Big Bang: Discovering the Reasons Behind Its Occurrence

[Chalnoth]

Again, time is the metric distance between two events on a worldline.

Now if you use for instance a Fermi normal coordinate chart in curved spacetime or simply a rest frame in Cartesian coordinates in flat space you can use time (which is then proper time) on one axis so it looks like it is a separate dimension. But just by using such a charts does not make it a dimension.

There is a distinction between the manifold and a choordinate chart and it is a mistake to assume that any of the dimensions of the manifold is time.

I don't see how this distinction is any different from any of the other dimensions. After all, which dimension on a manifold is "forward/backward"?

 

[Passionflower]

I don't see how this distinction is any different from any of the other dimensions.

What do you mean by "any of the other dimensions"? The manifold is 4-dimensional, but no singe dimension is a spatial or temporal. Only a coordinate chart maps (a region of) this manifold, with or without off-diagonal components, onto 4 dimensions of which one is temporal and three are spatial.

 

[finiter]

Dimensions are strictly mathematical. It may or may not represent the physical reality. The real world is just three dimensional. However, to analyze it, we can use one-dimensional or four-dimensional frames.

An expanding system requires a four dimensional frame. As time moves forward, the three space dimensions increase. The spherical surface of the expanding system, or the Gauzian surface described by the three space dimensions, encloses the spacetime. The spacetime can be regarded as the volume at a given time; it is the product of a volume factor and a time factor, ie, it is four dimensional. When the system contracts, the time factor decreases. Mathematically it is time moving back. But in real terms, the direction of time does not change, but the directions of the space dimensions are reversed.

 

[Chalnoth]

What do you mean by "any of the other dimensions"? The manifold is 4-dimensional, but no singe dimension is a spatial or temporal. Only a coordinate chart maps (a region of) this manifold, with or without off-diagonal components, onto 4 dimensions of which one is temporal and three are spatial.

Right. But your point about time being the metric distance between two space-time points for an observer is important, because the choice of dimensions is not completely arbitrary: the motion of an observer picks out a specific set of them. This indicates, for instance, that while no particular direction on the manifold can be identified uniquely as time, one cannot pick any direction as being time: there are some directions on the manifold which no observer can traverse (because it would mean moving faster than the speed of light).

 

[Chalnoth]

Dimensions are strictly mathematical. It may or may not represent the physical reality. The real world is just three dimensional. However, to analyze it, we can use one-dimensional or four-dimensional frames.

While true, the empirical evidence for four dimensional space-time is exceedingly robust.

The spacetime can be regarded as the volume at a given time; it is the product of a volume factor and a time factor, ie, it is four dimensional. When the system contracts, the time factor decreases. Mathematically it is time moving back. But in real terms, the direction of time does not change, but the directions of the space dimensions are reversed.

That would merely indicate that you chose a poor proxy for "increasing time", as increasing time should always be identified with increasing entropy.

 

[AC130Nav]

While true, the empirical evidence for four dimensional space-time is exceedingly robust.

Is it really evidence for FOUR-dimensional space-time? I think quantum mechanics indicates time must have at least two dimensions. After it's absolute nonsense that the observer determines and outcome.

The original evidence for the Big Bang was a uniform background radiation and seeming uniform expansion in all directions. The first was seen as making the formation of galaxies impossible and quickly non-uniformity was found. The second is based on the part of the universe we can observe. Is the world really flat?

 

[Chalnoth]

Is it really evidence for FOUR-dimensional space-time?

Yes.

I think quantum mechanics indicates time must have at least two dimensions.

Huh? No, not at all. All of quantum mechanics is based around a single dimension of time. And when people try to add a second dimension of time, they end up with closed timelike loops, which many people consider to be contradictory.

After it's absolute nonsense that the observer determines and outcome.

Yes, but that doesn't require more than one dimension of time. Everett explained how this works back in the 50's.

The original evidence for the Big Bang was a uniform background radiation and seeming uniform expansion in all directions. The first was seen as making the formation of galaxies impossible and quickly non-uniformity was found. The second is based on the part of the universe we can observe. Is the world really flat?

