Inflation - can someone please explain it?

In summary, inflation is a model of the universe's immediate state after the big bang where it rapidly expanded to explain various phenomena. There are theories on what caused it, but none have been proven yet. Some believe that the strong force may have played a role, but it is not a widely accepted explanation. Inflation is still an active area of research and there are ongoing efforts to find alternative explanations for the early universe puzzles it solves.
  • #1
edpell
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What is inflation (in big bang theory not economics)?
 
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  • #2
Inflation is a term used to describe a model of the universe immediately after the big bang where the universe underwent an exponential growth in size for a short time. The model seems to work in that many questions about the current state of the universe can be explained with the inflation theory, but could not be explained otherwise.
 
  • #3
Is it the case that having inflation happen makes explaining the current (or 12.9 billion years ago) state of the universe easier but we have no theory or experimental data on inflation? i.e what caused it, how fast it was, how long it took, how far it went, why is stopped, etc...
 
  • #4
I would love to hear a laymans explanation as well!

I find it very suspicious, that "coincidentally", just as as the universe cooled enough for some of the intense gravity to condense into nuclear strong force, that is when the inflationary epoch ran its course. I am in a minority camp, definitely not mainstream, that considers that nuclear strong force may have spacetime bending properties: that the creation of matter should cause space to expand, and conversion of matter to energy should cause spacetime to contract. I would very much like to hear any evidence decoupling spacetime inflation from nuclear strong force creation.
 
  • #5
edpell said:
Is it the case that having inflation happen makes explaining the current (or 12.9 billion years ago) state of the universe easier but we have no theory or experimental data on inflation? i.e what caused it, how fast it was, how long it took, how far it went, why is stopped, etc...
I think that is essentially right! It's often said that inflation scenarios merely replace one kind of problem by another.

Still, it's an interesting idea and worth learning about. You know about the currently ongoing accelerated expansion which is supposed to be driven by a constant "dark" energy density. It is a mild form of inflation, you might say.
You can figure out how that would work. (or ask at one of the forums) The dark energy density we have today is estimated to be about 0.6 to 0.7 nanojoules per cubic meter.

If you like math, look up "Friedman equations" in wikipedia. One of the equations has the second time-derivative of the scale factor. You have to make that positive to get acceleration. You can figure out how a constant energy density and the associated (negative) pressure could make the second derivative ä >0.

The main thing is that if the "dark energy" that we now hypothesize is gently accelerating expansion were much much stronger---like joules per cubic meter instead of nanojoules---we would be getting something like inflation.

So that vastly stronger "dark energy" is what they refer to as the scalar field or "inflaton" field that hypothetically drove inflation. But then we are left with the problem, as you suggested, of how do you turn the damn thing off??!

And exotic physics is needed just to allow the existence of the required field, as well as allowing it to decay.
===================

Nevertheless I'd say it's very interesting and worth thinking about. What seems unlikely and contrived to us now might someday seem quite natural, when we understand better. The fine tuning that now seems required (to get the desired amount of early expansion) may turn out to be automatic---provided by some mechanism not yet fully imagined.

It's also worth thinking about alternatives to inflation---other scenarios that would address the early universe puzzles (horizon problem, flatness problem, microwave fluctuation spectrum...) and so would allow us to dispense with inflation as unnecessary.

There are good people taking various different approaches and coming out with papers on different explanations for inflation, and also on ideas of how to dispense with inflation.
Just this year there was a paper by a prominent Loop cosmologist Abhay Ashtekar. And a paper by Nobel laureate Steven Weinberg. There've been others this year---just can't think of them at the moment.

It's an active area of research. The story's not over on inflation. Keep asking questions about it until you are satisfied.
 
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  • #6
Inflation solves a few problems. One is that different parts of the universe ought to be lumpy. You have one part of the universe. Light doesn't have time to reach another part of the universe. How come they are the same. Another problem is that there ought to be tons of weird particles like monopoles lying around.

So if assume that the universe expanded really, really rapidly, then these problems disappear. What happens is that the expansion smooths out the lumps, and then all of the tons of weird particles that were formed pre-inflation get spread out so that you do see them very often.

As far as what *caused* inflation. A lot of people tried a lot of models in the early 1980's that tried to get inflation to be the result of energy released when the universe fell below a certain temperature and nuclear forces froze, but that didn't work too well. So starting in the mid-1980's people stopped thinking up detailed models to explain inflation. One big problem with this models is that it's not that hard to start inflation, but the problem is that once the universe expands very, very rapidly, how do you get it to stop?
 
