I have read/seen about, the galaxies that we have observed are all moving away from eachother. Not only that, I think they said that they are accelerating? So my question is, if our Milky Way Galaxy is spinning (its arms) and our planet is spinning, how do we measure that its not just 'us' that's moving?
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I'll let others give you the detailed explanations, but here is a simple explanation of how it works.
The Big Bang and the Expansion of the Universe
The Big Bang and the Expansion of the Universe
Most, but not all galaxies are receeding from us with respect to our position in the Universe. We measure how rapidly galaxies are moving away from us (or towards us) using the Doppler Effect.
However, some galaxies are moving towards us like the Andromeda Galaxy.We are also moving towards a group of galaxies called the Local Group.
So while the Universe is expanding at an accelerating rate, some galaxies move towards each other due to gravitational attraction.
However, some galaxies are moving towards us like the Andromeda Galaxy.We are also moving towards a group of galaxies called the Local Group.
So while the Universe is expanding at an accelerating rate, some galaxies move towards each other due to gravitational attraction.
The most direct answer to your question is: we're moving in one direction, so if the universe weren't expanding, we'd see all the galaxies moving in the same direction - and at the same speed! Instead, we see them all moving in different directions (that is, whichever direction is away from us!) and at different speeds (the farther away, the faster the speed). Hope that helps.
Personally I've never seen an actual answer. Everything tend to be rather in the line of hypothesis, so you actually have to discuss it from that context.
Anyone has yet to confirm, in practice, what is going on in, and with, the universe, after all. This means that, in practice, nobody actually knows if the universe is expanding or not, it's just an interpretation of, our current "facts".
Anyone has yet to confirm, in practice, what is going on in, and with, the universe, after all. This means that, in practice, nobody actually knows if the universe is expanding or not, it's just an interpretation of, our current "facts".
What is difficult to understand, is the fact that the galaxies are in fact accelerating away from us not away from the center of the Universe.
The "regular" explanation of this fact as was able to find so far is that the space itself is expanding (as the distance between "quants" of space), so that if one would imagine a rubber band on which all the galaxies sit, and would begin expanding such band, the expansion would be uniform all the way along the band.
In this case however, I can't see how the Doppler shift can occur at all, as one could assume that the light would also be affected by such expansion, and it's wavelength would rise and fall accordingly as it goes through less and less dense Universe upon reaching us. So theoretically there should not be possible to detect such expansion at all (our instruments and measurements will also expand proportionally)
Any idea ?
regards,
fanda
The "regular" explanation of this fact as was able to find so far is that the space itself is expanding (as the distance between "quants" of space), so that if one would imagine a rubber band on which all the galaxies sit, and would begin expanding such band, the expansion would be uniform all the way along the band.
In this case however, I can't see how the Doppler shift can occur at all, as one could assume that the light would also be affected by such expansion, and it's wavelength would rise and fall accordingly as it goes through less and less dense Universe upon reaching us. So theoretically there should not be possible to detect such expansion at all (our instruments and measurements will also expand proportionally)
Any idea ?
regards,
fanda
There is a huge amount of information on this topic. Start with "Hubble Law".
There is even the CLEA exercise that gives you a chance to go through the measurement steps.
As an outline to hang your own reading:
When you take the spectrum of a galaxy it has narrow dips in brightness (=lines) at several wavelengths. These come from the cumulative spectra of all the stars in the galaxy. If a star or galaxy is moving toward or away from us the wavelength of these lines is shifted toward the blue (higher frequency) or red (lower frequency) end of the spectrum (see Doppler Effect). The faster the speed toward/away the bigger the shift in wavelength/velocity.
The analogy is with the pitch of a train whistle (or car engine) as the train approaches the pitch (frequency) is higher and lower as the train recedes. This is respect to if the whistle was standing still. It is the relative motion, so if you and the train were traveling along side by side you would not hear the same pitch as if both were standing still.
Starting in 1940's the spectra of galaxies have been measured. Only a few of the closest galaxies were found to be moving toward our galaxy. All the rest in every direction are moving away. What is more, the further the galaxy is away, the faster the speed of recession. There are a number of complementary ways of measuring distances to galaxies, which now agrees with each other. (look up cosmic distance ladder, or Chepied variable stars)
If it was just us moving then galaxies in front o us would be moving toward us and galaxies behind us would be moving away and we wouldn't see the pattern of further galaxies moving away faster. The other obvious idea is that our galaxy is at the center of the universe and everything is rushing away from the Milky Way. Cosmic BO or explosion. But looking around we don't see anything special about our galaxy. We can explain the measurements by having the whole universe expanding in all directions at once.
