Starship Asterisk*

by **Chris Peterson** » Tue Oct 04, 2011 4:50 am

Ann wrote:But like you said, gravity isn't a "force" at all.

Well, I didn't quite say that. "Force" is a broad term, and gravity can certainly be treated as a force. The question, though, is whether it is a particle mediated force like the others. Certainly, the GUT/QM folks want to think so; there is an elegance to the idea. But nobody has had any luck making a theory like that work... and they've been trying a long time.

by **aristata** » Tue Oct 04, 2011 4:10 am

This may be off topic, but, in my opinion, the background music was awful. I had to mute the sound. For all those who feel a need to add a soundtrack to similar astro-type videos, check out maschinegeist.org for ideas. Thanks a lot!

by **Ann** » Tue Oct 04, 2011 3:59 am

Chris Peterson wrote:

GR tells us that gravity isn't a thing at all, but merely a behavior observed in particles with mass that is a consequence of the way those particles distort space-time.

You're right of course, Chris.

When I wrote that post, I remembered a TV program I have seen that talked about the "strangeness" of gravity. A man was seen picking up a bunch of keys from the ground with the help of a magnet, and we were asked to consider the difference in strength between the force of electromagnetism and the "force" of gravity.

But like you said, gravity isn't a "force" at all.

Ann

by **neptunium** » Tue Oct 04, 2011 3:09 am

Wolf Kotenberg wrote:I am here to learn but today I think I went to the gym class by mistake

What do you mean?

wannabeastro wrote:I never miss an APOD

Not to be critical or anything like that, but how come you don't post here often?

by **wannabeastro** » Mon Oct 03, 2011 11:04 pm

Just an amazing concept, it sent chills down my spine. I never miss an APOD

by **Wolf Kotenberg** » Mon Oct 03, 2011 10:23 pm

I am here to learn but today I think I went to the gym class by mistake

by **Anthony Barreiro** » Mon Oct 03, 2011 7:40 pm

alphachap wrote:We are nowhere in this simulation. This simulation does not show the actual specific particular map of our universe, but it does show how a dark matter distribution can evolve naturally to reach a state similar, in a statistical sense, to our actual universe.

Thanks for the clarification. The explanatory paragraph could have been clearer about the difference between this simulation and the observed universe as well as the statistical similarity. I followed the links, but I don't know enough about astrophysics, cosmology, or the historical disagreements between Joel Primack and his critics to understand what they're talking about.

by **Chris Peterson** » Mon Oct 03, 2011 7:15 pm

NoelC wrote:Gee, with over 100,000 Xeon cores to the task I kind of expected the simulation to have a 4th dimension, but all it seems to be is a static 3D model being rotated.

What fourth dimension would you choose? I'm sure it was a four dimensional simulation, since it had to be propagated through time. This is intended to show how a universe that behaves like our own evolves. That is, it is a snapshot of the current state, which is naturally a three-dimensional image (projected to two, of course).

Anyone know where we are in that model?

As noted previously, it is a simulated universe, not ours, so it contains no actual structures. Presumably, "we" can be imagined at the center, since that is the position we occupy in the observable Universe.

by **NoelC** » Mon Oct 03, 2011 7:04 pm

Gee, with over 100,000 Xeon cores to the task I kind of expected the simulation to have a 4th dimension, but all it seems to be is a static 3D model being rotated.

The music was nice, though.

Anyone know where we are in that model?

-Noel

by **dduggan47** » Mon Oct 03, 2011 6:59 pm

neufer wrote:
dduggan47 wrote:
Is that anywhere close to current thinking or am I way off base?
All dark matter has mass as well as gravity and must consist in some small part of known matter such as neutrinos and 'free range' planets which are certainly not "all gravity and nothing else."

I think that means my second choice was correct ... as I suspected.

Well, it was fun to think about anyway. Thanks for the response, Art.

by **alphachap** » Mon Oct 03, 2011 6:15 pm

Anthony Barreiro wrote:Where are we in this simulation? It looks like there's a huge clump of dark matter in the middle, with various other accretions throughout. Can we see the Milky Way? The local group? The Virgo Supercluster? It would be nice to have a little arrow stating "you are here."

