Do we 100% know what the geometry of the Universe is?

[user079622]

If big bang existed, than universe must be sphere, because explosion expand in all direction..
I read that universe is maybe flat so how this is possible?

Why can we with telescope determine shape of universe?

 

[Orodruin]

If big bang existed, than universe must be sphere, because explosion expand in all direction..

This is a false assumption. The big bang was not an explosion in the usual sense of the word with matter being thrown out from a single event. It is what is described by the universe starting in a hot dense state in an isotropic, homogeneous, and spatially expanding setting.

Since the basic assumption is flawed, the rest of the question seems moot to me.

 

[PeterDonis]

If big bang existed, than universe must be sphere

No, that is not correct.

because explosion expand in all direction..

No, that is not what the Big Bang is.

I read that universe is maybe flat so how this is possible?

Because the Big Bang was not an explosion into a pre-existing space.

Why can we with telescope determine shape of universe?

We can, just not with 100% certainty. Our best current model is that the universe is spatially flat, but there are always finite error bars on real measurements.

 

[user079622]

We can, just not with 100% certainty. Our best current model is that the universe is spatially flat, but there are always finite error bars on real measurements.

All stars/planets in universe lay at same plane as earth? How thick is this plane?

 

[Orodruin]

All stars/planets in universe lay at same plane as earth? How thick is this plane?

That is also false and not what the theory actually says.

 

[user079622]

This is a false assumption. The big bang was not an explosion in the usual sense of the word with matter being thrown out from a single event. It is what is described by the universe starting in a hot dense state in an isotropic, homogeneous, and spatially expanding setting.

Since the basic assumption is flawed, the rest of the question seems moot to me.

Isn't theory in science only validate if can be proofed? Isnt that just one of assumptions?

 

[phinds]

That is also false and not what the theory actually says.

Nor what observation says. @user079622 you have a VERY incorrect view of cosmology. I suggest you do some basic reading (NOT pop-science presentations).

 

[user079622]

Nor what observation says. @user079622 you have a VERY incorrect view of cosmology. I suggest you do some basic reading (NOT pop-science presentations).

Are we talking about assumptions or proofed theory ?

 

[phinds]

Are we talking about assumptions or proofed theory ?

Either way, the view points you have expressed are so far off of reality they are "not even wrong".

 

[berkeman]

All stars/planets in universe lay at same plane as earth? How thick is this plane?

That's not what is meant by "flat" in the context of the Universe.

Here are a number of PF Insights articles that should help you learn more about the Big Bang:

https://www.physicsforums.com/insights/?s=big+bang

 

[PeterDonis]

All stars/planets in universe lay at same plane as earth? How thick is this plane?

"Flat" in this context does not mean "plane". It means "Euclidean 3-space".

 

[PeterDonis]

Isn't theory in science only validate if can be proofed? Isnt that just one of assumptions?

Are we talking about assumptions or proofed theory ?

Theories in science are never "proofed". You compare their predictions with observations, and this can give an increasing level of confidence in the theory, but you can never "proof" it.

Theories are also not assumptions. The statement that our universe is spatially flat is not an assumption; it's what our best current theory tells us with a high level of confidence. So is the statement that the Big Bang was not an explosion into a pre-existing space.

 

[DaveC426913]

I read that universe is maybe flat so how this is possible?

"Flat" in this context does not mean "plane". It means "Euclidean 3-space".

(I include this for clarity but I fear it will only deepen the OP's confusion.)

 

[PeterDonis]

View attachment 340951


(I include this for clarity but I fear it will only deepen the OP's confusion.)

The "flat" picture is of a cylinder, which is not the spatial topology of our universe in our best current model.

Where did you get these pictures from?

 

[Vanadium 50]

IBTL,

That is also false and not what the theory actually says.

Nor what anybody here has said.

@user079622 , you are replying to messages within seconds. While promptness is a virtue, it is possible to overdo it. It may help you to spend more time thinking about what other people are saying.

 

[Ibix]

All stars/planets in universe lay at same plane as earth? How thick is this plane?

No. There is matter in all directions in all three dimensions.

There are three possible cosmological models that come out of general relativity - all of them have space being more or less homogeneous in all directions. However, if you draw a large circle around a point you will not necessarily find that the circumference of the circle is $2\pi$ times the radius. If it is smaller the universe is called "closed". If it is larger the universe is called "open". If it is actually equal the universe is called "flat".

When we make measurements equivalent to this, we find that the universe could be any one of the three - either truly flat or so huge we can't see the curvature at our current measurement precision.

I repeat that "flat" does not mean two dimensional. All our models have stars in all directions from all points. In this context, flat is a technical term referring to the geometry of spacetime, not to the distribution of matter.

 

[user079622]

No. There is matter in all directions in all three dimensions.

Yes that make sense. When I look at sky I see stars at all directions.
Does all matter comes from single source, does all reversed matters path lead to same center?

 

[PeterDonis]

Does all matter comes from single source

I don't know what that means.

does all reversed matters path lead to same center?

The question doesn't even make sense. The universe doesn't have a "center".

 

[Orodruin]

Does all matter comes from single source, does all reversed matters path lead to same center?

This shows that you still suffer from the same misconception you had when you started this thread. You have to dispell that misconception if you are to get further.

 

[Ibix]

Yes that make sense. When I look at sky I see stars at all directions.

That's an interesting observation, because the stars you can see are part of the Milky Way galaxy that is disc shaped. It's quite a thick disc, but you get to intergalactic space in a much shorter distance in one direction from another. You are thinking on far too small a scale - it's the galaxies that are more or less uniformly distributed across the sky.

So you have to be very careful about drawing conclusions from the limited data directly available to you by direct observation without a very big telescope.