A 3D analogy is a raisin cake with each raisin being a galaxy, and the dough being the space of the universe (cake). As the cake expands the raisins are dragged along, all moving away from each other. There is also the 4D analogy with a balloon being blown up.
A neat little experiment. Take a sheet of paper, draw little circles representing galaxies randomly on it. Photocopy this drawing onto a clear overhead sheet, but slightly enlarges say 125%. Then overlap the overhead with your original centering it on a "galaxy" near the middle. You can see all the other "galaxies" are moved away from the central one. The further away the greater the shift. If the difference between the two was in time, the shifts could be converted to speeds. ie the further from the center the larger the velocity. You could even make a plot of velocity (shift) versus distance from the central galaxy (origin). Now pick another "galaxy" and center the overhead on it ! You'll see the same pattern of shifts but now with respect to the new origin.
The last part of the original question was about the universe accelerating.
Here's is a diagram taken from http://www.valdostamuseum.org/hamsmith/MRRsnIaSZgl.gif;
Instead of being a nice straight line the measurements follow a curve, with a lower slope (flatter) toward the right (larger distance). Further away means further back in time. The slope of the line gives the speed of expansion. So a lower (shallower) slope means a lower speed. So between then and now the speed has increased, so the rate of expansion has increased ie the universe has accelerated. (look up type 1a supernovae).
There is even the CLEA exercise that gives you a chance to go through the measurement steps.
As an outline to hang your own reading:
When you take the spectrum of a galaxy it has narrow dips in brightness (=lines) at several wavelengths. These come from the cumulative spectra of all the stars in the galaxy. If a star or galaxy is moving toward or away from us the wavelength of these lines is shifted toward the blue (higher frequency) or red (lower frequency) end of the spectrum (see Doppler Effect). The faster the speed toward/away the bigger the shift in wavelength/velocity.
The analogy is with the pitch of a train whistle (or car engine) as the train approaches the pitch (frequency) is higher and lower as the train recedes. This is respect to if the whistle was standing still. It is the relative motion, so if you and the train were traveling along side by side you would not hear the same pitch as if both were standing still.
Starting in 1940's the spectra of galaxies have been measured. Only a few of the closest galaxies were found to be moving toward our galaxy. All the rest in every direction are moving away. What is more, the further the galaxy is away, the faster the speed of recession. There are a number of complementary ways of measuring distances to galaxies, which now agrees with each other. (look up cosmic distance ladder, or Chepied variable stars)
If it was just us moving then galaxies in front o us would be moving toward us and galaxies behind us would be moving away and we wouldn't see the pattern of further galaxies moving away faster. The other obvious idea is that our galaxy is at the center of the universe and everything is rushing away from the Milky Way. Cosmic BO or explosion. But looking around we don't see anything special about our galaxy. We can explain the measurements by having the whole universe expanding in all directions at once.
A 3D analogy is a raisin cake with each raisin being a galaxy, and the dough being the space of the universe (cake). As the cake expands the raisins are dragged along, all moving away from each other. There is also the 4D analogy with a balloon being blown up.
A neat little experiment. Take a sheet of paper, draw little circles representing galaxies randomly on it. Photocopy this drawing onto a clear overhead sheet, but slightly enlarges say 125%. Then overlap the overhead with your original centering it on a "galaxy" near the middle. You can see all the other "galaxies" are moved away from the central one. The further away the greater the shift. If the difference between the two was in time, the shifts could be converted to speeds. ie the further from the center the larger the velocity. You could even make a plot of velocity (shift) versus distance from the central galaxy (origin). Now pick another "galaxy" and center the overhead on it ! You'll see the same pattern of shifts but now with respect to the new origin.
The last part of the original question was about the universe accelerating.
Here's is a diagram taken from http://www.valdostamuseum.org/hamsmith/MRRsnIaSZgl.gif;
Instead of being a nice straight line the measurements follow a curve, with a lower slope (flatter) toward the right (larger distance). Further away means further back in time. The slope of the line gives the speed of expansion. So a lower (shallower) slope means a lower speed. So between then and now the speed has increased, so the rate of expansion has increased ie the universe has accelerated. (look up type 1a supernovae).
