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Nereid
2008-Oct-09, 12:45 PM
In another Q&A thread (http://www.bautforum.com/questions-answers/79620-big-bang-questions.html), a BAUT member said this (post #87 (http://www.bautforum.com/1339557-post87.html)):

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.

GOURDHEAD
2008-Oct-09, 01:22 PM
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?

John Mendenhall
2008-Oct-09, 02:26 PM
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.



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?

Warren Platts
2008-Oct-09, 02:38 PM
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.

John Mendenhall
2008-Oct-09, 02:46 PM
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.

Ken G
2008-Oct-09, 02:55 PM
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.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.

Warren Platts
2008-Oct-09, 03:40 PM
Huh? Gravitational binding? That's not different, that's a result of the application of GR.
I was thinking of the apparent fact that stars on the edges of galaxies move too fast.

John Mendenhall
2008-Oct-09, 04:36 PM
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.

John Mendenhall
2008-Oct-09, 04:39 PM
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.



Not necessary and therefore not sufficient?

Warren Platts
2008-Oct-09, 05:35 PM
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.

Ken G
2008-Oct-09, 06:46 PM
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.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.

Ken G
2008-Oct-09, 06:47 PM
Not necessary and therefore not sufficient?Neither necessary nor sufficient.

Albohrt
2008-Oct-09, 06:55 PM
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

John Mendenhall
2008-Oct-09, 07:23 PM
Neither necessary nor sufficient.

I know. I was twitting. Sorry.

John Mendenhall
2008-Oct-09, 07:28 PM
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.



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

Celestial Mechanic
2008-Oct-09, 08:14 PM
"This isn't right; it isn't even wrong." - (Dirac?)Wolfgang Pauli. :)

Nereid
2008-Oct-09, 11:47 PM
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.

Ken G
2008-Oct-09, 11:53 PM
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.

Ken G
2008-Oct-10, 12:01 AM
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.

dodecahedron
2008-Oct-10, 12:11 AM
To Hannes Alfvén, the Big Bang was a fable - a fable devised to explain creation. "I was there when Abbé Georges Lemaitre first proposed this theory," he recalled. Lemaitre was, at the time, both a member of the Catholic hierarchy and an accomplished scientist. He said in private that this theory was a way to reconcile science with St. Thomas Aquinas' theological dictum of creatio ex nihilo or creation out of nothing.
— Anthony L. Peratt, 'Dean of the Plasma Dissidents', The World & I, May 1988, pp. 190-197.

 

Jens
2008-Oct-10, 01:37 AM
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.


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.

dodecahedron
2008-Oct-10, 02:19 AM
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?

Sam5
2008-Oct-10, 02:44 AM
To Hannes Alfvén, the Big Bang was a fable - a fable devised to explain creation. "I was there when Abbé Georges Lemaitre first proposed this theory," he recalled. Lemaitre was, at the time, both a member of the Catholic hierarchy and an accomplished scientist. He said in private that this theory was a way to reconcile science with St. Thomas Aquinas' theological dictum of creatio ex nihilo or creation out of nothing.
— Anthony L. Peratt, 'Dean of the Plasma Dissidents', The World & I, May 1988, pp. 190-197.

 

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.

Ken G
2008-Oct-10, 03:09 AM
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.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.


And I think the cosmological principle is basically what rules that 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.


Still the Big Bang is based upon creatio ex nihilo which is patently absurd by nature.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?

dodecahedron
2008-Oct-10, 03:21 AM
I noticed some years ago that the KJV

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


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 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.

blueshift
2008-Oct-10, 03:57 AM
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.

Sam5
2008-Oct-10, 03:57 AM
I'm not talking about some obscure edition of a holy book. I'm talking about the philosophical writings of Thomas Aquinas.



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?

Nereid
2008-Oct-10, 01:39 PM
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. [...](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.

Nereid
2008-Oct-10, 01:52 PM
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?
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! :mad:

Nereid
2008-Oct-10, 02:16 PM
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.
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""?

Nereid
2008-Oct-10, 02:26 PM
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.
Mea culpa ... 'the cosmological principle', in its standard meaning, is essentially mute on the scope of 'laws of physics' (it's about isotropy and whether we are in some sort of special place); what I wrote looks more like an extension of the strong equivalence principle.

As I wrote earlier, the point I'm trying to make, re the BBT's "axiom", is about the key scientific foundation of any BBT; in effect, what are the core assumptions, in a BBT, inherited from the fact that a BBT is a modern scientific theory (or theories) about cosmology?