Er, it wasn't that quickly. The non-uniformity of the CMB wasn't observed until the early 90's (with the COBE satellite), about 40 years after it was first observed. That isn't very fast in my book. But it was pretty obvious that such non-uniformity had to exist, it was just too small to detect until that time.

 

[finiter]

That would merely indicate that you chose a poor proxy for "increasing time", as increasing time should always be identified with increasing entropy.

I agree. In the case of universe that is the accepted opinion. But in the case of a theoretical system, is it not possible that entropy decreases with time? Here, I am tempted to question the concept of entropy itself. Is it not logical to take that the entropy of a contracting star decreases, while the entropy of the universe increases? Both are related and there would be symmetry. Matter contracts while universe expands and vice-versa, and expansion would thus be self limited. Of course, this would go against the existing concepts.

 

[Chalnoth]

I agree. In the case of universe that is the accepted opinion. But in the case of a theoretical system, is it not possible that entropy decreases with time?

The direction in which time increases is defined as the direction where entropy increases, so the answer is no. This is, by the way, the only way in which you have an arrow of time at all: if the entropy is constant (which would mean the system is at equilibrium), then there is no way to distinguish the past from the future, and there is no arrow of time.

Here, I am tempted to question the concept of entropy itself. Is it not logical to take that the entropy of a contracting star decreases, while the entropy of the universe increases? Both are related and there would be symmetry. Matter contracts while universe expands and vice-versa, and expansion would thus be self limited. Of course, this would go against the existing concepts.

Obviously one has to be careful when describing what one means by "entropy increasing". In general the direction of increasing time is the direction of increasing entropy only for a closed system. If it's an open system, we can still make the same identification, but it requires we take into account anything flowing into/out of the system, so the full statement becomes more complicated.

In general we actually have a rather poor understanding of exactly how entropy relates to gravitational systems, so we don't actually know how to write down the entropy of a contracting star. But we can write down the entropy of a diffuse gas, and we can write down the entropy of a black hole. The entropy of the black hole (which can be seen as a far extreme of the contraction of th star) vastly exceeds the entropy of the diffuse gas from which it came. From arguments like this we understand that the universe becoming more clumpy with time is a manifestation of increasing entropy. In fact, it is this fact, the clumpiness increasing with time, and not the expansion, that is the primary increase in entropy since the end of inflation.

If it were to be the case that our universe were to recollapse (which today seems manifestly unlikely), then we would still expect our universe to become more and more clumpy as it did so.

 

[finiter]

In general we actually have a rather poor understanding of exactly how entropy relates to gravitational systems, so we don't actually know how to write down the entropy of a contracting star. But we can write down the entropy of a diffuse gas, and we can write down the entropy of a black hole.

Is it not that the entropy of a black hole thus obtained is more speculative than factual?

 

[Chalnoth]

Is it not that the entropy of a black hole thus obtained is more speculative than factual?

No. Basically, a black hole is a much simpler system than, say, a star, and the entropy can be derived through a variety of independent methods, all arriving at the same result: the entropy of a black hole is proportional to the area of its horizon.

Ultimately, understanding the entropy of most systems where gravity is a significant factor (e.g. stars, galaxies) is likely to require knowledge of quantum gravity. But a black hole is just one of those special cases that is mathematically simple enough that we can be quite sure about its entropy already.

 

[finiter]

But a black hole is just one of those special cases that is mathematically simple enough that we can be quite sure about its entropy already.

Then, what about the black holes themselves? Are these not just a theortical stuff, that too more mathematical than physical?

One should be sceptical about mathematical models. Mathematics is a tool, in fact, an excellent tool, for analyzing. But of late, it has changed its role, it appears, and has become a shaping tool.

Coming back to black holes, does the scientific community accommodate the Doubting Toms even now, or do the Doubting Toms outnumber the others?

