Shouldn't the Big bang be physically impossible?

[Mr J]

TL;DR Summary

gravity black hole big bang and why Big bang occurs despite mass of universe.

I would appreciate if someone could clarify this simple contradiction with the Big Bang.

Assume black holes exist, and are caused by the collapse of large quantities of mass due to gravity. The resultant singularity is so massive that not even light can escape, much less material.

Yet if the Big Bang occurred, ALL matter that we observe today was at one point in space-time so concentrated that our universe must have been a black hole - as all matter that we observe clearly must be contained within a "universal event horizon" (In other words, take the entire mass of the universe today - which I believe is known - what then would be the event horizon of such a black hole, or at what point would all universal matter at some concentration become a black hole?).

But Black Holes - by definition - cannot 'explode' (meaning they cannot eject matter outside their event horizon). And if Black Holes can 'explode' - like the Big Bang - why are we not observing universes being created in space all the time, or at least quite often?

I assume that answer will be along the lines of gravity can also be repulsive, or matter is not 'exploding' rather space is expanding.
Any thoughts would be welcome. Thanks!

 

[Vanadium 50]

Yet if the Big Bang occurred, ALL matter that we observe today was at one point in space-time so concentrated that our universe must have been a black hole

There's your mistake. The initial conditions were not a black hole.

 

[Ibix]

A black hole is a region in space surrounded by vacuum, and there's a sense of "down" towards the centre. The universe at large was always filled more or less isotropically with stuff (matter, radiation, etc), so there can be no down, nor up. In other words, it looked nothing like a black hole.

 

[PeroK]

Any thoughts would be welcome. Thanks!

This question gets asked a lot. Most recently:

https://www.physicsforums.com/threads/schwartzchild-radius-and-the-big-bang.994811

 

[Mr J]

This question gets asked a lot. Most recently:

https://www.physicsforums.com/threads/schwartzchild-radius-and-the-big-bang.994811

Thanks - at least nice to know it was a reasonable question (more or less).

 

[DaveE]

I could be wrong, but it seems like you are thinking that the big bang was a collection of everything in an existing, otherwise empty universe. Then this stuff "exploded" and spread out into the universe.

I'm no cosmologist, but I am told this isn't what the big bang was. It was the expansion (or creation, if you like) of the universe. It isn't that stuff expanded into a universe that was already present. It was the universe that expanded. It was full of stuff before (i.e. spread out, not localized into a point), and remained full after.

 

[PeterDonis]

ALL matter that we observe today was at one point in space-time so concentrated that our universe must have been a black hole

No, this is not correct. A black hole is a static object. The very early universe was rapidly expanding. These are different spacetime geometries with different behaviors.

 

[phinds]

@Mr J you are clearly getting your "information" from pop-science presentations. These are notably unreliable, being as they are entertainment and not education. I suggest reading some basic physics.

 

[Drakkith]

@Mr J you are clearly getting your "information" from pop-science presentations. These are notably unreliable, being as they are entertainment and not education. I suggest reading some basic physics.

Unfortunately the details of different spacetime geometries go well beyond 'basic' physics.

 

[Scott Mayers]

I would avoid concern about inferring anything from black holes. As Drakkith just mentioned and can be extended upon, this is beyond 'basic' physics.

The Big Bang is one class of theories based on assuming a 'singularity' inferred AND that argues for a 'hot' origin. I'm not satisfied with this for other reasons philosophically but think that one has to ignore any complex issues that may or may not also be valid.

Personally, I think addressing Zenos paradoxes with respect to boundaries, such as a singularity refers to, might be a better means to enter skeptical inquiry into the grand Cosmological theories regarding origins. It is more 'philosophy' proper but then I think that "Theoretical Science" is itself a cross between philosophy, logic, and "science" proper. Regardless, colloquial discussion on this for a general audience needs to begin with how historical philosophy motivated and evolved to develop these theories. A lot of science is as equally 'political' to some degree which hides the means to understanding if we assume the present complexity of theory from the end rather than going back to the beginning sources of these theories.

 

[Scott Mayers]

Example inquiry to question the Big Bang:

How can you differentiate between the appearance of a singularity as being literally real versus a perspective illusion? [Like how do we make sense of witnessing the perspective of parallel lines in reality appear to converge to a point in the distance when they do not require meeting?]

