<p>Some cosmological models assume that the universe is isotropic and homogeneous and that is also flat and infinite. If the universe is infinite now it was infinite immediately after the big bang. If universe was infinite immediately after the big bang, what explains isotropy and homogeneity?</p>

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2020-06-28T19:48:13.180Z
The Lambda CDM model does not assume that the universe is flat or infinite.
ProfRob
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2020-06-28T20:29:10.713Z
The false presumption about ΛCDM assuming flatness and infinity aside, this is a brilliant question.
pela
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2020-06-29T07:27:13.860Z
@JamesK you're right I'll correct the "at". About homogeneity, I read the it (and isotropy) is explained by inflation because the entire observable universe has been causally connected long enough to come into thermal equilibrium. But if the universe was infinite soon after the big bang time, there are regions that was never causally connected. So what explains isotropy and homogeneity of the *entire* universe?
UFO
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2020-06-29T07:41:59.423Z
@RobJeffries in the [Lambda Nasa website][1] I read that there 6 independent parameters that completely specifies the cosmological model. 4 of them are related to matter content. Specifically: "The Ω parameters are defined as the ratio of the present day mean density of each component χ to the critical density: by definition ΣχΩχ = 1".  If ΣχΩχ=1 than the universe is flat and infinite (to be precise it could be also finite like a three-torus)  

(1): https://lambda.gsfc.nasa.gov/education/graphic_history/parameters.cfm
UFO
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2020-06-29T09:14:12.450Z
You can assume that $\Omega=1$ but there is no need to. Sometimes this is referred to as the "base $\Lambda$CDM model" or the "spatially flat $\Lambda$CDM model". to make it clear that this assumption has been made. Since $\Omega$ is consistent with 1, then the assumption is often made, since it is consistent with the predictions of inflation.
ProfRob
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2020-06-29T12:37:19.123Z
@RobJeffries thanks for clarifying, I'll rephrase the question
UFO
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2020-06-30T05:55:07.850Z
This may trigger some people, and I sincerely apologise in advance for that. But those who believe that the universe is finite are the equivalent of flat earthers.
White Prime
<cosmology><big-bang-theory><cosmological-principle>
Does the cosmological principle apply to the entire universe?
UFO
# Does the cosmological principle apply to the entire universe?

(Asked by: username_0 on 2020-06-28)

Some cosmological models assume that the universe is isotropic and homogeneous and that is also flat and infinite. If the universe is infinite now it was infinite immediately after the big bang. If universe was infinite immediately after the big bang, what explains isotropy and homogeneity?

**Tags:** cosmology, big-bang-theory, cosmological-principle

> **Comments:**
> **username_1**: The Lambda CDM model does not assume that the universe is flat or infinite.  
> **username_2**: The false presumption about ΛCDM assuming flatness and infinity aside, this is a brilliant question.  
> **username_0**: @JamesK you're right I'll correct the "at". About homogeneity, I read the it (and isotropy) is explained by inflation because the entire observable universe has been causally connected long enough to come into thermal equilibrium. But if the universe was infinite soon after the big bang time, there are regions that was never causally connected. So what explains isotropy and homogeneity of the *entire* universe?  
> **username_0**: @RobJeffries in the [Lambda Nasa website][1] I read that there 6 independent parameters that completely specifies the cosmological model. 4 of them are related to matter content. Specifically: "The Ω parameters are defined as the ratio of the present day mean density of each component χ to the critical density: by definition ΣχΩχ = 1".  If ΣχΩχ=1 than the universe is flat and infinite (to be precise it could be also finite like a three-torus)    (1): https://lambda.gsfc.nasa.gov/education/graphic_history/parameters.cfm  
> **username_1**: You can assume that $\Omega=1$ but there is no need to. Sometimes this is referred to as the "base $\Lambda$CDM model" or the "spatially flat $\Lambda$CDM model". to make it clear that this assumption has been made. Since $\Omega$ is consistent with 1, then the assumption is often made, since it is consistent with the predictions of inflation.  
> **username_0**: @RobJeffries thanks for clarifying, I'll rephrase the question  
> **username_3**: This may trigger some people, and I sincerely apologise in advance for that. But those who believe that the universe is finite are the equivalent of flat earthers.  

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## 0 answers