 

[Chalnoth]

Then, what about the black holes themselves? Are these not just a theortical stuff, that too more mathematical than physical?

There are different degrees of skepticism where black holes are concerned. Most today are largely convinced that they are the objects at the centers of galaxies, and also make up one member of certain binary systems. We have some observational tests in the works using extremely large baseline interferometry to actually observe the shape of the event horizon, so in any case we'll be quite sure whether or not these objects are black holes in a few years' time.

Coming back to black holes, does the scientific community accommodate the Doubting Toms even now, or do the Doubting Toms outnumber the others?

The scientific community doesn't kick anybody out. It's just that nobody listens to "Doubting Toms" that don't bring evidence to the table, or worse refuse to pay attention to the evidence we already have. This is the way it works in science: slowly more and more people become convinced of an idea as more and more evidence mounts in support of it. There typically remain some holdouts who continue to seek alternative explanations, and often even if we don't agree with them, the rest of the scientific community recognizes that they provide essential value to the scientific enterprise as a whole because there is always the possibility that we are wrong.

But at the moment my impression of people who do research in the area of black holes has been that the number of people who seriously doubt that black holes are real (or at least are not a very good approximation to reality) is vanishingly small.

 

[AC130Nav]

But at the moment my impression of people who do research in the area of black holes has been that the number of people who seriously doubt that black holes are real (or at least are not a very good approximation to reality) is vanishingly small.

I would expect most who spend time and money looking for black holes to believe in them. But how many have the earlier Hawking belief they are forever (might even be little universes), or adopt the Hawking revision which allows them to dissipate? Hopefully, there are yet other positions.

Obviously, something happens when too much mass gets in one place, ergo some kind of black hole. But math belongs in the experiment phase of the scientific method, not in theory.

 

[Chalnoth]

I would expect most who spend time and money looking for black holes to believe in them.

Well, no, this is a false characterization, because we're talking about people who are studying the most compact objects. Whether or not they are black holes is a crucial question that must be answered when studying these objects.

But how many have the earlier Hawking belief they are forever (might even be little universes), or adopt the Hawking revision which allows them to dissipate?

There is no question that black holes evaporate. If they don't evaporate, they're not black holes. It's intimately connected with the entropy calculation I mentioned above.

Hopefully, there are yet other positions.

There have not yet been any compelling alternatives to black holes presented.

Obviously, something happens when too much mass gets in one place, ergo some kind of black hole. But math belongs in the experiment phase of the scientific method, not in theory.

I hope you realize that this has been tested? That people haven't merely taken this on faith?

 

[DaveC426913]

I hope you realize that this has been tested? That people haven't merely taken this on faith?

I don't know what you mean by 'tested'. We can't test the physics of BHs, but our mathematical models of them do explain what we see via (albeit rather remote) observation.

 

[Chalnoth]

I don't know what you mean by 'tested'. We can't test the physics of BHs, but our mathematical models of them do explain what we see via (albeit rather remote) observation.

Well, theories are only really tested in science through the collection of a variety of independent sources of evidence. When the evidence from many directions all agrees with a given theory, and no compelling alternatives are presented, most scientists become convinced that the theory is at least approximately true. From what I gather from those who study this sort of thing, black holes passed that point some time ago.

Now, we may speculate wildly as to whether or not the objects we currently call black holes may be something that we haven't thought of yet. And they may be. But there isn't really a good way to actually go out and investigate the vague notion of, "something we haven't thought of yet," so it only makes sense to provisionally accept the theory we have until such time as evidence is presented that contradicts this.

Edit: And I'd like to point out that I purposely don't often express this level of skepticism, because it turns out that once a majority of scientists become reasonably convinced something is true within their own field of study, that is almost never overturned. In fact, I can't think of a single situation where this has occurred since the onset of modern science (say, about 150 years ago for physics, give or take). The possibility always remains, of course, but it's just unlikely.