We are nowhere in this simulation. This simulation does not show the actual specific particular map of our universe, but it does show how a dark matter distribution can evolve naturally to reach a state similar, in a statistical sense, to our actual universe.

by **Anthony Barreiro** » Mon Oct 03, 2011 6:04 pm

Where are we in this simulation? It looks like there's a huge clump of dark matter in the middle, with various other accretions throughout. Can we see the Milky Way? The local group? The Virgo Supercluster? It would be nice to have a little arrow stating "you are here."

by **Chris Peterson** » Mon Oct 03, 2011 4:15 pm

celestemekent wrote:This video proves nothing whatsoever it is a simulation based upon a fiction, while it is pretty and it does "flow" there is nothing there at all that offers evidence nor a proper explanation of how dark matter and dark energy could exist. Stop the entertainment and begin the process of gathering empirical proofs. Frankly mathematics is not a proper instrument for that proof either and this simulation is based completely on some mathematical equations thought up out of whole cloth it is nothing but speculation and fantasy.

This video is the result of a physical model, not a mathematical one. That means that a numerical simulation was operated using existing physical theory- theory that is supported by observations.

The fact that you don't get this, and that you believe it is "fiction", reveals a deep lack of understanding about the subject. Frankly, you are not qualified to have the opinion you do, and should be asking questions here, not making silly and unsupportable statements.

by **celestemekent** » Mon Oct 03, 2011 3:51 pm

This video proves nothing whatsoever it is a simulation based upon a fiction, while it is pretty and it does "flow" there is nothing there at all that offers evidence nor a proper explanation of how dark matter and dark energy could exist. Stop the entertainment and begin the process of gathering empirical proofs. Frankly mathematics is not a proper instrument for that proof either and this simulation is based completely on some mathematical equations thought up out of whole cloth it is nothing but speculation and fantasy.

by **neufer** » Mon Oct 03, 2011 3:46 pm

dduggan47 wrote:
Ann wrote:
Dark matter is certainly mysterious, because it is apparently "all gravity" and nothing else.
General Relativity says that gravity is the curvature of space-time due to the presence of matter.

As I understand (or misunderstand) Calabi & Yau, space-time can be curved even in the absence of matter. It seems to make sense then to think about such curvature as "all gravity and nothing else". It's all a matter of geometry.

Is that anywhere close to current thinking or am I way off base?

There are two kinds of curvature:

1) Weyl (saddle shaped) curvature that exists "even in the absence of matter or energy" and
2) Ricci (bowl shaped) curvature that exists only "in the presence of matter or energy".

An example of Weyl (saddle shaped) curvature is when an ISS astronaut tosses a tool off to the side and it comes back to conk him in the head a half orbit later.

Only (mass-less) gravitons (with Weyl curvature) might be considered "all gravity and nothing else."

All dark matter has mass as well as gravity and must consist in some small part of known matter such as neutrinos and 'free range' planets which are certainly not "all gravity and nothing else."

by **Chris Peterson** » Mon Oct 03, 2011 2:36 pm

Ann wrote:Dark matter is certainly mysterious, because it is apparently "all gravity" and nothing else.

But the gravity of ordinary matter is mysterious, too. It is unexplicably weak.

In what way is the weakness of gravity inexplicable? Certainly it is much weaker than the other forces, as you point out. But I can't think of any reason why it should or should not be so.

So many problems with understanding gravity come from trying to think of it as a "thing"- something that must exist in its own right, a flow of particles or a physical field, for instance. GR tells us that gravity isn't a thing at all, but merely a behavior observed in particles with mass that is a consequence of the way those particles distort space-time.

