I am curious about the fact that the universe is not only expanding, but expanding at an increasing rate.

If expansion was purely as a result of the inertia from the initial universe formation why would it be accelerating?

My geology studies have shown how magma accelerates as it leaves the high pressure of the mantle and moves into the lower pressure of the surface. Also artesian groundwater can exhibit great velocities as it leaves the confines of its aquifer and speeds towards the surface.

Could the universe be moving from an area of high pressure into regions of less dense space. This would explain the energy behind the increase rate of expansion.

If this is so, what sort of evidence should we look for?
If this is impossible, can someone tell me why?

This can't really explain what we are seeing, since the acceleration seems to have increased at a time long after the pressure should have dropped substantially.

When thinking about this, please keep in mind that the behavior of the entire universe (in the largest scale) might not map to any physical allegory on an Earthly scale.

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Forming opinions as we speak

Do you mean to infer that space itself can have variable density, or were you refering to a region of the universe with less matter per cubic planck length? I have tinkered with a concept that is well beyond my grasp that I have dubbed the Shmoo Field wherein I assume "convenient properties for my purposes" of a Higgs Field which permeates normal Euclidean geometric 3-space that is a mere place holder for all that the universe is and can do. All dynamism (mass and energy manifestations and the allowable paths for each) are the properties for the Shmoo Field which is capable of being of variable strength intensity (density), stretched, warped, crinkled, or deformed in whatever ways our expanding knowledge requires to maintain a reasonable level of occamism past the initial abuse. I go to great lengths and "construct elaborate epicycles" to avoid contemplating more than 3 space dimensions and dumping the needed characteristics, that observations will not let us deny, into the Shmoo Field in an extrapolation from Einsteins's assertion that "gravity is geometry".

Usual apologies to each Occam and Higgs---I'm not sure I owe an apology to Albert: if I do, so be it.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

Sorry gourdhead, but your a bit aver my head.

Are you agreeing with me?

What I'm asking is, could space have more pressure in the area we are coming from, than the area we are moving to, and could this unequal pressure, power the accelerated expansion of the universe?

Gourdhead was poking at your "less dense space" phrase where you meant space where the distribution of matter is less dense, as opposed to some aspect-of-the-space-that-makes-it-space being more dense.

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Forming opinions as we speak

In other words, Ozzy, what kind of "pressure" do you
have in mind? Can you describe the "pressure"?

-- Jeff, in Minneapolis

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http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

I got stuck on this statement. Isn't space defined by the universe?
And, even if you ignore that...The bigger the universe gets, the less dense it gets.
And, if there was "space" outside the universe, wouldn't density be zero?

People usually describe space that is expanding uniformly. There is no reason to think that the expansion is variable, or that our location is unique with all of the universe moving away from us (ie. not by us being located at the centre of the universe with `higher density` (more matter) space just beyond our visible universe pulling the matter away from us).

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`Irony` actually does mean `metal like`...

I'm glad Ozzy posted this - I was thinking of posing something similar

We often hear of "baby universes" and "Bubbles". When a bubble forms in water, it rises to the surface. As it rises, it gets larger, and it also accelerates (and so the size increase also accelerates).

Imagine an inhabitant in our water-bubble. He'd see the bubble accelerating in size, and could start looking for "dark energy" to explain this acceleration. Of course, this would be doomed to fail, since the expansion isn't driven by anything in the bubble, but by the difference in pressure between the bubble and the surrounding liquid.

So, we find ourselves in a universe that is increasing in size at an accelerating rate. Can we tell anything about what our "Universe-Brane" sits in, and could there be a parallel to the "bubble in water" allegory?

Well, as you imply, it can't be "purely as a result of the inertia from the initial universe formation." [Newton's First Law of Motion].

If there is such a "energy differential", I certainly hope it is not due to the Decay of the False Vacuum!
No, we don't want that.
As Antoniseb's 1st response said, one should not (in this case) be visualizing the universe as an expanding sphere, where the more distant space is expanding faster than nearby space. Recall that more distant means back in time, so from our viewpoint and considering the finite speed of light, the area of space that is expanding the fastest is right here in our neighborhood. So according to the idea you're asking about, the nearby space should be the least "dense" (meaning the space itself, not the density of matter within it). Off the top of my head, I can't think of any evidence that would show or disprove this, but I imagine there is such a test.....