 

[AWA]

I'd like to point out that I purposely don't often express this level of skepticism, because it turns out that once a majority of scientists become reasonably convinced something is true within their own field of study, that is almost never overturned. In fact, I can't think of a single situation where this has occurred since the onset of modern science (say, about 150 years ago for physics, give or take). The possibility always remains, of course, but it's just unlikely.

How about the aether, energetic versus atomic theory of matter (Boltzmann suicide included), blackbody radiation continuous emission, static universe... just to name a few that come to mind.
You haven't given your assertion much thought, have you?

 

[Chalnoth]

How about the aether, energetic versus atomic theory of matter (Boltzmann suicide included), blackbody radiation continuous emission, static universe... just to name a few that come to mind.
You haven't given your assertion much thought, have you?

Demonstrate that a majority of scientists were convinced of any of these things at one time.

Yes, there have been many ideas in science that turned out to be false. But it seems to me that the majority of them never passed the level of, "Well, maybe this explains things, let's test it!" Many others stem from pre-science concepts, and weren't overturned until we developed the ability to actually investigate them (thus they couldn't be rightly considered conclusions of science, and instead just suppositions).

 

[AWA]

Demonstrate that a majority of scientists were convinced of any of these things at one time.

Demonstrate? As if this was some math theorem? Go to a public library and read some History of Physics book. It is public knowledge available to anyone with a little interest in learning about past mistakes. That a majority of the academy favors one theory guarantees nothing about the validity of the theory.

Yes, there have been many ideas in science that turned out to be false. But it seems to me that the majority of them never passed the level of, "Well, maybe this explains things, let's test it!" Many others stem from pre-science concepts, and weren't overturned until we developed the ability to actually investigate them (thus they couldn't be rightly considered conclusions of science, and instead just suppositions).

You are describing models such as L-CDM in this paragraph "they are at the level of, "Well, maybe this explains things, let's test it!", or "waiting until we developed the ability to actually investigate them ,thus they can't be rightly considered conclusions of science, and instead just suppositions"(Dark matter, Dark energy...). Exactly like the examples I showed, and exactly like them they are supported by the majority of scientists. Certainly we have better technology now but our current technology is in its infancy to deal with such entities as black holes, dark matter and dark energy too so they can'be rightly considered conclusion of science, just suppositions.
Only time will tell which way they are considered in the future. Let's not be so naive as to believe everything is settled.

 

[Chalnoth]

Everything is certainly not settled. Dark energy I would also place in the same category of these things that later turned out to be false. But not dark matter or black holes. There's a lot we don't know about dark matter in particular, and a fair amount we don't know about black holes, but there really isn't much of a question any longer that both of these things exist (at least as a very good approximation to the real objects).

Perhaps more importantly, dark matter and black holes are supported by a wide body of mutually-corroborating evidence. This is something that none of your examples had.

With the aether, for instance, this was merely assumed to exist based on existing theory, and it quickly became apparent that its properties were becoming more more magical all the time. When it was finally looked for, though, the search came up empty, and so it was (correctly) dropped.

With the continuous black body spectrum, it was immediately obvious that something had to be wrong with that idea, as it predicted infinite emission of energy (I would also note that this one also turns out to be approximately correct, as it is accurate for low frequencies/long wavelengths compared to the temperature).

Given that observational cosmology began with Edwin Hubble, in essence, I don't see how a static universe could ever have been considered a majority view.

 

[AWA]

Everything is certainly not settled. Dark energy I would also place in the same category of these things that later turned out to be false. But not dark matter or black holes. There's a lot we don't know about dark matter in particular, and a fair amount we don't know about black holes, but there really isn't much of a question any longer that both of these things exist (at least as a very good approximation to the real objects).

There is not any question they exist as theoretical entities, not as physical entities. They could exist as physical objects, we don't know yet, believing on it is just a matter of personal election. Many physicists even those actively publishing papers on this subjects consider them theoretical constructs as long as no direct physical evidence for them is available and just think of it as the most plausible explanation so far.

Perhaps more importantly, dark matter and black holes are supported by a wide body of mutually-corroborating evidence.

