What does it mean to say the Universe is the same everywhere?

[bahamagreen]

Established theories rarely fall apart...

Hope it's OK to add a question here. I think to pertains to the topic.
What did cosmologists mean by "looks the same from everywhere"? Were they adjusting and calculating to compare "distant local" to "nearby local"? Or were they comparing raw values?

There was a time when both expansion theories of Steady State and Big Bang were being considered and developed. SS proposed the creation of matter so that a constant density was expected, and BB that the density decreased through time.
I'm not sure when cosmologists began thinking of the universe as "looking the same from everywhere" on a large scale, but the Steady Stated added "looking the same from anytime".
For three co-moving objects initially equal distant forming an equilateral triangle, the triangle's points labeled A B and C where C is the observer looking at A and B:

- at all times A and B would retain a subtended 60 degree separation observed from C because their light paths are co-linear with the radial expansion
- imagine an early universe observation of the arc between A and B would see no objects positioned on the arc between A and B
- imagine a present observational test for additional objects between A and B. all co-moving Z=4:
BB - Big Bang predicts the same lack of objects along the 60 degree arc between A and B
SS - Steady State predicts additional objects along that arc from matter creation

My example is poor, but it seems that observations of far enough distance and comparison of the density of distant and local space might have settled between SS and BB long before the CBR. Observation of "locally less dense there than here " would favor BB, and "locally same density there as here" would favor SS.

 

[PeterDonis]

What did cosmologists mean by "looks the same from everywhere"?

That if you were to look at the universe from any spatial point anywhere in the universe at the same cosmological time, it would look the same. Obviously we have no way to directly test this.

Were they adjusting and calculating to compare "distant local" to "nearby local"? Or were they comparing raw values?

I'm not sure what these mean, but I think the answer is "neither". As above, we have no way to directly test the cosmological principle. It is an assumption based on Occam's Razor.

 

[Ken G]

My example is poor, but it seems that observations of far enough distance and comparison of the density of distant and local space might have settled between SS and BB long before the CBR. Observation of "locally less dense there than here " would favor BB, and "locally same density there as here" would favor SS.

Yes you are correct, the main difference between SS and BB is that in BB, when you consider the largest scales we can observe, you do not expect things to look the same "everywhen". You do expect that in SS, so a good way to distinguish them observationally is to look for evidence that something has been changing with time. In the Big Bang model, the main thing that has been changing with time is the energy density in the prevailing radiation field, and the associated temperature of the radiation that goes along with it. So in BB, if you look distances away that correspond to 13.7 billion years of time, you should see a much hotter and denser universe, whereas in SS, if you look there, you should see the same cool and low density conditions the radiation field has today.

The reason the BB model is heavily favored over SS is primarily because of two pretty clear lines of evidence that the temperature of the universe as a whole is falling with time. The first is the ratio of helium to hydrogen, which has a clear explanation in terms of a history of nucleosynthesis that happens when the universe starts out extremely hot and cools as it expands during its first few seconds. That naturally produces the ratio we see, for reasons you can google ("Big Bang nucleosynthesis"). But in SS, the ratio is just a parameter of the model, you magically produce new matter with just that ratio, it has no reason to be that way. The second crucial line of evidence is the cosmic background radiation, which currently has a 2.7 Kelvin temperature. That also emerges naturally in a model that starts out very hot and expands at a rate consistent with what we observe, but is a completely free parameter in a SS model. Why would these two numbers come out so logically if we imagine expansion, yet be uncontrolled free parameters in the actual situation?

We never know that our explanations are the correct ones, but we prefer having explanations over saying "that's just how it is," whenever we can. The explanation is what fuels future advancements, by giving us deeper aspects of the model to test. That testing will no doubt continue, and someday something new will be learned that will completely shock scientists, such is the way. But in the meantime, we will always go with the theory that is as explanatory as possible, and as simple as possible.

 

[Ken G]

There is also other evidence that favors the BB over the SS model. Imagine placing our observations in a 3D space where farther objects from us are seen further back in time. That is a useful way to picture the complete story of what we can actually see, rather than imagining a full 3D rendition of how things are "now", and a different 3D version of "a billion years ago", etc., which is nothing we could ever observe so it requires filling in a lot of blanks. In the SS model, there would be no differences with distances on the largest scales, the things we see 10 billion years ago and 13 billion years ago would look the same. But in the BB, we should see clear differences; the farther away (and longer ago) things we see should show signature differences that expose the temporal evolution of the universe.

We do indeed see evidence of temporal evolution, because the galaxies we see farther away look different in telltale ways ("galaxy evolution"), and the large scale structure of voids and filaments also looks different (as though gravity were shaping it over time in ways that make sense in the BB picture). We also see some surprises, the most recent one being that galaxies seemed to have formed surprisingly quickly, but it is still not clear if that is a challenge to the model itself or just to the way we are interpreting the observations. I would not be surprised if something pretty fundamental is still out of whack in our models, and it will be fascinating once we have fixed the problem. It could take years, or centuries, the hardest thing to predict is the future, and that goes for the future of science too.

 

[phinds]

the hardest thing to predict is the future

Right. As Yogi Berra is reputed to have said, "It's hard to predict the future, especially when it's about stuff that hasn't happened yet"

 

[Ken G]

Though I suspect Yogi Berra never said half the things Yogi Berra said.

 

[DaveC426913]

Though I suspect Yogi Berra never said half the things Yogi Berra said.

"I never said half the things I said."
- Yogi Berra

 

[phinds]

in BB, when you consider the largest scales we can observe, you do not expect things to look the same "everywhen".

Nice. I hadn't heard it expressed that way before but I like that more than the normal "changes over time".

 

[phinds]

Though I suspect Yogi Berra never said half the things Yogi Berra said.

Yep, that does seem to be the case.