Simple questions about the Universe/cosmology

[zidane1strife]

Ah, you see, because there's about 10 times as much dark matter, it's galaxies which sit inside overdensities in dark matter, not dark matter which sits around galaxies. And it's not concentrated at the edges: most of the dark matter is in the center, tapering off as you go further and further. It's just that it goes out really far, and there's a whole lot of it.


Right, that's exactly what I described (and is described in Arjan's post above).

It can't be quite at the center, there's often a blackhole there. Besides the blackhole there's a heavy concentration of stars, if you had ten times the galactic mass near the center, mostly around a star heavy zone, or near a blackhole, one'd think they might cause multiple blackholes to occur due to the large amount of concentrated matter

Dark matter has been a nagging problem for astronomy for more than 30 years. Stars within galaxies and galaxies within clusters move in a way that indicates there is more matter there than we can see. This unseen matter seems to be in a spherical halo that extends probably 10 times farther than the visible stellar halo around galaxies.

The ghost universe of dark matter is a template for the visible universe, she said. Dark matter is 25 times more abundant than mere visible matter, so visible matter should cluster wherever dark matter clusters.

Therein lies the problem, Ma said. Computer simulations of the evolution of dark matter predict far more clumps of dark matter in a region than there are clumps of luminous matter we can see. If luminous matter follows dark matter, there should be nearly equivalent numbers of each.

sd

?

The ship itself may not have gravitationall pull working directly on it, but as the interior walls of the shell cave in, they will eventually reach the ship, and the pressures that drive them inwards will be applied to the ship too, eventually crushing it.

The outermost layer of your shell will be pulled inwards by all the interior layers of your shell, and the pressure of this pull will propagate inwards, applying everywhere within the interior. If you view your shell as a bunch of concentric layers, each layer will in this way apply additional pressure to all layers beneath it too. It is this pressure rather than gravity directly that causes the interior of the shell to cave in and destroy your spaceship.

It may be possible that you can have an intact spaceship in the center of the blackhole for a very short while, after the exterior of your shell has fallen into the event horizon but before it has fallen all the way into the center, but for a solar-mass blackhole, this period of time presumably amounts to only a few nanoseconds at most.

If it turns into a blackhole before the mass of the collapsing shell reaches it, it will be at the singularity, before the collapsing mass reaches it, the known laws of physics will breakdown.

 

[KimB]

It can't be quite at the center, there's often a blackhole there. Besides the blackhole there's a heavy concentration of stars, if you had ten times the galactic mass near the center, mostly around a star heavy zone, or near a blackhole, one'd think they might cause multiple blackholes to occur due to the large amount of concentrated matter

Well, not quite. Dark matter doesn't interact, so there's no reason for the "orbits" of dark matter to decay. That is, if you have a big cloud of gas, there's a whole lot of friction in that cloud. The loss of energy due to this friction can cause the cloud (if it's dense enough) to collapse into a star. Dark matter doesn't have this problem, and so stays pretty static.

But you're right that it can't be that close to the black hole: anything that gets too close to a black hole either gets ejected or absorbed, so there can't be any material, dark or otherwise, in some small region around the black hole for long.

 

[zidane1strife]

Well, not quite. Dark matter doesn't interact, so there's no reason for the "orbits" of dark matter to decay. That is, if you have a big cloud of gas, there's a whole lot of friction in that cloud. The loss of energy due to this friction can cause the cloud (if it's dense enough) to collapse into a star. Dark matter doesn't have this problem, and so stays pretty static.

But you're right that it can't be that close to the black hole: anything that gets too close to a black hole either gets ejected or absorbed, so there can't be any material, dark or otherwise, in some small region around the black hole for long.

While I do like dark matter and all, I think that subtle changes to the behavior of gravity that resolve these issues are more elegant, like this recent one.

 

[blakjedi]

As interesting and informative as this is, some scientists say we are still in the afterglow of the big bang and our universe is not yet mature. What would be really interesting is considering the "end of the universe" scenario and the future of man in it. "Heat death" and "cold death" interest me if only for that fact that if the outer fringe of galaxies are receding even faster away from us in our position, there may be no way to EVER reach the (lets say man comes to dominate this galaxy over the next million years). Maybe the future of heat death/cold death may be delayable/ reversible and mankind can stretch its legs so to speak throughout the universe...

 

[KimB]

While I do like dark matter and all, I think that subtle changes to the behavior of gravity that resolve these issues are more elegant, like this recent one.

Actually, so-called "MOND" (modified Newtonian dynamics) has been falsified pretty spectacularly recently. We just had a talk here at Davis yesterday about it.

Basically, if you ascribe to a model where gravity is modified, you expect the mass of objects to always trace where the most visible matter is.

Now, if we're talking about galaxy clusters, most of the matter is actually not in stars, but is in this hot x-ray emitting gas cloud that is diffused through the entire cluster (this x-ray cloud is more massive by a factor of about 6-8 than the stars). If two galaxy clusters collide, we expect the galaxies themselves, and the dark matter, to just pass right through undisturbed (for the most part). The hot x-ray clouds of the two clusters, however, will interact pretty strongly and get dragged inbetween the collision.