  • #7
CosmologyHobbyist said:
I find it very suspicious, that "coincidentally", just as as the universe cooled enough for some of the intense gravity to condense into nuclear strong force, that is when the inflationary epoch ran its course.

It's not a coincidence. Originally Alan Guth proposed inflation as the result of symmetry breaking of the strong force. However, after a few years of working on that idea, people gave up on using that as the cause of inflation because no one could get it to work right. One problem is that we know enough about the strong force to rule some things out.

I am in a minority camp, definitely not mainstream, that considers that nuclear strong force may have spacetime bending properties:

I don't think it's a minority view at all. There's a family of Kaluza-Klein and string theory models that model the strong force as geometric bends in space time. As with most things, it's really easy to say "strong force is the result of bends in space-time" but when you ask "what type of bends" you end up with physicists working for thirty years, and not being able to get something that works.

That the creation of matter should cause space to expand, and conversion of matter to energy should cause spacetime to contract. I would very much like to hear any evidence decoupling spacetime inflation from nuclear strong force creation.

OK. The idea that people tried in the 1980's was that when ice freezes it releases energy. So the transition between the disordered disorganized pre-symmetry breaking state and the ordered state should have released a lot of energy which then got pumped into the expansion of the universe. As with the idea of strong forces being ripples in space. it's a clever idea, but the hard part is getting the details right, and people spent a few years doing that before giving up, and saying "well it will be easier if we assume that inflation has something to with a mysterious thing that we don't know about, then to get it to work with strong force interactions."

If you are interested in this, I'm sure that there are some review papers that go through the modelling that was done in the early-1980's, as you see people come up with models, have them shot down, come up with other models, have them shot down, etc. etc. I think people went through three or four cycles before giving up.
 
  • #8
Dear 2, Wow! Thanks for the lowdown. Where can I search through the review papers? I haven't mastered that yet...
 
  • #9
So...what is an "inflaton?"

EDIT: I don't know any QFT, just looking for a sort of qualitative description.
 
  • #10
twofish-quant said:
There's a family of Kaluza-Klein and string theory models that model the strong force as geometric bends in space time.

Thank you for this insight. I have never come across this idea before.
 
  • #11
edpell said:
What is inflation (in big bang theory not economics)?
Inflation (of the cosmos) is the contemporary belief that the red shift noted while observing distant galaxies is Doppler related and that they are ballooning away from us into space at an ever accelerating pace. The more distant the galaxy, the faster it is receding.

Unfortunately, many seem to be moving at a pace faster than the speed of light and there is no apparent reason for their acceleration, yet, with esoteric sleight of hand, some cosmologists overlook or compensate for those problems in favor of the contemporary wisdom (and, of course, the opportunity to publish).

If you drop a white billiard ball into a container of cranberry juice, the deeper the container, the redder the billiard ball appears. There are several hypotheses to the effect that the red shift is not Doppler related, but stems from the nature of light as it traverses enormous distances.

Light bends in the presence of gravity. When light from a star passes another celestial body, a gravitational lens effect deflects the beam toward the mass. Light also bends when shone through a prism, spreading the colors apart into a spectrum. You will note that the trajectory of the red wavelength is the least affected by the interference while violet is cast the furthest from the original direction of travel. Every particle of mass in the universe has gravitational attraction. How many gravitational lenses are there between Earth and the stars of deep space? And when we observe those stars, why would we NOT expect to see some kind of color shift that increases with distance?
 
  • #12
Einbeermug said:
Inflation (of the cosmos) is the contemporary belief that the red shift noted while observing distant galaxies is Doppler related and that they are ballooning away from us into space at an ever accelerating pace. The more distant the galaxy, the faster it is receding.

I disagree what you are talking about is Hubble's observed expansion of the universe. Inflation is a extra feature added early in some big bang theories that allow a super fast, super large expansion for some period of time and then it stops and goes to the current leisurely pace (well changing pace, faster as time goes on).
 
  • #13
cepheid said:
So...what is an "inflaton?"

It's the mystery energy field that causes inflation. Since in quantum mechanics every field has an associated particle, the mystery energy field should have an associated particle, we now have a mystery particle.
 
  • #14
Einbeermug said:
Inflation (of the cosmos) is the contemporary belief that the red shift noted while observing distant galaxies is Doppler related and that they are ballooning away from us into space at an ever accelerating pace.

