Initial state based on current state?

[PhysDrew]

I was wondering if it were in actuality possible to assume the universe was created from a big bang, as it seems we used current understanding of the laws of physics as they apply to us now, and extrapolated back. Could it be possible that the boundary conditions we form our laws of physics upon could have changed somewhere along the way? So that the current universe looks a lot different to what it was, or what it was evolving to be?


EDIT: To highlight, I remember an example in a book I once read. Imagine one happens upon a room temperature glass of water. According to the laws of thermodynamics, it is impossible to determine an earlier state of the glass of water for certain. For instance was the glass initially hot, then cooled? Or was it always room temperature?

 

[mathman]

Much of the information leading to the big bang theory comes from observations of the past, since we can see things many billions of light years away, which means we are seeing into the past many billions of years.

 

[Chronos]

Most cosmologists will concede the big bang theory breaks down by the time we reach the initial 'singularity'. It is very effective afterwards. We could apply the same logic to human origins - that we arose from biological 'singularities', but, we know that is an over simplification. I'm convinced the origins of our universe are similarily complicated.

 

[Tanelorn]

Chronos I fully agree with you. I try but I cannot see how an infinite universe could ever have been infinitisimal. The universe surely cannot be infinite then?

 

[mathman]

Chronos I fully agree with you. I try but I cannot see how an infinite universe could ever have been infinitisimal. The universe surely cannot be infinite then?

We don't know if the the universe is infinite or not. I am inclined to not.

 

[Chronos]

Olber [of Olber's Paradox fame] deduced the universe cannot both be spatially and temporally infinite. His logic is still compelling, IMO. Logic is not absolute, but lacking mathematical proof, it is - and will always be - a powerful tool in the physics tool box. It is what compels us to critically examine our math for relevance and precision. Math can be both rigorously correct and irrelevant. In part, I think this was Godel's point.

 

[Tanelorn]

And some expect us to believe in the standard model with the universe starting as a singularity smaller than an atom because of mathematics and despite any common sense? Seems more and more like religion and the tooth fairy every day I am sorry to say. I am really thinking we have to look again for a more complete solution.

 

[mathman]

The big bang theory has a lot of physics behind it. The expansion of the universe; the H1, H2, He4, (etc.) ratios; cosmic microwave background. Mathematics is a tool putting things together - it is not the basis of the theory.

 

[Chronos]

I agree, mathman, but, I think we should avoid allowing math to blindly lead us into logical inconsistencies. I do not believe math will ever trump logic, but, often leads us in new directions.

 

[twofish-quant]

And some expect us to believe in the standard model with the universe starting as a singularity smaller than an atom because of mathematics and despite any common sense?

No. What happens at the pre-inflationary era is total guesswork and you can believe anything that you want about what happens then. The standard model of cosmology says *NOTHING* about what happened at t=0. You only have things that you can say at a small moment after t=10^-36 seconds when you have inflation.

Now there are people that are trying to guess what happens at/before t=10^-36 seconds, but none of that is part of the standard model of cosmology.

It's really important to point this out because people have very strong misunderstandings of what the standard model includes or doesn't include. As of 2010, the standard model says *nothing* about what happened at t=0.

Seems more and more like religion and the tooth fairy every day I am sorry to say. I am really thinking we have to look again for a more complete solution.

In the case of the inflationary era, what happens at t=10^-36 and 10^-33 seconds has some observable impact on the cosmic background radiation. As far as what happened before 10^-36 seconds, right people are guessing, but people are *trying* to correlate those things with CMB observations.

 

[twofish-quant]

I try but I cannot see how an infinite universe could ever have been infinitisimal. The universe surely cannot be infinite then?

One thing that I've learned is never let my lack of imagination dictate how the universe behaves.

 

[twofish-quant]

Olber [of Olber's Paradox fame] deduced the universe cannot both be spatially and temporally infinite. His logic is still compelling, IMO. Logic is not absolute, but lacking mathematical proof, it is - and will always be - a powerful tool in the physics tool box.

I don't see how Olber's Paradox is relevant here. Once you have a big bang, then his paradox breaks down. If you have an expanding universe in which all but a small portion is outside of of an event horizon, then you can have a universe that is both spatially and temporally infinite.

 

[twofish-quant]

I was wondering if it were in actuality possible to assume the universe was created from a big bang, as it seems we used current understanding of the laws of physics as they apply to us now, and extrapolated back.

And you can get back to about t=1 second, without having to think about any new physics. At t=1 second, you get temperatures and pressures that we can reproduce on earth, and then see what happens.

Also you don't have to assume current physics. You can assume new physics, and see what happens. Suppose there is some extra force that causes protons and neutrons to interact, what happens?

