Is the Big Bang Theory a Foolhardy Extrapolation?

[OOO]

Today I have been traveling across the night sky with Google Earth a bit. As always I'm surprised about how inspiring the cosmos is for thinking about physics. But I've always been quite skeptical about the big bang theory. Nowadays we're unable to simulate even a single hadron. And the observation of the sun seems to be a constant source of surprise about it's inner workings.

Yet people claim in detail about how the big bang took place and what happened when. I don't say the big bang is wrong because I'm not an expert in cosmology. But isnt't it a rather foolhardy extrapolation ? I have no philosophical (or whatever psychologically or religiously motivated) problem with a big bang apart from finding it a little strange that there should have been a singular event where such a huge amount of energy has been accidentally created from the vacuum. But if it was that way, well, okay.

What I do have a problem with is the assumption that the laws of physics have been the same for 13 billion years and/or over a distance of 13 billion lightyears. Who can say that ? Probably masses and coupling constants are just a matter of boundary conditions.

If I were to bet my son's life on the big bang, I would never ever do so.

So what makes you cosmologists so sure about that thing ?

 

[EL]

a little strange that there should have been a singular event where such a huge amount of energy has been accidentally created from the vacuum.

This is a common misunderstanding of the Big Bang theory. We cannot extrapolate all the way back to the "beginning" since we do not know what physical laws to use in that extrapolation. We can "only" go about 13,7 billion years back until the temperature of the Universe corresponds to energies we have measured in accelerator experiments. Before that we just have some fair speculations of what happened. What the Big Bang theory says is that the Universe ones was in a much hotter and much denser state than it is today.

What I do have a problem with is the assumption that the laws of physics have been the same for 13 billion years and/or over a distance of 13 billion lightyears. Who can say that ?

Measurements indicates so. Maybe someone's got a good reference?

If I were to bet my son's life on the big bang, I would never ever do so.

Please don't bet your son's life om anything.

 

[OOO]

Please don't bet your son's life om anything.

Of course I won't. That was just a metaphor for how carelessly we take some things for granted.

 

[OOO]

We can "only" go about 13,7 billion years back until the temperature of the Universe corresponds to energies we have measured in accelerator experiments. Before that we just have some fair speculations of what happened. What the Big Bang theory says is that the Universe ones was in a much hotter and much denser state than it is today.

So big bang cosmology is all about thermodynamics in a gravitational background ?

 

[EL]

So big bang cosmology is all about thermodynamics in a gravitational background ?

Well, you need to add atomic, nuclear and particle physics as well.

 

[EL]

Of course I won't. That was just a metaphor for how carelessly we take some things for granted.

In know.
But seriously, the Big Bang theory is very well founded. See e.g. http://en.wikipedia.org/wiki/Big_bang

 

[OOO]

But seriously, the Big Bang theory is very well founded. See e.g. http://en.wikipedia.org/wiki/Big_bang

I bet there has gone a lot of energy into it, but that doesn't necessarily mean it is well founded. Take this from http://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis as an example:

"During the 1970s, there was a major puzzle in that the density of baryons as calculated by Big Bang nucleosynthesis was much less than the observed mass of the universe based on calculations of the expansion rate. This puzzle was resolved in large part by postulating the existence of dark matter."

Know what ? There is less money on my bank account than I expected. This puzzle was resolved by postulating a giant white money-eating rabbit.

 

[yogi]

That same rabbit has been munching in my bank account

 

[OOO]

That same rabbit has been munching in my bank account

Probably we should try to convince this silly animal of becoming a macrobiotic. I mean, who does he think he is ?

 

[marcus]

If I were to bet my son's life on the big bang, I would never ever do so.

What about $50? Would you be willing to bet fifty dollars that it's wrong?

I'm curious how you imagine the bet would work. How would we decide who wins?

Or if you wouldn't be willing to bet against, would you bet fifty bucks that something like that has happened? And how would we decide?

 

[OOO]

What about $50? Would you be willing to bet fifty dollars that it's wrong?

I'm curious how you imagine the bet would work. How would we decide who wins?

Or if you wouldn't be willing to bet against, would you bet fifty bucks that something like that has happened? And how would we decide?

That's precisely the point.

 

[marcus]

That's precisely the point.

then maybe we are in at least partial agreement. I still wish you would spell out what you think is precisely the point of what. But I imagine that what you mean is something professional cosmologists know extremely well. They are the toughest skeptics of their own models, and by far the best informed doubters. Professionals know very well that one cannot verify some particular version of early universe cosmology.

