Comments about the FAQ: Is the big bang a black hole?

Following is a discussion about this Faq
In the last paragraph I explain my own opinion.

[Physics FAQ] [Copyright]

Updated by PEG 27 June 1997
Original by Philip Gibbs, 17 March 1997

Is the big bang a black hole?

This question can be made into several more specific questions with different answers.

Why did the universe not collapse and form a black hole at the beginning?

Sometimes people find it hard to understand why the big bang is not a black hole.
That is not so much the issue.
We (the people) have a certain idea about both "the beginning of the big bang" and a black hole. We compare the two and see if that fits the reality.
For us "the beginning of the Big Bang" is an object (Big Bang hole) with a tremendous mass of all the future galaxies collected in one "point". A Black hole is the same except that its mass is much smaller.
However there are three major problems with this model.
  1. The Big Bang hole is unstable (i.e. it explodes) while a Black hole is stable
  2. A Black hole collects matter and grows while the Big Bang hole radiates matter.
  3. A Black hole is in agreement with the laws of physics (i.e. mass attracts mass) while the Big bang hole is not.
The only way out is that the laws that describe both are very different.
After all, the density of matter in the first fraction of a second was much higher than that found in any star,
That is what we all agree about, except that the time period could be longer.

and dense matter is supposed to curve space-time strongly.
Here we are lost. How do we know that for the Big Bang space-time is strongly curved? What means space-time and curved?

At sufficient density there must be matter contained within a region smaller than the Schwarzschild radius for its mass. Nevertheless, the big bang manages to avoid being trapped inside a black hole of its own making and paradoxically the space near the singularity is actually flat rather than curving tightly. How can this be?
Why using words like: "manages to avoid", "being trapped", "of its own making" and "paradoxically". It does not sound very scientifically.
What is the definition of: flat ?
Two words: curved and flat, are introduced to explain something. Because both words are vaque they explain nothing.

The short answer is that the big bang gets away with it because it is expanding rapidly near the beginning and the rate of expansion is slowing down.
Is not current understanding that rate of expansion (acceleration) is increasing ?

Space can be flat while space-time is not.
Why this sentence ?
Why not write: Space is flat and space-time is not (i.e. is curved)

The curvature can come from the temporal parts of the space-time metric which measures the deceleration of the expansion of the universe.
Using one word like "temporal part" which is not clear, makes this whole sentence not clear

So the total curvature of space-time is related to the density of matter but there is a contribution to curvature from the expansion as well as from any curvature of space.
What means curvature of space ? Same remark as previous.

The Schwarzschild solution of the gravitational equations is static and demonstrates the limits placed on a static spherical body before it must collapse to a black hole. The Schwarzschild limit does not apply to rapidly expanding matter.
Why ?
The real issue is why does the Schwarzschild limit not apply to the Big Bang hole before it exploded.

What is the distinction between the big bang model and a black hole?

The standard big bang models are the Friedmann-Robertson-Walker (FRW) solutions of the gravitational field equations of general relativity. These can describe open or closed universes. All these FRW universes have a singularity at the origin of time which represents the big bang. Black holes also have singularities.
The issue IMO is that both the Big bang and a black hole are described by the same gravitational field equations of general relativity. The parameters of both are different giving rise to different solutions.
Singularities are typical a mathematical concept, they are an issue of certain mathematical functions (dividing by zero giving infinity), but they do not reflect a physical concept.
You can say that a black hole has a mass and a size but IMO you can not say that a Black hole (or any object) has a singularity.

Furthermore, in the case of a closed universe no light can escape which is just the common definition of a black hole. So what is the difference?
From a black hole no light can escape but that does not make it a closed universe. You can not compare a black whole with a universe.

The first clear difference is that the big bang singularity of the FRW models lies in the past of all events in the universe, whereas the singularity of a black hole lies in the future.
I do not understand. It is definitely not a clear difference. Words like past and future are clear. Concepts like "the past of all events" and "the future of all events" are not. You can not relate any of those two with the big bang or a black hole.

The big bang is therefore more like a white hole which is the time reversal of a black hole. According to classical general relativity white holes should not exist since they cannot be created for the same (time-reversed) reasons that black holes can not be destroyed. This might not apply if they always existed.
It is not wise in order to explain a black hole to introduce the concept of a white hole which is also not clear.
What is the defintion of time reversal ? Why introduce this concept.

But the standard FRW big bang models are also different from a white hole.
That seems reasonable, but again why going in this direction.

A white hole has an event horizon which is the reverse of a black hole event horizon. Nothing can pass into this horizon just as nothing can escape from a black hole horizon.
Both sentences seem correct, again why this information

Roughly speaking, this is the definition of a white hole. Notice that it would have been easy to show that the FRW model is different from a standard black or white hole solution such as the static Schwarzschild solutions or rotation Kerr solutions, but it is more difficult to demonstrate the difference from a more general black or white hole. The real difference is that the FRW models do not have the same type of event horizon as a white or black hole.
That may be so, but what does that explain. How do we know that in the first place.

Outside a white hole event horizon there are world lines which can be traced back into the past indefinitely without ever meeting the white hole singularity whereas in a FRW cosmology all worldline originate at the singularity. That being the case what does it explain?
Is that the answer on the question: "What is the distinction between the big bang model and a black hole?"
I do not think that "the relation between wordlines and a singularity" clarifies anything.

Question: What is the difference between the Big Bang hole and a Black hole ?
Both answers are not clear. They include concepts which are vaque, and as such explain nothing.

In the book:"Introducing Einstein's Relativity" at page 358 Ray d'Inverno writes:
There is an other difference about the initial singularity of cosmology, compared with the black hole singularities, in that the big bang singularity is in principle observable. And it is observation that is the linchpin (cornerstone) of cosmology.

I agree totally with the last sentence. By adding the word "in principle" the whole first sentence becomes vaque. Even without that word I would ask: How? How do we know this?
By using stronger telescopes we can see the Universe in more detail. However what we see is only a subset of all the events that happened in the past i.e. state of the Universe. As such I do not agree that at by using stronger telescopes we can always look further back in time (Assuming Big Bang "Universe" occupies a finite space).

The event that created the current microscopic background radiation happened maybe when the Universe was a 100 years old (or even earlier), when the Universe was very hot, giving birth to black body radiation of a very high temperature. Anyway when the Universe evolved and the temperature decreased, this black body radiation changed.
What we measure now is black body radiation of a cold Universe. To interpret this as information reflecting the state of the very early Universe can be misleading.


Sorry to say but this FAQ is not clear. A FAQ raises a question and gives an answer. A FAQ should be carefull which concepts it uses in order to explain the question.

The problem is that the Big Bang and "The beginning of the Big Bang" versus a Black hole are two completely different physical phenomena. To add the behaviour of a white hole only adds confusion. To add the behaviour of sun sized objects is realistic however also not easy and adds complexity: Why does a Sun sized object explode while a black hole does not?
Of course the biggest problem is why was there an explosion of something in the first place.
The description of that event (sequence of events) compared with the event (static behaviour) of a Black hole is totally different.
The mathematics of those events are also very different.

But mathematics per se do not explain anything. Mathematics are very powerfull for systems (experiments) of which there are many (like blackholes or the movement of the planets) but rather speculatif for single events like "the beginning of the Big Bang"
To describe "the beginning of the Big Bang" as an exploding singularity is misleading, because singularities do not exist.

A better understanding only comes when we better know what physically happened. That however is very difficult. Sometimes I think it is better to say: Sorry we do not know, in stead to offer speculations as a solution.



Created: 27 december 2001
Modified: 3 january 2002

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