The June 2007 issue of Astronomy, pages 64-65, contains some really bad astronomy.

And the "answer":

This answer is totally wrong. It isn't hard to find the right answer, if one consults some cosmology books.

For example, Cosmology by Edward Harrison, page 278:

Or The Universe at Midnight by Ken Croswell, page 75:

Or Cosmological Physics by John Peacock, pages 87-88:

This is not the first time Astronomy has mangled this question. In the February 2000 issue, the magazine tried to tell us that people are held together by gravity. You'd have to be REALLY fat for that statement to be true! (The magazine published a correction in the April 2000 issue.)

I saw that and was pretty incredulous...

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PW -- Plant Whisperer

eek! I shudder at the far reaching implications that statement would have had on modern physics and astronomy if that were true..... eeeeekkk... I wonder where they got thier data on that from??

-=Identity 4=-

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If my head got too full of stuff, would it collapse and stop at neutron-degenerate matter, or go all the way to a singularity??

Certainly one of the most misunderstood areas of cosmology. Expansion simply doesn't happpen in locally bound systems, as I understand it. Maybe one of the math people could give us some insights?

That´s what I´ve learned from many discussions on BAUT-Forums. And it seems to be very logical. Because otherwise we would not be able to take notice of any expansion. But is there a theory (SR, GR, etc) which predicts this? And how is locally defined? Galaxy, Cluster, Supercluster, The Great Wall, ????

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If everyone had even a basic grasp of scientific principles, this planet would be a better place (Phil Plait)

Die Lücke, die wir hinterlassen, ersetzt uns vollkommen [The gap we will leave behind will take our place entirely] (Carl Heinz Schroth)

1 + e^i*pi = 0

Observationally, the universe's expansion appears just beyond the Local Group: the nearest galaxy groups--the Sculptor group, the Maffei group, the Centaurus group, and the M81 group--are all moving away from the Local Group.

It may not be the local group that is coming towords us, but there are places like HDF are having some blue Galaxies. Seyferts, Linears, Starbursts, and interacting Galaxies show blue shifts. They are not considered as blue shifted Galaxies. On the other hand all the radio freq., Galaxies etc., are considered as redshifted ones. Are we not biased in this respect?

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=snp

Moved from Bad Astronomy Stories to Small Media at Large.

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Everything I need to know I learned through Googling.

Is it we have to consider it, that it is only expanding? There can be many ways, if we put together all the facts....

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=snp

There can be other theories, which can allow both Blue and Red shifted galaxies in any proportion. It not needed, that we support only SR and GR...

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=snp

Bodies move dynamically in orbits. There can be multiple bodies moving around a common center . Some can go nearer and some go far. These localized systems can be there anywhere. Like a Children’s Giant wheel which is located at a distance to an observer. It can be in any plane of rotation. Some buckets go away and some come near.....

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=snp

The bodies don’t collapse. Just because the nature says all my masses are not equal and there is no uniform density at any scale. Uniform density is considered for simplification of mathematical equations for solving cosmological problems. Which in-turn created singularities!

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=snp

I have heard it stated that bound systems settle into equilibrium at an ever so slightly larger size due to the expansion, than they would settle at were there no expansion. That everything from galaxy clusters all the way down to the atoms in our rulers would be affected in this way. Is this possibly correct?

It would seem plausible when considering the "Big Rip" scenario.

Gravitational attraction forces bind bound systems. When they try to move towards each other due to gravity, centrifugal forces come into picture and they will try to move about each other. In a multi body system, these forces can be called taught ness forces. When these taught ness forces are more, the system will contract; otherwise the system will expand….

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=snp

So in a "Big Rip" scenario, everything stays at exactly the same size regardless of the rate of expansion, until such a time that the rate of expansion is so large that it overwhelms gravity and atoms suddenly fly apart? Is this the suggestion?

Sir, please explain me a little about big-rip scenario...

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=snp

In Big Rip, the rate of acceleration of expansion keeps increasing so that it eventually overwhelms everything - not just gravity, and the effects become obvious at smaller and smaller scales. So other galaxies disappear, then the galaxy itself flies apart, then the solar system (what's left of it - this would be a long time in the future), then planets, then atoms and nuclei.

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I say there is an invisible elf in my backyard. How do you prove that I am wrong?

Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

So is it not possible that the metric expansion of space happens everywhere, but in gravity bound systems everything settles into equilibrium at a slightly larger size than it would without any expansion? Everything from the size of galaxies to the size of atoms? Gravity overcomes expansion (for now!), but does gravity hold objects in the same orbit or position with expansion as it would without?

As I said I have heard it stated thus - but that we cannot check for this because our rulers are also affected.

I'm asking because this came up in another thread and I can't seem to find a definitive answer or general consensus. Everyone just says where gravity binds objects there is no expansion. But this is not expansion as such, it is atoms/matter/planets/stars/galaxies (or more precisely the "gaps" in between the objects that they are made from) settling at a slightly different size depending on the rate of expansion. Possible? Probable? Stupid?