 

[Ibix]

The Big Bang is one class of theories based on assuming a 'singularity' inferred AND that argues for a 'hot' origin.

Big Bang theory does not assume a singularity. The singularity falls out of the maths if you assume an isotropic and homogeneous universe with non-zero mass density.

How can you differentiate between the appearance of a singularity as being literally real versus a perspective illusion?

Nobody expects the singularity to be real. Its appearance in the maths is most likely an artefact of us needing a quantum theory of gravity to accurately describe the very early universe.

 

[PeterDonis]

The Big Bang is one class of theories based on assuming a 'singularity'

No, the Big Bang model does not assume a singularity.

A particular idealized model, if extrapolated backward far enough, predicts an initial singularity; but that just means that nobody actually uses that model extrapolated that far backward. Cosmologists only extrapolate it backward as far as the hot, dense, rapidly expanding state that is the earliest state of the universe for which we have good evidence. What came before that is an open question, although the current front runner is some kind of inflation model; note that inflation models do not have to have an initial singularity, and the "eternal inflation" models that appear to be currently favored do not.

 

[PeterDonis]

How can you differentiate between the appearance of a singularity as being literally real versus a perspective illusion?

A singularity is present in a GR solution if there are geodesics that cannot be extended to arbitrary values of their affine parameter. This is a perfectly well-defined condition that has nothing to do with "illusion". Most physicists do not consider a solution that contains a singularity to be physically reasonable, but that also has nothing to do with "illusion".

 

[PeterDonis]

how do we make sense of witnessing the perspective of parallel lines in reality appear to converge to a point in the distance when they do not require meeting?

What you are describing here is a coordinate singularity, like lines of longitude meeting at the north and south poles. That is not the kind of singularity that shows up in idealized cosmological models; there are no inextendible geodesics at a coordinate singularity.

 

[haushofer]

The Big Bang is one class of theories based on assuming a 'singularity'

I guess that's similar to saying that Newtonian gravity with point particles "is one class of theories based on assuming a 'singularity'", simply because Newtonian gravity goes like 1/r^2. But of course, the fact that in the Newtonian context two point particles can experience an infinite force (i.e. a "singularity") is a consequence of the equations of motion. Furthermore, this singularity is only problematic if you assume that the theory holds at all energy scales.

 

[jbriggs444]

I guess that's similar to saying that Newtonian gravity with point particles "is one class of theories based on assuming a 'singularity'", simply because Newtonian gravity goes like 1/r^2. But of course, the fact that in the Newtonian context two point particles can experience an infinite force (i.e. a "singularity") is a consequence of the equations of motion. Furthermore, this singularity is only problematic if you assume that the theory holds at all energy scales.

This "singularity" even includes geodesic incompleteness. The Newtonian n-body problem with point-like masses includes the possibility of ejecting a particle to infinity in finite time. See this review article.

 

[epenguin]

@Mr J you are clearly getting your "information" from pop-science presentations. These are notably unreliable, being as they are entertainment and not education. I suggest reading some basic physics.

For the first sentence – that's all most of us are ever going to have. For the second

Unfortunately the details of different spacetime geometries go well beyond 'basic' physics.

is the point.
Is it not the case that if you are going to account for the Big Bang you have to go beyond 'basic physics' and the guidelines for doing it are that theories try to be as natural a generalisation of it as possible and fit all the evidence (e.g. that there was a Big Bang)?

Now it is said that these popularisations are unreliable. But many of the popularisations are by the people who have participated in creating the subject and the theories. So can anyone do better? Can you?

 

[weirdoguy]

So can anyone do better?

As hundreds of threads here show - yes, a lot of people can do better.

 

[epenguin]

As hundreds of threads here show - yes, a lot of people can do better.

OK I confess I hardly ever come to this section of the forum, and only happened on it today because it was in my emails of posts of the week. I cannot go through hundreds of threads, can you recommend a couple of posts?

 

[PeroK]

OK I confess I hardly ever come to this section of the forum, and only happened on it today because it was in my emails of posts of the week. I cannot go through hundreds of threads, can you recommend a couple of posts?

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

 

[weirdoguy]

Insights are one thing, but I think that people like @PeterDonis, @PeroK, @Ibix, @phinds and MANY others do a tremendously better work in their postings then most of popularisators. I won't recommend any particular post, because I don't know what is interesting for you, just stay in touch with this section and you'll see

 

[PeterDonis]

For the first sentence – that's all most of us are ever going to have.