They are suported by theoretical models, and are not contradicted by observation (basically they can't be with current technology, so as long as this state of things doesn't change, they are actually unfalsiable and therefore not real science for the moment, they might be in the near future)

With the aether, for instance, this was merely assumed to exist based on existing theory, and it quickly became apparent that its properties were becoming more more magical all the time. When it was finally looked for, though, the search came up empty, and so it was (correctly) dropped.

You are describing again something very similar to things like strings, WIMPS,etc, let's give it some time and they may or may not be dropped.

With the continuous black body spectrum, it was immediately obvious that something had to be wrong with that idea, as it predicted infinite emission of energy (I would also note that this one also turns out to be approximately correct, as it is accurate for low frequencies/long wavelengths compared to the temperature).

Well, "inmediately" took at least 30-40 years. Certainly little in geological terms but not so little in the history of modern science.

Given that observational cosmology began with Edwin Hubble, in essence, I don't see how a static universe could ever have been considered a majority view.

You lack in imagination and notions of science history, a static universe was not a majority view, it was the only view since the first concepts of a universe outside the Earth was formed by ancient astronomers to 1922 when a universe of changing radius was theoretically hypothesized by Friedmann, and certainly it was stil the majority view until a few years after the 1929 Hubble observation of the redshift-distance law. When Einstein in 1917 proposed the first modern model of universe he reccurred to the static universe even if his equations allowed other solutions because that was the standard view at the time, this can be read in most books on the matter.

 

[Chalnoth]

They are suported by theoretical models, and are not contradicted by observation

This is a statement that is true about any theoretical model ever produced that has yet to be falsified. The point remains that a wide body of observations have consistently followed the expected results based upon these theoretical models. This is what science is, this is how it works, this is how we can be reasonably confident that they are at least approximately correct.

And yes, we are talking about real objects here. There is some possibility that what we know of as a black hole is in reality some other compact object. But there is no question that these compact objects we think are black holes exist. With dark matter there is very little known about its specific nature, but there is no serious question that there is some form of non-luminous matter out there, and none of the standard model particles fit. With dark energy we are less sure, but that's largely because it's an observationally difficult problem.

It is true that there are dissenters for both dark matter and black holes, but they are a strong minority. Most in the requisite subfields of cosmology/astrophysics are quite convinced that dark matter and black holes are real (caveat: I'm sure this is true about dark matter, but black holes are a bit outside my field, so I am less sure they're this confident...I think it's true, but it's a more vague impression on my part, sadly).

(basically they can't be with current technology, so as long as this state of things doesn't change, they are actually unfalsiable and therefore not real science for the moment, they might be in the near future)

This isn't correct. There are a number of observations that could, in principle, have falsified black holes and dark matter. They haven't. Now, we may not have the ideal observations that some people might prefer, but this is often not an option in science. We work with the evidence we have, not the evidence we wished we had. Yes, it would be nice to get a picture of a black hole's event horizon (this may happen relatively soon, actually, which would be rather exciting). Yes, it would be nice to detect and measure the properties of whatever particle it is makes up dark matter. But it is foolish to think that without a specific sort of observation that we might like to have that we can't be reasonably confident as to whether or not the model is at least approximately accurate.

Well, "inmediately" took at least 30-40 years. Certainly little in geological terms but not so little in the history of modern science.

Huh? I'm pretty sure the ultraviolet catastrophe was known about from the start, even though they didn't know the solution at the time.

You lack in imagination and notions of science history, a static universe was not a majority view, it was the only view since the first concepts of a universe outside the Earth was formed by ancient astronomers to 1922 when a universe of changing radius was theoretically hypothesized by Friedmann, and certainly it was stil the majority view until a few years after the 1929 Hubble observation of the redshift-distance law. When Einstein in 1917 proposed the first modern model of universe he reccurred to the static universe even if his equations allowed other solutions because that was the standard view at the time, this can be read in most books on the matter.

