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This question already has an answer here:

Disclaimer: I'm not asking whether the Big Bang happened at a point. I'm asking whether the fact that the universe is isotropic and that the Big Bang happened contradict each other.

To be honest I am just starting to learn General Relativity and cosmology but I feel like this question has a profound basis. If we assume that the universe "started" by the Big Bang (which I naively imagine as just the expansion of a sphere) then surely it must have started out from a certain point, which is still in our sphere universe. However we also know that the universe is isotropic, that there are no "special" points. How can these two facts coexist since if the Big Bang really did happen then there must be a characteristic point in our universe, a special point, which contradicts the principle of isotropy. So what am I missing, if I'm missing anything?

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marked as duplicate by knzhou, Javier, GiorgioP, Ben Crowell, Kyle Kanos Mar 16 at 9:57

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Possible duplicate of Did the Big Bang happen at a point? $\endgroup$ – knzhou Mar 15 at 18:21
  • $\begingroup$ @knzhou I believe my question is not a duplicate of the one that you are referring to. $\endgroup$ – user212860 Mar 15 at 18:24
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    $\begingroup$ If so, you should edit into your question the reason why the answers to the proposed duplicate don't satisfy you, so you get new answers. $\endgroup$ – knzhou Mar 15 at 18:53
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    $\begingroup$ I'm confused. You said you know the BB didn't happen at a point, but later you say "surely it must have started out from a certain point". $\endgroup$ – PM 2Ring Mar 15 at 19:15
  • $\begingroup$ Your reference to a "sphere universe" suggests that you may be misinterpreting the curvature of spacetime: I believe that the temporal component in that curvature is, per the Cosmic Microwave Background data, generally considered to comprise nearly all of it, even though what appears as space to one observer may, under circumstances greatly complicated by gravity, appear as time to another. The most important of those complications is the gravitational collapse that occurs when large stars run out of fuel, which leaves the volumes they had occupied observable only indirectly. $\endgroup$ – Edouard Mar 15 at 20:00
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Your assumption of a three dimensional sphere is wrong, because we are in the General relativity framework in cosmology, i.e. we have space time, and the Big Bang happened , in the original model, at a (0,0,0,0) four dimensional point.

The three space dimensions are like the surface of a sphere ,in a two dimensional analogue, and time is at the radius of the analogue. This means that all space points now were at the origin of the Big Bang.

The current Big Bang model posits a fuzzy beginning due to quantum mechanical uncertainties, in effective quantizations of gravity.

bb

I'm asking whether the fact that the universe is isotropic and that the Big Bang happened contradict each other.

The quantum mechanical fuzziness and the inflation period in the plot generate the isotropy. Before introducing the inflation period there could be no thermodynamic equilibrium, due to the light cone separations at the early universe.

Take a usual balloon and assume it started expanding from a point, where all the surface was concentrated at a point. All points at the surface of the balloon were at the origin where expansion started. This is an analogue of the BB eartly universe in two space dimensions and one time.

If at the early universe there were no light cone then yes, you would expect a thermodynamic equilibrium . But it is General Relativity and four vectors that describe the universe, and at the very early times it is the light cone that does not allow thermodynamic equilibrium, and the inflation period has been included in the BB model to fit the observed isotropy.

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  • $\begingroup$ I see how my assumption of a sphere is wrong but you still mentioned a special point, an origin (0,0,0,0). How can this coexist with the principle of isotropy? $\endgroup$ – user212860 Mar 15 at 18:52
  • $\begingroup$ The quantum mechanical fuzziness and the inflation period in the plot generate the isotropy. Before introducing the inflation period there could be no thermodynamic equillibrium, due to the light cone separations at the early universe. There is no special point in our universe . all points were at the beginning of 0,0,0,0 $\endgroup$ – anna v Mar 15 at 18:55
  • $\begingroup$ take a balloon and assume it started expanding from a point, where all the surface was concentrated at a point. All points at the surface of the balloon were at the origin where expansion started. It is the light cone separations that could not be reconciled with the isotropy. $\endgroup$ – anna v Mar 15 at 18:58
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    $\begingroup$ I mostly agree with this answer. However, if global curvature of the universe is <= 0, then space is infinite, and it has always been infinite, ever since t>0. Current Big Bang theory doesn't really address the state of the cosmos at t=0, since that's a mathematical singularity in standard GR. $\endgroup$ – PM 2Ring Mar 15 at 19:20
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    $\begingroup$ @annav it doesn't answer anything because it is completely wrong. $\endgroup$ – MBN Mar 17 at 16:21
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Your intuition is faulty because you are imagining universe’s expansion as similar to the expansion of gases from a bomb blast. Something like the “front of the universe” is expanding and occupying some sort of imaginary space, moving away from a central point. A wrong intuition that may come from the name “Big Bang”.

When we say the universe is expanding, we mean the distance between two adjacent points in universe is becoming bigger, just like a rubber band with 2 marks that is being stretched.

Imagine a container with a lot of balloons having a small amount of some chemical reactants whose reaction produces gases. After some time, the balloons will start expanding and occupying a bigger volume. However this expansion isn’t starting in a particular point, it is coming from everywhere since each of the balloons is pulling over the ones around it. This is the same type of expansion that happens when you boil a cup of rice.

For a better intuition, imagine the entire universe as a 4D Mobius “strip” balloon(1), inflating from a zero size volume.

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Einstein assumed space was isotropic and homogeneous because he believed the universe was closed.

And it simplified the model.

Also, an isotropic and homogeneous space is needed for constant curvature.

Cosmology principals assume the universe is isotropic and homogeneous - basically has a "stream line flow".

In fact, the cosmological principle reduce a 10 parameter theory to a single function and discrete parameter which characterizes Robertson-Walker space time.

However to determine the structure of the universe, we need an accurate measure of distance - which currently doesn't exist.

For instance, Google for the "Fingers of God" - which is a plot of all the measured red shifts in a quadrant of the sky - for instance the Virgo Galaxy. It looks like there's a big finger pointing back to the Earth.

And another interesting plot is the "Pancakes of God" - which suggests the universe is just a web of Machian objects.

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