Unlike either the (Copernican and successor) cosmological principle or the (strong) equivalence principle, such core assumptions would indeed be axioms, i.e. not testable, even in principle.

speedfreek
2008-Oct-10, 02:44 PM
The universe needs no centre or edge.

Ken G
2008-Oct-10, 04:10 PM
If the focus is on identifying the uber principles that lead us to adopt one particular axiom set or another, rather than the axiom sets themselves, then I would say the uber-principle must address the question, when we see variations with distance and time (they can't be separated without the axioms), will we interpret that as a dynamical universe that changes in time but not space, or a spatially diverse universe that changes in place but not time, or neither, or both?

Occam rules out the last of those until we absolutely need it, which means we do not assert we are in a special place and time, but look for one or the other, or neither. Furthermore, there are three spatial dimensions but only one time dimension, so if we are in a special place but not a special time, we have three degrees of freedom to account for, but if we are in a special time but not a special place, we have only one. So that rules out special place until we need it (although note, there is another possibility here which doesn't even have a name-- where we allow specialness in a single spatial coordinate, the radial distance from us, but is steady in time. Let's leave that off because there doesn't seem to be a reason to single out the radial coordinate over the azimuthal ones, but we should not overlook this completely). Now we are down to either being in a special time, or not. That's pretty much the BBT versus the Steady State model.

So the way we choose between those is, are we going to allow for completely new physical processes, or just modifications to the parameters of our existing physics and our existing inventory of matter. If we allow the former (expressly, ongoing matter and energy creation mechanisms), we should seek to eliminate all specialness in space and time, and use the Steady State model. If we constrain ourselves to stick with the physics we have, then we have to allow the specialness of time, and we then have the BBT, in the most general sense Nereid refers to of any theory that involves a dramatic temporal evolution of the universe.

Thus, the scientific community's choice of the BBT over the SST reflects, I would say, a preference to use only the demonstrated physics that we already have at out disposal when we attempt to explain the overall behavior of the universe. We allow ourselves unconstrained parameters like a dark matter component and a cosmological constant, but stay within the theories we currently have to do that (with some modification to the standard model of particle physics, but it already suffers from incomplete symmetry principles so some would have wanted to try to modify it anyway).

I don't think this choice says that we think our current theories are more likely to be correct than some unknown alternatives, I think it simply reflects the most scientific uber-principle of all-- we shall adopt no theory until we need it, independently of our preconceived biases about how we'd like the universe to be.

Tim Thompson
2008-Oct-12, 04:25 AM
I don't think of Big Bang Cosmology (BBC) so much as a single scientific theory, but rather as a family of scientific theories which all share a single, common metaphysical principle that the universe has a unique beginning. If you want a single axiom, and you want to stick to classical (i.e. non quantum) physics, then I would say some thing like this: All world lines terminate on the same past singularity. That should differentiate BBC from competitors like QSSC.

But science is always a moving target with a certain amount of squishiness to it. The unique beginning certainly can go away if one appeals to quantum physics. Then we are awash in a sea of cyclical universes, bouncing cosmologies and colliding branes and it's a free-for-all. How you go about making that neatly axiomatic escapes me at the moment, but once I figure it out I will be sure to make a note in the margin of one of my books that it's too long to fit there. :)

RussT
2008-Oct-12, 09:17 AM
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.

Simply put, the "Cosmological Principle" 'should' be applied to Our Universe "BEFORE" any theory is applied.

We see/Observer Clusters of galaxies that are seperated by Huge Voids between those clusters as far as we have been able to progressively detect.

SO, let's see how many "assumptions" are really 'built into' Nereids simplistic 'one fundamental axiom'.
I will do that by making a simple statement, adhering to nereids, and watch what happens.

It has become pretty obvious from the Nuker Teams work and many others that Galaxies are most likely 'born' in one of several different theorized ways all having to do with the galaxies "Cores".

SO, IF our Milky Way Galaxy is ~13 billion years old (I am even going to leave out Ellipticals here!!!), then any galaxy that is very similar to the Milky Way, should be ~13 billion years old as well, so when we see a Milky Way type Galaxy that is 10 billion Light Years Away, THEN, keeping "c" as a Constant, that galaxy should be 13 Billion Years Old, where it was when it emitted that light, BUT the Universe would have to be at least 23 Billion Years Old, AND IF there are galaxies Past That....Etc Etc Etc...

Galaxies in Clusters Between Huge Voids IS what we see and that should be the Cosmological Principle!

Here come the Fire Works:eek:

timb
2008-Oct-12, 11:36 AM
Mea culpa ... 'the cosmological principle', in its standard meaning, is essentially mute on the scope of 'laws of physics' (it's about isotropy and whether we are in some sort of special place); what I wrote looks more like an extension of the strong equivalence principle.