So, if you find such a collision, and can measure where the mass is by using gravitational lensing, then you can see whether the majority of the mass stayed with the galaxies, or continues to trace the majority of the normal matter which is found in these gas clouds.

A couple of such clusters have been found, and the matter follows the galaxies almost exactly (not the x-ray cloud that is left between the two), confirming that most of the matter in the universe is dark matter, not "normal" matter.

 

[blakjedi]

Outside of implicit behaviors how do you measure non-baryonic matter? I mean is there dark matter surrounding earth that we aren't aware of? If so how could we have so much dark matter everywhere else and not right here...

 

[KimB]

Outside of implicit behaviors how do you measure non-baryonic matter? I mean is there dark matter surrounding earth that we aren't aware of? If so how could we have so much dark matter everywhere else and not right here...

Well, it's not that it's not right here, but rather that it's very diffuse, and very weakly-interacting (or non-interacting). The galaxy is mostly empty space, with a star here and there. So a diffuse cloud of matter can easily go undetected until you look at large-scale gravitational behavior.

And that's what you do: you look at gravity. The first way this was done was to look at the speeds of stars within a galaxy: they were moving too fast to be explained by normal General Relativity. Now we mostly use gravitational lensing, which allows us to get reasonably direct measurements of the masses of very far-away objects.

 

[blakjedi]

The galaxy is mostly empty space, with a star here and there. So a diffuse cloud of matter can easily go undetected until you look at large-scale gravitational behavior.

Hmm so its like accounting for the mass of neutrinos while admitting that they are virtually massless and ver weakly interacting... So dark energy is essentially ether...

 

[KimB]

Hmm so its like accounting for the mass of neutrinos while admitting that they are virtually massless and ver weakly interacting... So dark energy is essentially ether...

Well, we're sure that dark matter is not made of neutrinos. Basically, their mass is too light: while one could conceivably explain the current dark matter density with neutrinos, they would give an entirely different picture of structure formation in the early universe. So, dark matter must be made up of massive, weakly-interacting particles (similar to neutrinos, but more massive).

Now dark energy, that we know precious little about. My current research is in this field, actually. Come back in 5 years and see if we can say anything about it then

 

[Frank]

It would be quite interesting to put an egg sized blackhole at the surface of the earth and watch it go. I assume it oscillates to a standstill at the earth's centre and then starts eating lunch.

How much does an egg sized black hole weigh at the earths surface ?

Well, a whole lot more than the Earth itself. If you turned the whole Earth into a black hole, it would be about 9 millimeter across.

 

[Frank]

Things Of Interest - How to destroy the Earth

 

[KimB]

Things Of Interest - How to destroy the Earth

That's just awesome

 

[zidane1strife]

Chalnoth, according to that article(which appeared on the net just yesterday.), there are some anomalies with regards to the location of dark matter and some regions of space. So both mond and dark matter would've trouble since the former is falsified*(even this latest mod falls in?) and the latter is not entirely in accordance with all the observations.

Here's another chunk of that other article and the comments on dark matter issues

They have created a formula that allows gravity to change continuously over various distance scales and, most importantly, fits the data for observations of galaxies. To fit galaxy data equally well in the rival Dark Matter paradigm would be as challenging as balancing a ball on a needle, which motivated the two astronomers to look at an alternative gravity idea.

Legend has it that Newton began thinking about gravity when an apple fell on his head, but according to Dr Zhao, "It is not obvious how an apple would fall in a galaxy. Mr Newton's theory would be off by a large margin - his apple would fly out of the Milky Way. Efforts to restore the apple on a nice orbit around the galaxy have over the years led to two schools of thoughts: Dark Matter versus non-Newtonian gravity. Dark Matter particles come naturally from physics, with beautiful symmetries and explain cosmology beautifully; they tend to be everywhere. The real mystery is how to keep them away from some corners of the universe. Also Dark Matter comes hand- in-hand with Dark Energy. It would be more beautiful if there were one simple answer to all these mysteries".

Dr Zhao, a PPARC Advanced Fellow at University of St Andrews, School of Physics and Astronomy, and member of the Scottish Universities Physics Alliance (SUPA), continued "There has always been a fair chance that astronomers might rewrite the law of gravity. We have created a new formula for gravity which we call 'the simple formula', and which is actually a refinement of Milgrom's and Bekenstein's. It is consistent with galaxy data so far, and if its predictions are further verified for solar system and cosmology, it could solve the Dark Matter mystery. We may be able to answer common questions such as whether Einstein's theory of gravity is right and whether the so-called Dark Matter actually exists".

"A non-Newtonian gravity theory is now fully specified on all scales by a smooth continuous function. It is ready for fellow scientists to falsify. It is time to keep an open mind for new fields predicted in our formula while we continue our search for Dark Matter particles."

I also don't like the idea of the universe coming to an end, hope they find a few galaxies with even greater redshift(right now we've seen galaxies to about several hundreds m years after the bb) to see what they've to say.