Nope. Inflation involves early universe.

There are several hypotheses to the effect that the red shift is not Doppler related, but stems from the nature of light as it traverses enormous distances.

Yup, and after thinking about it between roughly 1950 and 1970 those theories were abandoned by most people. The trouble is that if light is getting scattered, then distant galaxies ought to look blurry. If it is getting absorbed then the energy should get re-emitted at other frequencies, and you should see a warm glow. Also it's pretty easy to think of something that blocks blue light, it's pretty hard to think of something that *shifts* light and keeps all of those spectral lines nice and sharp.

How many gravitational lenses are there between Earth and the stars of deep space?

Not very many. If there were lots of gravitational lens then looking at the universe would be like looking through a dense fog. Couldn't that cause the cosmic microwave background? Maybe. Again, people thought about that during the 1960's. The problem is that if the CMB was caused by scattered sources, you run into the opposite problem with things being too smooth.

And when we observe those stars, why would we NOT expect to see some kind of color shift that increases with distance?

The trouble with gravitational lens is that its a lens and doesn't shift frequencies. There are lots of things that change light. There is absorption, scattering, lensing. The trouble is that you have to pick one.
 
  • #15
twofish-quant said:
It's the mystery energy field that causes inflation. Since in quantum mechanics every field has an associated particle, the mystery energy field should have an associated particle, we now have a mystery particle.
And not just a mystery particle, but some mystery mechanisms.

1) What caused inflation to occur, and to procede at just the right rate to solve our current cosmological problems?

2) What caused inflation to "switch off" everywhere in the Universe at the same time after it had expanded 1026 times or more? If it hadn't done so, there would be anisotropies in matter distribution as we survey the high-redshift domains in various directions.
 
  • #16
turbo-1 said:
And not just a mystery particle, but some mystery mechanisms.

And also there is the problem with reheating. If the universe expanded really, really fast it should have cooled, right? So how did the universe get hot again?
 
  • #17
twofish-quant said:
And also there is the problem with reheating. If the universe expanded really, really fast it should have cooled, right? So how did the universe get hot again?
Obviously, the degeneration of inflatons into ordinary matter is a highly exothermic process. :devil: See? All fixed! :tongue:
 
  • #18
twofish-quant said:
And also there is the problem with reheating. If the universe expanded really, really fast it should have cooled, right? So how did the universe get hot again?
Reheating occurred when the energy in the inflaton field which drove the expansion was released into thermal energy.

In the standard inflationary picture, this happens naturally when the inflaton field reaches the bottom of its potential: it starts to oscillate around the minimum, which causes it to decay.
 
  • #19
twofish-quant said:
Nope. Inflation involves early universe.
My misunderstanding. I was equating inflation w/expansion. I've been away from astronomy/cosmology for some 35 years and have not kept up with the current vernacular.
Yup, and after thinking about it between roughly 1950 and 1970 those theories were abandoned by most people. The trouble is that if light is getting scattered, then distant galaxies ought to look blurry. If it is getting absorbed then the energy should get re-emitted at other frequencies, and you should see a warm glow. Also it's pretty easy to think of something that blocks blue light,
So if you look at a light source while standing in the red 'shadow' of its spectrum it appears fuzzy? What about someone observing that source from its violet 'shadow'? Do they assume the galaxy is approaching? I would rest easier if cosmologists didn't use the expansion of the universe to explain away velocities greater than C. What is the current data on distant galaxies when observed in frequencies above/below the visible spectrum. Is the Doppler effect apparent there, also?
...it's pretty hard to think of something that *shifts* light and keeps all of those spectral lines nice and sharp.

If there were lots of gravitational lens then looking at the universe would be like looking through a dense fog.
Presumably the ratio of matter to aether in local space is 1 atom/m3 - and there are a lot of 'm's btween here and the more remote galaxies. We don't know what elemental gasses occupy deep space and even with that minute density, the more distant light sources may appear red for much the same reason that the sky is blue.
Couldn't that cause the cosmic microwave background? Maybe. Again, people thought about that during the 1960's. The problem is that if the CMB was caused by scattered sources, you run into the opposite problem with things being too smooth.

The trouble with gravitational lens is that its a lens and doesn't shift frequencies. There are lots of things that change light. There is absorption, scattering, lensing. The trouble is that you have to pick one.
If there was a BigBang, then we should be able to determine an epicenter somewhere in the known universe. If galaxies appear to be receding at a relatively even pace at 360° originating at our point of view, the matter of BB is HIGHLY suspect.
 