As I wrote earlier, the point I'm trying to make, re the BBT's "axiom", is about the key scientific foundation of any BBT; in effect, what are the core assumptions, in a BBT, inherited from the fact that a BBT is a modern scientific theory (or theories) about cosmology?

Unlike either the (Copernican and successor) cosmological principle or the (strong) equivalence principle, such core assumptions would indeed be axioms, i.e. not testable, even in principle.

I thought the cosmological principle was homogeneity and isotropy. Both are testable to an extent, aren't they? Is homogeneity challenged by the existence of structures at the 109 ly scale? (eg "Great Walls")

As to axioms, they are usually something you look for once a theory is well settled and we wish to express it at its logical minimum. I don't think the BBT is at that point yet. Maybe "assumptions" would be a better word.

Seiryuu
2008-Oct-12, 12:18 PM
SO, IF our Milky Way Galaxy is ~13 billion years old (I am even going to leave out Ellipticals here!!!), then any galaxy that is very similar to the Milky Way, should be ~13 billion years old as well, so when we see a Milky Way type Galaxy that is 10 billion Light Years Away, THEN, keeping "c" as a Constant, that galaxy should be 13 Billion Years Old, where it was when it emitted that light, BUT the Universe would have to be at least 23 Billion Years Old, AND IF there are galaxies Past That....Etc Etc Etc...

Doesn't this assume that time passes at equal rates everywhere in the universe? If time passes faster in one part, similar galaxies CAN have different ages, although then you can wonder if our measurements in years has any meaning at all if it doesn't reflect the rate at which time passes.

In the latter case, I reckon you'd have to make a distinction in time as the measurement of the duration to change position and time as the measurement of the duration at which time actually passes.

Cougar
2008-Oct-12, 10:27 PM
...the core idea is that the universe evolves by expansion, ruled by the equations of general relativity.

Someone said the equations of general relativity can accurately describe a large number of possible cosmic evolutionary scenarios, so that wouldn't seem to be so limiting. But as to the universe evolving by expansion, what other conceivably possible view can explain the primal abundances of the elements? So is this an 'assumption,' or a solid deduction grounded on our observations of the physical laws here on Earth?

Nereid
2008-Oct-13, 12:08 AM
Some thoughts about the assertion rcglinsk actually made ...

"Axiom" calls to mind Hilbert's 6th problem, now over a century old:
Mathematical Treatment of the Axioms of Physics. The investigations on the foundations of geometry suggest the problem: To treat in the same manner, by means of axioms, those physical sciences in which already today mathematics plays an important part; in the first rank are the theory of probabilities and mechanics.
The ensuing century's work by mathematicians and physicists has surely made us wiser and somewhat chastened, even if the Clay Millennium Prize problems include one from classical physics (Navier-Stokes equations) and one from quantum physics (Yang-Mills theory). So, in choosing "axioms", rcglinsk may be reflecting an older, and at least somewhat inaccurate, view of the nature of physics (rcglinsk has, in other posts, clearly shown a fondness for an extremely narrow version of Popper's (naive) falsificationism, so at least this seems consistent).

Then there's "The "so called big bang theory"" ... is there one? Or is this, perhaps, merely a convenient shorthand, especially outside the technical literature? The leap from something like "the Big Bang theory says that the universe began as a single point 13.7 billion years ago" to "the Big Bang theory assumes that the universe originated 13.7 years ago" is very easy to make, but it's a bit of a surprise to see someone with an apparent legal background falling for such a logical error, particularly as he goes on (it seems) to conflate "assumes" with "takes as true and not testable, even in principle".

So if we apply the description of science by Ken G, in a post in a now vanished thread, concerning explanatory and predictive power, to the BBT (or Big Bang Cosmologies, a meta-class, as Tim Thompson wrote just a while ago), perhaps the closest, meaningful, question we could ask is something like "what are (all) the physics theories which successful, contemporary cosmological models incorporate?"

And that's been answered, several times already ...

Nereid
2008-Oct-13, 12:42 AM
[...]
We see/Observer Clusters of galaxies that are seperated by Huge Voids between those clusters as far as we have been able to progressively detect.
The isotropy and homogeneity aspects of any cosmological principle can, of course, be tested, by detailed observation ...

There are several ways to measure, quantitatively, the degree to which a chunk of the universe is homogeneous; for example: divide it into N equal-sized smaller chunks, measure (estimate) the average (mass-energy) density in each, and calculate a measure of how much these averages differ; repeat, using a chunk of the universe that is 1000 times bigger (say); repeat ... plot the 'variation in averages' against 'size of chunk' ... the extent to which the former decreases as a function of the latter gives an indication of how the universe tends towards homogeneity (or not)*.

