# measuring the speed of the Earth

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I've been told by someone on another forum that scientists measure the speed on the Earth by comparison to the background microwave radiation. I asked how one finds a "fixed point" in the BMR and the reply was that we measure its red shift.

How credible is my source? Does this make sense?

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yeah makes sense. the difference in redshift from one direction to another would allow you to calculate the speed. or, if you just need the local speed then you can take the distance from the sun and the time taken to go round it.

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Okay,

So how do we figure out the red shift? Doesn't this require assuming that we know the speed of its source? Is this just the origin of the BB? Do we deduce this point by observing the direction that all galaxies are receding?

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ok now i'm not sure. i ain't no fizzycist.

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Okay,

So how do we figure out the red shift? ?

the redshift doesnt have to be "figure out", it can be directly measured as a temperature difference

I can get some more exact numbers and links to original papers, if you want. But the main thing is there is a BLACK BODY CURVE that is characteristic of the mix of wavelength in the glow from any warm or hot object. If you study the mix of wavelength you can determine the temperature of the glow very exactly---by fitting a blackbody curve to your data.

the first thing they did was to take average over whole sky and fit curve, and get the AVERAGE cmb temp. the fit to blackbodycurve was amazingly good. that was circa 1990 altho it had been done some before also.

the temp was something like 2.76 Kelvin.

then they started measuring more precisely and mapping temp by DIRECTION instead of averaging the whole sky

and they found that the temp of the cmb was A FEW MILLIKELVINS WARMER in the direction of the constellation Leo

and that it was a few millikelvins cooler in the direction 180 degree opposite to Leo in the sky.

all the wavelengths coming from Leo direction were just a tiny percentage shorter, and those from the opposite direction were just the same percentage longer.

you figure it out. what would you have said, to explain it?

Doesn't this require assuming that we know the speed of its source? Is this just the origin of the BB? Do we deduce this point by observing the direction that all galaxies are receding?

this part of your post has a mistaken assumption in it. somebody else can address this if they want. I want to wait and address it AFTER I hear back from you about the first thing.

Do you want to know the exact speed the solar system is moving relative to cmb restframe? I can get link to the circa 1995 paper if you want, it gives the exact millikelvins temp diff, and it translates that into kilometers per second of speed.

IIRC it is around 350-370 km/sec-------sort of like a thousandth of the speed of light.

so working backwards that would imply that the temperature difference was around 3 millikelvins (a thousandth of the overall average 2.7 Kelvin).

any questions about that much so far?

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Thanks Martin, makes sense.

I guess it's like the doppler effect - if you measure the pitch of sound coming at you and then measure it after it's passed by you, then you can pretty much deduce its absolute frequency by taking the average.

No need to post links. I was really just curious. But thanks anyway.

I assume these measurements took into account what season it was (our velocity relative to the sun) and also our motion relative to the center of the galaxy. This brings up another question (just out of curiocity again). How much displacement of the Earth would we need in order to deduce the point of origin of the cmb? I mean, how much would the Earth have to change its position in order to notice a change in where the cmb is most "warm" (from Leo to somewhere else)? Would one half-orbit around the Sun do it? Would one half-orbit around the center of the galaxy do it?

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I assume these measurements took into account what season it was (our velocity relative to the sun) and also our motion relative to the center of the galaxy.

Thanks for replying. Yes! these measurements took account of the earth's motion around the sun. That speed is about 30 km/sec, so it is roughly 8-10 percent of the sun's speed towards Leo.

so what you actually measure is sometimes some percent faster or some percent slower, but over the year it averages out.

One can also find out the ABSOLUTE MOTION OF OUR GALAXY relative to cmb. It is just a vector calculation because we know sun's speed relative to galaxy center----and we know sun's speed relative to cmb----so we can calculate speed of galaxy center relative to cmb.