  • #20
Einbeermug said:
So if you look at a light source while standing in the red 'shadow' of its spectrum it appears fuzzy? What about someone observing that source from its violet 'shadow'?

If it's due to scattering, yes. If you look at the moon through a haze that forms halos, the moon looks blurry.

What is the current data on distant galaxies when observed in frequencies above/below the visible spectrum. Is the Doppler effect apparent there, also?

Yes. Lyman alpha is a very strong line in the ultraviolet, and you start detecting tons of quasars and hydrogen gas clouds. There are things like the Lyman alpha forest and the Gunn-Peterson trough. You have a sharp emission line by a quasar. And then you have a bunch of absorption lines on one side of that line. The standard interpretation is that you have a highly redshifted emission line, and then in between the galaxy and the Earth you have a bunch of gas clouds at smaller redshift whose shadow you can see.

Presumably the ratio of matter to aether in local space is 1 atom/m3 - and there are a lot of 'm's btween here and the more remote galaxies.

And you can calculate cross sections and absorption coefficients. If you shine a beam of light through hydrogen/helium gas, there are only about a dozen or so known things that can happen to them.

We don't know what elemental gasses occupy deep space and even with that minute density, the more distant light sources may appear red for much the same reason that the sky is blue.

1) Yes we do. It's a hydrogen/helium mix with trace amounts of other stuff
2) Distant light sources don't appear red. They are shifted. If you have a UV object and you shift that, it looks blue. Also you get back to the scattering problem

If there was a BigBang, then we should be able to determine an epicenter somewhere in the known universe. If galaxies appear to be receding at a relatively even pace at 360° originating at our point of view, the matter of BB is HIGHLY suspect.

Big bang happened everywhere.
 
  • #21
twofish-quant said:
Lyman alpha is a very strong line in the ultraviolet, and you start detecting tons of quasars and hydrogen gas clouds. There are things like the Lyman alpha forest and the Gunn-Peterson trough. You have a sharp emission line by a quasar. And then you have a bunch of absorption lines on one side of that line. The standard interpretation is that you have a highly redshifted emission line, and then in between the galaxy and the Earth you have a bunch of gas clouds at smaller redshift whose shadow you can see.

And you can calculate cross sections and absorption coefficients. If you shine a beam of light through hydrogen/helium gas, there are only about a dozen or so known things that can happen to them.
It is my understanding that signature absorption lines can be shifted by differentials in the temperatures of the media traversed by a beam before it is observed. Discounting the possibility that the effect of space is neutral (actually its properties may be so subtle as to not be detectable by current methodology) then over vast distances wouldn't a material density of even 1 atom per CuM be enough to result in the same shift as a 'cloud' would render in a local venue? Wouldn't each photon suffer multiple resonant scatterings.
Big bang happened everywhere?
A simple axiom, implication and conclusion:
Axiom: Something must exist before it can change or be changed.
Implication: Cause and effect (change) is derived from existence
Conclusion: No phenomenon can be derived from its own derivative, so the phenomenon of existence was not the result of any process - cause and effect, change, creation, whatever you wish to call it.

If existence is not a function of time, then the 'age' of the cosmos is not 'temporary', and that which is not temporary is 'eternal'. The 'age' of BB seems to be guaged at only 14B years. If the cosmos were going to suffer an entropy death, it seems it should have happened by now.
 
  • #22
Einbeermug said:
It is my understanding that signature absorption lines can be shifted by differentials in the temperatures of the media traversed by a beam before it is observed. Discounting the possibility that the effect of space is neutral (actually its properties may be so subtle as to not be detectable by current methodology) then over vast distances wouldn't a material density of even 1 atom per CuM be enough to result in the same shift as a 'cloud' would render in a local venue? Wouldn't each photon suffer multiple resonant scatterings.
The frequencies of absorption lines are almost completely unaffected by the intervening medium. What you've written here only holds true for continuous spectra, where, for instance, a cool intervening medium will absorb some higher-frequency light and emit more low-frequency light, lowering the average frequency of the spectrum as a whole. But since the entire process is a process of absorption and reemission of light, it just doesn't affect the frequencies of emission lines.
 