So it would be senseless to propose a bet like that.

All you can do is use the best most accurate theory of gravity we have, so far, and fit the data to it as closely as you can, making the fewest assumptions you can to get a close fit to observation, and see what that says.

You don't BELIEVE theories, you test them and use them (always provisionally in case a better theory comes along, and always skeptically.) Conclusions should always be qualified: "if suchandsuch model is correct, then..."

Yet people claim in detail about how the big bang ...
So what makes you cosmologists so sure about that thing ?

I am curious where you are getting your ideas about who these people are and what they claim. I hope you are not talking about popular journalism or pop-sci books. I'd like to have a link to somewhere a professional cosmologist makes some unqualified claim about events and conditions.

Assertions really ought to be qualified by reservations like "according to the usual LCDM model" or "according to GR"

GR is the most precise theory of gravity we have, it fits observations to many decimal places, but I think any working cosmologist would tell you that it can't be relied on in extreme situations. You don't believe GR, you apply it where it works and keep mental reservations about where it doesn't.

Conventional cosmology is, of course, entirely based on GR. So it is subject to the same reservations.
==================

 

[Wallace]

The issue, as marcus suggests, lies not with Big Bang theory but in understand the nature of scientific 'knowledge' in general. Modern Cosmology has a pretty detailed theory of how the Universe has evolved that is referred to commonly as 'the Big Bang', although the theory is much more complex than the simple name implies.

The point though is that your initial problem is a straw man argument, professional cosmologists do not hold the view that you are arguing against. Based on the best evidence we have we attempt to construct what appears to be the most likely explanation for what we observe, and that is about all we can ever say about a scientific theory, that it is supported by the evidence.

The evidence for the Big Bang is overwhelming, but that doesn't mean that it is an unassailable truth in scientific cannon.

Pop-sci writers such as Richard Dawkins, or more relevant to cosmology, Simon Singh, sometimes, in my view, go a bit over the top in terms of the certainty of scientific knowledge. It's probably a forgivable sin, but it does lead to misunderstanding and confusing, such as that expressed by the OP.

 

[Dan Tee]

What the Big Bang theory says is that the Universe ones was in a much hotter and much denser state than it is today.

Would it be a stupid question to ask the size of the universe at this early stage?

 

[russ_watters]

Of course I won't. That was just a metaphor for how carelessly we take some things for granted.

The big bang is not taken for granted.

But evidence shows clearly that everything in the universe is flying apart. So what does that imply about the past...?

 

[russ_watters]

Would it be a stupid question to ask the size of the universe at this early stage?

That's a very difficult question. The best that can be said is it must have been very hot and very dense.

 

[pervect]

Would it be a stupid question to ask the size of the universe at this early stage?

We are pretty sure from the CMB readings that the entire observable universe originally occupied a very small size, because the temperature of the CMB appears to be the same in all directions.

However, the entire universe may be and probably is much bigger than the observable universe. For more on this, see for instance

http://map.gsfc.nasa.gov/m_uni/uni_101bb2.html

Please avoid the following common misconceptions about the Big Bang and expansion:

The Big Bang did not occur at a single point in space as an "explosion." It is better thought of as the simultaneous appearance of space everywhere in the universe. That region of space that is within our present horizon was indeed no bigger than a point in the past. Nevertheless, if all of space both inside and outside our horizon is infinite now, it was born infinite. If it is closed and finite, then it was born with zero volume and grew from that.

 

[EL]

Would it be a stupid question to ask the size of the universe at this early stage?

I see you have already got some anwers to your question, but I'll just add some words.
No, it is not a stupid question, but mearly slightly unprecise. It depends on what you mean by "the universe". Usually in the scientific community "the universe" is a synonym to "the observable unvierse", that is the part of the universe from where light has had time to reach us. In that case "the universe" was very tiny at the moment the temperature corresponded to the energy scales we have been able to study in labs. In principle it should be possible to calculate the size of "the universe" at that moment. At least a rough estimate shouldn't be a problem, but I do not have the numbers in my head.
If by "the universe" you mean also the parts which are unobservable it gets more complicated, and more philosophical. See pervect's reply.

 

[OOO]

I still wish you would spell out what you think is precisely the point of what.

I was comparing cosmology with the ability to make long-term predictions in other branches of physics. Take the weather forecast as an example and compare it's time scale (days) with cosmology (13 billion years). Add to this the assumption that we don't even understand the standard model very well, let alone theories beyond the standard model.