IIRC it is around 500 km/sec and towards the tiny southern constellation Calix and more generally (roughly) in direction of Hydra Centaurus

the professional journal articles give all this with errorbars and in terms of coordinates----like standard celestial, or like galactic coordinates.

Anyway, yes they do take account of other steady motions

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

the cmb is the ultimate rest frame in cosmology. they also talk about it in different language, as "the Hubble flow". before people had the cmb rest frame they had an equivalent. Hubble detected that in the Leo direction the galaxies are not speeding away from us quite so fast, but in the opposite direction he detected that the galaxies were speeding away faster than average. so he deduced that the solar system was moving RELATIVE to the regular expansion of the universe, which people started calling "motion relative to the Hubble flow" meaning motion relative to the expansion of space.

the cmb gives the same information and is easier to measure precisely.

This brings up another question (just out of curiocity again). How much displacement of the Earth would we need in order to deduce the point of origin of the cmb? I mean, how much would the Earth have to change its position in order to notice a change in where the cmb is most "warm" (from Leo to somewhere else)? Would one half-orbit around the Sun do it? Would one half-orbit around the center of the galaxy do it?

there is no point of origin of the cmb.

there is no special point in space where the BB occurred (all equally)

there is no special point in space from which the galaxies are speeding away (from all equally)

the cmb is an approximately uniform bath of radiation

the U could be infinite or it could be finite but very large----at present observation cannot distinguish

suppose it is infinite, then it is like an infinite unbounded volume of raisinbread dough which is rising by yeast action

the raisins are the galaxies and they are getting farther from each other

at every stationary point it looks like you are at the center of expansion

accidental little motions of your raisin, like Milky's 500 km/sec don't count they are not part of this raisinbread story

you can never determine a special point from which expansion started because it does not exist, all space expands. When all space began expanding, it was not "anywhere" to start with because there is AFAWK no larger surrounding space for it to be in. the space we know is the only "where". So you are asking about a point that is not defined (according to present mainstream cosmology.)

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the redshift doesnt have to be "figure out", it can be directly measured as a temperature difference

I can get some more exact numbers and links to original papers, if you want. But the main thing is there is a BLACK BODY CURVE that is characteristic of the mix of wavelength in the glow from any warm or hot object. If you study the mix of wavelength you can determine the temperature of the glow very exactly---by fitting a blackbody curve to your data.

the first thing they did was to take average over whole sky and fit curve, and get the AVERAGE cmb temp. the fit to blackbodycurve was amazingly good. that was circa 1990 altho it had been done some before also.

the temp was something like 2.76 Kelvin.

then they started measuring more precisely and mapping temp by DIRECTION instead of averaging the whole sky

and they found that the temp of the cmb was A FEW MILLIKELVINS WARMER in the direction of the constellation Leo

and that it was a few millikelvins cooler in the direction 180 degree opposite to Leo in the sky.

all the wavelengths coming from Leo direction were just a tiny percentage shorter, and those from the opposite direction were just the same percentage longer.

you figure it out. what would you have said, to explain it?

I would contend that the all matter, visible universe seems to be acting as if it were a pulse of ideal liquid that began to lose momentum 3.5 billion years ago, and thus transitioned into an increasingly turbulent state, which we quite correctly have been interpreting as an accelerating expansion of the all matter, visible universe we are embedded in.

I am contending that the visible, all matter universe is not the result of an isometric, spherical expansion, but is the result of a pulse of matter being ejected from a much reduced but still exant BB event/body and the high likelyhood that a similarly massive ejection of anti-matter occured at the other pole of the still exant BB event/body.

This proposed model is a version of the oscillating universe model. The only fundamental change from the standard model is that it presumes that the pre-inflationary epoch BB event/body displayed a high degree of angular momentum.

aguy2(amen)

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you can never determine a special point from which expansion started because it does not exist, all space expands. When all space began expanding, it was not "anywhere" to start with because there is AFAWK no larger surrounding space for it to be in. the space we know is the only "where". So you are asking about a point that is not defined (according to present mainstream cosmology.)

