Red Shifts and the expanding universe

[WendyE]

I have a question about the light observed from galaxies that are moving away from us due to the expansion of the universe. I understand that we can tell they are moving away because the light from these galaxies is red shifted. I was wondering how this red shift is generated, exactly. Is the light red shifted because the space that the light travels through is expanding? Or is it red shifted for the same reason that a sound source moving away from you is shifted to lower tones?

 

[GeorgeSol]

I'd like to know, too. There seems to be different models to explain it.
Some of the redshifting is caused by the Doppler effect, which, as you mentioned, is similar to sound shifting. This component is due to the motion of the stars as the move about their galaxy and, even more so, the motion of the galaxy about the c.g. of its glactic cluster.

But the more distant galaxies exhibit large redshifts and are mainly due to the expansion of space. Some say the light's wave gets stretched as space expands. I find this difficult to grasp, but it can be useful in mathematical models, apparently, which yield valid results.

 

[DavidBektas]

GeorgeSol is right, the main factor for the redshift varies with distance:

1. For small distances the photon only travels a relatively short time and therefore the redshift due to the expansion of space is small as well. The main factor in this case is the velocity of the object.

2. For large distances it´s the other way around: The redshift due to the expansion dominates the "common" Doppler-Effect due to the velocity.

Another cause for redshift is the gravitational redshift, but I don´t know if that has a considerable effect on the overall redshift (maybe for nearby, massive and relatively slow bodies?).

 

[Jorrie]

GeorgeSol is right, the main factor for the redshift varies with distance:

1. For small distances the photon only travels a relatively short time and therefore the redshift due to the expansion of space is small as well. The main factor in this case is the velocity of the object...

Take into account that at small distances, the spectral shifts are blue shift and red shift, i.e., fairly random motions. At larger distances there is only red shift. This supports the cosmological expansion as the cause of the red shift at cosmological distances.

 

[chronon]

From http://math.ucr.edu/home/baez/physics/Relativity/GR/hubble.html

Are the light waves "stretched" as the universe expands, or is the light doppler-shifted because distant galaxies are moving away from us?

In a word: yes.
In two sentences: the Doppler shift explanation is a linear approximation to the "stretched light" explanation. Switching from one viewpoint to the other amounts to a change of coordinate systems in (curved) spacetime.

See also http://www.astro.ucla.edu/~wright/cosmology_faq.html#MX

Are galaxies really moving away from us or is space just expanding?


This depends on how you measure things, or your choice of coordinates. In one view, the spatial positions of galaxies are changing, and this causes the redshift. In another view, the galaxies are at fixed coordinates, but the distance between fixed points increases with time, and this causes the redshift.

So the two ways of looking at things are essentially the same. Beware of distinctions between redshift due to movement and redshift due to expansion of space: they're the same thing.

My opinion is that the expanding space viewpoint is the wrong way of looking at things, see:
http://www.chronon.org/Articles/stretchyspace.html

 

[DavidBektas]

My opinion is that the expanding space viewpoint is the wrong way of looking at things, see:
http://www.chronon.org/Articles/stretchyspace.html

That would imply that the redshift is either due to gravitational or velocity redshift, which leads to a big problem for large distances.

If you interpret the redshift as the "common" Doppler-Shift, you would get velocities v>c for objects at large distances (plus, you would need to explain why there are no blueshifts). From Wiki:

"In very distant objects, v can be larger than c. This is not a violation of the special relativity however because a metric expansion is not associated with any physical object's velocity."

http://en.wikipedia.org/wiki/Hubble_expansion

I hope I didn´t misinterpret you, but I think the stretching of the photon due to the cosmic expansion is the only possible explanation for such enormous redshifts.

 

[chronon]

If you interpret the redshift as the "common" Doppler-Shift, you would get velocities v>c for objects at large distances (plus, you would need to explain why there are no blueshifts).

No, because you would be using a different coordinate system in which all velocities are <c, and you would use the special-relativistic formula for Doppler shifts.
See http://www.chronon.org/Articles/milne_cosmology.html

 

[Jorrie]

No, because you would be using a different coordinate system in which all velocities are <c, and you would use the special-relativistic formula for Doppler shifts.
See http://www.chronon.org/Articles/milne_cosmology.html

I'm under the impression that interpreting the red shift by means of special relativistic Doppler shift gives a different answer for things like the distance-red shift relationship than what the cosmological expansion view gives. Cosmological red shift simply says how much space has expanded since the light left the source. Doppler shift, in contrast, says how fast the relative velocity between us and the source was when the light was transmitted. Or do I have it wrong?

