Is it the increase of space-time between two objects or an actual increased (or increasing) distance between two objects?

I'm torn because in my mind Redshift is a way to measure the impact of space-time and not necessarily the Doppler effect of expanding distances between two objects.

The reason i feel this way is in its simplest form a Photon has no mass and thus travels from point a to b across the fabric of space-time and every curve thereof. Wouldn't the measurement of redshift from galaxies further out show the increasing impact of space-time (or increase thereof)?

At least if you apply it to space time you don't have to ignore gravity for the most part. I'm no cosmologist but reading up on Hubbles Constant gravity plays very little role and in fact it mentions that the Hubble Constant " does not account for cosmological effects like the curvature of space-time due to gravitation."

Obviously i don't have any math to back up what i feel but in a way it almost makes sense to let gravity win.

I put it in the ATM just because i didn't know if was appropriate to ask a question an insinuate why i feel the way i do without coming off as sounding like i'm against standard thinking.

Which way is it

Both. You're describing cosmological redshift when you talk about the expansion of space increasing the wavelength of light. You're describing Doppler redshift when you talk about light signals exchanged by objects that are moving apart because they have different velocities.
They're both valid ways of producing redshifts, and you need to think about both of them (as well as gravitational redshift) when you consider the redshift that occur in the light of distant astronomical objects, like galaxies and quasars.

Grant Hutchison

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Can anyone explain why a doppler redshift is the preferred explanation for astronomers?

Thanks for the tips.. i have a lot of learning to do here

I guess i'm trying to step back from the molecular hydrogen substitute for dark matter and find out how/why the Doppler red-shift is the preferred redshift.

Off to google i go

It isn't. Doppler is used for nearby objects in relative motion; cosmological is used for redshifts generated by the expansion of space. People who know what they're doing never confuse the two. But sometimes pop-sci sources loosely refer to the cosmological redshift as "Doppler", and treat it as if it can be converted directly to a relative velocity, rather than a change in scale factor. (The two work out approximately the same for cosmologically "nearby" objects, but diverge significantly at large redshifts.)

Grant Hutchison

You propose two options:

1. Increase in space-time
2. Increase in space

There's a third option:

3. Increase in time

Increase in time? Does that make any sense.

The solution is that the light traveling distances where Hubble redshift is observed under go a decrease in velocity.

Lower velocity = increase in duration of light's journey between galaxies separated by cosmological distances


velocity of a wave:
v = fw

f = 1 / t

v goes down, f goes down, and t goes up.
ight.

This is observed, and is predicted by a Monadic cosmology:

http://www.cloudmusiccompany.com/paper.htm


Hubble redshift doesn't indicate the expansion of space.

It indicates the limits of our current theories of l

It seems pretty clear from the OP that byronm wants to understand the current mainstream view of the cosmological redshift; which is that it does indeed arise from the expansion of space.

You're of course very welcome to your own view.

Grant Hutchison

I guess i'm trying to understand how a photon could measure velocity of an object with mass as an expansion of the universe if a photon itself contains no mass.

I mean theoretically gravity could be the lens that describes everything we see thus giving the appearance of expansion or contraction based upon the fluctuations of that gravitational lens.

Could you theoretically test this by say launching a spacecraft as far out and as fast as voyager towards a redshifted galaxy and then get a spectral analysis of the incoming photons at the currently velocity of the expansion and thus repeat the process until humanly possible to measure any increasing or decreasting velocity thereof relative to the observers point.

I mean technically if you achieve the expansion velocity and its still redshifted wouldn't that mean that the light you are observing is forever gravitationally shifted and entirely upon the frame of view vs redshifted because of a doppler/cosmological effect?

sorry if i'm running off on tangets.. i haven't researched the measured velocity of expansion so i'm taking a wild guess on how you would test the redshift thereof against a body moving it at the same speed/direction thereof.

OK , I can think about both.:-)

I think I know what byronm is asking about.

When we say Expansion of Space does it mean that you strech the existing space or you add some space in between (2 objects, an object and an observer)?
I asked this question some time ago here and nobody answered.
Now I know why. Nobody knows he he.
There is a debate running (source: arxiv) on Expansion of Space as obviously Space is presented as nothing in SRT (not sure about GRT :-), so, what is expanding actually?

