In another Q&A thread, a BAUT member said this (post #87):

The "so called big bang theory" is not nearly so complex as you describe. It has three basic axioms:

1. The universe is finite in volume
2. That volume is increasing
3. The universe is finite in time, it has an "age"

This is, of course, quite mistaken.

However, it does raise the question of just what the postulates ("fundamental axioms"?) of modern LCDM cosmological models actually are.

That's what this thread is about, and I hope rcglinsk will actively participate in the discussion.

To kick things off, I'm going to offer an extremely brief, and possibly inaccurate, answer:

There is only one axiom in 'the big bang theory', namely a cosmological principle to the effect that 'the laws of physics' are the same everywhere and everywhen in the observable universe.

As stated your axiom is very attractive. Is it sufficiently disambiguating? Would it not hold true under postulates that exclude the big bang or big bangs?

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And all the rest of mainstream cosmology falls out from this one axiom?

Hmm . . . this is going to be an interesting thread.

What about initial conditions? When do we start? Before or after inflation, if there is such a thing?

Besides, what laws you have to obey depend on how big you are and like if you've been charged before. Also, apparently, there are different laws for crowded places versus wide open spaces.

Huh? Gravitational binding? That's not different, that's a result of the application of GR.

I think this is related to the key axiom, but as GOURDHEAD points out, it is not enough by itself because it does not single out the Big Bang theory, and ruling out that axiom would not necessarily defeat the Big Bang theory in some modified form.

Nor can one say that the Big Bang theory asks for both the laws to be the same, and for there to be no new laws we don't already know about, because both dark matter and dark energy represent unknown physics. Ruling out, for example, the Steady State theory on the grounds that it requires that new matter be created all the time in an unknown way seems disingenuous alongside allowing the LCDM cosmology to invoke dark energy and dark matter.

So what is the key defining characteristic of the Big Bang model? I would say it has to do with the interpretation of redshift. If the physics of redshift is the physics of general relativity, then we get the Big Bang, inescapably. So I would modify Nereid's axiom into a single other axiom:

The "law of physics" that accounts for cosmological redshifts is general relativity.

The rest of the LCDM is just tinkering with parameters and confronting tests, if we allow the dramatic predictions about dark matter and dark energy to be considered "tinkering". The more fundamental the change in the physics (like, say, allowing for the laws to evolve with time), the more one can argue if it is "really the same theory as the Big Bang", but to me the core idea is that the universe evolves by expansion, ruled by the equations of general relativity. The rest are ramifications of that assumption.

I was thinking of the apparent fact that stars on the edges of galaxies move too fast.

If I recall correctly, there was at least one GR specialist that felt that was explained by GR also. I'll edit in a link, if I can find it.

Regards, John M.

Not necessary and therefore not sufficient?

Sufficiency does not imply necessity. A gas stove is sufficient to boil water, but it is not necessary to have a gas stove to boil water.

I think that paper is generally regarded as bogus, for the simple reason that Newtonian mechanics should obviously be sufficient to explain the motions of stars in galaxies. The argument is then seen to be tantamount to saying "here's a very complicated and error-prone calculation that should come out the same as a much simpler one, but doesn't, so I conclude that something in the complexity means it should not come out the same as the simple one." As opposed to the much more obvious: they made a mistake.

Neither necessary nor sufficient.

I agree, "the laws of physics are the same everywhere and everywhen in the observable universe" - Nereid

dark matter and dark energy are not unknown physics, rather byproducts of physical operations most haven't chosen to think about yet. I'm again putting this out there: big bang theory - all the matter in currently known space compiled into one point then exploding out everywhere. What do black holes do? Compile matter into one point until...? I'm theorizing they explode within themselves and create new pockets of space from scaled down matter which would then create three axioms founded in the one axiom stated by Nereid, "the laws of physics are the same everywhere and everywhen in the observable universe": 1. The finite (measurable) universe is dependent upon the infinite (unmeasurable) properties of existence.
2. The volume of the universe is both increasing and decreasing through the power of existence creating cyclical gravity & pressure systems acting in accordance with 'physical laws' filling up space with matter and recycling it down into relatively smaller pieces of matter expanding (but space non-the-less!).
3. The universe is finite (measurable) with an indeterminable age, while existence is infinite.

One axiom

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I know. I was twitting. Sorry.

"This isn't right; it isn't even wrong." - (Dirac?)

Wolfgang Pauli.

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The bar has been lowered for the promotion of ATM ideas; the bar for the acceptance of ATM ideas must remain high.