Thanks to SpaceTas for the Dopler Effect explanation.
But the Dopler effect will only exist if the density of spacetime fabric does not change. So it's not space itself that is expanding, but only the matter that "sits" on the top of the spacetime fabric.
I remember however reading somewhere that properties of spacetime fabric would also change during the Universe expansion. Or am I wrong ?
But the Dopler effect will only exist if the density of spacetime fabric does not change. So it's not space itself that is expanding, but only the matter that "sits" on the top of the spacetime fabric.
I remember however reading somewhere that properties of spacetime fabric would also change during the Universe expansion. Or am I wrong ?
TheOldName":4tizua0a said:
Well, what sort of confirmation would you like, "in practice?" Someone to take a spaceship a few hundred thousand light years out and see if things are expanding? What would an "actual answer" be? I find it curious you put "facts" in quotation marks. We see galaxies moving away from us in every direction, at a speed (to first order) proportional to their distance from us. It's quite simple, really.
fanda":37x3paxj said:
Hi Fanda - first off, as far as we know there is no physical "spacetime fabric" that has a density. At least, if there is, it's not necessary for the mathematical description we currently have of it. The idea of spacetime having a "density" is taking the analogies a bit too far, so be careful thinking about it that way.
Now, yeah, spacetime does "expand" - that's what the expansion of the universe is, it's really not galaxies moving so much as spacetime's expansion pulling them apart. That's right. But the Doppler effect works just as well - it's just a little more subtle
As it turns out, if you have light moving through an expanding spacetime, it'll get redshifted as well. Think about it like this: light waves have a certain wavelength, but as space expands, the light gets stretched by the expansion - causing the wavelength to get longer. Incredibly, the Doppler shift that you see from this picture of it is the exact same as the Doppler shift that you see if spacetime isn't expanding but the galaxies are moving the same way.Responding to the insightful comments by TheOldName and by fanda:
1. See The Classical Theory of Fields by Landau and Lifshitz, page 369 in the chapter on "Relativistic Cosmology": "If we regard this [the observed redshift of distant galaxies] as a Doppler shift, we arrive at the conclusion that the galaxies are receding." These fellows are accepted as outstanding authorities in this field, and you'll note their conspicuous use of the word "If" here.
2. If, as general relativistic cosmologies would have it, it is space itself that is expanding, does that imply a Doppler shift? A definite "maybe" on that one.
3. General relativity needs 2 adjustable parameters to fit the SNe 1a data. Hence dark energy (and dark matter) are needed, as well as an adjustable value of the Hubble constant. That requires one to bring a lot of faith to the table. Not a problem, if your career is founded on the truth of the present understanding.
A detailed explanation of the SNe 1a data that fits observations quantitatively (and beautifully) is to be found at http://www.3rdRelativity.org, pages 56-70. See in particular the graphical representation on page 67. There are no adjustable parameters anywhere in this approach, none. Moreover, the Hubble constant is explained as originating in a scaling of spacetime, as an intrinsic property of electromagnetic interactions. And it is evaluated explicitly in terms of known fundamental constants: See page 62.
It would appear that the universe is not expanding. Rather, spacetime scales with distance. The real beauty of this understanding is that a simple class of experiments can test it directly. See page 24 of http://www.3rdRelativity.org
The problem here is not science, it's sociology.
(The second author's name is Lifchitz, almost. Replace the "c" with an "s" to get the correct spelling. He was one of the outstanding Russian physicists of the 20th century. Someone with a problem has deleted it and won't let me replace it.)
Edit by Meteor Wayne: Actually no one deleted the name, unfortunately it tripped the automated filter. We'll tray and get that fixed on Monday when the guru returns.
1. See The Classical Theory of Fields by Landau and Lifshitz, page 369 in the chapter on "Relativistic Cosmology": "If we regard this [the observed redshift of distant galaxies] as a Doppler shift, we arrive at the conclusion that the galaxies are receding." These fellows are accepted as outstanding authorities in this field, and you'll note their conspicuous use of the word "If" here.
2. If, as general relativistic cosmologies would have it, it is space itself that is expanding, does that imply a Doppler shift? A definite "maybe" on that one.
3. General relativity needs 2 adjustable parameters to fit the SNe 1a data. Hence dark energy (and dark matter) are needed, as well as an adjustable value of the Hubble constant. That requires one to bring a lot of faith to the table. Not a problem, if your career is founded on the truth of the present understanding.
