Question about the big bang?

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I don't know much about the big bang but if there was an explosion and the universe started expanding from a small region how could it expand at all the Gravitational field would be immense. If all our energy in the universe today was put into a small region it would be like a black hole and even light could not escape. And it seems like the big bang would have taken a long time to happen because time would be moving pretty slow around all that energy.
Any input will be much appreciated
 

marcus

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Cragar,
in my signature field, at the end of each post, I have a link to an excellent Scientific American article by Charles Lineweaver that answers many questions like that.

Give it a try, the first page is blank so you have to scroll down. The link has the name
"Charley" in it.

He is a recognized world-class cosmologist. He explains that the BB should not be thought of as an explosion. There is clear language and lots of illustrations.
 
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thanks for the link marcus
 

marcus

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Thank you both for the positive response. If Lineweaver's article is right for you (interesting, not too confusing, but not entirely satisfactory) then before long you may be back with more questions, ideally ones people here including myself can answer directly. I hope you will keep asking about cosmology. If people didn 't it would get awfully quiet here. :biggrin:
 
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When they talk about the redshift of the photon, they say it is because space itself is expanding and it is not a Doppler shift. So the photon actually loses energy on its way to earth, Is this similar to Gravitational redshift? It seems weird that the photon would get shifted just because space is expanding.
 
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i am in big trouble again i need only one answr about boubt that is there a chance to time become zero matter and energy denstity infinite and nothing moment occurs ? as i came to know about LQG it says that matter and energy never becomes zero is it my right under standing i am in big trouble please reply me via email answer in detail please my email id is mehul_dangar9@yahoo.com
 

marcus

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When they talk about the redshift of the photon, they say it is because space itself is expanding and it is not a Doppler shift. So the photon actually loses energy on its way to earth, Is this similar to Gravitational redshift? It seems weird that the photon would get shifted just because space is expanding.
Pedagogically it's a contentious issue how that should be explained. People have long arguments and get doctrinaire about what you should tell students.

It can be analyzed and pictured in different ways.

Doppler analysis does not work if you just consider the recession speed at the moment of emission and at the moment of arrival. I think that is the message Lineweaver is getting across. t behaves as if the wavelength was gradually extending all during the travel. Because in the end the wavelength is expanded by the same factor that distances have expanded.

But you can set up a long series reference frames and a long series of small doppler shifts and do the analysis and get the right answer. So some people like to understand it that way.
Others find that too long and involved and they prefer to think of the wavetrain being elongated as the distance between here and there increases---by the same z+1 factor.
It is "AS IF" the wavetrain were being stretched by expanding space---but that is just a pictorial way to describe it. Space is not a stretchy material.
I think if one were to write out Maxwell's equation for EM wave propagation in a geometry with expanding distances it would work out as if the waves were being stretched while they were in flight. But anyway people argue about how this should be explained.

It is a smart question to ask, but I'm embarrassed to say I don't have a satisfactory answer.
I don't like the explanation with the long series of little doppler shifts, a long chain of imagined intermediate observers. It is too elaborate for my taste. I like the stretch picture but I accept that space is not a material substance, so it is just a picture.
 

marcus

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i am in big trouble again i need only one answr about boubt that is there a chance to time become zero matter and energy denstity infinite and nothing moment occurs ? as i came to know about LQG it says that matter and energy never becomes zero is it my right under standing i am in big trouble please reply me via email answer in detail please my email id is mehul_dangar9@yahoo.com
LQG is not proven yet. It's predictions must be checked.
It was rash of you to get into an argument about an unproven theory.
I don't do email about this stuff. I think you are asking about the LQG bounce.

Yes it is your right understanding! According to LQG there is a bounce instead of the big bang. And energy density does NOT become infinite.
And space does not shrink down to zero.

There is a contraction which reaches very high density and then quantum effects make a rebound and expansion begins.

The best paper I think is by Abhay Ashtekar (a highly regarded world-class physicist)
Here is the brief summary:
http://arxiv.org/abs/1005.5491
Here is the full article as PDF, for download:
http://arxiv.org/pdf/1005.5491
The Big Bang and the Quantum
by Abhay Ashtekar

If you got into an argument with somebody it was very reckless of you, because this theory has not been proven yet. I think it has a good chance, but there must be observations by space instruments to check it, and this takes time. And it may be wrong and be disproved too. That is possible.
 

Chronos

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I am unconvinced redshifted photons 'lose' energy. They are also time dilated, so, the total energy eventually gets here. Energy is not conserved in GR, but, energy-momentum is conserved in GR.
 
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sir please tell me can we ever enable toknow both the things simulteniously position and velocity ?
and from according to quantum gravity it is came toknow that big bang is justnot bigbang originally but it is bigbounce universe back and forth continuously but it states that we cant know any thing about the previous uni. is it right?if it is right then why please reply me via email my address is mehul_dangar9@yahoo.com
 
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i would be thankful if u will explain me this statement @Marcus or anyone else

In Einstein's general theory of relativity, space is dynamic.It can expand ,shrink and curve without being embedded in a higher dimensional space.

whats the meaning of this ?
 

marcus

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In Einstein's general theory of relativity, space is dynamic.It can expand ,shrink and curve without being embedded in a higher dimensional space.

whats the meaning of this ?
Means just what it says.

Around 1850---back in the middle of the 19th century---mathematicians discovered how to describe geometry, curvature, shape and such-like stuff entirely from the inside without having need of any surrounding space.

So you can have curvature without the need for any surrounding environment. You can have expansion, likewise. These features of changing and developing geometry are definable by internal observers moving around and measuring within the space itself (not outside). They can tell for example if it is curved or not, by measuring triangles inside.
 

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