# The Naked Scientists Forum

### Author Topic: Is the density of the Universe Uniform?  (Read 1769 times)

#### CliffordK

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##### Is the density of the Universe Uniform?
« on: 02/02/2011 03:37:20 »
Ok, not on a small scale, but on a cosmologic scale.  Perhaps a cubic megaparsec or 1000 cubic megaparsecs.  Is the density more or less uniform?  Background Density?

I'm trying to visualize the "big bang" expansion of the universe beginning from a point source.  I can think of 3 models.
• Spokes on a wheel.
Stars shot out from the middle, and continuing motion at more or less a constant velocity.
One would anticipate that over time, the density of the outer layers would fall off at the cube of the distance.
If the matter is being constantly replenished, the density would be weighted towards the middle.  Except, in that case, you would also see the age of matter weighted towards the middle.  Otherwise, see the "shell".
• A "shell" in which we are likely located somewhere in the shell (where most of the matter is located).  Relatively less matter is located at the center of "explosion".  From edge to edge, one would likely see:
Low density towards the outer edge due to a "normal" distrbution of velocities with relatively few ultra-high velocity particles.  Also the "spokes on a wheel" expansion lowering the density of matter by the cube of the distance as above.
High Density weighted towards the inner side of the "shell", which is related to the typical velocity of movement of matter from the middle.
Low Density weighted towards the point of origin of the "Big Bang".  The slower movement of matter that just didn't make it that far, assuming a Normal Distribution of original velocities.
Perhaps another spike in density at the core relating to matter with the lowest velocity of expansion, and collapsing back upon itself, perhaps orbiting around the core
• "Bubble" with uniform density.  This would be hard to explain on a cosmological scale with a "big bang".  Consider sticking a straw into the center of a balloon and blowing it up through the straw.  The particles at the middle expanding outward would have to be travelling at a higher velocity than those at the outer edge of the bubble which have to be decelerating with respect to those in the middle.  On a micro-scale, the shape of a bubble and density within is maintained by pressure exerted on the outer edges, as well as random space-filling movements of the particles.
You might get at least portions of uniform density if all, or part of the universe was actually collapsing back on itself (see the core above).
How much of the physical universe is actually the visible universe is hard to tell.  Perhaps it is only a small fraction of it.  Perhaps it is essentially the whole universe.  If we were located at the core of the universe, collapsing back upon itself as in the last part of the second bullet, then we might see the whole thing, but would still anticipate variable density looking outward.

Now, introducing the partial, or complete curvature of the trajectory of light around the center of mass boggles my mind...  If we actually saw a loop of light, then we might be seeing the same galaxy, or even our own from a different direction.  Yet we might not recognize it 10 billion years ago, unless we could see it at the same stage of development from multiple directions.

#### Soul Surfer

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• keep banging the rocks together
##### Is the density of the Universe Uniform?
« Reply #1 on: 02/02/2011 08:48:11 »
The answer to the headline question is yes but the scale needed to show this now is probably very vast but the uniformity was incredible at the time of the cosmic microwave background as we can still measure.  as far as your visualisations ids concerned the first two are very wrong the "big bang" was most definitely not any sort of explosion but an incredibly smooth expansion.  your bubble is the best that you are offering but it does not have any edges.

#### The Naked Scientists Forum

##### Is the density of the Universe Uniform?
« Reply #1 on: 02/02/2011 08:48:11 »