A detailed explanation of the SNe 1a data that fits observations quantitatively (and beautifully) is to be found at http://www.3rdRelativity.org, pages 56-70. See in particular the graphical representation on page 67. There are no adjustable parameters anywhere in this approach, none. Moreover, the Hubble constant is explained as originating in a scaling of spacetime, as an intrinsic property of electromagnetic interactions. And it is evaluated explicitly in terms of known fundamental constants: See page 62.
It would appear that the universe is not expanding. Rather, spacetime scales with distance. The real beauty of this understanding is that a simple class of experiments can test it directly. See page 24 of http://www.3rdRelativity.org
The problem here is not science, it's sociology.
(The second author's name is Lifchitz, almost. Replace the "c" with an "s" to get the correct spelling. He was one of the outstanding Russian physicists of the 20th century. Someone with a problem has deleted it and won't let me replace it.)
Edit by Meteor Wayne: Actually no one deleted the name, unfortunately it tripped the automated filter. We'll tray and get that fixed on Monday when the guru returns.
Spearshaker":37by7lcw said:
Really? Just a "maybe"? I don't know, if you actually use GR you'll see that redshift is a pretty straightforward result of an expanding spacetime.
ramparts:
I appreciate the explanation, but if you could detail for me the following statement:
if you actually use GR you'll see that redshift is a pretty straightforward result of an expanding spacetime.
If the distance has expanded from D to D+Delta due to the space expansion, the time it takes EM waves to traverse this distance will not change ( case time T would shrink to T-Delta). I know it's a dangerous assumption, but we do know the instances when this happens. The gravity affecting the time is one of them. The space does change, as Gravity-B has shown us. In fact it can be dragged by a mass, and twisted by it. Materials without density can't do that.
If the EM wavelength has "expanded" from F to F + delta, we will still measure it at F, cause our measure of distance has expanded due to the space expansion.
I still can't see how redshift is possible under this assumptions ?
I appreciate the explanation, but if you could detail for me the following statement:
if you actually use GR you'll see that redshift is a pretty straightforward result of an expanding spacetime.
If the distance has expanded from D to D+Delta due to the space expansion, the time it takes EM waves to traverse this distance will not change ( case time T would shrink to T-Delta). I know it's a dangerous assumption, but we do know the instances when this happens. The gravity affecting the time is one of them. The space does change, as Gravity-B has shown us. In fact it can be dragged by a mass, and twisted by it. Materials without density can't do that.
If the EM wavelength has "expanded" from F to F + delta, we will still measure it at F, cause our measure of distance has expanded due to the space expansion.
I still can't see how redshift is possible under this assumptions ?
Hi. You can find the relevant math here:
http://en.wikipedia.org/wiki/Redshift#E ... n_of_space
Note that you'll need a decent grasp of calculus for that. Anywho, I don't know what you mean by "our measure of distance has expanded due to the space expansion." That's not really true - distances get bigger, and we can absolutely measure that, otherwise the expansion wouldn't be something physically meaningful. Think about it like this: a photon with a wavelength of 5 millimeters gets redshifted to say, 6 millimeters because of the expansion of spacetime, as it travels from its host galaxy to Earth. We measure it having a redshift of 6 millimeters, but since the atom which produced it hasn't changed with the expansion (the laws of physics stay the same), we know that it should still be at 5 millimeters. So we've measured the redshift after all.
http://en.wikipedia.org/wiki/Redshift#E ... n_of_space
Note that you'll need a decent grasp of calculus for that. Anywho, I don't know what you mean by "our measure of distance has expanded due to the space expansion." That's not really true - distances get bigger, and we can absolutely measure that, otherwise the expansion wouldn't be something physically meaningful. Think about it like this: a photon with a wavelength of 5 millimeters gets redshifted to say, 6 millimeters because of the expansion of spacetime, as it travels from its host galaxy to Earth. We measure it having a redshift of 6 millimeters, but since the atom which produced it hasn't changed with the expansion (the laws of physics stay the same), we know that it should still be at 5 millimeters. So we've measured the redshift after all.
Thanks for the link, I will take a look
Well, there is no disagreement here. I am talking about the spacetime fabric expansion, where the relation between distances would also change. This change is by definition undetectable by anyone living within this Universe, as there is no stable reference that does not change itself.
