Friedmann's Assumption: Understanding the Universe's Uniformity

[wabbit]

So although experimentally universe must be somehow curved.

I wouldn't go that far. It is my personal view as a layman that this is more likely than not, but "must" is a big word, there is as far as I know no proof either way.

but practically its exactly flat.

If you mean by that "as good as flat for all (or most) practical purposes" then yes, its curvature is currently estimated to be too low to have any practical consequences.

 

[Chronos]

The CMB is depicted using Mollweide projection, which renders a spherical surface flat with minimal distortion. It looks oval when depicted in this fashion.

 

[Chalnoth]

That image portrays a finite universe as viewed by an external observer - very misleading. Observational evidence suggests the universe in which we reside is isotropic and homogeneous in every direction. In other words, yes, we see the same number of galaxies in every direction. The only variable is distance, which disappears if you look 'far' enough into the universe.

It's the other way around.

The observable universe is extremely homogeneous and isotropic on large scales. But there's no reason to believe that the isotropy and homogeneity extends far beyond the observable universe.

Take inflation, for example. In simple models of inflation, the universe is made to be smooth because of the rapid early expansion. This rapid early expansion doesn't eliminate any variations that existed early-on, it just made it so much larger than the observable universe that we can't detect them. If it were possible to take a sort of "bird's eye" view of the universe on scales much larger than the part we observe, the natural expectation is that pretty large deviations from homogeneity and isotropy would appear.

Of course, it is possible to assume that the universe, not just the observable part, truly is homogeneous and isotropic. But that's an assumption, and I don't think any realistic model of how our universe could have formed produces a perfectly homogeneous or isotropic universe.

 

[wabbit]

It's the other way around.

The observable universe is extremely homogeneous and isotropic on large scales. But there's no reason to believe that the isotropy and homogeneity extends far beyond the observable universe.

Take inflation, for example. In simple models of inflation, the universe is made to be smooth because of the rapid early expansion. This rapid early expansion doesn't eliminate any variations that existed early-on, it just made it so much larger than the observable universe that we can't detect them. If it were possible to take a sort of "bird's eye" view of the universe on scales much larger than the part we observe, the natural expectation is that pretty large deviations from homogeneity and isotropy would appear.

Of course, it is possible to assume that the universe, not just the observable part, truly is homogeneous and isotropic. But that's an assumption, and I don't think any realistic model of how our universe could have formed produces a perfectly homogeneous or isotropic universe.

I would however argue that it is more likely than not for the universe to be homogeneous at a significantly larger scale than our observable universe : it would seem to be an extraordinary coincidence for the homogeneity scale to just match precisely our observable scale - since we know that it is greater, it sounds more reasonable to assume that they are of different magnitude that just too numbers that happened to be close.

This is of course a layman view and perhaps there are arguments that support the idea that the two scales are bound to almost coincide, I just haven't seen them yet.

 

[microtech]

IF the CMB is the light from "the last scattering" at T₀+378,000 years, I don't quite understand how there can be something "behind" that (as the image above implies, where "our" Universe is inside a 46.5 Gly sphere)... Have scientists finally come to their senses, and given up on the "big bang"? That would (should?) have made big news, but I haven't heard a thing... Can someone please explain? Here's my adaptation of a Wikipedia illustration of the "observable" universe, a "zoom in" on the image above. These are the Universe numbers presented in Wikipedia (en.wikipedia.org/wiki/Observable_universe):

Diameter: 93 billion light years (Radius 46.5 Gly)
Volume: 4×10⁸³ liter
Mass (Baryonic): ⁵³ kg
Density: 9.9×10^-30 g/cm³ (equivalent to 6 protons per m³ of space)
Age: 13.798 ± 0.037 billion years

Note that the density given in Wikipedia is for a Universe "a mere" 13.8 Gly radius, whereas the volume is for a 46.5 Gly radius, which would have the much thinner density of 2.45×10^-31 g/cm³ (equivalent to 0.15 protons per m³ of space).

The image above indicates a much larger Universe... Very confusing... Can someone who knows what this is supposed to mean please explain? Bye bye big bang? Hello infinite Universe? What?

https://sites.google.com/site/microtechnonstop/f/Visible%20vs.%20Observable%20Universe%20.png

 

[microtech]

Take inflation, for example. In simple models of inflation, the universe is made to be smooth because of the rapid early expansion. This rapid early expansion doesn't eliminate any variations that existed early-on, it just made it so much larger than the observable universe

This is an oft claimed feature of "inflation," but I have yet to see an explanation of how this would work. What, exactly, becomes "much larger than the observable universe," and at what timeframe? Alan Guth's original (simple?) claim was an expansion factor of "at least 10²⁶," which would make the universe 0.1 m across after 1/10³² second, how can a one meter difference in radius "smooth out" anything?

 

[microtech]

Mass (Baryonic): 53 kg

As I don't know (yet) how to edit earlier posts, I comment my own post: The Mass was of course given as 10⁵³ kg.

 

[Chronos]

I fail to see your point, microtech, Logic does not trump observational evidence and math.