And this would impact upon my point how? Obviously before Hubble, there was no observational cosmology, so there could be no science-based conclusion on whether or not the universe was static. This seems to me a case of science overcoming the presumptions that came before, not a case of science giving the wrong answer.

 

[AWA]

Look, you made a silly statement (or bad informed)in post 52, and I pointed it out, you might as well admit it, if you don't I couldn't care less, to go on would be a waste of time since it all should be clear for anyone with a minimum discernment.

 

[Chalnoth]

Look, you made a silly statement (or bad informed)in post 52, and I pointed it out, you might as well admit it, if you don't I couldn't care less, to go on would be a waste of time since it all should be clear for anyone with a minimum discernment.

I don't think it was a silly statement at all. It may have been a bit short and poorly-explained, but I don't think it was silly.

Every field of science has had a period of "growing pains" where people were just settling into the field and learning how things work. Lots of bad ideas abounded and it took time to expunge them with experimental data. But once that initial period of "growing pains" passed, as happened with cosmology some time ago, I'm not aware of a single instance of an evidence-supported theory being widely-held as true that was overturned.

There are some important qualifiers there, of course, and you may argue that those qualifiers are invalid/inconsistent, but I have seen you present nothing that makes that case.

 

[AC130Nav]

I don't think it was a silly statement at all. It may have been a bit short and poorly-explained, but I don't think it was silly.

Every field of science has had a period of "growing pains" where people were just settling into the field and learning how things work. Lots of bad ideas abounded and it took time to expunge them with experimental data. But once that initial period of "growing pains" passed, as happened with cosmology some time ago, I'm not aware of a single instance of an evidence-supported theory being widely-held as true that was overturned.

There are some important qualifiers there, of course, and you may argue that those qualifiers are invalid/inconsistent, but I have seen you present nothing that makes that case.

So cosmology is fine and just needs a little tweaking? Isn't that what they said about epicycles? Again a case where math tried to trump reason.

 

[Chalnoth]

So cosmology is fine and just needs a little tweaking? Isn't that what they said about epicycles? Again a case where math tried to trump reason.

People have been making this sort of accusation for some time about dark matter and dark energy in particular. But I have been quite confused as to why they think the situation is in any way reasonable.

To take the example of epicycles, for instance, there were some massive conceptual problems. The first one is that when the observations became more and more precise, more and more epicycles had to be added to make them work. To contrast this with dark matter, the simplest model for dark matter, a perfectly collisionless, zero-temperature massive particle, has passed with flying colors every observation that it so far would have impacted. There are potential issues where our understanding of the physics in question is otherwise poor (such as the centers of galaxies), but in every case where the physics is relatively well-controlled, where we have a reasonably good handle on the systematic errors, this simplest model of dark matter works fantastically.

Of course, for theoretical reasons we don't expect any real dark matter to actually have zero temperature or be perfectly collisionless. And we hope to one day be able to exploit these non-idealities in order to determine more explicitly the nature of dark matter. But there most definitely hasn't been a need to continually add new properties to dark matter to explain observations.

With dark energy you might have more of a point, but there I'm starting to think that our original assumption that the cosmological constant just had to be zero was based on faulty thinking. Since we actually have no indication from theory that this parameter should have been zero, we should have expected a non-zero value, and thus should, had we paid attention to our equations, have expected to at some point observe some accelerated expansion.

Edit: I'm also confused that when claiming that we can be pretty sure dark matter is a WIMP, and dark energy is the cosmological constant, that this is equated to "everything being okay" with cosmology. No, not at all. Provided you are talking in terms of knowing everything about cosmology, just knowing that dark matter is some sort of WIMP doesn't in any way say what dark matter actually is, it just narrows down the possibilities. Stating that dark energy is actually the cosmological constant doesn't in any way say why it takes this particular value, something which definitely would require new physics to explain. It just narrows the focus to what sorts of new physics we should be looking for. Observational cosmology is by no means close to finished, but we have answered some questions.

 

[finiter]

And this would impact upon my point how? Obviously before Hubble, there was no observational cosmology, so there could be no science-based conclusion on whether or not the universe was static.