 

[chronon]

I'm under the impression that interpreting the red shift by means of special relativistic Doppler shift gives a different answer for things like the distance-red shift relationship than what the cosmological expansion view gives. Cosmological red shift simply says how much space has expanded since the light left the source. Doppler shift, in contrast, says how fast the relative velocity between us and the source was when the light was transmitted. Or do I have it wrong?

In the (0,0) case - that is an expanding universe without deceleration due to gravity or a cosmological constant - the two ways of looking at things are equivalent, so that you can consider the redshift as being due to the velocity of recession when the light was emitted, according to the rules of special relativity. If there is non-zero deceleration or acceleration then this will no longer the case - whatever is causing the deceleration or acceleration will also act on the light 'in flight'. However, the difference from the (0,0) case is very small (undetectable before the 1990's), which I don't consider sufficient to give up the view that the redshift is due to recession.

You might like to take a look at http://arxiv.org/abs/gr-qc/9303008 for further information on this matter

 

[Jorrie]

You might like to take a look at http://arxiv.org/abs/gr-qc/9303008 for further information on this matter

Thanks Chronon, I took a peak, but the idea of cosmological redshift taken as special relativistic Doppler shift still sounds awkward to me, because it implies movement through space. I agree that one can take a given cosmological redshift and say that it is the same as the redshift caused by some velocity v through flat spacetime, but IMO, that does not make it a valid interpretation of cosmological redshift.

 

[oldman]

A paper by Davis and Lineweaver may be useful in this thread. It is at http://arxiv.org/abs/astro-ph/0310808 , and the following extract from it gives the flavour of the matters that are discussed:

In special relativity, redshifts arise directly from velocities. It was this idea that led Hubble in 1929 to convert the redshifts of the “nebulae” he observed into velocities, and predict the expansion of the universe with the linear velocity-distance law that now bears his name. The general relativistic interpretation of the expansion interprets cosmological redshifts as an indication of velocity since the proper distance between comoving objects increases. However, the velocity is due to the rate of expansion of space, not movement through space, and therefore cannot be calculated with the special relativistic...

A general difficulty I have is that despite the numerous references made in cosmology to "expanding space", I haven't yet come across a believable definition of "space" itself, so I wish people would stop using a concept that is not defined.

Frustrated, I have great sympathy with the views Chronon expresses at: http://www.chronon.org/Articles/stretchyspace.html . I've also concocted my own definition of space, namely "Space is what you can swing a cat in". But I'd prefer something more scientific. Any offers?

 

[Chronos]

I still like the rubber sheet analogy. It is a neat way of portraying conservation of energy [energy as a fixed quantity, whereas the volume of space that contains it is variable]. You could tweak the math to conserve space, but energy [redshift] is more measurement friendly.

 

[SpaceTiger]

No, because you would be using a different coordinate system in which all velocities are <c, and you would use the special-relativistic formula for Doppler shifts.
See http://www.chronon.org/Articles/milne_cosmology.html

The Milne cosmology is an actual model of the universe, not just a different coordinate system. It hasn't been consistent with the data for quite some time...

 

[Garth]

The Milne cosmology is an actual model of the universe, not just a different coordinate system. It hasn't been consistent with the data for quite some time...

Some would disagree...
A Concordant “Freely Coasting” Cosmology.

Garth

 

[SpaceTiger]

Some would disagree...
A Concordant “Freely Coasting” Cosmology.

You disagree that the Milne cosmology is more than a change in coordinate system? I don't see what this has to do with your model, other than the fact that both are "coasting".

 

[Garth]

You disagree that the Milne cosmology is more than a change in coordinate system? I don't see what this has to do with your model, other than the fact that both are "coasting".

Sorry, I should not have included all your quote!
I was referring to your statement:

It hasn't been consistent with the data for quite some time...

and comparing it with the Freely Coasting Model (FCM)

I was not referring to my SCC model, which is different to the Milne model and the FCM of Kolb ( A coasting cosmology ) & the Indian team's interest.