Does all matter comes from single source, does all reversed matters path lead to same center?

No. In our current models, if you run them backwards the density of matter increases everywhere. There is no single point from which everything came.

 

[phinds]

Yes that make sense. When I look at sky I see stars at all directions.

And, as @Ibix has pointed out, they are ALL in the (relatively) flat disk of the Milky Way Galaxy. All of the stars you can see with the naked eye are in that disk but basically all of the stars in the Observable Universe are NOT in the Milky Way.

That is, the number of stars in the Milky way is trivial (basically, a rounding error in the 12th decimal place) compared to the number of stars in the Observable Universe (we just can't see any of the other ones without a telescope).

 

[user079622]

No. In our current models, if you run them backwards the density of matter increases everywhere. There is no single point from which everything came.

But how can this be physically possible?
Example with fluid, if you decrease distance between each molecule of air, shouldn't it come to one point?

 

[phinds]

But how can this be physically possible?
Example with fluid, if you decrease distance between each molecule of air, shouldn't it come to one point?

Not if it started off as something bigger than a point, as is the case with the Universe. Even the Observable Universe does not shrink to a point if you run time backwards, although it DOES go down to something quite small. I've heard estimates all the way from the size of an atom to the size of a golf ball.

 

[PeterDonis]

But how can this be physically possible?

Because this is General Relativity, not Newtonian physics. Your Newtonian intuitions do not work in this domain.

 

[Ibix]

Example with fluid, if you decrease distance between each molecule of air, shouldn't it come to one point?

If you assumed that the geometry of spacelike slices of spacetime remained static then you would be correct. Spacetime geometry does not work that way, however.

Note that something does go wrong if you run a simple model of the universe backwards - density rises to infinity (although it does it everywhere, not just at a point). This is the "Big Bang singularity", and finding a model that doesn't go wrong in this way is a topic of active research. Alan Guth's inflationary theory is the current leader of the field.

 

[user079622]

Because this is General Relativity, not Newtonian physics. Your Newtonian intuitions do not work in this domain.

Obviously that's the problem, hmm. So we are in math world.

 

[Ibix]

Obviously that's the problem, hmm. So we are in math world.

No - you are interested in bits of physics where we can no longer pretend that Euclidean geometry is an accurate model of the world. That you have an unquestioned belief in the accuracy of Euclidean geometry is part of why you struggle with what we're saying.

Don't naively accept one mathematical model just because it's a decent approximation of the medium-scale world we live in day to day.

 

[user079622]

Note that something does go wrong if you run a simple model of the universe backwards - density rises to infinity (although it does it everywhere, not just at a point). This is the "Big Bang singularity", and finding a model that doesn't go wrong in this way is a topic of active research. Alan Guth's inflationary theory is the current leader of the field.

Ok so that mean we are still finding what is really happening. So that mean all above members wrote in posts maybe is not 100% correct?

 

[DaveC426913]

The "flat" picture is of a cylinder, which is not the spatial topology of our universe in our best current model.

Where did you get these pictures from?

Yeah, they're not the best. There's plenty of diagrams out there. I just hoped to start a discussion for the OP about what curvature is, as opposed to what it's not.

 

[Ibix]

So that mean all above members write in posts maybe is not 100% correct?

Depends what you mean. It's all 100% certain implications of the premises of our models. And our models make extremely accurate predictions of the outcome of experiments that we do. But are they necessarily 100% accurate models of the universe? Almost certainly not - they're just enormously better than anything anyone else has come up with.

All of science is contingent on the possibility that we'll discover something new tomorrow. Honestly, all of knowledge is contingent on it. Anybody who tells you something is 100% certain (except in terms of mathematical deductions from premises, as above) is probably selling you something.

 

[PeterDonis]

Alan Guth's inflationary theory is the current leader of the field.

Inflation models don't necessarily eliminate the initial singularity.

 

[PeterDonis]

Obviously that's the problem, hmm. So we are in math world.

Newtonian physics is "math world" just as much as GR is. Newtonian physics is just different math, whose predictions are now known to not be as accurate as those of GR.

so that mean we are still finding what is really happening.

Depends on what you mean. We don't have good knowledge of what happened before the hot, dense, rapidly expanding state that we call the "Big Bang" (which is not the same thing as the "initial singularity" that appears in some models). We do, however, have good knowledge of the current spatial geometry of the universe--that it is flat to within a very good approximation (i.e., if it is actually curved, the radius of curvature is much, much larger than the radius of our observable universe).

So that mean all above members wrote in posts maybe is not 100% correct?

As has already been pointed out, we never can be certain that anything in science is "100% correct".

However, that is not the same as saying that we know nothing at all or that we don't know that the naive Newtonian model you have been implicitly using in your intuitions is not correct. We do know that that model is wrong, to an extremely high confidence: its predictions are nothing like what we see.

 

[Jaime Rudas]

In this context, flat is a technical term referring to the geometry of spacetime

And, specifically, to the geometry of space.

 

[PeterDonis]

flat is a technical term referring to the geometry of spacetime

And, specifically, to the geometry of space.

More precisely, to the geometry of spacelike hypersurfaces of constant FRW coordinate time (or, equivalently, constant proper time for comoving observers) in the overall FRW spacetime geometry.

 

[PAllen]

But how can this be physically possible?
Example with fluid, if you decrease distance between each molecule of air, shouldn't it come to one point?

Suppose simply that a current cosmological time state of the universe is unbounded Euclidean 3 space. Then every earlier state, however many doublings of density involved, is also unbounded. The cosmological time exactly zero is not actually part of the model (even in the idealized mathemetical model). Every time later than zero (even e.g. $1/(((10^{100})^{100})^{100})$ seconds after zero) is still an unbounded state. That is just the nature of infinity.