Is it not better to take that observational cosmology started from Galileo? He was the first one to use a telescope for that purpose. In that case, static universe can be taken as a science-based conclusion of the past.The expanding universe of ours may be a part of still bigger static entity, the Ensemble containing the multiverses. Or, being static can be a concept of the future also.

 

[Chalnoth]

Is it not better to take that observational cosmology started from Galileo? He was the first one to use a telescope for that purpose. In that case, static universe can be taken as a science-based conclusion of the past.The expanding universe of ours may be a part of still bigger static entity, the Ensemble containing the multiverses. Or, being static can be a concept of the future also.

That would have been astronomy. Modern cosmology didn't really start until we had an inkling that our universe extended beyond our own galaxy, which was what Hubble's observations showed.

Yes, in a way cosmology was what Galileo was dealing with, but that was only because at the time, they thought our universe was smaller than our own solar system turned out to be! Modern cosmology didn't really start until we started to get hints as to the true extent of our universe.

 

[budrap]

To take the example of epicycles, for instance, there were some massive conceptual problems. The first one is that when the observations became more and more precise, more and more epicycles had to be added to make them work.

To make the analogy between Ptolemaic Cosmology and Modern Cosmology more precise you only need to substitute ad hoc modifications for epicycles in the second sentence above. The ad hoc modifications of MC would be Inflation, Dark Matter and Dark Energy all three of which have been added to compensate for predictive failures of the model.

Also analogous is the failure of MC to reexamine it's underlying premises as a consequence of those predictive failures. PC was premised on an Earth centric cosmology with all extraterrestial objects orbiting the Earth in perfect circles.

MC is premised on the existence of a Universe, meaning the existence of a singular space-time reference frame encompassing all of the observable and unobservable cosmos and a corollary assumption that the observed cosmological red-shift is the product of a recessional velocity.

...the simplest model for dark matter, a perfectly collisionless, zero-temperature massive particle, has passed with flying colors every observation that it so far would have impacted.

This is hardly surprising since Dark Matter is by design without interactive properties other than the gravitational (and otherwise unobservable) mass necessary to bring the MC model into conformance with observation.

 

[Chalnoth]

To make the analogy between Ptolemaic Cosmology and Modern Cosmology more precise you only need to substitute ad hoc modifications for epicycles in the second sentence above. The ad hoc modifications of MC would be Inflation, Dark Matter and Dark Energy all three of which have been added to compensate for predictive failures of the model.

By this logic, anything that we ever learn that is new would be just another epicycle.

For dark matter and inflation in particular, they were each proposed as a potential solution to a particular puzzle. Dark matter was initially proposed because galaxy clusters appeared to be far, far more massive than could be explained by their stars. Inflation was initially proposed to solve the flatness and homogeneity problems inherent in the big bang theory. Each made very specific predictions about future observations, predictions that could, in principle, have turned out some other way, but were nevertheless borne out by observations.

This is the exact opposite of epicycles, because the simplest version of the original idea has been continually borne out by observation (for inflation, that's chaotic inflation, for dark matter, that's cold, collisionless massive particles).

Dark energy is rather more difficult, just because it's an extremely weak effect. However, our best test of this that we know of so far will come in the relatively near future with weak lensing observations that measure the growth of structure (that is, how quickly the universe becomes more clumpy). This sort of observation turns out to be very sensitive to the nature of dark energy. It's also a difficult observation to do correctly, so we should expect it to take a little while to hammer out all of the systematic errors.

This is hardly surprising since Dark Matter is by design without interactive properties other than the gravitational (and otherwise unobservable) mass necessary to bring the MC model into conformance with observation.

Nobody expects dark matter to be completely collisionless. Many models of dark matter, for instance, interact with the weak nuclear force in the same way that neutrinos do. It just turns out to be rather difficult to rule out any particular model. Fortunately, however, current dark matter searches are actually starting to rule out some significant regions of parameter space. Perhaps in a few years we'll get lucky. We'll see.

