Question about gravity and space

[Sir Doom]

Fact is, the Moon is receding from the earth ever, ever so very slightly; about an inch and a half a year.

Yeah, I was just reading about that actually.

Not so much that it's expected, but what science has found to be the case. However, in "this case"---the Earth/Moon situation---the expansion is not taking place. It's explained in this little comment from an article on the universe' expansion.

"One famous analogy to explain the expanding universe is imagining the universe like a loaf of raisin bread dough. As the bread rises and expands, the raisins move farther away from each other, but they are still stuck in the dough. In the case of the universe, there may be raisins out there that we can’t see any more because they have moved away so fast that their light has never reached Earth. Fortunately, gravity is in control of things at the local level and keeps our raisins together"
source
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And it does, which is why the expansion is not uniform. See my remark to you in post 8.

Yeah, sorry I didn't mean to marginalize your response there. Thanks

 

[Penumbra]

For those wondering just how fast the Universe is expanding, it was published in 2013 that the rate is about 67.8 kilometers per second per megaparsec. This means that objects 1 megaparsec (about 3.26 million light-years) away from us will appear to recede at a speed of 67.8 kilometers per second. The observed speed is approximately proportional to the distance (at low redshift values, that is), so at 10 mpc the velocity is 678 km/s and at 100 mpc it is 6,780 km/s. Assuming it also works linearly for very small distances, then the Alpha Centauri system (at 4.3 light-years) would be receeding from us at ~300 meters per hour (ignoring the effects of gravity). The Moon would be receding at ~100 meters per millennium (once again, ignoring gravity).

That's a useful number, and shows why some objects are not receding at all.

The expansion between the Alpha Centauri system and the Solar System is 300 meters per hour, but solar systems can move through space much faster than that, and so could potentially move towards each other if they happen to travel in that direction faster than 300 m/s for example.

 

[Saint_of_Me]

First my disclaimer: I am no physicist or scientist of any kind, nor have I had any instruction in these matters by any institute of higher learning. Thus, my grasp of these concepts is going to be dubious at best.

From what I understand gravity warps space itself, especially around super-massive objects like stars. We see light traveling from distant stars change course around planets in a phenomenon called gravitational lensing.

Also, we know from observation of the known universe that the galaxies are all receding from one another which indicates that space is expanding in all directions at all times.

So, my question is:

What happens to the expansion of space in the warped space around large gravitational sources?

On the contrary, Doom: you sound pretty knowledgeable about the Cosmological principal of Curved Space-Time and also of the Doppler Effect, or Universal Expansion/Acceleration.

And your question is a fairly common one, and a good one. Since universal expansion and gravitational MOA (massive object attraction) do seem to sort of contradict each other.

So let me see ii I can help a little bit.....And I promise to keep the math to a bare minimum!

OK, so, we know everything about the universe changes when you go faster, right? That is not so, there are some quantities that are invariant under the transformations that occur due to a faster or slower speed. One example is the speed of light.

Another example is the distance between two events, but we must also include time in this. This is because with the ideas that the speed of light is constant in all reference frames and that the laws of physics are the same in all reference frames, space and time mix together and create such effects are time dilation. ( BTW; GPS uses this), and also length contraction (experimentally verified, since muons reach Earth), and the loss of simultenaity.

Here's the ONE equation I will use: The space time distance between two events is... s^2 = -(ct)^2 + x^2 + y^2 + z^2. (There's another definition that flips all the signs, but the following reasoning holds for either definition). Our commonly used distance formula is only the sum of the squares of the component " ts".....so, why then is there a minus sign?

This is because space is not "nice" like Euclidean space used in mathematics. There are ways of working with spaces that are not "nice" that require mathematical objects that describe the curvature of space. These mathematical objects result in a different formula for length, one that includes the minus sign as seen in the formula for the spacetime interval between events.

General relativity says that the presence of mass curves space and changes the definition of distance. This allows for the curving of light around massive bodies (confirmed) and the perturbations of some orbits--like with Mercury. These predictions come from first of all special relativity (replace the spacetime equation with a different one)-- and from the equivalence between gravitational fields and accelerating reference frames.

So why does space have the property of warping due to the presence of mass? Blame Einstein! LOL. Since It follows from the axioms of his relativity. Two axioms are needed to add time as a dimension and to suggest that space is not "nice" when it comes to distance, and one or two more axioms suggest that the presence of mass further makes space not "nice" when it comes to finding distances.

 

[Thief]

been watching for decades a s the experts bang this back and forth.....

I say.....space( having no substance) cannot be influenced by gravity.

Gravity is that force acting between masses......and cannot affect space.

Observation at any level cannot be tweaked.
numbers can be elusive AND misleading.
as noted in this topic

the effect of gravity is always to the next object having mass.
motion is effected if the two objects happen to be moving.
calculations display the effect of shape and velocity of the mass influenced.....nothing more.

 

[Thief]

Thanks for the corrections and the detailed description!

I think my problem is rearing its ugly head again, though unfortunately.

So, take the Earth and the Moon for example. They are too close for spacial expansion to overwhelm gravitation, so they stay at the distance they stay at (more or less). So, if we can't observe the expansion of space in this local area, why do we expect its happening at all?

Additionally, considering that gravity has an observable effect on space itself, shouldn't gravity also slow, stop or even reverse spacial expansion depending on the strength of gravity involved?

I can't really reconcile that gravity has the effect of warping space without modifying the expansion as well.

you cannot observe space.
only the measured distance.
measure is a cognitive device created by Man to serve Man.