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Everyone is entitled to his own opinion, but not his own facts.

1) if in the past more matter was closer together and the combined gravitational hold was like a confining pressure. As matter escaped this pull, it speeded up.

2) The extremities of the universe are closer to a true vacuum than the interior. The differential pressure drives the expansion.

Sorry Jeff thats the best I can describe it. I havent passed astrobabble 101 yet!

A geometrical description of the effects of gravity on stars
and galaxies is very different from a geometrical description
of the effects of a rubber balloon on the contained gas, or the
weight of Earth's upper atmosphere on the lower atmosphere.
I would not want to try to equate the two by calling the effect
of gravity "pressure".

"Pressure" means something pressing or pushing. What is
between stars and galaxies that presses or pushes them apart,
opposite to the force of gravity pulling them together?

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Pressure may be the wrong terminology, I dont know if differential "pressure" is even possible in space. Is space a true vaccuum? I didnt think it was. Just close to it?

Is our galaxy expanding as a mass away from other galaxies, or is it expanding outwards from within?

I think that depends on what you mean by "pressure". Also, the
amount of such pressure is important. If there is not enough of
the pressure to account for the effect you are trying to explain,
then hypothesizing that such pressure exists doesn't do anything
for you. If you hypothesize that Earth's oceans are kept from
flying off into Space by the pressure of the atmosphere, but the
pressure needed to do that is 50 times what the pressure actually
is, then the hypothesis fails.

Ignoring light, neutrinos, and dark matter, interstellar Space
(between stars within the Galaxy) has an average density of
about one particle per cubic centimeter. The large majority
of those particles are lone hydrogen atoms. Most of the rest
are helium atoms and diatomic hydrogen molecules. There is
about one dust particle (made of silicon, oxygen, iron, and
the like) for every thousand hydrogen atoms.

Between galaxies the average density is more like one particle
per cubic meter, or one particle per million cubic centimeters.

At sea level, air contains about 100,000,000,000,000,000,000
molecules per cubic centimeter.

In the atmosphere, molecules constantly bump into their
neighbors, which causes pressure. Each molecule has millions
of collisions every second. In Space, with a density of one
particle per cubic centimeter, atoms or molecules are so far
apart that they can fly in straight lines for years or
millenia without bumping into another particle.

Our galaxy is not expanding.

Distant galaxies -- more than about ten million light-years
away -- are seen to be moving away from us. In general, the
more distant a galaxy is, the faster it is moving away.
Galaxies closer than about ten million light-years do not
share this motion. Instead, they are moving randomly. Some
move toward us, and some move away. The distant galaxies
also move randomly relative to their neighbors. Every galaxy
sees its neighbors moving randomly, and more distant galaxies
moving away from it, with a speed proportional to distance.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

This is a very complex issue that has been well addressed by minds greater than mine. . . If you take the bother of reading all of the thoughtful replies then you will be informed. The trouble with this question is that we do not know the answer. I can not say that the reason the universe is expanding at a ever increasing rate is because of the........
We could try gravity but we know not what might be pulling this universe to bits. Could it be a mistake. Its not actually happening. It just looks like it is from here. No, I did not think so either. I do not know. And I want to.

The force which causes the universe to currently expand at an ever increasing rate has been labelled "dark energy". It's also known as "the cosmological constant" because of the way it's used in the equations used to describe general relativity.

What it "actually" is we don't know.

Is dark energy expanding at an ever increasing rate? Is it the opposite of gravity?

The fact that it is the only the really distant galaxies that appear to exhibit this phenomenon makes me wonder if the apparent acceleration may just be an optical illusion? Is that possible, or has this been discounted by evidence like doppler shifts?

The idea is, it is an expansion due to an inherent antigravity property of space, such that test particles in empty space with no significant gravity of their own will still "fall" away from each other. However, you need a lot of space (billions of light years), and it still takes a very long time (billions of years).

So are you saying that space exhibits anti gravity properties. So space is the opposite of gravity? Or is dark energy a property of space, and the dark energy is the repulsive force?

Sounds kinda tao, yin and yang (I like it!)

Your questions are now at the level of the research frontier on dark energy. Stay tuned for another decade or two!

See what four months of Bad Astronomy Forums can do????!!!

Thanks everyone.