There is only one axiom in 'the big bang theory', namely a cosmological principle to the effect that 'the laws of physics' are the same everywhere and everywhen in the observable universe.

Clearly, the 'laws of physics' needs to be better described, but note that the axiom, as I have written it, does not constrain what these are, in any way (other than their universality); the key aspect of this "axiom" is in eliminating the need to visit every part of the universe, everywhen, with fully equipped tools (etc) of physics, to actually do the experiments (everywhere and everywhen) to show that it is so.

Of course any such 'laws of physics' may contain parameters whose values must be put in 'by hand' ... I think there are ~25 such constants in the best versions of the 'laws of physics' in today's textbooks.

Indeed you do need more than one or two "axioms" to have a successful theory - "initial conditions" perhaps - but they are surely a different kettle of onions from the "axioms", aren't they?

But for me one of the most powerful things about choosing this "axiom", plus some ancillary thingies, is that it makes cosmology secondary to physics: the BBT is 'merely' the application of physics to the universe ... and the most fundamental questions then relate to what the "laws of physics" are, and how to work them out.

It sounds like you are favoring a scheme where the most important thing is that the laws be consistent, whatever they are. But I think what the laws are is very much the axioms we must identify-- I see consistency as an attribute of the chosen laws, rather than as an uber-law. Any law could include some variability as part of the law, but it has to be a law-- the axiom has to specify what the law is. That's why I see the laws of general relativity as the crucial axiom that underpins the Big Bang theory, and that law includes consistency in space and time as part of its own description.

I should also add that the "cosmological principle" may be viewed as a secondary postulate of the theory, more along the lines of a boundary condition. That principle does not assert that the laws are the same everywhere, it asserts that the mass distribution and composition is more or less the same everywhere. It's not a required element of the Big Bang theory, it is more an allowed element, and we of course adopt it because we can-- a la Occam's Razor.

 

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I think you're correct, that that is really the sole axiom. The important axiom for the big bang (and other cosmological theories) is that "what we see is really what is there." In other words, one has to reject the silly (but impossible to discount) argument that the universe was created by a deity a minute ago, and everything was conveniently arranged so that we would believe it. Or for example, that we are just looking at a projection of some sort, like a movie. And I think the cosmological principle is basically what rules that out.

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Still the Big Bang is based upon creatio ex nihilo which is patently absurd by nature. What came before the big bang? Are scientists going to take it on faith that there was nothing and always nothing which suddenly decided LOL SPACETIME and BAMF it's universe everywhere?

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If only it was as easy to soothe my hunger by rubbing my belly.
- Diogenes of Sinope
Interdum Taurus Est Victor
- Old Joke

I noticed some years ago that the KJV doesn’t say “out of nothing”. It says, “...that what is seen was not made out of what was visible.” I don’t know how Lemaitre’s Catholic version translates that passage.

But it's not actually necessary to rule that out, it isn't the job of science to rule out nonscientific perspectives or beliefs, it is only its job to identify what is scientific. A theory is not a negative thing, it does not say what could not be true. It doesn't even need to say what is true, only what could be true in the general approach taken by science, i.e., in a way that organizes and unifies the observations we have, and makes testable predictions and guides the next suite of observations. That is the sole thing that separates it from deity-driven models, what it does for us, not what it rules out.

I can understand why you might look at it this way, but again this can be heard in a way that is actually backwards logic. Principles never rule anything out, we don't use theory to inform and constrain uniquely new observations about how they have to come out, that isn't the role of a principle when we are dealing with what is fundamentally unknown and with observations that have never been done before. Instead we use such uniquely new observations to inform and constrain the attributes of a theory that can organize the observation into the existing body of observations, and to tell us what might be a good thing to look for next. And that's what the Big Bang theory does for us, and its axioms should reflect that specifically.

See, this is the kind of objection we open ourselves up to when we are not careful to be painstakingly clear about what the actual goals of science are. As soon as we open the door to saying that a mental attitude can tell the universe what it is allowed to do, other people will feel free to do the same thing, and why shouldn't they?

I'm not talking about some obscure edition of a holy book. I'm talking about the philosophical writings of Thomas Aquinas.

I have no objections at all with that. I'm the kind of person who'd rather watch the world burn. See Alfred's monologue about the thief in The Dark Knight.