  • #23
twofish-quant said:
Big bang happened everywhere.
If galaxies are in motion, they have trajectories. Although complicated by the possibility of the expansion/inflation (I hope you are not inferring creation) of space between celestial bodies, from any point of reference each trajectory ought to be tracable back in time to produce at least some approximation of its position 14B years ago. And even if the point of observation is not fixed, compensating calculations can be derived to accommodate those dynamics.

If space, itself, is expanding, why is it not measurable locally but only at enormous distances?
 
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  • #24
Einbeermug said:
If space, itself, is expanding, why is it not measurable locally but only at enormous distances?

What can be viewed as the cumulative effect of many small local expansions is large enough to detect and is, in fact, measured.

I will try to answer your question carefully, please tell me anything that doesn't make sense to you.

The current rate of expansion of distances is 1/140 of one percent every million years. Given limited time and the limited accuracy of distance measurements, the local detection of expansion is not practical. You can do some numbers and see for yourself.

And there is a deeper problem. Our context is General Relativity, and since there is nothing in General Relativity called "space, itself" it must not be "space, itself" that is expanding. What expands, in this context, are distances (defined a certain way) between observers who are at rest with respect to the ancient matter that gave off the cosmic microwave background.

You are at rest relative to the CMB if your microwave sky is approximately all the same temperature, with no doppler hotspot. At rest can only be determined with finite accuracy. Another reason local measurements are a limited option.

It has been determined that the solar system is moving about 370 km/s in the direction of Leo. This motion relative CMB can be taken out of observational data, so that our corrected observations are then from the standpoint of an observer at rest relative to the CMB.

We see the universe, in this case, from the standpoint of an observer at rest with respect to the hot ionized gas of the early universe, and relative to the ancient light which that gas emitted.

The Hubble law v = Hd is valid for distances between stationary observers. By definition the distances are as if measured by freezing the expansion process and timing light signals.

When you see v = Hd, remember that d is such a distance, and v is the current rate that distance d is expanding.

In a universe where geometry is governed by General Relativity none of us have the right to assume that the distance between stationary observers will not change, will not increase or decrease. Gen Rel says that Euclidean geometry can't be taken for granted. Euclid may be approximated in some situations, but is not to be relied on. Same with Special Rel geometry. When and where it applies, there will be some reason that causes a standard flat non-expanding geometry to be (approximately) applicable.

=============================

You asked about local detection. Assume that a galaxy sends us some light and that while the light is in transit, distances expand by a factor of 2. When the light arrives, the galaxy will be twice as far as when it emitted the light.

The wavelength of the light will also be twice what it was when emitted.

You can think of that as a detection of the cumulative effect of many infinitesimal (local) lengthenings that occurred along the way. So although anyone (local) lengthening would have been too small to detect, one is in effect detecting the whole lot of them by measuring their cumulative result.

The wavelength of the light is increased by exactly the same factor as distances at large are increase. Hopefully since the wavelength is a small distance, this will go some ways towards as satisfactory response to your question.

=================
As a footnote I should mention that there are several different definitions of distance that find uses in cosmology. The one I'm using here is the one appropriate to the very basic Hubble law, and to the commonly used Friedmann model. You imagine freezing the expansion process at the present, or at some given moment, and timing light signals, essentially radar-ranging with expansion stopped. The idea of being at rest with respect to CMB (or equivalently, before we had the CMB, with respect to the expansion process) helps make simultaneity precise here.
 
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  • #25
Einbeermug said:
It is my understanding that signature absorption lines can be shifted by differentials in the temperatures of the media traversed by a beam before it is observed.

That's different from my understanding. It's very hard to shift an absorption line, because any sort of change involves an interaction with the medium which causes something much more complex than a simple frequency shift from happening.

Discounting the possibility that the effect of space is neutral (actually its properties may be so subtle as to not be detectable by current methodology) then over vast distances wouldn't a material density of even 1 atom per CuM be enough to result in the same shift as a 'cloud' would render in a local venue? Wouldn't each photon suffer multiple resonant scatterings.

If it's made up of ionized hydrogen then no, since cross sections for interactions are really low. Now if the photons are moving through neutral hydrogen, then yes the light will interact with the medium, and we see this happening with the most distant quasars (the Lyman alpha forest and the Gunn-Peterson trough). But when it goes through a cloud, what happens is that you get new absorption lines, you don't get previous lines shifting.

Also the cross sections for light moving through hydrogen is pretty well studied, since you can pass light through hydrogen on Earth and see how it behaves. There aren't any known effects that would case frequency shifts. Scattering, yes. Absorption, yes. Shifting, no.

