Question about the expansion of space.

The expansion refers to the space between galaxies. Within galaxies, gravity and electromagnetic force hold things together.

 

Perhaps with a qualification... If I understand it right, there are some closer galaxies in our region which form a thing called the "Local Group", which are called that because they are close enough that their gravity keeps them together. Is that right?

 

This is correct. Basically, using General Relativity we can both predict that overall there will either be expansion or contraction (empirically, it's expansion), but that locally dense regions (whether a solar system, a galaxy, a galaxy cluster, or larger object) will tend to stay stable.

 

In a word yes.

 

Um, no. As already explained, local objects do not expand in a universe that is (on average) expanding.

 

The expansion "force" is extremely weak - far weaker than gravity. Anything gravitationally bound will overcome it. The only circumstances where objects are weakly bound enough for the expansion to overcome gravity are in the vast open spaces between galactic clusters.

Imagine a bunch of pennies glued to a balloon. If you inflate the balloon, you would not expect the pennies to tear apart into dust. It is apparent that the forces holding a penny together vastly outstrip the strength of the glue.

 

Could you point me to where it says this?

 

To the contrary it is gravity that is weak.When I hear of galaxy's speeding away from us at near the speed of light powered by inflation I think of an extremely strong force. Nothing to do with the strong force (gluons) holding protons & neutrons together and how would you know that the penny isn't expanding anyway? What would be your measuring stick? Relative to what? Because you would be expanding along with it! Actually I think there are just too many assumptions on this subject right now and I think it would be wise to wait for the evidence to come in on dark energy ect...

 

True. And the cosmological expansion is weaker.

An extremely weak force can produce quite an effect when it accumulates. With mass, like gravity. With distance, like CE.

Not relevant to the current discussion.

Agreed. You should wait until you know more. There are plenty of books on the subject. :tongue:

 

I have been interested in this subject for over 60 years and I have read a lot of books on it over that time I think that my conclusion of the assumptions is as good as anyone elses,maybe you need to read more on the subject because there are a lot more alternative views to yours or mine.

 

Bluey,

You really should listen to what people here have to say. Your 60 years of reading is pretty irrelevant as much of the current cosmological consensus has come in recent years.

It is pretty clear that while gravity is the weakest of the fundamental forces it can also be the strongest given certain circumstance.

It is also pretty clear (from the years of evidence and reams and reams of data) that expansion on a cosmoligcal scale only affects bodies of mass that are not gravitationally bound strongly enough - such as expansion on scales larger than the local group.

We could all do with reading more but remember some of the people who comment on this forum are experts in there field and they will echo the points that are being made.

 

Unfortunately, I don't know of any popular sources that describe this, but it comes from the perturbation theory expansion of a homogeneous, isotropic universe, and is in any standard, modern cosmology textbook.

 

As Cosmic Novice said, your attack on Dave is a reflection of your own ignorance. Dave is a very helpful contributor on this forum and he DOES know what he's talking about.

By the way, when I say "ignorance" I do not mean to be rude and I certainly do NOT mean "stupid". We're all ignorant on lots of stuff but on this particular subject, Dave's level of ignorance is way less than yours.

 

This makes no sense, as written.

Whatever laws/processes of physics are at work between Galaxies are also at work between him and his monitor.

I do not see how you could possibly dispute this, else questions like 'where is the boundary of a Galaxy' and 'how small can a Galaxy be before it is counted as being 'between' other Galaxies', and such like.

Laws of physics are Universal, not Galactic. This is harking back to days before the 'crystal spheres' was blown up as explaining why astronomical bodies hang in the sky without falling down!!! Whaat!?! The same gravity is at work here on earth as it is in the heavens!?!? Preposterous!!!

I suspect visitors to Westminster Abbey are currently wondering what that scraping noise is - as Newton rolls in his grave!?

 

I think you meant to say '..but it can also be the dominant force, given...&c.'.

It cannot be both the weakest and the strongest, simultaneously!

 

I think the most informative answer, at this level of question, would be to say; 'yes, the same physics is at work, but the effects of the expansion of space are so small on the you-monitor scale that it would be impossible to discriminate them, by orders of magnitude, over local effects'.

 

Well, no, the effects of the expansion are simply not there when you have a gravitationally-bound system. If you place a gravitationally-bound system, such as a solar system, inside a space-time which is, overall, expanding, its behavior just doesn't change.

Dark energy does change things slightly, because dark energy actually does add an extra repulsive force between objects based upon distance. But its effect is so small for gravitationally-bound systems that we just don't care for most situations (though it does have a significant impact on the sizes of systems that eventually do become bound, so that our most sensitive proposed test of dark energy is observing how systems join together over time to form larger bound systems).

 

No. It's not true.

If it were true, that means that the expansion would also be happening at larger (but still sub-intergalactic) scales. It would mean we would detect that the galaxies themselves are expanding along with space. They're not. Which also means anything smaller (more strongly bound) than galaxies are also not expanding.

The expansion only takes over where gravity is virtually zero to many decimals - in the voids between whole clusters of galaxies.

[D'oh. Chalnoth beat me.]

 

Actually it makes perfectly good sense.

Indeed they are. The dark energy "force" or whatever it is, is so incredibly weak that it is trivially easy for gravity to overcome it anywhere near large bodies.

Your saying that this doesn't make sense, or implies different laws of physics is the same as saying that a kid pushing on a tank doesn't move the tank but that If the kid were pushing on his little red wagon, he would move the wagon so you object that he should be able to move the tank.

Different level forces produce different results.

The fact that far distant galaxies are receeding from us FTL makes lots of folks think that dark energy is very strong and would have a noticible local effect. Not true.

I heard it best said once like this: Even though the universe is expanding, it's still going to be hard to find a parking place. Sounds silly, but the point behind it is this. If you magically draw parking space lines in intergalactic space, how long does it take before there's room for another car? The answer is billons of years. BUT when you add up all those unbelieveable scadzillions of parking space sized areas over billions of light years, the result is amazing.

The result between a guy and his monitor would be infinitesimal but non-zero if it were not for gravity, but because of gravity, it is zero