomorrow but if not, one of us can describe it---it wouldnt be so difficult to describe the collision.

 

hey it IS way cool, one of the neatest astronomy observations that's happened all year

 

I am so proud that I spent some time with Maxim Markevitch (even if it was vacation not science). he's a neat guy.

[Bettina]

probably you can get it tomorrow but if not' date=' one of us can describe it---it wouldnt be so difficult to describe the collision.

 

hey it IS way cool, one of the neatest astronomy observations that's happened all year

 

I am so proud that I spent some time with Maxim Markevitch (even if it was vacation not science). he's a neat guy.[/quote']

 

I finally got to see the .mpg movie and read some of the questions posed in the articles. I particulary liked questions 31 and 35 that I pasted below.

 

31. Acolyte on Aug 21st, 2006 at 6:19 pm

 

....So, the question is: how certain are we that the thing that did the nudging is right there close to the galaxies and not somewhere else? Is it perhaps that there is a unique solution to the bending of light from all the background galaxies, so that it could only be there?

 

35. Paul on Aug 21st, 2006 at 7:47 pm

 

Since you guys ignore the EM force which is 10^39 greater than G I think you guys should wake up! This animation shows exactly what you would expect from an EM Field interaction and the X-Rays would be the natural conclusion. This isn’t Proof of Dark Energy or Dark Matter. It is just the illogical conclusions of people who keep refusing to see the obvious electromagnetic interactions of the Universe. It qualifies more as Garbage In and Garbage Out of a computer simulation than the realization that standard Maxwell interactions predict this and nothing else really does. Of course Red Shifts will not be velocity either. They are simple optical tuning effects in space. That will crank out all of the nonsense of a failing theory.

 

So, Martin, is what we just witnessed really dark matter?

 

Bettina

[Martin]

 

we just saw several theories of "modified gravity" (like the famous MOND and the variant called TeVeS) falsified

 

the way the game is played is by eliminating guesses

 

the fact that galaxies turn so fast that they would fly apart if they only had the mass of the matter we can see (and related facts about galaxy clusters, and about lensing) have been explained so far in two main ways

 

A. there is an invisible particle that doesnt interact with the others

B. the law of gravity is different, like MOND

 

other explanations are possible and some have cropped up but those are the main ones on the table, maybe for completness one should include

 

C. neither of the above

 

======================

 

what this observation mainly does it it totally kills MOND and all the simple modifications of Newton law and Einstein GR gravity law that have been proposed

 

but that doesnt prove there is an invisible dark matter particle

 

there is still very much more exciting work and discoveries to do regarding this puzzle of the too-fast whirling galaxies and the stability of clusters and the lensing and all

 

one has to learn to take one's excitement in small doses in a process of gradual elimination of bads and narrowing down on goods

 

Answer: no, we didnt just see dark matter, but it was a big step anyway

 

I'm happy MOND can be ruled out. I thought it had a real good chance so they didnt just knock off something trivial, they shot down something significant and that is progress.

 

==================

 

maybe my thread headline was misleading. Yes it is EVIDENCE pointing in a definite direction, but it is not yet FINAL PROOF.

 

BTW those New Scientist articles are great. appreciate your calling attention to them.

[J.C.MacSwell]

How much stronger gravity would be required, in the average galaxy, to hold it together?

 

Put another way, if we assume that the mass that we can observe is all there is in each galaxy, how much smaller would the average galaxy have to be to hold it together?

 

Could our mass estimates be correct, but our distance measurements be off enough to allow for the difference?

 

Could this be evidence of space curvature over a certain scale? (If I was on the North Pole, could see the equator along the curve etc. and correctly estimated the distance to the equator but thought the Earth was flat, I would overestimate the length of something lying along the Equator)

[darkangel199]

could it be a type of "shadow of something from a higher dimension? like if we stick out hands in a two dimensional world we will also appear to pass through objects without interacting with them at all and our hands would certainly seem "exotic and new" because of the amazing things we can do. Couldn't it just be some type of higher dimensional form of matter or some kind of higher dimensional thing?

 

and isnt it theorized that gravity passes through dimensions, which is why its so weak? maybe this higher dimensional thing is effected by gravity because gravity passes through dimensions? (sorry for my very unscientific sounding idea.)

[Martin]

How much stronger gravity would be required' date=' in the average galaxy, to hold it together?