I suppose if Universe is expanding, it is not (or not simply) a fabric of space expansion, otherwise we would not need the dark energy to explain the expansion. I've seen a few articles however where scientists are confusing both
I guess that's likely explains it.
regards,
fanda
Well, there is no disagreement here. I am talking about the spacetime fabric expansion, where the relation between distances would also change. This change is by definition undetectable by anyone living within this Universe, as there is no stable reference that does not change itself.
I suppose if Universe is expanding, it is not (or not simply) a fabric of space expansion, otherwise we would not need the dark energy to explain the expansion. I've seen a few articles however where scientists are confusing both
I guess that's likely explains it.
regards,
fanda
fanda":97vsidll said:
The expansion cannot overcome the nuclear, strong electrostatic or strong gravitational attraction. So the universe is expanding but not individual objects, solar systems, galaxies, or graviationally bound galaxies.
I suppose if Universe is expanding, it is not (or not simply) a fabric of space expansion, otherwise we would not need the dark energy to explain the expansion. I've seen a few articles however where scientists are confusing both
Dark energy is not what is driving the expansion of the universe, dark energy is what is driving the accelleration of the universe. No one knows what dark energy is. Some energy must be driving the acceleration of the universe, but it is not known what that energy is so it is called dark energy.
regards,
fanda[/quote]
It would seem then that the only thing affected by the expansion of space is space itself. Seems even to contradict the idea of dark matter if you think about it because even if the expansion is so small as to not overcome even gravity itself on a galactic scale (which is when it is weakest, over great distances) then it should still have a measureable effect AGAINST holding galaxies together which is why "dark matter" was dreamed up because apparently there isn't enough gravity to hold them together WITHOUT compensating for expansion.
Jerromy":3bjibn30 said:
No that is not quite right. Dark matter was not 'dreamed up', as you say, because of expansion. Dark matter has nothing to do with expansion per se. The rotation of the spiral arms in galaxies and the way galaxies interact indicates that there is much more matter than can be seen in the galaxies. This is the rational for dark matter. not expansion of the universe.
It should also be noted that, given the current expansion rate of the universe, gravity on galactic scales is absolutely strong enough to counteract the expansion. Dark matter or no, the expansion isn't going to pull our galaxy apart any time soon. In fact, it's not even strong enough to counteract the gravity between nearby galaxies - that's why our galaxy and our biggest neighbor, the Andromeda Galaxy, are on a collision course.
Here is an non-dark energy explanation to the accelerating expansion of the universe.
1. Expansion of space-time was initiated at the BB and continues unabated.
2. Expansion of space-time accelerates and has always accelerated.
3. The expansion of space-time is not driven by a force, but is analogous to gravity which is a distortion of space-time and not a force.
4. Matter initially expanded at about the same rate as space-time, but gravity and other forces emerged which caused matter to expand at a slower rate, and allowed stars and galaxies to form.
5. Galaxies continued to expand away from each other, and the effects of gravity gradually decreased as the distance separating them increased.
6. And finally, galaxies began to separate at accelerating rates that more closely align with the ever accelerating expansion rate of space-time as the effects of gravity faded.
Any opinions?
1. Expansion of space-time was initiated at the BB and continues unabated.
2. Expansion of space-time accelerates and has always accelerated.
3. The expansion of space-time is not driven by a force, but is analogous to gravity which is a distortion of space-time and not a force.
4. Matter initially expanded at about the same rate as space-time, but gravity and other forces emerged which caused matter to expand at a slower rate, and allowed stars and galaxies to form.
5. Galaxies continued to expand away from each other, and the effects of gravity gradually decreased as the distance separating them increased.
6. And finally, galaxies began to separate at accelerating rates that more closely align with the ever accelerating expansion rate of space-time as the effects of gravity faded.
Any opinions?
FlatEarth":2vqavdia said:
This is your conjecture and is at odds with the scientific community. Your conjecture is not supported by current theory or observation.
Yes, it is conjecture on my part, but you make an incorrect statement when you say it is not supported by observation. What I've done is to try to explain logically what is observed without resorting to an unknown and likely non-existent force. I wonder why scientists don't consider this possibility, and there may be good reason (or there may not). That's what I hope to get by posting this again. The last time all I got was basically "This is not what is taught, therefore you are wrong." I'm looking for a more intelligent discussion than that.origin":uenxi2ug said:
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