anti Dark Matter?

[chasmanian]

thank you so much nFA, for all you wrote.

it helps very much.
I am going to forget about what the wiki says.

"the metric expansion of the FLRW is happening everywhere. local clumps of matter can cause local deviations from expansion that retard it. the metric expansion is accelerating due to DE so that less bound clusters will stop holding together and only the tighter clusters remain."

would you please expand (har har) on this a little?
where is expansion retarded by local clumps of matter?
is space expanding right where I stand now?
certainly my body is not expanding, right?
and where is the expansion not retarded by local clumps of matter?

also, I would guess that DM is being thinned out by the expansion of space.
am I right about that?

[notFritzArgelander]

thank you so much nFA, for all you wrote.

it helps very much.
I am going to forget about what the wiki says.

"the metric expansion of the FLRW is happening everywhere. local clumps of matter can cause local deviations from expansion that retard it. the metric expansion is accelerating due to DE so that less bound clusters will stop holding together and only the tighter clusters remain."

would you please expand (har har) on this a little?
where is expansion retarded by local clumps of matter?

where there are dense enough clumps. DE is everywhere. so there is DE everywhere that is pushing expansion everywhere. normal matter and DM are clumpy and when they are dense enough oppose the expansion.

is space expanding right where I stand now?

yes. there is DE in the volume that you occupy that is driving expansion.

certainly my body is not expanding, right?

because the chemical bonds in your body are strong enough to hold it together despite the expansion.

and where is the expansion not retarded by local clumps of matter?

where chemical bonds or gravity are too weak to counteract the expansion.

also, I would guess that DM is being thinned out by the expansion of space.
am I right about that?

in part. DM comes in clumps too like in galaxy halos and clusters. where the density is high enough DM will resist the expansion, where it isn't it won't.

[chasmanian]

thank you so much, omg.

ok, so inside my body, the expansion is not happening.

but right next to my body, outside of it, is the expansion happening?

(which reminds me of a Groucho Marx line:
outside of a dog, a book is a man's best friend.

inside of a dog, its too dark to read.)

[notFritzArgelander]

thank you so much, omg.

ok, so inside my body, the expansion is not happening.

but right next to my body, outside of it, is the expansion happening?

i'd have to do a calculation to be sure but there is DE inside your body so expansion is being pushed, chemical bonds are keeping your body in place resisting the expansion. i don't know that there is enough mass energy in your volume to resist the expansion. :shrug: that's where i'd have to calculate to be sure. your body isn't being held together by gravity but by EM/chemical bonds. for larger objects that are in hydrostatic equilibrium (gravity dominates chemical bonds) then the expansion is surely effectively resisted.

so unless you are a large spherical planet (like Ceres, Vesta, or Pluto ), i'd have to say that the expansion is still there but chemical bonds can dominate. i'd have to do a calculation to be sure.

(which reminds me of a Groucho Marx line:
outside of a dog, a book is a man's best friend.

inside of a dog, its too dark to read.)

inside a dog all cats are black.

[notFritzArgelander]

PS don't confuse the expansion of spacetime with the motions of matter. i think that's what the author of the Wiki got confused about. the FLRW metric defines the comoving coordinates for the universe as a whole. matter follows geodesic curves within spacetime and needn't be "expanding" with spacetime. other forces are at play.

even distant galaxies have their own peculiar motions and do not mark out the expansion of space time.

[chasmanian]

thank you so much nFA.

you're a joy.

[chasmanian]

I always get confused when I see the words comoving coordinates.

I googled for a while, and finally found this.
I like it.
scroll down past the picture of Edwin Hubble:

https://geol105.sitehost.iu.edu/images/ ... iverse.htm

[notFritzArgelander]

I always get confused when I see the words comoving coordinates.

I googled for a while, and finally found this.
I like it.
scroll down past the picture of Edwin Hubble:

https://geol105.sitehost.iu.edu/images/ ... iverse.htm

Yes, I can see how that would be confusing. It's only OK "on the average" which is to say NEGLECTING peculiar motions of the galaxies w.r.t the FLRW coordinates. The galaxies in the illustration give the impression of being rigidly attached to the sphere of comoving coordinates. They aren't. They have their own motions with respect to the comoving coordinates defined by the FLRW metric. Thankfully the galaxies aren't stretching like dots painted on a balloon though! A better analog than galaxies painted onto a balloon would be ants free to walk about to different values of the comoving coordinates.

