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madjack
2007-Jan-11, 10:29 PM
That's it! Thatís my question. It seems logical that if the universe started as a Singularity or Big Bang, why did it expand in the first place and not just collapse under its own gravity?

Peter Wilson
2007-Jan-12, 12:58 AM
What fun would that have been?

Jeff Root
2007-Jan-12, 01:05 AM
I think you mean to ask "why didn't it just stay collapsed?"

Good question.

-- Jeff, in Minneapolis

max8166
2007-Jan-12, 01:22 AM
Whats to say it did?

I would propose Against the mainstream that that is just what the Universe is. If the gravity well of the universe extended beyond the singularity that it was it would have to expand into something, there is nothing other than the universe so beyond the universe is nothing, 0 dimensions. Therefore the premise that we have expanding space, big bang etc, is meaningless outside our universe.

This could mean that what we consider to be black holes could, in fact, be large areas of invisible space whose galaxies are beyond our view as there light can never get out of their universe!

By using Einstiens theory of GR which shows that Mass bends Space-time you could perhaps have another universe within our own. But this is very AGM so I shall say no more.

Ken G
2007-Jan-12, 01:39 AM
It seems logical that if the universe started as a Singularity or Big Bang, why did it expand in the first place and not just collapse under its own gravity?

This is a very important question, because it brings up the element of the Big Bang that many people miss-- the need for an initial condition. Like all dynamical models in physics, the Big Bang requires an ad hoc initial condition that is entirely outside the theory. Here, the initial condition is rapid expansion. So the very strong gravity is trying to dynamically collapse everything into a black hole, but black hole solutions apply to stationary mass-- not expanding mass. The universe has "slipped out of the net", so to speak, entirely due to this initial condition. At present, there is no explanation for the initial condition, and although some are trying some pretty creative solutions, I suspect there will always be an ad hoc initial condition here. It seems to be a fundamental limitation of science, though not all scientists would agree with that. (Welcome to the forum madjack.)

Blob
2007-Jan-12, 02:12 AM
Hum,
i suppose the standard answer is to say that `random quantum fluctuation in the quantum foam (http://en.wikipedia.org/wiki/Quantum_foam) created the bigbang,` and that for many previous universes they did just recollapse.

But there is a new branch of physics - M Theory (http://en.wikipedia.org/wiki/M-theory) that many hope to have answers to just those questions. i would reckon that most string theorists would be happy with the colliding branes theory (http://arxiv.org/abs/hep-th/0103239) as the initiator od the BB.

And the recent work of Neil Turok (http://www.damtp.cam.ac.uk/user/ngt1000/) and Roger Penrose (http://en.wikipedia.org/wiki/Roger_Penrose) seem promising,

Maksutov
2007-Jan-12, 02:15 AM
This is a very important question, because it brings up the element of the Big Bang that many people miss-- the need for an initial condition. Like all dynamical models in physics, the Big Bang requires an ad hoc initial condition that is entirely outside the theory.Nothing wrong with that. Sort of like we need Maxwell in addition to Newton. Re ad hoc, it boils down to working from the basis that the Universe is knowable. All else derives from that.
Here, the initial condition is rapid expansion. So the very strong gravity is trying to dynamically collapse everything into a black hole, but black hole solutions apply to stationary mass-- not expanding mass. The universe has "slipped out of the net", so to speak, entirely due to this initial condition. At present, there is no explanation for the initial condition, and although some are trying some pretty creative solutions, I suspect there will always be an ad hoc initial condition here. It seems to be a fundamental limitation of science, though not all scientists would agree with that...Most wouldn't. Those who would are applying an artificial roadblock to increasing scientific knowledge. One of the nice things about science is anything is open to question and everything is fair game. No limits, except those that are self-imposed by a few individual scientists. Concerning the need for an "initial condition"*, it's already being worked on based on the increasing evidence that we've been collecting through observations of the macro and micro-Universe. (http://scienceline.org/2006/08/21/ask-snyder-bang/)

*Actually prior condition is more concise and appropriate, since, if we are to find out what was going on before the Big Bang, then we need to look for something more than a moment of no duration.

Meanwhile, welcome to the BAUT, madjack! Read the FAQs, which contain the board rules, etc., and keep asking questions and closely evaluating the answers you receive.