...[/quote']

 

MacSwell and darkangel, the questions you both ask are interesting and provocative but i am not prepared to respond (partly because of other things happening) and hope someone else will step in.

 

MacSwell just to clarify---you may already understand this perfectly so it may be a repetion---when one looks at a spiral galaxy edge on, one can compare the redshift on the RHS with the redshift on the LHS and from the DOPPLER component of that, one can measure the SPEED that one edge is going away and the other edge is coming towards

 

so by DOPPLER one constructs what is called a ROTATION CURVE which tells the speed that stuff is revolving around center, at various distances.

and from that one can infer that there must be more mass (several times more mass than one can see) just to keep it from flying apart

 

just like the solar system would, if the sun were suddenly only 20 percent as massive as it really is.

from the speed that stuff circles around one can calculate what extra mass there must be in the galaxy

 

analogous calculations can be made in other situations---like galaxies that are not seen edge on but only partly tilted---like clusters of galaxies

 

the required "dark matter" always seems to be several times like 3-10 times what the ordinary matter is that one can see or infer is there.

 

I cant be more precise at the moment, or evaluate alternative explanations. It is pretty clear that it is going to lead people to some new physics, possibly quite radical new physics----I think quantum gravity is going to enter in.

[J.C.MacSwell]

MacSwell and darkangel' date=' the questions you both ask are interesting and provocative but i am not prepared to respond (partly because of other things happening) and hope someone else will step in.

 

MacSwell just to clarify---you may already understand this perfectly so it may be a repetion---when one looks at a spiral galaxy edge on, one can compare the redshift on the RHS with the redshift on the LHS and from the DOPPLER component of that, one can measure the SPEED that one edge is going away and the other edge is coming towards

 

so by DOPPLER one constructs what is called a ROTATION CURVE which tells the speed that stuff is revolving around center, at various distances.

and from that one can infer that there must be more mass (several times more mass than one can see) just to keep it from flying apart

 

just like the solar system would, [b']if the sun were suddenly only 20 percent as massive as it really is.[/b]

from the speed that stuff circles around one can calculate what extra mass there must be in the galaxy

 

analogous calculations can be made in other situations---like galaxies that are not seen edge on but only partly tilted---like clusters of galaxies

 

the required "dark matter" always seems to be several times like 3-10 times what the ordinary matter is that one can see or infer is there.

 

I cant be more precise at the moment, or evaluate alternative explanations. It is pretty clear that it is going to lead people to some new physics, possibly quite radical new physics----I think quantum gravity is going to enter in.

 

Thanks Martin

 

So if say the mass was estimated correctly but the distances overestimated by a factor of 2 (thus galaxy volume by 8 and gravitational force by 4) then "3 masses" worth of dark matter would be "found" (no longer needed)

 

Or in your example of a 20% mass sun if the distances were reduced by 55% then everything would stay in balance.

 

Is there any correlation between "dark matter required" and the distance away?

[Severian]

So I assume that means that Dark Matter can interact with other Dark Matter through means appart from gravity?

 

It can interact through weak interactions. The important thing is that it shouldn't be charged. By definition, anything charged interacts with (ie. emits) light, so any charged matter is not 'dark'.

 

Therefore by definition, dark matter must be a neutral particle.

 

If it didn't interact with any other particles it would be pretty much impossible to produce in colliders. But since all the dark matter candidates do have weak interactions, we hope to produce them at the LHC. (The prime candidate is still a nuetralino.)

[J.C.MacSwell]

It can interact through weak interactions. The important thing is that it shouldn't be charged. By definition' date=' anything charged interacts with (ie. emits) light, so any charged matter is not 'dark'.

 

[b']Therefore by definition, dark matter must be a neutral particle.

[/b]

If it didn't interact with any other particles it would be pretty much impossible to produce in colliders. But since all the dark matter candidates do have weak interactions, we hope to produce them at the LHC. (The prime candidate is still a nuetralino.)

 

I should know this, but neutrons themselves aren't candidates as they radiate via their charged quark components, right?

[Severian]

Yes, that's right.

[[Tycho?]]

What is a charged quark component?

[Martin]

'']What is a charged quark component?

 

I think MacSwell didnt mean a component of a quark, he meant that the nucleons can radiate via their quarks, which are charged.

 

even if overall the thing is neutral, if it has some part which is charged then it can radiate.

 

think that's what he meant, so no serious confusion hopefully