 

[budrap]

By this logic, anything that we ever learn that is new would be just another epicycle.

No, what we learn when the model fails is that it is inadequate. It is the ad hoc hypothesis that is "just another epicycle". This doesn't make the hypothesis necessarily wrong but unless the model's underlying premises are simultaneously reexamined to see if they are still sound in the light of new data then there is always the danger that the MC model will suffer the same fate as the Ptolemaic - wrong for a millenium but still capable of cranking out a right answer.

Like the Earth centric perfect circles of Ptolemy the "Universe" as a singular container of the cosmos is a vestigal concept from deep in our cultural history. It desperately needs to be reconsidered.

 

[AC130Nav]

Nobody expects dark matter to be completely collisionless. Many models of dark matter, for instance, interact with the weak nuclear force in the same way that neutrinos do. It just turns out to be rather difficult to rule out any particular model. Fortunately, however, current dark matter searches are actually starting to rule out some significant regions of parameter space. Perhaps in a few years we'll get lucky. We'll see.

At least we get decent answers from you--not true in other portions of the Physics Forum where the current dogma is viciously defended.

However, I do not see how anything with mass (unlike the poster to whom you responded) could fail to to have collisions. Mass implies a gravitational field, and this is what collisions are about not surfaces.

Question: I have read that the "expansion" of the universe has been found not to be uniform. (I put it in quotes not because I don't believe in it, but because I don't think that's the right word--more like rapid decompression in aeronautical terms). Is it uniform at all distances from us?

 

[Chalnoth]

However, I do not see how anything with mass (unlike the poster to whom you responded) could fail to to have collisions. Mass implies a gravitational field, and this is what collisions are about not surfaces.

Collisions among matter which we are familiar with are driven by the electromagnetic force, not the gravitational force. Basically, when atoms get squeezed together, the electrostatic repulsion of the electrons prevents too much compression, or the atoms passing through one another, even though atoms are mostly empty space.

We already have a relatively familiar particle, after all, that has mass, interacts gravitationally, and yet passes through matter all the time: the neutrino. Neutrinos don't have enough mass to explain dark matter, but we expect that whatever particle actually makes up the dark matter would turn out to be much like the neutrino in its interactions, just with more mass.

Question: I have read that the "expansion" of the universe has been found not to be uniform. (I put it in quotes not because I don't believe in it, but because I don't think that's the right word--more like rapid decompression in aeronautical terms). Is it uniform at all distances from us?

The expansion of our universe is quite uniform to within experimental tests. Here's one measure using supernovae as a test:
http://www.pnas.org/content/101/1/8/F3.expansion

This diagram, termed a "Hubble diagram", which is a diagram of velocity vs. distance, shows a linear relationship out to about 650Mpc, or a little over 2 billion light years. That's pretty uniform. We also have measures that go even further out, but this is one that I found quickly and felt it was good enough to drive the point home.

 

[Chalnoth]

No, what we learn when the model fails is that it is inadequate. It is the ad hoc hypothesis that is "just another epicycle".

But the accusation of inflation, dark matter, and dark energy being "ad hoc" is entirely specious. Yes, they were each initially proposed as a possible explanation for a particular inadequacy in the model. If you'll look at the history, there were a number of competing models for each of these things. But those competing explanations have been knocked out one by one by subsequent observations, and now only inflation, dark matter, and dark energy have survived (though granted, modified gravity still remains a possibility for the explanation of the accelerated expansion, it isn't looking good for that possibility).

You might wish to claim that maybe there is some explanation that we haven't thought of yet, but that isn't science. One of the primary driving forces of scientific work stems around simple practicality. You can't do work on, "maybe it's something we haven't thought of yet," as you have to have a specific idea to at least get started. If you have a specific idea of an alternative, then great! Work on it! Flesh it out! See if it is self consistent, see if it is consistent with past observations, see if it provides testable predictions for future observations.

But don't just stand on the sidelines and say, "Yeah, I think it's something else." That isn't science.