[chasmanian]

cool and awesome and good.
thanks alot nFA!!

[chasmanian]

nFA,

do you please possibly have any thoughts about the Cosmological Constant Problem?

https://en.wikipedia.org/wiki/Cosmologi ... nt_problem

[notFritzArgelander]

nFA,

do you please possibly have any thoughts about the Cosmological Constant Problem?

https://en.wikipedia.org/wiki/Cosmologi ... nt_problem

I have no worthwhile thoughts about it. There are no plans for a surprise visit to Stockholm. I do have thoughts about how approaches to the problem occasioned by reading your link. In summary, my views haven't changed much since the last time we discussed this on the old forum. Maybe some more but marginal and unimportant details?

The first thing that struck me as remarkable in your link was that Walther Nernst thought about it back in 1916 10-11 years before QM was put on a sound physical basis. If I knew this I had forgotten it. I may have to check it out. I discovered I can ALMOST read the German. So that's nice.

The second thing is that almost all of the proposed solutions are speculative handwaving nonsense which can be safer ignored. They are toy models which do not have the backing any ideas about quantum gravity and so must be treated with extreme skepticism.

There is one approach that I kinda sorta like that I mentioned on the old forum that the Wikipedia article utterly fails to mention. Which leads to my third thought and a video that I posted on the old site.

In 1990 Rafael Sorkin estimated the cosmological constant by assuming that unknown physics (quantum gravity) wants to make the cosmological constant zero but that it can't be exactly zero because of Heisenberg uncertainty considerations. Using causal set theory, the magnitude of the cosmological constant can be estimated as deviating from zero by an amount that agrees with observations. I think this is the best handwaving argument and the only successful one IMO. The fact that we don't know if causal set theory will give us a complete quantum gravity yet is an objection that is not as serious as the success of the estimate. The unknown details of QG are swept under the rug by assuming that that drives the cosmological constant to zero and the causal set estimate is then independent of the details of the QG theory.

Here's the Fay Dowker video where she starts out talking about BH entropy and at 48 minutes in switches to that other event horizon, the edge of the observable universe.



For more background on causal sets see https://en.wikipedia.org/wiki/Causal_sets

[chasmanian]

nFA, thank you so much for all you wrote.

you are so generous and kind.

will reply more soon.

[chasmanian]

nFA,

thank you again.

I find it very interesting and a bit entertaining that the Cosmological constant problem is:

"described by physicists as "the largest discrepancy between theory and experiment in all of science"[1] and "the worst theoretical prediction in the history of physics."

and that Einstein considered the Cosmological constant to be his biggest blunder.

[chasmanian]

I want to sing this:

How do you solve a problem like the Cosmological Constant?


to the tune of this

How do you solve a problem like Maria?

[chasmanian]

nFA,

it strikes me as rather odd, funny, strange, kooky, flabbergasting, mystifying, nigh on unfathomable,

that we live in an Observable Universe
that consists of:

- of the total of Matter (regular and Dark), 85% is the kind that is rather unobservable.
or at least we cannot hold it in our hands, or put it on a table in front of us to directly check it out.

- of the total of stuff (all Matter and Dark Energy), 95% is rather unobservable.

any thoughts about that?

[notFritzArgelander]

nFA,

it strikes me as rather odd, funny, strange, kooky, flabbergasting, mystifying, nigh on unfathomable,

that we live in an Observable Universe
that consists of:

- of the total of Matter (regular and Dark), 85% is the kind that is rather unobservable.
or at least we cannot hold it in our hands, or put it on a table in front of us to directly check it out.

- of the total of stuff (all Matter and Dark Energy), 95% is rather unobservable.

any thoughts about that?

In this case it is Newton who was right:

“What we know is a drop, what we don't know is an ocean.”

We should not feel excessively proud of what we have learned. A little humility before Nature is a better attitude. The unknown should not be rejected because we don't know it.

Shakespeare too:

There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.”

I have nothing educated, let alone intelligent, to say since so little is known. Except that Nature is teaching us how little we know.

[chasmanian]

thank you for your super wonderful reply nFA.

how little we know, ah yes.

made me think of the beautiful Sinatra song.

How Little it Matters (How Little We Know)
I could write another verse,

hmmmmmmmm, let me see.



how little we know, about all this Dark Matter

well,its a start