When non-uniform masses formed into a multi-body system, the internal distances will vary according to their positions in the their orbits of movement. The system will be in dynamic equilibrium. Some time it may be expanding and some time it may be contracting and always it is changing.

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=snp

Yes, I understand that, but it doesn't really answer the question.

The system will be in dynamic equilibrium, but would the internal distances be the same if there were no expansion? Is the rate of expansion one of the dynamics involved in the equilibrium?

Remember, I'm not saying everything is expanding, I'm saying that everything settles into equilibrium at an ever so slightly larger distance than it would were there no expansion. As the rate of expansion changes, so does the distance at which objects settle into equilibrium. If the expansion continues to accelerate, cosmologists predict that eventually even atoms themselves will be ripped apart by it.

If the accelerating rate of expansion can eventually overcome gravity completely and destroy the orbits of planets around a star, surely that means any rate of expansion would have some kind of effect on the size of an orbit. So would the size of the orbit change as the rate of expansion changes? Of course we cannot measure this change locally as our rulers would also be affected.

If a force works against gravity, an object subject to that force is not held in the same place by gravity until the force overcomes it. If a rocket cannot escape Earth's gravity because it doesn't have enough thrust, it gets to a certain height and then falls back. With less thrust it doesn't reach as high - with more thrust it can escape. If the expansion of space is so small that it is easily overwhelmed by the gravity which holds systems together, those systems obviously cannot escape, but does the "height they reach before they fall again" change with the rate of expansion?

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Never attribute to malice what can be adequately explained by ignorance or stupidity.
Isaac Asimov

Please imagine earth is rotating about sun situation. Earth follows an elliptical path. Here some times it goes away from sun, some time it comes near sun,,, When it is going away from sun, the distance between sun and earth appears to be increasing, may be accelerating. After some time it goes back to sun. May be decelerating, or distance appears decreasing, coming nearer.

Now in a multi-body scenario also, there may be many bodies appear to go away from each other, or accelerate away. And after some time, they all appear to come near or may decelerate from each other.

Here bodies wont expand them selves..

What do you say..

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=snp

I would say you describe a classic multi-body scenario where any object is slowest at its apoapsis and fastest at its periapsis.

When I realised that with a current rate of expansion of 77 km/s per Megaparsec, that equates to 1.66 mm an hour per 1.9 million miles, I can see that we needn't worry about that rate having any measurable effect on the size of atoms in the near future, but it might be an issue in the far distant future. Perhaps theoretically it might be more accurate to suggest that space is expanding everywhere, but has no measurable effects within gravity bound systems?

Space is not expanding in multi body system. Bodies move about each other. Some ma come near and some go far…

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=snp

You keep saying it, and you are indeed correct, but I think we are having a problem with semantics.

I have heard cosmologists say space is expanding everywhere, but it only causes an increase in distance between non-bound systems. Within bound systems, the expansion of space has no measurable effect on that system, as gravity holds those systems in place relative to each other, against that expansion. You might consider space simply to expand past, around or through that object, only having an measurable effect on distant objects outside that system.

You are working from the viewpoint that there is no expansion (no increase in distance) within bound systems (which seems to be true). I am working from the viewpoint that space is expanding everywhere, but there is no increase in distance where gravity is at work.

Both viewpoints seem equally valid, when compared to our observations. But if the acceleration of expansion continues as it is, there with come a point where the expansion of space does have a measurable effect within bound systems. Is it not worth considering that the process is occurring everywhere, but is having no effect within bound systems due to their gravity overcoming it?

Do we consider that the phenomenon/process which causes metric expansion, supposedly accelerated by negative or dark energy, is only happening between non-bound systems and the process is not trying to happen anywhere else, that the process somehow knows where gravity is and doesn't even try to expand where there is gravity, or that space is expanding at the same rate everywhere in the universe, but has no effect within bound systems?

Think about it...

Gravity is getting weaker, expansion is getting stronger. At some point they will cross...

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

May be we shall go to ATM for further discussions...

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=snp

That ATM reminder was for Philip, who has a hypothesis about ether foam and bubbles.

What I am referring to is not ATM, but is simply a question of interpretation. Is the process behind the metric expansion of space happening everywhere? Is space always expanding, but within bound systems the space expands past, around or through the objects within that system, having no effect on their relative distances because their gravity easily overwhelms that expansion? Or is the process simply not happening at all within those systems?

The proposed Big-Rip scenario seems to imply the former, that the process behind expansion is trying to happen everywhere, but at the present time only causes distances to increase outside of bound systems. As time goes on, the accelerating expansion will have more and more of an effect within those bound systems as the expansion overwhelms the relatively weak interactions at progressively smaller scales scales until there comes a time when the expansion eventually pulls atoms apart.

In essence, we only observe the results of the process. We see the distant galaxies receding, due to the expansion of space between us and them. We see no recession of objects within our galaxy because the gravity within our galaxy is too strong for the expanding space to cause any increases in distance. But is the process occurring within our galaxy, and not having an effect on distance, or is the process simply not occurring at all?