A simple axiom, implication and conclusion

I'm very dubious about using "word logic" to say things about physics because it turns out that words tend not to be precise enough to allow for clear thinking. When you say something "exists" what are you saying? It turns out that when you use words, you are saying something quite complex and fuzzy. Does a vacuum exist?

Axiom: Something must exist before it can change or be changed.

I've seen stranger things happen so I don't trust "word logic" when it comes to these sorts of things. In any case, we are talking about physics and not math, and axioms don't work in physics.
 
  • #26
Marcus, thank you for your reply.
marcus said:
What can be viewed as the cumulative effect of many small local expansions is large enough to detect and is, in fact, measured.

I will try to answer your question carefully, please tell me anything that doesn't make sense to you.

The current rate of expansion of distances is 1/140 of one percent every million years. Given limited time and the limited accuracy of distance measurements, the local detection of expansion is not practical. You can do some numbers and see for yourself.

And there is a deeper problem. Our context is General Relativity, and since there is nothing in General Relativity called "space, itself" it must not be "space, itself" that is expanding. What expands, in this context, are distances (defined a certain way) between observers who are at rest with respect to the ancient matter that gave off the cosmic microwave background.
:confused: :Distance is a measurement - a quantitative assessment or comparison of something(s) with physical presence in the universe. Does the definition in GR propose that a distance can increase without either expanding or adding to that which is being measured?
You are at rest relative to the CMB if your microwave sky is approximately all the same temperature, with no doppler hotspot. At rest can only be determined with finite accuracy. Another reason local measurements are a limited option.

It has been determined that the solar system is moving about 370 km/s in the direction of Leo. This motion relative CMB can be taken out of observational data, so that our corrected observations are then from the standpoint of an observer at rest relative to the CMB.

We see the universe, in this case, from the standpoint of an observer at rest with respect to the hot ionized gas of the early universe, and relative to the ancient light which that gas emitted.

The Hubble law v = Hd is valid for distances between stationary observers. By definition the distances are as if measured by freezing the expansion process and timing light signals.

When you see v = Hd, remember that d is such a distance, and v is the current rate that distance d is expanding.

In a universe where geometry is governed by General Relativity none of us have the right to assume that the distance between stationary observers will not change, will not increase or decrease. Gen Rel says that Euclidean geometry can't be taken for granted. Euclid may be approximated in some situations, but is not to be relied on. Same with Special Rel geometry. When and where it applies, there will be some reason that causes a standard flat non-expanding geometry to be (approximately) applicable.
Are you saying that within the frame of reference of GR, the past trajectory of a very distant object cannot be calculated or even approximated with any degree of certainty? Any idea how fast the error factor grows in relation to distance?
 
  • #27
Einbeermug said:
:confused: :Distance is a measurement - a quantitative assessment or comparison of something(s) with physical presence in the universe.

Distance really isn't a measurement. When you do the experiment, you are taking doing different experiments and coming up with numbers which you then process. The experiment can involve bouncing radio waves off the object, putting a ruler to the object, measuring how long it takes for sound/light to come from the object, etc. etc. Once you've done that experiment, you then take the results process them, and then you can fit them to the same scale.

What happens is that in our nice cozy Euclidean world, the results of these different experiments can be processed in a way that you end up with some number of meters that happens to be the same.

This doesn't happen in the world of GR. You run different experiments, and you find that you can't put all of the results on the same scale. What ends up happening is that you end up with several different definitions of "distance" which come from performing different experiments. All of those different definitions give the more or less the same numbers at earth-based experiments, but they don't when you do cosmological experiments.

Does the definition in GR propose that a distance can increase without either expanding or adding to that which is being measured?

There isn't one definition of distance in GR. There are several commonly used definitions for "distance" all of which are different from each other, and each of them seem perfectly reasonable.

Are you saying that within the frame of reference of GR, the past trajectory of a very distant object cannot be calculated or even approximated with any degree of certainty?

The trajectory of an object can be defined and calculated within GR. The trouble is that when you ask "what is the distance to that object?" it's not clear what you mean by the question since it's not obvious what you mean by "distance".

This is why I don't trust "word logic" arguments. It turns out that to even talk about a "distance" you are making a huge number of assumptions about how the universe works, which happen to be true for walks down to the local chemist, but false when you talk about distant galaxies.
 