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A patriot must be ready to defend his country against his government
- Edward Abbey, A Voice Crying in the Wilderness
If only it was as easy to soothe my hunger by rubbing my belly.
- Diogenes of Sinope
Interdum Taurus Est Victor
- Old Joke

The laws of physics are 1.) Mass...the resistance to a change in motion. The law of inertia. 2.) Force = mass X acceleration and 3.) Forces come in pairs, often misnamed "for every action there is an equal and opposite reaction".

These laws are only valid in inertial reference frames. They cannot be measured in accelerating reference frames.

4.) The laws of electromagnetism and uniform motions are valid in all inertial reference frames and they do not change with time. This is why patents for perpetual motion machines are rejected by the patent office. If the laws of physics do change with time, then no patents can be offered since any machine invented within the present laws of physics will no longer function when those laws change. All warranties could not be made and would be invalid.

5.)Entropy and the Second Law of Thermodynamics...I list this one since it is something alternative theories have to attack when attacking the BBT. For if entropy does increase as time passes, then it must decrease as one progresses into the past. If one extrapolates into the past and upholds the Second Law, it must point to a singularity. I have a feeling this is right where Nereid was focused.

6.) The laws of quantum physics and the particular characteristic that, from experiments in inelastic scattering, the most fundamental particles (quarks) are reproduced by the stretching and snapping of gluon fields. In BB cosmology the conversion of energy into matter was done by an inflation field which only needed one meson and its own force to reproduce the universe.

7.) The laws of physics are the same in all acceleratiing reference frames as well as inertial reference frames. General Relativity.

Anyway, that is my two cents. There are other axioms and laws but these stand out for the moment.

The KJV is well known to Protestants. Thomas Aquinas is not. If you would like to tell me what the Catholic translation of Hebrews 11:3 is, I would be interested in knowing it.

I haven't read Lemaitre's religious views on the subject, only your claim about his religious views. I have an English copy of his 1927 BB paper, but nothing regarding his religious views on the subject.

Have you read an English translation of his 1927 paper?

(emphasis added)

This seems to be one of the more persistent myths (or misunderstandings, take you pick) of the "big bang theory" ("BBT") ... as I read it, the actual scientific theory - or theories, or classes of theory - that are given the name BBT do not claim to address the mutual inconsistency of GR and the core quantum mechanics/quantum field theory components of the SM (standard model of particle physics); the 'singularity' if you will.

Said another way, what is usually meant by the BBT does not include anything concerning the origin of the (current) observable universe.

If the Alfvén quote in your earlier post is accurate (and does not omit something important), then it's very sad to see someone of his obvious intellect making such a gross blunder ... perhaps he simply didn't take enough time to understand the limited scope of GR-based cosmological models?

And this post (that I'm quoting) points to another thing or three that irritates me no end: the strawman ("the Big Bang is based upon creatio ex nihilo"), the use of the logical fallacy of argument from incredulity ("which is patently absurd by nature"), and the (wilful?) misconstruing of the purpose/nature of (contemporary) science ("Are scientists going to take it on faith that there was nothing and always nothing which suddenly decided LOL SPACETIME and BAMF it's universe everywhere?").

But worst of all (so far anyway) is the (blind?) repetition of the mistake rcglinsk made: creatio ex nihilo or The universe is finite in time, it has an "age" are not, repeat NOT, axioms/core assumptions/whatever of any BBT!

You've put your finger on an important aspect, I think.

Any answer to the question "what are the 'laws of physics' which BBTs, as a class, incorporate?" (or similar questions) must include GR and the SM (standard model of particle physics), and I doubt there's any significant disagreement over such answers.

And in turn that could be re-phrased as an answer to different questions, ones that ask about something deeper than just 'incorporate'.

However, the question in the OP - poorly worded and vague as it is - is aimed at something different; it asks about what is the BBT "über"? (Perhaps it should ask whether there is any such "über", or does it make any sense to even ask such a question?)

In one way, of course, a theory of cosmology which assume the universality of 'the laws of physics' but which incorporates something other than 'GR+SM' would not be called a BBT ... or would it? As is becoming clear (to me at least), "BBT" is thought of, by many people, as a description of the solution (the observable universe evolved from a hot, dense state ... or something similar) rather than a handle for 'application of GR and the SM to the universe as a whole'. Thus any cosmological theory which predicts 'a big bang' would get the moniker 'BBT', even ones that explicitly exclude both GR and the SM!

So perhaps it's time to clarify what is meant by the two key terms in the question? Can we - the participants in the discussion in this thread - agree on what we mean by "the Big Bang theory"? ditto, re ""axioms""?