For clarification: Milne is empty, FCM assumes the Milne linear expansion for empirical reasons, but with matter, however, it is not able to suggest a mechanism to deliver this, except Kolb's "K-matter" (DE??)).

Both Milne/FCM and SCC are "Freely Coasting", R(t) \sim t; however the Milne model, and the FCM, have k = -1, whereas SCC has k = +1.

They do coincide for BBN where the curvature term is not dominant, however the difference between FCM and SCC is SCC does not fit the Type Ia SN data so readily, therefore it requires these not to be standard candlers, on the other hand, the SCC model is conformally flat and therefore fits the WMAP data better.

I hope I have cleared up the misunderstanding.

Garth

 

[chronon]

The Milne cosmology is an actual model of the universe, not just a different coordinate system. It hasn't been consistent with the data for quite some time...

Indeed. My argument isn't that that is the way the universe is, but that it is not qualitatively different, i.e. it is close enough to serve as a useful approximation when discussing things like redshift.

In addition to this the Milne cosmology serves as a null hypothesis. Hence it is a useful exercise to see what would allow the data to fit this hypothesis, as is done in the paper quoted by Garth.

 

[SpaceTiger]

Indeed. My argument isn't that that is the way the universe is, but that it is not qualitatively different, i.e. it is close enough to serve as a useful approximation when discussing things like redshift.

The concept of the celestial sphere is useful for qualitative discussions of the night sky, but when people ask about what's actually going on, I don't tell them that the stars are pegged to a glass sphere!

Honestly, though, I can't think of any situation in which the Milne model is useful for conceptualizing the universe we live in. It may be useful for practicing the mathematics of cosmology or understanding how the field developed to its current state, but that's about it.

 

[Chronos]

I agree very much with ST's position. He is not advocating, or denouncing any view, just dealing with the bare facts and only ruling out the most basic bad theories - at least IMO. Scientists are slippery creatures who freely admit their errors within error bars [physics 101]. That assertion still puzzles me. . . and watching judge Judy did not help matters.

I still like Garth's model. He does not beat around the bush or hide behind skirts. He tells it like he sees it, and I respect that. I don't foresee Garth making any excuses if GPB falls short of his predictions. That is the mark of a true scientist, IMO.

 

[ValenceE]

Hello to all,

Again I must state that I'm very much a layman who's always been marveling at our magnificent universe, trying to comprehend what I can about its majesty and my own relationship with it.

I don't always have the scientific knowledge to validate thoughts, ideas and perhaps 'revelations' that come to mind, but I hope, actually I'm sure, that the different forums and all it's participants will help greatly in this quest...

Now here’s an attempt that might just go down in flames, but it’s consequent with what I just said…

Could the red shifts be dependant on both our Earth’s position in our own multi rotational reference frame coupled with the far away galaxies or light sources in their own rotational frame, both moving in opposite directions ?

The velocity of light remains c but we would be moving away from the source as it is moving away from us in our respective rotational frames thus creating a perceived red shift… not necessarily because of expansion.


Does this make any sense?




VE

 

[fgosborn]

Hello ValenceE, I am new to this forum. I have been reading about Halton Arp and his theories about red shifts, if you do a search on him and also the electric universe model you might find it interesting. I was looking for some discussion on these models, but have yet to find any.

 

[SpaceTiger]

Hello ValenceE, I am new to this forum. I have been reading about Halton Arp and his theories about red shifts, if you do a search on him and also the electric universe model you might find it interesting. I was looking for some discussion on these models, but have yet to find any.

You won't find much discussion of these models on PF, as they have long since been dismissed by mainstream science.

 

[fgosborn]

You won't find much discussion of these models on PF, as they have long since been dismissed by mainstream science.

Whats PF? Is that in reference to Halton Arp or Electric Universe (sorry, not the sharpest tool in the shed)?

 

[SpaceTiger]

Whats PF?

Physics Forums.

Yes, I was referring to both of those models.

 

[fgosborn]

Electric Universe

<<

Physics Forums. >>

<<Yes, I was referring to both of those models.