The idea of dark matter isn't mysterious at all; we already understand nonbaryonic matter, which has mass but interacts only weakly or not at all with the electromagnetic forces. What makes dark matter mysterious is that we haven't identified the specific nonbaryonic particle or particles that make it up. Its gravitational behavior, however, is perfectly well understood, which is why simulations like the one in today's APOD are possible.

by **BenevolentGuest** » Mon Oct 03, 2011 2:17 pm

simonate wrote:i view APOD everyday, i love those images, but i can not view any video on apod, i do not know why, and can't find answer on internet, i come from china, i realy want to view those amazing video , please help me

my software
WIN XP SP2
IE8.0

thank you

You must install flash player for browsers here: http://get.adobe.com/flashplayer/

Or best, first download and install firefox here: http://www.mozilla.org/firefox/ and then flash player here: http://get.adobe.com/flashplayer/

Because IE8 is not really good web browser.

by **alphachap** » Mon Oct 03, 2011 1:31 pm

I suspect we can not see this dark matter because it is in a parallel universe. This parallel universe may be very close to our, millimeters or picometers, but along a fourth space dimension. If gravity can span this fourth dimension, then we would gravitationally feel this matter but not see it.
This would be in accordance with string theory.
In string theory, gravity is mediated by closed strings whereas the other forces are mediated by open strings. Open strings have their ends attached to submanifolds (the so-called branes) whereas closed strings may propagte freely in all dimensions. If our universe is one of those branes, this would explain why gravity appears weaker: the force is "spreading" out in all dimensions. The other forces mediators are confined within our brane and do not leak out. This would also explain why we feel the gravity from other branes.

by **orin stepanek** » Mon Oct 03, 2011 1:13 pm

I enjoyed the videos; especially this one. http://www.youtube.com/watch?v=TFgpi9qj46s#! The simulations seem to be quite accurate.

by **casusbellus** » Mon Oct 03, 2011 12:54 pm

What a colossal waste of CPU time. Making a video of something we know so little about. The "standard concordance cosmology" has so many weaknesses, and dark matter and energy so far from being understood (if they exist) that making such a video is akin to a committee of blind people describing a sunset. Actually, they could do a more accurate job.

by **dduggan47** » Mon Oct 03, 2011 11:15 am

Ann wrote:Dark matter is certainly mysterious, because it is apparently "all gravity" and nothing else.

Ann

Science disclaimer: I read a fair amount about it but I understand fairly little.

General Relativity says that gravity is the curvature of space-time due to the presence of matter.

As I understand (or misunderstand) Calabi & Yau, space-time can be curved even in the absence of matter. It seems to make sense then to think about such curvature as "all gravity and nothing else". It's all a matter of geometry.

Is that anywhere close to current thinking or am I way off base?

by **saturn2** » Mon Oct 03, 2011 10:45 am

The dark matter is a good topic.
The dark matter is in the Universe is very important.
I thinkthat the dark matter has many planets, more that in the normal matther of the Universe.

by **simonate** » Mon Oct 03, 2011 8:05 am

i view APOD everyday, i love those images, but i can not view any video on apod, i do not know why, and can't find answer on internet, i come from china, i realy want to view those amazing video , please help me

my software
WIN XP SP2
IE8.0

thank you

by **Ann** » Mon Oct 03, 2011 6:16 am

Dark matter is certainly mysterious, because it is apparently "all gravity" and nothing else.

But the gravity of ordinary matter is mysterious, too. It is inexplicably weak. Just think of the fact that you can lift a bunch of keys from the ground with the help of a magnet - so the magnetic force of the small magnet exerts a stronger force on the keys than the gravity of the entire Earth!

I read somewhere - and I'm sure this is all speculation, mind you - but like I said, I read that some people speculate that gravity is something that "leaks" into our universe from, well, from another universe, I guess! Or from another dimension, whatever that means.

So I realize that we absolutely don't know anything about this, but it's interesting to speculate that ordinary matter may be a sort of "lightning-rod" leading gravity into our universe through the bosons and quarks and charges and forces it is made up of. And maybe dark matter is a lightning-rod that is doing the same thing, leading gravity into our universe, but without the attending bosons and quarks and charges and forces.

Ann

by **bystander** » Mon Oct 03, 2011 4:48 am

NASA Supercomputer Enables Largest Cosmological Simulations
NASA Ames Research Center | 2011 Sept 29

Scientists release most accurate simulation of the universe to date
University of California, Santa Cruz | 2011 Sept 29

Introduction: The Bolshoi Simulation
Publications: The Bolshoi Simulation

NASA HECC: Pleiades Supercomputer

Starship Asterisk*

APOD: Dark Matter Movie from the Bolshoi... (2011 Oct 03)

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