To byronm:
Doppler redshift occurs when you have two objects moving with different velocities as Grant says, but with that we say that the objects have kinetic energy mv^2/2.
SRT and GRT do not allow v>c and if you would interpret the redshift as Doppler you will end up with the observation of v>c, which is contradictory.
It will ultimately imply that "something" kicked a galaxy or group of galaxy so hard that they achieved a velocity of near c, c or even more then c. No source of energy of that scale is known.
Except for Bing Bang.
The very first version (help with source needed, Gamow?) of Bing Bang stated just that.
In the beginning was there just a big ball of neutrons that exploded (?).
(OK, working hypothesis, we will not go in debth with that)
In that way the matter aquired the kinetic energy and hence redshift, which in the beginning was Doppler redshift. Later, the situation worsened as we could observe objects with z>1. With that, the idea of Doppler redshift as a cosmological redshift was burried deep, very deep. With that went the big ball of neutrons, too.
New equation(relativistic) was found to acoount for a redshift and the Expansion of Space was introduced as the cause of it.
But, as you can read on arxiv, 50 years later it's still not clear what is expanding.
The problem lies in that, if you apply some properties to the space you will contradict GRT,
promoting Space to Aether.
Someone will say, Space does not expand only distances between objects increase(?)(whatever that means)
OK, if I go with that, how do you achieve movement of galaxies?
Kinetic energy? No? (of course not)
Space is bearing the galaxies as in "Matter tells space how to curve, space tells matter how to move"? How nothing can communicate with anything? Are you saying that GRT is effectively an Aether theory ?(I don't have any problems with that).
I can imagine that gravity communicates to space via gravitational field and gravitons but I'm not aware that GRT and extensions say anything of another way around. If you introduce zero-point energy et al. , what are carriers of that energy? Is it Space or something filling the space?
Another thing.
Light leaving gravitational body appear to be redshifted to a distant observer.
But so does the light leaving "antigravitational body" i.g. bobble of expanding space which has curvature with oposite sign of that of the gravity?! Light is redshifted in both cases!


What's wrong with this thought experiment in the era of Dark Energy:
A bobble of space is expanding with accelerated rate (due to Dark Energy).
When we observe test photon leaving an object on the oposite side of the bobble (we are not gravitationaly bound, gravitational and Doppler redshift are negligable) we will see
that photon gets redshifted up to the centre of the bobble. After that, Dark Energy is actually pushing the photon towards us and the photon aquire blue shift. (c =const)
Netto shift should be : zero.

Similarely, is this correct?:
Beam of light strikes a mirror placed on a massive body.
All other causes for redshift are negligable except gravitational. We can observe the frequency of the light at the time of emission f_e and at the time of apsorption f_a.
We observe that f_e = f_a.

I think the reason no-one knows is that it is an ill-posed question: you're inserting an assumption that there's a thing called "space" which must behave in one of these two ways.
In fact, the phrase "space expands" is just a metaphor for the mathematics, which describe a set of coordinates expanding, so that points within the coordinate frame move apart as time goes by.

By way of analogy: suppose I take a "mind-expanding" drug. During my mind-expanding experience, does extra mind appear, or does my existing mind simply get bigger? Most people would feel that such a question makes no sense, since they recognize that in this context both "mind" and "expanding" are metaphors for something we don't properly understand. I think the analogy with "space expanding" is quite tight, apart from the fact that we us a mathematical theory to describe "space", and subjective knowledge to describe "mind".

Grant Hutchison

No, I don't think your analogy is in place. Why would scientists use metaphore?
No scientist working on drugs will use words "mind-expanding" while performing some experiment (on him/her selves for instance) and ceartanly not in a scientific journal.
Expansion of space causes a real effect - redshift. You can't back off in the realm of mathematics on this issue. It's like to say that Schrödingers cat causes neutron decay.

I don't expect that anyone says what space (or gravity or time..) is, but I do expect that someone put the finger on "Expansion of space" because it's a behaviour cruical to photons. They expirience it. We have cause and effect relationship here.
We have a new force: space-force (dark energy) acting on both, matter and radiation.
Naming it force is apropiate especially now when that force causes accelerated
(but collisionless!) movement of massive bodies.
One more interesting thing:
In the regime of accelerated expansion, is it properly accounted for a redshift?
Haven't think about it much, but sounds to me as it could be nonlinear realtionship. (which troubles (1+z)^4 )
(Isn't the gravitational redshift non-linear?)
An excersize (for Jerry I guess :-)) which result could end in love-hate relationship (yes, there are no time dilation and yes, SNIa data are correct :-)).

Because physics is metaphor. It's our current best way of telling ourselves about the Universe; it shouldn't be confused with what the Universe actually is.

No backing off is involved. Something certainly happens that causes cosmological redshift. Our best model of that "something" involves the mathematics of an expanding metric. End of story.
You don't need to ask many successive questions about any aspect of physics before you get to a point at which we have to say: "Well, here's the maths that matches the observations. That's all we know." Whether it's expanding space or quantum mechanics, our knowledge reduces to mathematical stories about reality, not reality itself.

Grant Hutchison

And as far as I understood it, cosmological redshift is an artefact of the expanding coordinates. Use non-expanding coordinates and all the observed redshift becomes Doppler, and super-luminally receding galaxies become sub-luminal. I'm still not 100% sure on that (despite learning it from Ned Wright's cosmology tutorial) because knowledgeable people like you always seem to shy away from agreeing with it

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I think this needs it's own thread.

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I guess, I understand it now.
The idea is based on an imaginary three dimensional spacetime. So, why three? The idea is to treat time as dependent on the three other dimensions. So time would be a real movement - in an imaginary space. This can't be seen at the place where we are, since that is the way we define ourselfs. At our spot, it's a movement in time.
Now think about the lightcone. Our past light cone is what we can see. Further away means something like nearer to the outside. In that imaginary space it would mean an angle increasing with distance. That angle makes out of our advance in time a movement in space.