Yes, I understand what you mean, but I was thinking the cmb might be able to tell us where the "center of mass", so to speak, was for all matter (all raisins) in the universe. I realize we can't see passed 15 or so billion years and there might be more galaxies beyond this distance, but if the direction of the cmb changes relative to the Earth depending on where it is relative to the Sun or the center of the Milky Way, this might be able to tell us where the "center of mass" of the universe is.

I really need a diagram to explain this, but I guess I should ask a question first. When you say that the cmb is a "uniform bath of radiation", do you mean to say that it travels in the same direction no matter where you measure it from (all its photon travel in parallel), or would its direction change depending on where you measure it from (all photons radiate out from a point somewhere beyond Leo). Or does it not travel at all (which confuses the heck out of me since I would think all electromagnetic radiation travels at c).

If it travels radially, we should be able to find the center point from where these "radial lines" (I guess you could call them) diverge from. That would be the "center of the universe" in a manner of speaking.

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If it travels radially, we should be able to find the center point from where these "radial lines" (I guess you could call them) diverge from. That would be the "center of the universe" in a manner of speaking.

I'm not sure you understand the cmb. It was emitted simultaneously at every point in the universe (which wasn't all that large at the time). It was not emitted at one small point.

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In BB theory the Universe emerged from a tremendously dense and hot state where the photons was forced to interact with the plasma, until space expanded causing the plasma to cool, so eventually the photons began to travel freely through space. Those photons where emitted in all directions, from uniformly everywhere inside Universe and they are the relic radiation we call CMBR.

The raisins are on the surface of the bread dough, I guess one could say that in this analogy space is the surface, so the center of mass should be somewhere in the middle of the dough, which would be outside of our 3D-space dimensions.

I once speculated in another thread that time could be this 4th dimension but mainstream says: "center could only be reached by traveling in a 4th spatial dimension, not the time dimension of 4D spacetime, but there is no evidence that this 4th spatial dimension exists" and I still don't know the explanation for ruling out the time-dimension.

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The raisins are on the surface of the bread dough, I guess one could say that in this analogy space is the surface, so the center of mass should be somewhere in the middle of the dough, which would be outside of our 3D-space dimensions.

...

my first astro teacher years ago used that raisinbread analogy, it must be a good one:-) I'm still using it. you are using it. (maybe people just like raisinbread!)

but anyway, if you use that image there is no "center of mass" and there is no surface

for the analogy to work, it is important to picture the raisins as all throughout the dough

each individual raisin sees all the other raisins receding away from it.

in mainstream cosmology there is no central distinguished point to the expansion of space (the idea wouldn't have any meaning, it would be like asking what is the center of an infinite flat sheet of paper:-) )

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You are correct Martin, shame on me.

I only had some faint memory of the Raisin bread model and thought it was a variant of the Ant on a Balloon model...

Which model is favored for describing the rate of expansion with respect to current observations ?

(Does more distant stars recede with the same factor as closer, (X * distance), or more/less ?)

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I'm not sure you understand the cmb. It was emitted simultaneously at every point in the universe (which wasn't all that large at the time). It was not emitted at one small point.

In all directions - I see.

You'll have to forgive my lack of knowledge on this stuff. It can sometimes be hard to get the right visualization without knowing all the details.

I hope you don't mind my asking, but there is one tiny little point of confusion left in my mind. When Martin talks about the cmb being "warmer" or more red-shifted in the direction of Leo, how is this to be interpreted if the cmb is radiating from all points in space in all directions? It should be uniformly "warm" in all directions, shouldn't it?

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Never mind - it suddenly all came together

Stupid me - I suddenly realized the red-shifting was solely the effect of the Earth motion relative to the cmb.