>>

Thanks, I'm probably missing something obvious then, but;

I’ve read material that plasma makes up 99% of all matter in the universe. That plasma remains electrically charged in space. That it isn’t a perfect conductor so the magnetic fields are not frozen or locked (have only a vague idea what that means).
If I understand the issue, it’s that modern Astronomy accepts a gravity-dominated universe. But it’s based on the assumption that matter is electrically neutral. So gravity would dominate. But if this is not so, then gravity must be a secondary influence. If electromagtic fields dominate, driven by Birkland currents and Z pinch effects, there are would be no missing mass problems or black holes, neutron stars, dark matter ect.

In Halton Arp’s photographs he shows high red shift quasars connected to low red shift galaxies

 

[SpaceTiger]

That plasma remains electrically charged in space.

The presence of positive and negative charges in electromagnetism causes large-scale plasmas to indeed be neutral, rendering electrostatic forces insigificant for driving the motions of celestial bodies (particularly stars, planets, etc.). Magnetic fields, however, are still important driving forces in the interstellar and intergalactic media.

In Halton Arp’s photographs he shows high red shift quasars connected to low red shift galaxies

It was long ago shown that there is no excess of high-redshift quasars around low-redshift galaxies. The photographs you saw were just chance alignments.

 

[Mike2]

I have to wonder how much of the redshift is due to photons from earlier times having to climb out of a deeper gravity well because the universe was denser when they were emitted. What's this effect called again?

 

[SpaceTiger]

I just received a book in the mail by Donald E. Scott, The Electric Sky and he discusses gravitational lensing and Einstein’s Cross and how that the four quasars that surround the galactic core are one gravitationally lensed distant quasar located at a far distance (based on red shift). But the idea is based on perfect alignment of Earth the galaxy core and the distant quasar. It was pointed out that only two images should be present. For four images it would require four objects in perfect alignment; the Earth the galaxy the two distant quasars.

The author seems to be stuck on the simple point mass models of gravitational lensing. In more complex potentials, near-perfect alignment is not necessarily required to produce four images. As I already said, fgosborn, this is not the place to discuss either the electric universe or Halton Arp's models. Please take such discussions elsewhere on the web.

I have to wonder how much of the redshift is due to photons from earlier times having to climb out of a deeper gravity well because the universe was denser when they were emitted. What's this effect called again?

I'm not aware of any such effect. It would be very small, if it even existed.

 

[fgosborn]

Please take such discussions elsewhere on the web.


I searched the archives and found this link posted:

http://www.tim-thompson.com/grey-areas.html

Wish I had done that before I ordered the book. I think I'll refocus my studies.

 

[hellfire]

I have to wonder how much of the redshift is due to photons from earlier times having to climb out of a deeper gravity well because the universe was denser when they were emitted. What's this effect called again?

In a denser past the universe was also homogeneous and isotropic on large scales and therefore the photons were not forced to climb out of any potential wells against any special direction. Local inhomogeneities, however, may lead to a redshift or blueshift of photons emitted beyond them.

 

[Mike2]

In a denser past the universe was also homogeneous and isotropic on large scales and therefore the photons were not forced to climb out of any potential wells against any special direction. Local inhomogeneities, however, may lead to a redshift or blueshift of photons emitted beyond them.

Yes, I think we're talking about the Sachs-wolfe effect described at:

http://en.wikipedia.org/wiki/Integrated_Sachs_Wolfe_effect

I'm not so sure. Wouldn't a photon climbing out of a less dense region be redshifted less than one climbing out from near the surface of a black hole with the same mass? So if the redshift depends on density, than wouldn't the expansion cause a less of redshift with time? If a photon looses energy because by the time it leaves a galaxy cluster that cluster has become more compact, then wouldn't there be a similar effect from the average density decreasing with time?

As an after thought, is it because the galaxies are not gravitationally bound their increasing distance does not have gravitational effects of tranversing photons?

 

[hellfire]

I'm not so sure. Wouldn't a photon climbing out of a less dense region be redshifted less than one climbing out from near the surface of a black hole with the same mass?

Yes, the amount of gravitational redshift depends not only on the mass but also on the distance (to the mass) from which the photon is emitted.

If a photon looses energy because by the time it leaves a galaxy cluster that cluster has become more compact, then wouldn't there be a similar effect from the average density decreasing with time?

No, as far as I know. Both scenarios are different. For gravitational redshift to take place there must be a potential well. The photon climbs out of it to the region where the gravitational interaction is weaker getting redshifted. In a perfectly homogeneous and isotropic universe there is no such a situation.

