Could a universal "Oort cloud" structure cause expansive acceleration?

superdave

Could there be a giant sphere of invisible non-reactive matter (neutrinos?), left over from the big bang, so massive that it's gravity is pulling galaxies outward?

I know the Oort cloud doesn't pull planets outward, but that's because the mass is small compared to the Sun, and not concentrated but distributed relatively evenly around the solar system.

But if there was 73% of the universe's matter (the estimate I saw for the amount of mass-energy in the form of Dark Energy) as a thin sphere on the edge of the 'expansion radius' of the big bang, could it explain the acceleration we see?

As the initial velocity of the 'cloud' slows due to the gravitational force of the inner universe, the distances between galaxies and the cloud would decrease and the gravitational force between galaxies and the cloud would increase, causing what would look like a continued outward acceleration (until everything smacked into each other, turning the universe into one giant hollow shell).

Obviously I have no scientific basis for this hypothesis, it's just curiosity if it was even possible. I teach HS physics and we got into a discussion about this.

phinds

There is no such thing, so the answer to your question is no.

superdave

But why? We can measure our galaxies position relative to other galaxies. Are you saying there is no place in the universe where other galaxies are not evenly distributed in all directions?

phinds

I am saying that the universe has no center and thus there IS no such thing as a "radius of expansion"

superdave

So it if has no center, then in every galaxy in the universe, there should be a nearly equal distribution of other galaxies in all direction. So do astronomers assume that this is the case? That no matter where you are in the universe, matter is ultimately even in all directions?

I'm trying to understand what this means.

phinds

Yes, this is one of the two fundamental properties of the universe. It is called istropy (the universe looks the same in all directions, from all points in it). The other is homogeneity (at large scales, a given volumn of the universe contains the same amount of matter as an equal volume elsewhere in the universe).

Edit: you really should read the FAQ in the cosmology seciton.

Mark M

As phinds said, this is correct. When we say that the universe is expanding, we mean that the distance in between these galaxies gets larger over time, without the galaxies themselves actually moving.

In the very early universe, rather than a homogeneous distribution of galaxies, the universe was filled with a homogeneous plasma and extremely hot radiation. The evolution of the universe from this state of hot, dense, plasma everywhere is the big bang model. Contrary to its name, it has nothing to do with an explosion, or the universe beginning at a single point. Just that the universe was filled with a very hot, very dense, homogeneous plasma.

superdave

The FAQ didn't tell me anything that I didn't already know.

Isotropy and Homogeneity follow from the cosmological principal. Which is really just saying that in any large sample of the universe, the amount and type of matter will be the same (homogeneity) and that in all directions the same things should be observable (isotropy). But every source I've found says that isotropy applies to the laws f the universe (i.e. at any point in the universe, any galaxy that is 10 billion light years away should look like a 10 billion year-old galaxy), not about the actual matter distribution of the universe.

Right, the universe was a quark-gluon plasma, very hot and dense. Density = Mass/volume. Volume had to increase from a very small number. Because the Mass/energy amount is assumed be constant. As volume increased, due to expansion, the universe cooled, the quarks began to stick together (strong force), asymmetrical annihilation, helium, hydrogen (EM force), stars (gravity), etc...

But okay, let's ignore the idea of an edge to matter in space. (which is a different idea than an edge to space)

Could there be a large scale local phenomena, where there is a large shell of neutrinos or other type of matter that is largely non-interactive except through gravity, pulling our observable universe outward through gravity alone, not through dark energy?

The test for this hypothesis would be to see if the expansion acceleration is increasing for galaxies/clusters further away. Do we have accurate enough measurements of the dark energy expansion to determine that?

chill_factor

the simple answer is no.

the complicated answer is still no, but do refer to any modern day introduction to cosmology textbook (math based, for scientists, not the popsci ones). i'll just give you a hint: in newtonian mechanics even, a spherical shell of matter exerts no net force on something on its inside.

superdave

Ah true, I had forgotten all about that. I remember having to solve that same problem for EM. Case closed.

phinds

Yes, I can see that.