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... I suddenly realized the red-shifting was solely the effect of the Earth motion relative to the cmb.

that is right. sounds like everybody is all right with this.

one tiny point of language though in your post just before this you say

[...cmb being "warmer" or more red-shifted, in the direction of Leo...]

you meant, I think, to say

[...cmb being "warmer" or more blue-shifted, in the direction of Leo...]

in astronomy blue is the hot color and red is the cold color----blue-white stars are hotter than red stars.

actually the cmb is radio-waves and doesnt have any color! so this talk about redshift and blueshift is just astronomer SLANG. it is jargon.

but in the direction of Leo the radio waves are a little shorter, corresponding to a few millikelvin hotter temp, the photons (if you can picture radiofrequency photons!) are a little higher frequency, more energetic, shorter wavelength. So astronomers, in their jargon, call them "blueshifted"

You have the essential idea right. They are blueshifted by the earth's motion towards them!

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one tiny point of language though in your post just before this you say

[...cmb being "warmer" or more red-shifted, in the direction of Leo...]

you meant, I think, to say

[...cmb being "warmer" or more blue-shifted, in the direction of Leo...]

Yes, I did. Thanks Martin.

I think I need to take my brain out and put it back in the vat. It obviously isn't ready to face the day today.

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Yes, I did. Thanks Martin.

I think I need to take my brain out and put it back in the vat. It obviously isn't ready to face the day today.

you must be joking but come on!

you are doing great

the important thing is to be stubborn and keep asking questions

your persistent reasoning is the kind of thing that keeps me visiting SFN

thanks gib

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gib may want to take a break, or maybe not.

in any case, for anyone, gib or someone else, who wants to think more along these lines

the redshift factor of the cmb is about 1100

that means the wavelengths of the cmb light are longer, by a factor of 1100, than they were at the moment the light was released-----i.e. scattered the last time from the last hot gas ion

that means the TEMPERATURE of the universe is now less, by a factor of 1100, than it was at the moment the cmb light was released or scattered for the last time

you can find out the temperature. multiply 2.76 kelvin by 1100.

the cosmological redshift IS NOT A DOPPLER EFFECT it is a stretching out effect

space itself has stretched out by a factor of 1100 since that time

and in the course of stretching (via the effect of gradual stretching on maxwellequations) it has stretched out the lightwaves, so they have longer wavelength.

that should be hard to understand the first time you encounter it (or anyway I think so, does anyone find it easy to understand?)

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that should be hard to understand the first time you encounter it (or anyway I think so, does anyone find it easy to understand?)

The stretching out of the cmb waves is easy enough to visualize - you add more space, the wave occupying that space gets stretched out.

What I'm having difficulty with now is how I'm supposed to visualize the structure of space. I've heard of two models: 1) the surface of a 4D sphere (similar to the raisins on the crust of the bread loaf that Spyman alluded to - which Martin said was not quite right) and 2) the raisin throughout the bread load (which Martin said was right). 1) makes sense out of the ubiquitous nature of the cmb - if it was emitted in the early stages of the universe all over the universe, and if space has just been expanding at all points since then, then there would be nowhere for the cmb to go (at least, not if it wanted to escape us). But 2) is a little harder to understand. First, I'd have to understand if it means that space is finite like the loaf (i.e. there would be a definite "end-of-space" that could be reached if one travelled far enough). Or is this just an imperfection in the loaf metaphore? Second, I'd have to understand if the cmb is expected to eventually leave the universe (or at least, distance itself from all the matter in the universe). I mean, if we are to visualize the universe as just a 3D continuum of space that doesn't wrap around itself (like the surface of the 4D sphere) then the cmb should eventually travel out into empty space. Even if space is finite, I'd assume it would be expanding at c (otherwise, the cmb would "hit" the edge of the universe - whatever that means) which means that at the outskirts of the mass of material bodies would be an outer region of empty space that's perpetually growing and will eventually be the final resting place of the cmb.

Is this making any sense?

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I don't think Martin favored any model, (thats why I asked him), he only pointed out that they are different and I had mixed them.