Ken G
2007-Jan-12, 04:13 AM
But there is a new branch of physics - M Theory (http://en.wikipedia.org/wiki/M-theory) that many hope to have answers to just those questions. i would reckon that most string theorists would be happy with the colliding branes theory (http://arxiv.org/abs/hep-th/0103239) as the initiator od the BB.


But then the question will be, where did the branes come from? There's no escape from needing an initial condition to physical dynamics, it's just the way science is done.

Ken G
2007-Jan-12, 04:21 AM
Most wouldn't.Oh, have you polled them? Or is this just a subjective belief of yours?

Those who would are applying an artificial roadblock to increasing scientific knowledge.And that is obviously a subjective belief. Personally, my subjective belief is that an understanding of the limitations of science is quite crucial for extracting the most from the scientific process-- it's limitations are its strengths, like someone who is good at dancing spending more time dancing than painting.


One of the nice things about science is anything is open to question and everything is fair game. No limits, except those that are self-imposed by a few individual scientists. You really believe that? A strength of science is that it has no limits? Have you ever seen the definition of science? Science is all about imposing limits. I like Feynman's angle, which basically says that science is a set of rules to help keep you from fooling yourself, given that you are the easiest person to fool. If you think that means "no limits", then you have fooled yourself already.

Actually prior condition is more concise and appropriate, since, if we are to find out what was going on before the Big Bang, then we need to look for something more than a moment of no duration.You see, you resort to ATM language to even have a point here. The most honest interpretation of the current scientific data tells us there was no "prior" condition to the Big Bang, as time itself is to have originated there. It might become possible to extend our understanding of time and our universe, just as it might become recognized that the Earth is really sitting on the back of a turtle, but it is not the honest interpretation of existing observations. But then, who cares about observations, I forgot that science has "no limits"!

Cougar
2007-Jan-12, 07:38 PM
That's it! Thatís my question. It seems logical that if the universe started as a Singularity or Big Bang, why did it expand in the first place and not just collapse under its own gravity?
Negative pressure vacuum energy. (http://en.wikipedia.org/wiki/Cosmic_inflation)

Fazor
2007-Jan-12, 07:46 PM
Maybe the universe did collapse into a black hole and this is what came out the other side :thinks: and perhaps the universe is expanding because matter/energy/whatever is still being sucked through the black hole and coming out into our universe! Indubidibly!

(no i don't actually believe this. but kinda a fun thought).

Ken G
2007-Jan-12, 07:51 PM
Negative pressure vacuum energy. (http://en.wikipedia.org/wiki/Cosmic_inflation)

This is actually not an explanation of the Big Bang, it is an explanation of a phase of the Big Bang. It is not thought to have initiated the expansion, it required that there be expansion in the first place. Hence is does not replace the need for an initial condition. However, it is quite significant if it is true because it means that a lot of initial conditions would have turned out looking more of less indistinguishable from our universe.

Jeff Root
2007-Jan-12, 08:16 PM
According to arguments that I accept, all the matter involved in
the Big Bang was extremely dense and extremely hot at first, and
cooled as it spread apart and became less dense.

Ordinary gases at densities and temperatures we can observe
expand when they are hot, and cool when they expand. I gather
that the high temperature of the material in the Big Bang does
not similarly explain the expansion of that material. Why not?
Why could the high temperature not cause the expansion?

On the other side of the coin, repeating a question asked before,
why did gravity slow the expansion? If the matter filled the
entire Universe, the gravitational pull on every particle would be
the same in all directions, so there should be no net effect of
gravity. It should not slow the expansion at all. The matter
should continue expanding at its original rate forever.

-- Jeff, in Minneapolis

Peter Wilson
2007-Jan-12, 08:33 PM
A black hole is a gravitational potential well, with a well-difined center. The early universe had no center--still has no center, come to think of it.

Thus, the early universe's expansion is not equivalent to a BH expanding, or matter coming out of a BH. The density of a BH is similar to early universe, but not the geometry of the situation.