There's a great chart somewhere on the SDSS website that shows this ...


[...]
SO, IF our Milky Way Galaxy is ~13 billion years old (I am even going to leave out Ellipticals here!!!), then any galaxy that is very similar to the Milky Way, should be ~13 billion years old as well,(bold added)

rcglinsk, if he were not banned, would jump in here and point out that this is an example of a rather common logical fallacy (crudely, "all cats are black, this animal is black, therefore this animal is a cat" ... or is it?)

so when we see a Milky Way type Galaxy that is 10 billion Light Years Away, THEN, keeping "c" as a Constant, that galaxy should be 13 Billion Years Old, where it was when it emitted that light, BUT the Universe would have to be at least 23 Billion Years Old, AND IF there are galaxies Past That....Etc Etc Etc...
[...]
Among other things, this conclusion requires that the evolution of galaxies (well, those 'very similar to' the MW anyway) be sufficiently well understood that 'looks like MW' -> 'is ~13 billion years old'. AFAIK, as a research topic, the evolution of galaxies is pretty hot these days, with only limited consensus and a great many, highly pertinent, questions wide open ...

* note to the pros: I know this is crude and over-simplified; if you can describe P(k) (or similar) more clearly, please do so ...

Ken G
2008-Oct-13, 12:51 AM
But as to the universe evolving by expansion, what other conceivably possible view can explain the primal abundances of the elements? So is this an 'assumption,' or a solid deduction grounded on our observations of the physical laws here on Earth?Strictly speaking, axioms don't represent deductions, they lead to deductions, and then we test the deductions. In practice, we start inductively, building up a set of observations we know we will need to fit, then we find axioms that foot the bill, and then reverse direction and explore all the deducible ramifications of those axioms. So what makes them axioms is simply that we explore their logical ramifications to make new hypotheses and check the consistency of the old ones. Nereid wants to look at this from the most general perspective, and then gradually hone in on the more specific axioms. I think it is like starting at the trunk of the tree, and asking, where does the first "branching" occur, and then follow up that branch to the smaller and smaller branches and twigs as you go along. Ultimately you have the LCDM model, which is really just one leaf at the end of one twig.

RussT
2008-Oct-13, 01:03 AM
Doesn't this assume that time passes at equal rates everywhere in the universe? If time passes faster in one part, similar galaxies CAN have different ages, although then you can wonder if our measurements in years has any meaning at all if it doesn't reflect the rate at which time passes.

In the latter case, I reckon you'd have to make a distinction in time as the measurement of the duration to change position and time as the measurement of the duration at which time actually passes.

Doesn't this assume that time passes at equal rates everywhere in the universe?

If I answer this yes, here in Q&A, I will either be banned, OR this will be moved to ATM.

Ummm, so I guess, there was an "Inherent" 'built in' assumption in the Big Bang Cosmological Principle.

If time passes faster in one part, similar galaxies CAN have different ages,

The 'reasoning' behind galaxies having 'faster star formation rates' at the far distances of the ~13 billion light year sphere/visable Universe, has always been that they were much closer together when they formed, causing more profound perterbations. BUT, that has changed dramtically, which has been talked about VERY little.

The older galaxy formation paradigms were either "Top Down" or "Bottom Up" merger scenarios, BUT, the reason I included the Nuker Team and Others, in my original post, is that with the newer Galaxy Birth scenario, which include the formation of SMBH's, in mainstream, and alternative ECO/MECO type Massive Objects, in the Cores of galaxies/dwarf galaxies, that actually presents quite a problem that has not really been properly addressed...;)

Seiryuu
2008-Oct-13, 09:23 AM
I see. Thanks for clarifying :)

RussT
2008-Oct-13, 10:34 AM
I don't think of Big Bang Cosmology (BBC) so much as a single scientific theory, but rather as a family of scientific theories which all share a single, common metaphysical principle that the universe has a unique beginning. If you want a single axiom, and you want to stick to classical (i.e. non quantum) physics, then I would say some thing like this: All world lines terminate on the same past singularity.



I don't see it as strange. For the photon or the wave front, the speed of light is infinite. Zero distance from point a to point b (because it takes zero time to get there). Not hard then to see the Universe as a singularity from that frame of reference.

And, It appears, that Thorkil's* quote he put in this thread in Q&A is 'built in' as well?
http://www.bautforum.com/questions-answers/79973-black-hole.html

*Thorkil proved that he is Very knowledgable in SR in grav's Lorentz Contraction 2 thread!