# Big bang and BH

1. Sep 8, 2004

### blue_sky

Why the univers is not a big black hole?

2. Sep 8, 2004

### EL

Why should it be?

3. Sep 8, 2004

### humanino

Because it is an elegant hypothesis ?

4. Sep 9, 2004

### blue_sky

The universe expansion means that in the past all the mass was concentrated; why that mass didn't collapse in a huge BH?

5. Sep 9, 2004

### Chronos

The rate of expansion was incredibly fast in the very early universe. To put this in perspective, by the time the universe was 3 minutes old, it had expanded to a size of ~30 light years. Gravity, weakling that it is, could barely slow the thing down, much less stop it. The battle was about as evenly matched as a tug-of-war between an ant and a locomotive. While the inexorable tug of gravity has been slowly gaining ground [actually, it may have started losing ground again awhile back], expansion is still winning the battle to this day. Current observational data suggests this trend will continue indefinitely.

6. Sep 9, 2004

### franznietzsche

Maybe this only sounds odd to me, but....

$$\frac{30 light years}{3 minutes} = \frac{10 light years}{1 minute}$$

Now i'm assuming that you were aware of this when you posted it chronos, so please explain...

edit: taking a shot at your location...Iowa? or Missouri maybe...

7. Sep 9, 2004

### humanino

so ?
What is the contradiction here ?

8. Sep 9, 2004

### EL

OK, now I understand what you mean...
chronos gave a good explanation.

Last edited: Sep 9, 2004
9. Sep 9, 2004

### humanino

I still don't get it. This is definitely the worse day in a long time for me. Inflation is certainly not a linear process is it ?

10. Sep 9, 2004

### EL

Sorry humanino. I of course ment CHRONOS gave a good explanation. Not franznietzsche (who I have no idea what he is talking about...)

11. Sep 9, 2004

### humanino

Well thank you very much for this precision EL, I was beginning to think that reason was failing me. Hard day

12. Sep 10, 2004

### Chronos

1] Modern theory calls for a superluminal inflationary epoch in the early universe. While, at a glance, this appears to violate relativity, it actually does not. It was 'empty' space that was expanding. Hubble's constant gives us a yardstick to measure the historical rate of expansion.
2] about an hour from St Louis.

13. Sep 13, 2004

### blue_sky

Sorry, but I don't understand. Considering the overall mass of the universe it seems to me that going back in time the universe should be all confined inside the event orizont. So, whatever was the "superluminal inflationary epoch", nothing can escape from a BH event orizont... or I miss something?

14. Sep 13, 2004

### humanino

I am trying to answer, and since I am likely to go wrong, I would appreciate to be corrected. Thanks for helping me in that case.

As far as I understand, Chronos last post above answers your question : it is the space-time that is expanding. It is probably the most difficult thing to understand, because this misconception occurs all the time. For instance, the universe could be infinite at the begining of time. Imagine an infinite pudding in which expansion makes the berries getting far from each other : if you go back in time, the pudding never needs to be a single spot. It can be infinite all along, yet the local density can become infinite too.

An horizon occur where "light rays turn in circle". Inside the horizon, space and time are sort of "swapped". No such thing happens in the early universe, because inflation is such a violent phenomenon. Inflation is meant produce enough room to explain the homogeneity of the CB. If some sort of inflation occured inside a BH, the singularity and the horizon would eventually disappear.

15. Sep 13, 2004

### hellfire

It seams to me that if the whole energy of matter in the current universe were confined nearly into a point, then you may be right.

But the total energy of the universe before inflation could have been zero or nearly zero (it doesnt really matter). During inflation the scalar field driving the expansion (inflaton) accumulated energy in its vacuum state (its energy density was constant and the volume increased).

If one wants to take into account energy conservation, this energy might have been compensated by the energy of its own gravitational field (this is a negative value). After inflation the accumulated energy was transferred to the matter field in a process called reheating.

I think this is an accepted possibility, but to me it seams very speculative and not necessarily correctly defined.

Last edited: Sep 13, 2004
16. Sep 13, 2004

### Chronos

Of course it is speculative. Even some professional scientists question big bang theory [BBT]. It is, however, the best explanation to existing observational evidence: of which there is a great deal. If you think BBT is weird, try quantum physics. Now that stuff is downright 'spooky'. Still, a lot of people believe it because it works really, really good.

17. Sep 13, 2004

### humanino

:rofl:
Seriously, is there any serious competitor to BBT ?
AFAIK, none has been as succesful on several facts, among which :
redshift
CMB
relative abundance

Of course, Einstein himself questionned BBT, and was even against at the beginning.

18. Sep 14, 2004

### hellfire

I did not question the big-bang theory, but the hypothesis that the energy accumulated into the gravitational field compensates (more or less exactly) the energy of matter in the universe.

19. Sep 14, 2004

### Chronos

Apologies, I misunderstood your question. The concept of gravity as negative energy has to do with the conservation of energy law and how the universe has managed to avoid being in violation. Stephen Hawking gave this explanation:

"The answer is that the total energy of the universe is exactly zero. The matter in the universe is made out of positive energy. However, the matter is all attracting itself by gravity. Two pieces of matter that are close to each other have less energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together. Thus, in a sense, the gravitational field has negative energy. In the case of a universe that is approximately uniform in space, one can show that this negative gravitational energy exactly cancels the positive energy represented by the matter. So the total energy of the universe is zero."