That depends on what you want. If you want to actually understand the physics, you can't do it from pop science presentations alone. Pop science presentations can get you interested (and good ones will do that), but they can't actually teach you the physics.

Is it not the case that if you are going to account for the Big Bang you have to go beyond 'basic physics' and the guidelines for doing it are that theories try to be as natural a generalisation of it as possible and fit all the evidence (e.g. that there was a Big Bang)?

You are misunderstanding what was meant by "basic physics". It doesn't mean a "basic" theory that more complicated theories are generalizations of. It means a basic presentation of the physics, assuming as little previous background knowledge on the reader's part as possible. You then build from that basic presentation to more complicated and comprehensive presentations that require more background knowledge and are more difficult to understand. Where you stop depends on what you intend to do with your knowledge; if all you want is enough understanding to be able to follow high-level discussions, that's one thing; if you want to be a professional cosmologist, that's quite another. But all of the presentations are of the same theory, just at different levels of difficulty.

many of the popularisations are by the people who have participated in creating the subject and the theories. So can anyone do better? Can you?

It's not a question of different people doing better. The same people who write some of the popularizations have also written textbooks and peer-reviewed papers. The reason the latter are good sources while the former are not is that in textbooks and peer-reviewed papers, what the authors write is reviewed by other experts, who will require revisions or corrections if the authors say something that is not justified by the underlying physics they are using. In popularizations, that editorial checking is not there; and experience shows that without it, scientists, even ones who are leaders in their field, can't resist making claims that go beyond the underlying physics and are simply their own personal opinions or their own preferred interpretations. And unless you yourself are an expert, you can't tell from reading their popularizations which parts are actually the underlying physics and which parts are their own personal opinions or preferred interpretations; and that means you can't, as a non-expert, learn the actual physics from a popularization, even if it's by a leading scientist in the field.

 

[elcaro]

Summary:: gravity black hole big bang and why Big bang occurs despite mass of universe.

I would appreciate if someone could clarify this simple contradiction with the Big Bang.

Assume black holes exist, and are caused by the collapse of large quantities of mass due to gravity. The resultant singularity is so massive that not even light can escape, much less material.

Yet if the Big Bang occurred, ALL matter that we observe today was at one point in space-time so concentrated that our universe must have been a black hole - as all matter that we observe clearly must be contained within a "universal event horizon" (In other words, take the entire mass of the universe today - which I believe is known - what then would be the event horizon of such a black hole, or at what point would all universal matter at some concentration become a black hole?).

But Black Holes - by definition - cannot 'explode' (meaning they cannot eject matter outside their event horizon). And if Black Holes can 'explode' - like the Big Bang - why are we not observing universes being created in space all the time, or at least quite often?

I assume that answer will be along the lines of gravity can also be repulsive, or matter is not 'exploding' rather space is expanding.
Any thoughts would be welcome. Thanks!

It is a popular misconception comparing the highly dense situation of a black hole with a similar high density near the big bang. But for one thing do not confuse the big bang with something like a dense very small point, I think there is where you go wrong.

A better understanding would be to imagine a 2D rubber sheet (representing 3D space) which is infinitely stretchable, and imagine that a very small circle (representing the size of the observable universe in a very early time) grows larger and larger as the rubber sheet gets uniformly stretched in all directions.

I hope this helps in seeing how the two situations differ and can not be compared.

 

[bhobba]

The inflationary model is a much better way of looking at the modern theory of how the universe began:
https://ned.ipac.caltech.edu/level5/March05/Guth/Guth_contents.html

Thanks
Bill

 

[timmdeeg]

Interesting article, thanks.

One thing, it is mentioned "In inflationary models, on the other hand, any original curvature of the early universe would have been stretched out to near-flatness as the universe underwent its rapid expansion" and thereafter "during the inflationary epoch,

gets driven rapidly to 1".

Because of |

-1|

1 / (aH)2 "near-flatness" makes sense. This leaves the sign of the curvature open before inflation, the shape of the universe could e.g. be a very large 3-sphere. But

= 1 seems to require spatial flatness before inflation which would exclude the 3-sphere. So it seems that either the one or the other (almost flat or flat) can be true.

Perhaps I'm misinterpreting something. Kindly let me know your thoughts.