  • #28
Twofish - Tks for the post. Seems the more answers I get, the more questions I have.
twofish-quant said:
originally posted by einbeermug
it is my understanding that signature absorption lines can be shifted by differentials in the temperatures of the media traversed by a beam before it is observed.
that's different from my understanding. It's very hard to shift an absorption line, because any sort of change involves an interaction with the medium which causes something much more complex than a simple frequency shift from happening.
discounting the possibility that the effect of space is neutral (actually its properties may be so subtle as to not be detectable by current methodology) then over vast distances wouldn't a material density of even 1 atom per cum be enough to result in the same shift as a 'cloud' would render in a local venue? Wouldn't each photon suffer multiple resonant scatterings.


if it's made up of ionized hydrogen then no, since cross sections for interactions are really low. Now if the photons are moving through neutral hydrogen, then yes the light will interact with the medium, and we see this happening with the most distant quasars (the lyman alpha forest and the gunn-peterson trough). But when it goes through a cloud, what happens is that you get new absorption lines, you don't get previous lines shifting.

Also the cross sections for light moving through hydrogen is pretty well studied, since you can pass light through hydrogen on Earth and see how it behaves. There aren't any known effects that would case frequency shifts. Scattering, yes. Absorption, yes. Shifting, no.
If I read you correctly, Doppler is the ONLY known cause of a frequency shift that would not alter the position of elemental signature lines. Correct?
A simple axiom, implication and conclusion

I'm very dubious about using "word logic" to say things about physics because it turns out that words tend not to be precise enough to allow for clear thinking. When you say something "exists" what are you saying? It turns out that when you use words, you are saying something quite complex and fuzzy. Does a vacuum exist?
I would define an existence as an irreducable (not comprised of independent components) physical presence (not necessarily material) within the comos. Qualifications would necessitate it having quality (even inertness is a quality), quantity (no matter how small) and location in space. Unfortunately 'exist' has another connotation. The English language does us a disservice when it allows us to say conditions 'exist'. Conditions OCCUR. Physical presences EXIST.

Depending on your definition, vacuum may be considered a condition OR an existence. It commonly applies (as a condition) to a vessel that has had all material substance removed. It may also be applied to that vast portion of the cosmos that has no property of mass. If that portion of the universe didn't exist, all matter would be coalesced into a super-critical mass with nowhere to explode. But according to 'uncle Al' even mass is just a condition, one that is convertable into energy.
Axiom: Something must exist before it can change or be changed.

I've seen stranger things happen so I don't trust "word logic" when it comes to these sorts of things. In any case, we are talking about physics and not math, and axioms don't work in physics.
If you are referring to observations of the microcosm that are interpreted as "two things occupying the same space" and "things moving from point A to point B without traversing the points in between", or "something that appears to be partially in one location and the rest in another", yes mother nature is a master of legerdemain. There are viable natural explanations for those phenomena, interpretations stemming from a different perspective of the standard model that probably classify as conjecture not suitable for scientific discussion.

I must note; however, that other than limited adolescent experimentation with recreational substances, I've never seen anything that didn't exist - much less observe any change in its condition.:wink:
 
  • #29
Einbeermug said:
If I read you correctly, Doppler is the ONLY known cause of a frequency shift that would not alter the position of elemental signature lines. Correct?
Or a gravitational redshift.
 

1. What is inflation?

Inflation is a general increase in the prices of goods and services in an economy over a period of time. It is usually measured as a percentage increase in the price level in a given economy.

2. What causes inflation?

Inflation can be caused by a variety of factors, but the most common cause is an increase in the money supply. This can happen when the government prints more money, or when banks lend out more money than they have in reserves, leading to an increase in demand for goods and services.

3. How does inflation affect the economy?

Inflation can have both positive and negative effects on the economy. A moderate level of inflation (around 2-3%) can stimulate economic growth by encouraging consumer spending and investment. However, high levels of inflation can lead to a decrease in the value of money, which can cause economic instability and negatively impact businesses and consumers.

4. How is inflation measured?

Inflation is typically measured using a price index, such as the Consumer Price Index (CPI) or the Producer Price Index (PPI). These indexes track the changes in prices of a basket of goods and services over time.

5. How can inflation be controlled?

Governments and central banks use various monetary policies to control inflation. This can include adjusting interest rates, regulating the money supply, and implementing fiscal policies such as taxes and government spending. The goal is to maintain a stable and moderate level of inflation to support economic growth and stability.

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