 

[Mike2]

Yes, the amount of gravitational redshift depends not only on the mass but also on the distance (to the mass) from which the photon is emitted.


No, as far as I know. Both scenarios are different. For gravitational redshift to take place there must be a potential well. The photon climbs out of it to the region where the gravitational interaction is weaker getting redshifted. In a perfectly homogeneous and isotropic universe there is no such a situation.

What I guess I'm wondering about is doesn't the gravitational well get deeper for more dense homogeneous isotropic distributions? Isn't a photon in the middle of a very dense distribution of dust in a deeper well then such a fine distribution of dust? What exactly is the equation for the gravitational well for an even distribution, and how does it change with lighter density?

 

[Mike2]

What I guess I'm wondering about is doesn't the gravitational well get deeper for more dense homogeneous isotropic distributions? Isn't a photon in the middle of a very dense distribution of dust in a deeper well then such a fine distribution of dust? What exactly is the equation for the gravitational well for an even distribution, and how does it change with lighter density?

The gravitational potential, U, can be calculated at any point, \[<br /> {\rm{\vec r}}<br /> \], for a mass density distribution, \[<br /> {\rm{\rho (r)}}<br /> \], using the formula:

\[<br /> {\rm{U = - }}\int_{{\rm{all space}}} {G\frac{{{\rm{\rho }}({\rm{\vec r&#039;)}}}}{{\left| {{\rm{ \vec r - \vec r&#039; }}} \right|}}} \,\,\,d^3 {\rm{r&#039;}}<br /> \].

See:
http://scienceworld.wolfram.com/physics/GravitationalPotential.html

Integrating over the same region with a lesser fixed desity means the potential is less. And photons would be blue shifted as the density is decreased. But since things are not static, galaxies are moving apart, I wonder how that would affect the calculation. I suppose you'd have to integrate over time as well from when the photon was emitted to when it was received. Perhaps the region of integration would increase with expansion, or maybe the region remains out to the event horizon for a given time. But since even the most distant galaxies are still within our view, I suppose we would have to feel their gravity as well.

 

[hellfire]

My understanding of gravitational redshift is based on the derivation that is usually done based on the Schwarzschild geometry, so I may be missing something in your argument. But even the SW and ISW effects are derived in this way, making use of the Newtonian approximation. This means that for (this kind of) redshift to take place you have always a inhomogeneous distribution of energy density in the line of sight. Consider a photon emitted from x_0 and traveling on a direction x in an homogeneous and isostropic expanding space. There will be never an inhomogeneous distribution of energy density in the line of sight and thus it is not possible to find any region so that the photon may "feel" any kind of attraction during its journey from x_0, along x to the observer. It seams to me that you claim that the photon "feels" the attraction of the energy density that existed in past, but I fail to make any sense of this.

 

[Mike2]

My understanding of gravitational redshift is based on the derivation that is usually done based on the Schwarzschild geometry, so I may be missing something in your argument. But even the SW and ISW effects are derived in this way, making use of the Newtonian approximation. This means that for (this kind of) redshift to take place you have always a inhomogeneous distribution of energy density in the line of sight. Consider a photon emitted from x_0 and traveling on a direction x in an homogeneous and isostropic expanding space. There will be never an inhomogeneous distribution of energy density in the line of sight and thus it is not possible to find any region so that the photon may "feel" any kind of attraction during its journey from x_0, along x to the observer. It seams to me that you claim that the photon "feels" the attraction of the energy density that existed in past, but I fail to make any sense of this.

I don't know. It seems obvious to me. Given a potential


\[<br /> {\rm{U = - }}\int_{{\rm{all space}}} {G\frac{{{\rm{\rho }}({\rm{\vec r&#039;)}}}}{{\left| {{\rm{ \vec r - \vec r&#039; }}} \right|}}} \,\,\,d^3 {\vec{r&#039;}}<br /> \].

if the density decreases so does the potential. A homogeneous, isotropic universe only means that rho is constant over space. But if rho decreases with time, then the potential that a test particle (or photon) would feel will also decrease with time. And a photon feeling less of a potential will be blueshifted compared to the prior potential that it felt. There might be some complications when trying to apply this to an expanding universe, but I think just this much would indicate that it should be considered, right?