 

[microtech]

observational evidence and math

What observational evidence and math would that be? When I do the math, the "Guth Inflation" blows up a very small universe to one with a radius of 10 cm in a very short time. That is not "logic," it is simple math. How do these 10 extra cm do anything to solve the flatness and the horizon problems? And who was there to observe anything, 13.8 billion years ago? Interpretations of data from observations made today is not evidence of what might have taken place 13.8 Gyr ago.

 

[wabbit]

How do these 10 extra cm do anything to solve the flatness and the horizon problems?

You need to look at the maths / model to see how that question is answered by inflation and determine to what extent that answer is satisfactory. One thing is sure, inflation does address this issue - the key point is probably that what matters for this is not the size of the universe at the end of onflation per se, but the ratio of the size after and before inflation, and this ratio is something like 10^30 or greater in such models, which is big enough to have the required impact (actually that number comes from the required impact, which is a constraint on the model).

Inflation is probably the leading theory for explaining flatness, horizon etc. but not the only game in town either, and it has its own issues. But as far as I can tell, these seem to have to do more with such things as the need to fix parameters at some arbitrary looking value etc, than with observations not matching predictions (inflation seems pretty good at that).

Edit : Regarding such questions, more knowledgeable members directed me to this : http://cern.ch/lesgourg/Inflation_EPFL.pdf , and this http://lanl.arxiv.org/abs/astro-ph/0305179, both are pretty good introductions to inflation, with different style etc.

 

[bahamagreen]

There may be confusion about the meaning and mechanics of homogeneous and isotropic.
The first refers to the overall spacing of things as observed from anywhere (looks to be the same throughout).
The second refers to the lateral but not longitudinal view of things as observed from a particular place (looks the same in every direction).

There are problems about this; first, they both use the phrase "looks the same", which is troublesome.

Taking the second one, saying that the universe "looks the same" in any direction has two possible meanings; it might mean that in any direction you chose the result will be the same as compared to another direction (lateral aspect), but it might also be thought to mean that the results "look the same" in any particular direction comparing segments of that direction to other segments of that same direction - that the observed spacing of objects is constant with increasing distance (the spacing of things in the universe looks the same no matter how far you look in anyone direction)... but this is not true, per Hubble, et al. In this sense the universe does not look the same in every direction; it seems to be changing uniformly with respect to distance in every direction (longitudinal direction)... but doing this the same way for every direction.

The first one says the universe looks the same throughout, but that might mean either it appears so or is inferred to be so after correction for time delay. This is a primary confusion if one knows that the universe is expanding with time... seeing more distant regions as they were from progressively younger more dense states. If they appear to be the same throughout by visual inspection, this suggests that the time delayed views of more distant objects represent some intrinsic correction for their having originated from an earlier scale factor in order to match that of the local observer. On the other hand, if these views are different but accounted for or corrected by knowing of the expansion, the inference is that the universe is the same throughout. It may not be clear whether "looks the same throughout" means "the same as observed" or "the same after correcting for expansion"... it is rarely explained which method is used to develop this snapshot of the universe.

 

[Chronos]

There is no good alternative to inflation that can reasonably explain the extraordinary uniformity of the CMB temperature, since these regions could not have otherwise been in causal contact, thus inflation solves the horizon problem. Inflation also smooths out quantum fluctuations that would otherwise have grown to enormous proportions in the modern universe, thus it also solves the flatness problem. Scientists believe the universe is larger than the observable universe because it is perfectly flat [the largest triangles we can 'draw' in the universe add up to precisely 180 degrees] to the limit of our measurement ability. If the entire universe were a sphere the size of the observable universe, the largest triangles would add up to measurably less than 180 degrees. See http://arxiv.org/abs/1101.5476 for discussion.

 

[Chalnoth]

Have scientists finally come to their senses, and given up on the "big bang"?

Seriously? Don't be a douche.

 

[Chalnoth]

The image above indicates a much larger Universe... Very confusing... Can someone who knows what this is supposed to mean please explain? Bye bye big bang? Hello infinite Universe? What?

https://sites.google.com/site/microtechnonstop/f/Visible%20vs.%20Observable%20Universe%20.png

This graphic is incorrect.

The visible universe and the observable universe are essentially the same thing. The visible universe (that is, the universe visible by using photons) terminates at the surface of last scattering, the surface that emitted the CMB some 13.7 billion years ago or so. This surface is currently about 47 billion light years away. When the CMB was emitted, it was about 43 million light years away. The rapid early expansion carried the light from the CMB away from us at a high speed for billions of years. Since then, the expansion has slowed dramatically, and the light has had a chance to gain ground and start to reach us.

If we ever manage to observe the universe through something other than photons (e.g. neutrinos or gravity waves), we might be able to see to earlier times, which would also give us a window into matter that is currently further away.

 

[Drakkith]

IF the CMB is the light from "the last scattering" at T0+378,000 years, I don't quite understand how there can be something "behind" that (as the image above implies, where "our" Universe is inside a 46.5 Gly sphere)... Have scientists finally come to their senses, and given up on the "big bang"? That would (should?) have made big news, but I haven't heard a thing... Can someone please explain?

Sure, what exactly are you having trouble understanding?