Could it be possible that the boundary conditions we form our laws of physics upon could have changed somewhere along the way? So that the current universe looks a lot different to what it was, or what it was evolving to be?

It's possible. So what you do is that you assume different physics and see what happens. It turns out that it's *really* difficult to do this. If for example, you change the fine structure constant a little bit, then the universe looks *really* different assuming that you get a universe at all.

EDIT: To highlight, I remember an example in a book I once read. Imagine one happens upon a room temperature glass of water. According to the laws of thermodynamics, it is impossible to determine an earlier state of the glass of water for certain. For instance was the glass initially hot, then cooled? Or was it always room temperature?

Yup. But if you have a video tape of that glass of water then you can figure out more, and we happen to be able to look into the past.

 

[Tanelorn]

Twofish-Quant, do you agree with Penrose that the obervational universe is a tiny part of the whole (10^30 larger), but still finite? Or perhaps even infinite?

Due to the CMBR I am willing to accept that our observable universe was once very small and hot, but before last scattering, 300,000 years old, I am uncertain. I just don't know the evidence well enough. Presumably Penrose's very large universe would also have been a part of this very small hot universe as well, so the whole universe at this time would still have been enormous? Is it required that all parts of Penrose's universe would still have had to be in thermal contact to explain the smoothness in the CMBR or just some of it?

 

[twofish-quant]

Twofish-Quant, do you agree with Penrose that the obervational universe is a tiny part of the whole (10^30 larger), but still finite? Or perhaps even infinite?

I have no reason to agree or disagree with him. I disagree with the idea that current data shows evidence of pre-big bang activity, but it's possible that he is "right by accident".

Due to the CMBR I am willing to accept that our observable universe was once very small and hot, but before last scattering, 300,000 years old, I am uncertain. I just don't know the evidence well enough.

There are a lots of bits of evidence that gets us back to 10^-36 seconds. One of the major ones is the fact that the universe is made up of 25% helium. We can be reasonably certain about what happens there because we can run experiments in nuclear reactors to see what happens, that gets you to about t+1 second. As you go back in time there is more and more guesswork, and right now 10^-36 seconds gets you to the point where you have to throw up your hands and say that we really don't know what happens next.

Weinberg's book "The First Three Minutes" is old but it's a classic and gives you some a pretty good background for why we think we understand what we understand. One curious thing is that Hawking's "A Brief History of Time" is a rather bad book for understanding the big bang. The problem is that Hawking is interested in "weird parts of the universe that we don't understand" so he talks a lot about that, and he doesn't talk so much about "parts of the universe that we think we know pretty well."

One other odd thing is that we think we understand more about what happened at t=3 minutes than what happened at t=500,000 years. Once the universe becomes transparent, you enter the "dark ages" at which point it's rather hard to figure out what is going on sense you can't see it directly.

Presumably Penrose's very large universe would also have been a part of this very small hot universe as well, so the whole universe at this time would still have been enormous?

Don't know. Penrose is stabbing in the dark. That's not necessarily a bad thing, but it's not part of the standard story so I'm not too familiar with it.

Is it required that all parts of Penrose's universe would still have had to be in thermal contact to explain the smoothness in the CMBR or just some of it?

Again don't know.

 

[PhysDrew]

It's possible. So what you do is that you assume different physics and see what happens. It turns out that it's *really* difficult to do this. If for example, you change the fine structure constant a little bit, then the universe looks *really* different assuming that you get a universe at all.

Ah this reminds me of something I read a few months ago (haven't heard anything since). According to some new research, the value of the fine structure constant may be changing. It has received a lot of criticism though, and a lot of it was debunked on the basis of incomplete or inaccurate measurements. Have you read anything related to this? What do you think?

I found the reference, it was reported in NewScientist, 23rd October 2010.

 

[Chronos]

'New Scientist' is a grab bag - some articles are good, some are crackpot. Any article claiming variation in the fine structure constant is . . . dubious. The criticism is well deserved.

 

[PhysDrew]

'New Scientist' is a grab bag - some articles are good, some are crackpot. Any article claiming variation in the fine structure constant is . . . dubious. The criticism is well deserved.

Yeah that's what I thought. I read through it, was amused, and promptly forgot about it! Reading the above comment jogged my memory.

 

[sri sharan]

I was wondering if it were in actuality possible to assume the universe was created from a big bang, as it seems we used current understanding of the laws of physics as they apply to us now, and extrapolated back. Could it be possible that the boundary conditions we form our laws of physics upon could have changed somewhere along the way? So that the current universe looks a lot different to what it was, or what it was evolving to be?

Well that's a part of the assumptions of the standard model ,isnt it?Every physical theory has assumptions.We can never truly say if the assumption is true or false.We can only experiment and see if the evidence supports the assumption.In this case it does seem to support