I take it you do not believe in the Cosmological Principle.

marcus

Basically what you are talking about has been a popular research line earlier in the last decade. Here is a 2007 REVIEW article with dozens of references to earlier work.
http://arxiv.org/abs/astro-ph/0702416
The Accelerated Expansion of the Universe Challenged by an Effect of the Inhomogeneities. A Review
Marie-Noëlle Célérier (Observatoire de Paris-Meudon)

The idea is that if you give up on the uniformity assumption (homog and isotrop distribution of matter on scales > 150 Mpc or so) you can imagine lots of different UNEVEN distributions of matter which MIMIC the tiny observed "acceleration".

In fact as the author of this review article observes one does not have to mimic acceleration as such, what one has to do is match the SUPERNOVA DATA. The fact that SNe at a given redshift z are dimmer (in a regular way) than one expects them go be on the basis of a standard FRW model with largescale uniformity.

The author concludes that the problem is NOT to explain the dimmer-than-expected SNe observations but, she says, the problem will be to SELECT the right non-uniform largescale matter distribution and several that seem cabable of explaining the obsrervations.

I remember hearing quite a bit about these research attempts around 2007 and 2008, and not so much lately.

One of the most capable and prominent advocates back then IMHO was David Wiltshire and I think he is still writing articles about this, challenging the cosmological constant.

It's been gone over so much it seems a bit Quixotic to me, tilting at the Lambda windmill. But he is very smart. So you might want to read what he has to say about it recently.
http://arxiv.org/abs/1102.2045
Gravitational energy as dark energy: Cosmic structure and apparent acceleration
http://arxiv.org/abs/0912.5236
Gravitational energy as dark energy: Average observational quantities
http://arxiv.org/abs/0912.5234
Gravitational energy as dark energy: Towards concordance cosmology without Lambda

My impression is that this particular gambit (challenging Lambda using non-uniform matter distribution) has gotton old and is petering out, fizzling so to speak. But it is always respectable to challenge assumptions, in science, and people do it regularly and win notice and approval for doing it. I admire Wiltshire for the quality of his work on this and his persistence. But I notice that his most recent paper is NOT about this. Perhaps he is moving on somewhat.

superdave

I meant to say 'didn't already know', but I'm sure you realized that.

As for the cosmological principle, it just seems like more of matter of convenience. It's basically, "Well, we can't predict things about the rest of universe unless it's all like our part of it, so we will assume it is"

Still, even if you assume isotropy, observations in only certain directions, like dark flow, show that it's only the laws that are the same in all directions, not specific movement and structure.

marcus

Here is a special issue of the journal Classical and Quantum Gravity (August 2011) which is entirely devoted to inhomogeneous cosmological models
http://iopscience.iop.org/0264-9381/28/16

The David Wiltshire article I just gave you the link for (so you can read it if you want) is included in this special issue of CQG.
Most of the other article that appear in that TOC are either free to read at that website, or if not there, would be available in electronic preprint version on the arxiv. I think you would be well advised to learn what the experts have come up with as alternative explanations of the actual observations, without using Lambda the cosmological constant.
they are apt to be somewhat more "expert" than your giant "oort cloud" idea. they have detailed knowledge of the data to be fitted by the model, and the most plausible realistic-seeming type of inhomogeneity to be employed to get the best fit.

But my impression is that their ideas are still not catching on, and the thing may be petering out.

superdave

Thanks, it's pretty interesting stuff. I like it because it seems a lot simpler than adding this unknown, undetected Dark Energy. Occam's razor

The question is, is it petering out because new data doesn't match, or just because it's not popular because accepting inhomogeneity would invalidate a lot of people's models? Is homogeneity the new ''Steady State" model, made more complicated with dark energy to make it fit the data? Is dark energy analogous to Ptolemy adding ellipse after ellipse to planetary orbits to avoid doing away with an Earth centric solar system?

Mark M

No. The difference is that dark energy is heavily supported by evidence, but the steady state model was not.