Ken G
2007-Jan-12, 09:41 PM
Ordinary gases at densities and temperatures we can observe
expand when they are hot, and cool when they expand. I gather
that the high temperature of the material in the Big Bang does
not similarly explain the expansion of that material. Why not?Because you need more than high pressure to get expansion-- you need a pressure difference, a high-pressure center and a low-pressure outside. That is what is lacking-- the high pressure was everywhere, and the theory requires no pressure difference because the expansion itself is part of the initial conditions. That's why the Big Bang is not a model of an explosion, contrary to popular belief.
On the other side of the coin, repeating a question asked before,
why did gravity slow the expansion? If the matter filled the
entire Universe, the gravitational pull on every particle would be
the same in all directions, so there should be no net effect of
gravity.It turns out that gravity, unlike pressure, makes its presence felt even if it is everywhere. It's general relativity-- it causes spacetime curvature even if it is homogeneous (especially if it is homogeneous!). This is deeply related to why gravity is not a force, and pressure is (in the fluid description).

publius
2007-Jan-12, 11:28 PM
You know, it might be helpful here to discuss just what is meant by "intitial conditions" so we can better understand what they are.

Basically, the laws of physics can be expressed as differential equations of varying order and complexity. There are two kinds there, ordinary differnetial equations in one variable, and partial differential equations involving more than one. The latter are usually about field equations, and you want to know the field as a function of space and time. These get complicated......to say the least.

Consider an ordinary one variable differential equation. One gets "a" solution. Generally, there is more than one solution. Just like algebraic equations can have more than one solution, a differential equation can have more than one function that solves it. Consult any text on the subject and you'll learn the details. There are a lot of such details.

Anyway, the solution to these equations consists of the sum or other combination of these solutions that involve *arbitrary constants*.

It is the initial conditions that determine these arbitrary constants. If time is the indepedent variable -- that is our differential equation is asking for some time evolution of some variable -- to pin down those constants, we need to specify what the system was doing at some particular time.

Simple example: I "drop" a ball in a uniform Newtonian gravitational field: The solution to differential equation describing height as a function of time is this:

z(t) = 1/2 gt^2 + v0 t + z0

The two constants are z0, the initial height, and v0, the initial velocity. The motion can be very different depending on those constants.

If the indepedent variable is a spatial variable, say how something varies with position, then "boundary condition" is usually used rather than initial, but it is the same thing. We have arbitrary constants in our solutions, and we need some data points to pin those constants down.

Now, go to partial differential equations. Things get even more "arbitrary". Solutions to partial differential equations generally involve arbitrary functions, not just constants. For example the solution the simple partial differential wave equation is this:

A(x, t) = f1(x + ct) + f2(x - ct)

where f1 and f2 are completely arbitrary functions. Those functions are completely arbitrary. An function of the term x +/ ct will solve the wave equation.

To get a specific solution there, we need to specify an entire function for our intial and/or boundary conditions. Above, note we have a space and time coordinate there. Solutions of that depend on what it was doing at some initial time everywhere and what is doing for all time at one or more particular spots. And there are several different ways this can be done depending on the particular system. For example if A above is the displacement of a vibrating string, we might hold it fixed at two ends, then pluck it in the middle. That would involve a boundary condition on the ends, plus initial conditions in the middle. Or we might fix one end and drive the other. That will involve different combinations. Consult a text for the details.

Now, the Einstein Field Equation is very complex set of partial differential equations. The more variables and the higher the order, not to mention the non-linearity, make for a complex class of solutions, and equally complex initial and boundary conditions to pin down that solution.


The laws of physics are about how systems evolve. There are infinite classes of such evolution equations. You've got to specify how the system started out (which in general, is just specifying a snapshot of it at some particular time), and in general place other constraints (boundary conditions) to get a particular solution.

The Big Bang metric is no different. We've got initial conditions and boundary conditions to pin down those solutions to the EFE.

-Richard

Ken G
2007-Jan-13, 04:53 AM
Yes, that's a good summary. In general, dynamics require initial conditions, and even time-steady solutions (an important approximation in physics) require boundary conditions. These are a pretty inescapable element of what the scientific process requires, and have to be prepared or assumed manually and independently of any fundamental theory, yet many here of the "science gives all answers" persuasion (on other threads, not this one) seem to have their head in the sand a bit on this matter.