I think that the usual GR derivation assumes a none interacting dust, but I'm asking what happens if the dust has a gravitation interation.

 

[hellfire]

A homogeneous, isotropic universe only means that rho is constant over space. But if rho decreases with time, then the potential that a test particle (or photon) would feel will also decrease with time.

It does not "feel" any potential because it is immersed in a homogeneous and isotropic distribution of matter.

 

[Mike2]

It does not "feel" any potential because it is immersed in a homogeneous and isotropic distribution of matter.

I suppose my question is: why does the integral in post 36 not apply? Or why is it constant for temperal change in the spatially invariant mass density? If this is a new consideration that has not been mathematically proven irrelevant, then it has the potential to remove the cosmological constant from the supernova data. For a correction of redshift due to changing potential energy might offset the acceleration we see in the supernova data.

 

[Mad_Morlock]

Big questions.

1. Why is Virgo the only cluster of galaxies massively blue-shifted?

2. Why are galaxies in the Draco cluster supermassively redshifted?

3. Does the metric expansion of space imply a decrease or implosion across a 4th dimension?

 

[Chris Hillman]

Milne universe distinct from Minkowski vacuum?

Hi, spacetiger,

I am also puzzled by your assertion that "the Milne cosmology is an actual model of the universe, not just a different coordinate system", probably because I don't fully understand what you have in mind when you say "model of the universe".

My own view (which is the mainstream view, at least in classical gravitation) is that a cosmological model is a spacetime model (Lorentzian manifold) together with some tensor fields (spinor fields, whatever) describing some nongravitational physics. Strictly speaking, such a model should in some sense should be consistent with the idea that it is describing very large scale physical phenomena, possibly in a highly idealized fashion.

To fix ideas: a familar example of a cosmological model in gtr, in the sense I have in mind, would be the FRW dust solution with E^3 hyperslices orthogonal to the world lines of the dust particle (in MTW, a marginally open matter-dominated zero Lambda FRW model), which is a Lorentzian universe equipped with a tensor field describing a (pressureless) perfect fluid. But as this example illustrates, in fact it often suffices to give the metric tensor in some coordinate chart, adding only that we consider this spacetime model a perfect fluid solution in gtr, since the stress-energy tensor of our perfect fluid and the world lines of the fluid particles can then be obtained from the EFE, by computing the Einstein tensor directly from the given metric (which also shows that the pressure measured by observers flowing with the fluid is zero).

In more complicated situations, of course, simply specifying that we consider a given Lorentzian manifold to be a solution in gtr of a given type (e.g., perfect fluid plus Lambda plus source-free EM field plus a minimally coupled massless scalar field) might not be quite enough to deduce the intended nongravitational physics from the EFE alone. In any case, whatever deductions are possible are most conveneniently discovered by employing an appropriate frame field (called an "anholonomic basis" in MTW).

From this viewpoint, it seems to me, it is natural to consider the Milne model to be nothing but a certain vacuum solution (the Minkowski vacuum) equipped with a timelike congruence (which can be naturally if not quite uniquely extended to a frame field corresponding to a family of inertial non-spinning observers) which we consider to model the world lines of galaxies. But in this cosmological model, galaxies are treated (in gtr) as test particles, unlike the FRW models and more interesting models, in which they produce a nonvanishing gravitational field. If you drop the somewhat dubious assertion that a particular family of test particles (timelike congruence) is distinguished, in gtr the Milne model does then reduce to the "Milne chart" for the Minkowksi vacuum solution.

Maybe the distinguished timelike congruence is the additional structure you had in mind?

 

[Chronos]

But isn't the Milne model an oversimplification designed to test other hypotheses? In my mind, Milne is the poor man's null hypothesis in an N simulation.

 

[SpaceTiger]

From this viewpoint, it seems to me, it is natural to consider the Milne model to be nothing but a certain vacuum solution (the Minkowski vacuum) equipped with a timelike congruence (which can be naturally if not quite uniquely extended to a frame field corresponding to a family of inertial non-spinning observers) which we consider to model the world lines of galaxies. But in this cosmological model, galaxies are treated (in gtr) as test particles, unlike the FRW models and more interesting models, in which they produce a nonvanishing gravitational field.