The Raisin bread model is not supposed to have an edge or center - it's a problem within the analogy.

Objects with a redshift factor above about 2 is receding from us faster than light, highest confirmed galaxy redshift being z = 7.0 and the CMBR has a z about 1100.

I don't know how to make the calculations but the points from where the CMBR was emitted is moving away from us several times faster than c due to the expansion of space in between them and us.

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... points from where the CMBR was emitted is moving away from us several times faster than c due to the expansion of space in between them and us.

yes! It is amazing when you first run into that.

Ned Wright's calculator will find the current distance to the region of last scattering (where the CMB we are now seeing originated)

I will get a link actually now he has TWO or more versions, I didnt know that when I posted earlier

http://www.astro.ucla.edu/~wright/CosmoCalc.html

http://www.astro.ucla.edu/~wright/DlttCalc.html

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

he uses the parameters H = 71

Omegamatter = 0.27

Omegavac = 0.73 (other people sometimes tag this parameter with a Lambda)

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

there is another cosmology calculator which may work better for us. it is by Siobhan Morgan

IIRC you have to type in parameters like H=71, Omega= 0.27, Lambda = 0.73

but once you do that it gives the same answers as Wright's

here is Siobhan's page

http://www.earth.uni.edu/smm.html

here is her bunch of astronomy JAVA applets

http://faculty.cns.uni.edu/~morgan/ajjar/

here is her cosmology calculator (based on a program supplied by Sky and Telescope magazine, probably same as Wright used)

http://faculty.cns.uni.edu/~morgan/ajjar/Cosmology/cosmos.html

I typed in 0.27 for omega, 0.73 for lambda, 71 for Hubble, and 1100 for redshift

I got that a typical region from where CMB is coming to us at present is 45.5 billion LY away (present distance)

and is currently receding from us at 3.3 times speed of light

and at the time it emitted the light it was receding from us at 56.95 time speed of light

see if you get the same results.

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Martin's right - that is amazing.

I think you can guess my next question - concerns cosmic speed limit violations!

I recognize that there might be a difference between an object's motion relative to another object and the expansion of space between two objects. I'm wondering if this accounts for the FTL recession of the cmb? That is, can an object (like the cmb) appear to be travelling FTL because the space between it and the observer is expanding FTL? The object wouldn't really be moving in that case since it is fixed to a particular point in space (as is the observer), and it's just that more and more "space points" get inserted between them.

Does this make any sense?

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...

Does this make any sense?

yes it certainly does.

at the moment I can't think how to say it better.

two things can be stationary (each sitting still in space)

but the distance between can be increasing at a certain rate

(called the recession speed) caused by the expansion of the space between them

in Gen Rel the distance function (the "metric") is dynamic. it changes.

fortunately only noticeably for very large distances

cosmologists make a distinction between two sorts of speed: recession speed and speed of motion (nothing can ever catch up to and pass a photon, that would be motion speed, it would be illegal to go faster than a photon, but very long distances can gradually get longer, leading in the case of very very long distances that they get longer at a rate that can be several times more than c)

I didnt have to rephrase, you said it right already

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yes it certainly does.

at the moment I can't think how to say it better.

two things can be stationary (each sitting still in space)

but the distance between can be increasing at a certain rate

(called the recession speed) caused by the expansion of the space between them

in Gen Rel the distance function (the "metric") is dynamic. it changes.

fortunately only noticeably for very large distances

cosmologists make a distinction between two sorts of speed: recession speed and speed of motion (nothing can ever catch up to and pass a photon, that would be motion speed, it would be illegal to go faster than a photon, but very long distances can gradually get longer, leading in the case of very very long distances that they get longer at a rate that can be several times more than c)

You may be seeing no problem with this type of violation of the C limit and of course presuming, as you seem to, in a isometric, spherical expansion this phenomenon would likely be the case, but a more limited pulse/jet model could very well avoid what I would see as a violation of C.

aguy2

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