 

[KimB]

Chalnoth, according to that article(which appeared on the net just yesterday.), there are some anomalies with regards to the location of dark matter and some regions of space. So both mond and dark matter would've trouble since the former is falsified*(even this latest mod falls in?) and the latter is not entirely in accordance with all the observations.

As far as I know, cold dark matter is entirely consistent with all current observations, and is also quite consistent with n-body simulations of the evolution of structure in the universe (see the Millenium Run). The only thing unpleasant about dark matter is that we don't yet know what particle (or particles) make up the dark matter in the universe.

As for dark matter and dark energy, I don't think there's any reason to link the two. These are just two things which make up most of the energy density in the universe, but we don't know what they are. They behave entirely differently, however, and so it's unlikely that the two would be solvable within one framework.

I also don't like the idea of the universe coming to an end, hope they find a few galaxies with even greater redshift(right now we've seen galaxies to about several hundreds m years after the bb) to see what they've to say.

Well, you can only go back so far. Looking far away is the same as looking back in time. We can only see out as far as the age of the universe. And the galaxies actually formed at some point, so before some time, we won't see any galaxies, because they won't have formed yet.

Edit:
Oh, and by the way, about the Millenium Run, you can view their press release, which includes some videos and images here:
http://www.mpa-garching.mpg.de/galform/press/

 

[zidane1strife]

As far as I know, cold dark matter is entirely consistent with all current observations, and is also quite consistent with n-body simulations of the evolution of structure in the universe (see the Millenium Run). The only thing unpleasant about dark matter is that we don't yet know what particle (or particles) make up the dark matter in the universe.

As for dark matter and dark energy, I don't think there's any reason to link the two. These are just two things which make up most of the energy density in the universe, but we don't know what they are. They behave entirely differently, however, and so it's unlikely that the two would be solvable within one framework.


Well, you can only go back so far. Looking far away is the same as looking back in time. We can only see out as far as the age of the universe. And the galaxies actually formed at some point, so before some time, we won't see any galaxies, because they won't have formed yet.

Edit:
Oh, and by the way, about the Millenium Run, you can view their press release, which includes some videos and images here:
http://www.mpa-garching.mpg.de/galform/press/

Suppose they found a galaxy with a greater redshift than should theoretically be possible(aka, too near to or greater than the calc age of the universe.)... wouldn't that put an interesting spin on things?

 

[KimB]

Well, that would only affect what we know about structure formation. Of course, there's just no way that we'd find a galaxy at a redshift beyond that of the surface of last scattering (which is where the CMB is).

 

[Fred]

Well there are some problems with CDM at smaller scales, see for instance

http://arxiv.org/abs/astro-ph/0205464

and the dynamics of structure formation is still wide open. But yea we know that at least the three variants of observed neutrinos would only account for a small percentage of darkmatter and most of the other variants (for instance sterile neutrinos) would 'freeze out' all wrong.

Most people, at least from my side of physics (high energy particle physics) believe the most likely dark matter candidate would be some sort of SUSY particle, but there again there is something a little fishy going on.

Astrophysics in general is a very tough and touchy subject, b/c while some of us are used to experiments with a very large amount of precision, in astronomy you are very happy if you match theory to within an *order of magnitude* Its only very recently with the advent of Wmap and a few other huge projects that there is any hope of figuring out things precisely and throwing out ideas with any degree of confidence.

 

[KimB]

Well, while dark matter is not my field of research at the moment, I do have to mention that 2002 is a long time ago in the field of cosmology right now. Currently, the amount of data we are collecting is doubling every year. That means that every year, we are collecting as much information about the cosmos as was collected in the entirety of human history before.

I don't yet know if more up to date n-body simulations agree better with the data or not, but bear in mind that the results of the Millenium Run, which I posted about earlier, were published last year.

Anyway, personally I don't think we're going to be able to expect our models of dark matter to properly fit experiment until we get some idea as to what dark matter actually is (note that it's at small scales where you'd expect your n-body simulations to have the most trouble). My reasoning here is simply that it takes a tremendous amount of computing power to do a proper simulation of structure formation down to the galaxy level, so much so that it's basically impossible to find by accident the correct makeup of the universe by running just the right simulation such that it fits experiment exactly: the best we can hope to do is show approximate consistency, if that.

 

[nutball]

My reasoning here is simply that it takes a tremendous amount of computing power to do a proper simulation of structure formation down to the galaxy level, so much so that it's basically impossible to find by accident the correct makeup of the universe by running just the right simulation such that it fits experiment exactly: the best we can hope to do is show approximate consistency, if that.

Exactly. Plus it strikes me that the ways and means of comparing the models with reality (power spectra for example) are pretty blunt instruments which can hide a great many sins.

From my (very sketchy) understanding of the inner workings of the N-body codes I get the impression that some of the small-scale structure in the Millennium Simulation should be taken with a pinch of salt.

 

[KimB]

Right, because if you go small enough, you'd only have a resolution of a few particles. Now, supposedly, the Millenium Run had enough particles to get down to the 10kpc scale, which is pretty good, as it gets you down to the galaxy scale, but shouldn't be able to say anything about the structure of galaxies.