All I'm saying is that you can't simply take an FRW model and change the coordinate system to get the Milne cosmology and, thus, cannot interpret redshift as a doppler shift. A Minkowski vacuum with timelike congruence is not consistent with the data. Even further, to treat the galaxies as non-gravitating test particles requires new physics (or, at least, the rejection of old physics), since these galaxies have been independently measured to have cosmologically significant masses.

 

[heusdens]

I hope I didn´t misinterpret you, but I think the stretching of the photon due to the cosmic expansion is the only possible explanation for such enormous redshifts.

There are of course other possible candidates for explaining redshift-distance correlation. For instance, the light on it's long way (billions of years) could interact with interstellar atoms, ions, molecules, dust or other particles/material objects, and get redshifted because of that.
That none of these have turned out to be right is another question.

 

[Pavel]

Hi, sorry to bring this thread back, but this is a very interesting topic. I read some references and I think ended up more confused than enlightened. So, if you could answer just one simple thought-experiment question to me, I'll be clear and grateful.

So, I have a star and the Earth in the same reference frame - no motion relative to each other. The star emits a photon in the direction of the Earth. The photon has frequency x. ( I assume a single photon has frequency. After all, its energy level is defined by e=hv, right?) Now, some time during its journey to the earth, space between the star and the Earth starts expanding. Then the expansion stops while the photon is still travelling! Now the star and the Earth in the same frame again. The photon finally reaches the Earth. My question is, does the photon have the same frequency x when it arrived or did it get redshifted because there was a period of expansion on its journey.

If the answer is the same x, things would get pretty hot right now if the expansion of the universe stops (despite the fact that radiation got deluded), wouldn't they?

In advance, thanks!

Pavel

 

[Mike2]

So, I have a star and the Earth in the same reference frame - no motion relative to each other. The star emits a photon in the direction of the Earth. The photon has frequency x. ( I assume a single photon has frequency. After all, its energy level is defined by e=hv, right?) Now, some time during its journey to the earth, space between the star and the Earth starts expanding. Then the expansion stops while the photon is still travelling! Now the star and the Earth in the same frame again. The photon finally reaches the Earth. My question is, does the photon have the same frequency x when it arrived or did it get redshifted because there was a period of expansion on its journey.

The photon frequency would not change. The change in frequency is due to the Dopler effect, due to the preceived velocity. With initially no recession velocity, then some, then none again, by the time the photon got here, the stretching due to velocity would be compensated for. It would be as though the photon was first accelerated away from us, and then accelerated towards us. The final distance to the star would have changed, but not its velocity at the time of reception.

If the answer is the same x, things would get pretty hot right now if the expansion of the universe stops (despite the fact that radiation got deluded), wouldn't they?

I don't think things would get hotter because the density has been reduced quite a lot since it initial start of expansion.

 

[Pavel]

I don't think things would get hotter because the density has been reduced quite a lot since it initial start of expansion.

Well, if the frequency of the photon is the same when emitted and received, then it follows that all the light that we receive today would be of higher energy, some much higher, if the Universe would stop expanding today. What would happen to the CMBR? Also, what about Olbers's Paradox? Isn't [pretty much] the only solution to the paradox the expansion of the Universe? If the expansion stops, the sky would be lit, wouldn't it?

That's what makes me think things would get hot indeed. But I'll go with the experts' opinions.

Pavel.

 

[hellfire]

Cosmological redshift is not due to Doppler effect (relative motions), but due to stretching of space (change of geometry). I think that the photon in your Gedankenexperiment would get redshifted.

 

[Jorrie]

We won't fry

Well, if the frequency of the photon is the same when emitted and received, then it follows that all the light that we receive today would be of higher energy, some much higher, if the Universe would stop expanding today.
Pavel.

I think Hellfire has effectively answered the questions indirectly, but to make it clearer, think about cosmological redshift as equivalent to the ratio (or factor) that the universe has expanded by between the time the photon has left the source and the time the photon reaches the observer. The wavelength of the photon has been stretched and stays stretched. Hence, if the universe would hypothetically stop expanding, the redshifts we measure will not change, neither will we fry.

Actually, loosely speaking, one can say that the cosmological redshift on its own does not really show that the universe is expanding, only that it has expanded!

Jorrie