They are the toughest skeptics of their own models, and by far the best informed doubters. Professionals know very well that one cannot verify some particular version of early universe cosmology.

That's fine, so maybe I just got the wrong impression of the whole thing.

I am curious where you are getting your ideas about who these people are and what they claim. I hope you are not talking about popular journalism or pop-sci books. I'd like to have a link to somewhere a professional cosmologist makes some unqualified claim about events and conditions.

Star Trek certainly is not one of my sources, although sometimes I think that it could be helpful...

I admit that to a large extent I'm referring to the view of popular science journalism. That's because I always avoided cosmology while learning GR (shouldn't have done this, I guess).

Maybe I just haven't found an adequate attitude towards pop journalism yet. It's the same with my own field, theoretical particle physics (in which I consider myself a novice): Everywhere the public is informed about how fantastic the standard model is and how cool physicists are. But if you look more closely it boils down to some highly accurate predictions, some quite reasonable ones, and the bloody rest.

 

[EL]

Add to this the assumption that we don't even understand the standard model very well

What do you mean? The standard model is extremely well tested!

 

[EL]

let alone theories beyond the standard model.

And that is why we can just speculate about what happened at temperatures corresponding to energies higher than what we have observed in experiments.

 

[OOO]

But evidence shows clearly that everything in the universe is flying apart. So what does that imply about the past...?

The evidence you speak of is an indirect one. Nobody has ever seen a galaxy fly away. And what's more, we can't do experiments with galaxies in their natural environment. So we have to rely on the assumption that physics is everywhere the same in the universe like in that part where we live in and which we can explore by experiments. And even the theories we design to explain these experiments change on a timescale that's infinitesimal compared to the timescale of the universe.

So even without knowing in detail about how the direct evidence is gained, I think there is much to be careful about. But, as marcus has already affirmed, cosmologists are well aware of the critical role of assumptions in their models.

 

[OOO]

What do you mean? The standard model is extremely well tested!

Yeah sure, that's the reason why theorists sit around, yawning and waiting for something exciting to happen...

 

[EL]

Yeah sure, that's the reason why theorists sit around, yawning and waiting for something exciting to happen...

I do not get what you mean?

 

[OOO]

I do not get what you mean?

Can you name an example of an "extremely well tested" prediction of the standard model that is more or less directly responsible for a corresponding prediction of cosmology ?

 

[EL]

Can you name an example of an "extremely well tested" prediction of the standard model that is more or less directly responsible for a corresponding prediction of cosmology ?

You should turn it the other way around: Our knowledge of the laws of physics up to temperatures (energies) of around 100 GeV enables us to extrapolate the history of the Universe back to that temperature scale.
The predictions of the standard model are extremely well tested in accelerator experiments.

 

[OOO]

You should turn it the other way around: Our knowledge of the laws of physics up to temperatures (energies) of around 100 GeV enables us to extrapolate the history of the Universe back to that temperature scale.
The predictions of the standard model are extremely well tested in accelerator experiments.

The universe is not an accelerator. I think we agree that most matter in the universe we see today consists of hadrons. The extrapolation you refer to reaches up to a point when there were no hadrons. What about the step inbetween ? Can you explain the formation of hadrons with the standard model ? If you can, I'll buy that extrapolation.

 

[EL]

The universe is not an accelerator. I think we agree that most matter in the universe we see today consists of hadrons. The extrapolation you refer to reaches up to a point when there were no hadrons. What about the step inbetween ? Can you explain the formation of hadrons with the standard model ? If you can, I'll buy that extrapolation.

Yes the point is of course that we also know about what physical laws hold at energies lower than 100 GeV. The quark-hadron transition is described by QCD (but I don't know to what detail). Note also that we need not know exactly every detail of every possible reaction that may occur, since we can treat the story statistically.

 

[OOO]

Note also that we need not know exactly every detail of every possible reaction that may occur, since we can treat the story statistically.

Yes, of course, like we can treat the weather forcast statistically...

 

[EL]

Yes, of course, like we can treat the weather forcast statistically...

Except that the "prediction" is backwards, and the weather we are "predicting" is the mean weather of the whole Universe, and not just some local fluctuation.
What is crusial though is that the Big Bang theory predicts a lot of stuff we can go on and controll. For example the now measured CMB and helium over hydrogene ratio was predicted long before it was confirmed. See e.g. http://en.wikipedia.org/wiki/Big_bang#Observational_evidence