Updated by PEG 27 June 1997
Original by Philip Gibbs, 17 March 1997
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
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.
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
The real issue is why does the Schwarzschild limit not apply to the Big Bang hole before it exploded.
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
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.
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.
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.
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