How comes the universe is said to be "homogenous at large scales"?

[Chillheimer]

Maybe I understand it wrong, but one glance at the microwave background  disproves this:



This sphere around us, the farthest reaches we can actually measure has since expanded.
And with it so have the differences in the above image.
This is not flat or homogenous on the very largest scales. It's the opposite imho.

Any thoughts?

[blob]

As I understand it the fluctuation is extremely small and is supposed to illustrate quantum randomness. What I've got a hard time with however is the inflation theory which is the currently accepted explanation for that background radiation and its homogeneity. During an infinitesimally small period of the first second of the supposed big bang the universe would have inflated in size from much smaller than a proton to a size bigger than 30 billion light years across (since all those microwave photons supposedly emanating from shortly after the big bang are still reaching us from all directions)...  

[kram1032]

The cosmic microwave background has a temperature of 2.72548±0.00057 K. That is, between 2.72491 (the red dots) and 2.72605 (the blue dots) K.
That's pretty much spot-on 2.725K. The differences are tiny.

[Chillheimer]

they might be tiny, but they are there.

if we say the differences aren't significant because they are tiny, there is no point in putting so much effort to image the cosmic microwave background in the first place.

[Sockratease]

they might be tiny, but they are there.

if we say the differences aren't significant because they are tiny, there is no point in putting so much effort to image the cosmic microwave background in the first place.

But the word be "homogenous" doesn't mean "Identical" or "with no variation" - here's a quick Dictionary definition:


Definition of homogeneous
        1:  of the same or a similar kind or nature

So being as close as it is  (0.00057 K is a very tiny variation, indeed!)  I'd say it qualifies as homogenous.

Then we also have to be sure if they said homogenous, or homogeneous.  Very similar words with very slight differences in meaning.

Homogeneous, which derives from the Greek roots homos, meaning "same," and genos, meaning "kind," has been used in English since the mid-1600s. The similar word homogenous (originally created for the science of genetics and used with the meaning "of, relating to, or derived from another individual of the same species")

I'm not sure anyone ever meant to imply that there is no variation in a homogenous (or even a homogeneous) sample of anything.  It seems to me that no matter how hard you try, you will never be able to get a completely "free from variation" sample of anything.  I don't recall anybody ever quantifying at exactly what level of variation a sample can be said to be homogenous (or even homogeneous) so maybe that would be a good thing to research?

[Chillheimer]

I didn't know it had several meanings.
But that doesn't matter sooo much, because (as usual) my approach is fractal cosmology-aspect.

https://en.wikipedia.org/wiki/Fractal_cosmology
The usual counterargument against a fractal cosmos is that
"The universe has been observed to be homogeneous and isotropic (i.e. is smoothly distributed) at very large scales.."
The line where it starts to be homogenous has been again and again pushed further back.
Now it's drawn at around 100 Megaparsecs (roughly 325 million lightyears)
But then theres several Large quasar groups that are far larger, with the largest https://en.wikipedia.org/wiki/Hercules%E2%80%93Corona_Borealis_Great_Wall spreading over 10 billiojn lightyears - contradicting the standard model.

I find it reasonable to go rightaway to the furthest view of our universe that we have, the cosmic microwave background.
And that does not look smooth at all. The structures continue all the way down.

Maybe I chose the wrong words or misunderstand the terms homogenous and isotropic. But it's these terms that are the main argument against a fractal cosmos. And I can only repeat, the cosmic microwave background does very obviously not look smooth or without large structures.

[Chillheimer]

The cosmic microwave background has a temperature of 2.72548±0.00057 K. That is, between 2.72491 (the red dots) and 2.72605 (the blue dots) K.
That's pretty much spot-on 2.725K. The differences are tiny.

I've got to go back to this point, as it doesn't make sense in my opinion.
Of course the temperature-differences are tiny after billions of years of cooling, they all converge to the same point.
They will become evermore tinier, as we approach absolute zero. When the actual Bigbang happend the differences naturally were much larger - at least in my understanding.

I hope the attached picture illustrates my point.

[M Benesi]

  The idea of homogeneity comes from looking for a "source" direction for the Big Bang.

  If one direction was hotter (and thus denser) than another, and became steadily hotter and denser the further away we looked, that might be construed as evidence for a "source" direction for the BB. 

  So the homogeneity of the Universe on large scales basically is evidence that we don't have evidence that pinpoints a locale that spacetime started expanding from. 

  Of course if the Universe is a creation of our minds, rather than the other way around, we aren't going to pinpoint a source of our experiences by looking away from our or other minds- the further from mind you look, the colder you are in that game. 

[Chillheimer]

  The idea of homogeneity comes from looking for a "source" direction for the Big Bang.

  If one direction was hotter (and thus denser) than another, and became steadily hotter and denser the further away we looked, that might be construed as evidence for a "source" direction for the BB. 

  So the homogeneity of the Universe on large scales basically is evidence that we don't have evidence that pinpoints a locale that spacetime started expanding from.   

Ah, that really helped to understand the term, thanks!

But: Why is homogenity used as a argument against the cosmos being fractal? 
a source-direction is no concept that makes sense in this context, fractals don't need/have a source direction.

  Of course if the Universe is a creation of our minds, rather than the other way around, we aren't going to pinpoint a source of our experiences by looking away from our or other minds- the further from mind you look, the colder you are in that game. 

yep.

I like the image of the expanding universe as a balloon thats slowly filled with air, expanding and points on its surface move away from each other.
Every point has the perspective that it is the absolute center and everything moves away from it.

Same for a zoom into a fractal. no matter which point you zoom into, every other point will move away, the further, the faster.
Same in the universe.

[M Benesi]

Ah, that really helped to understand the term, thanks!

But: Why is homogenity used as a argument against the cosmos being fractal? 
a source-direction is no concept that makes sense in this context, fractals don't need/have a source direction.

  It's used as evidence for the FLWR and other BB type models of the Universe's evolution.  Not sure that it is evidence that the Cosmos is not fractal.

  4d spacetime sort of ensures a certain amount of self similarity on different scales- propagation of forces (or simply distortions in the underlying geometry of space) will result in certain geometric patterns at different scales of interaction (think spherical interaction ranges from inverse square laws). 

  If we look at the spherical and pseudo-cyclical interactions that follow inverse square laws on various scales, we are going to see similar geometry at the different scales, so we aren't going to step around self similarity of some form... unless things get REALLY weird at the quantum scale.  like weird weird.

[youhn]

Would a truly homogenous thing even exists in reality? I think not.
The word itself is somewhat of a simpel model for a property we experience. This experience in mainly due to our perceptional resolution. You see 1 color, even though the light has a spectrum of different light waves (actually different colors, do colors even exists).

Roughness is a very nice subject. Let's image a smooth flat surface.
If we add 0,01 mm sized ripples, it feels rough but it's still flat.
If we add 10 mm sized ripples, it feels smooth but it's not flat anymore.

Jump a few scales and you see that the term homogenous has everything to do with our human scale of perception (in time and space).