5 ms·

How do we know that galaxies are accelerating away from us and not moving at constant speed? There's a more basic question: How do we know the galaxies are mov

by narag 3y ago

How do we know that galaxies are accelerating away from us and not moving at constant speed? There's a more basic question: How do we know the galaxies are moving? It seems (I haven't seen any other response, like... ever) that we have one and only one way to measure the speed of galaxies: the red shift. It's impossible to triangulate those huge distances and the time scale would also be a barrier, so no way of confirming the red shift calculations with a different method. That means that if the red shift was caused by any other effect, say the light "degrading" after millions of years of travelling the void, all the calculations would be invalid. I've asked about this many times and the answers are in the line of "we don't know any other reason for the light to red shift" and "the current theoretical frame is consistent", even if there isn't any other measure to be consistent with. There was a prediction (expansion is related to Big Bang) that the far away galaxies, being younger, would have a different composition. This prediction seems to be failing, but advances in instrumental could give us a more precise answer in the future.

k7sune 3y ago
“Degrading” sounds very intuitive to me. Can the frequency of the light waves simply slow down over a very long distance/time? Or maybe the speed of light simply slows down over an unimaginably long distance? We don’t have any model to describe such behavior, but everyday objects around us all slow down one way or another, what makes light so different?
- Sprocklem 3y ago
  IIRC, this was one of the explanations proposed when the existence of a red shift was first noted: that the light is somehow slowly losing its energy over very long distances, becoming “redder” as it did so. It ultimately lost out to the dark energy / space-time expansion theory, although I do not recall why. Presumably there was some observation that precluded “degrading” light from being the sole explanation.
  - hnfong 3y ago
    https://en.wikipedia.org/wiki/Tired_light https://en.wikipedia.org/wiki/Tired_light
  - semi-extrinsic 3y ago
    There are several challenges for "tired light", or indeed any theory that's an alternative to expansion of the universe. The theory has to explain why the light gets redshifted, but does not get blurred, and the spectral lines do not get broadened. This severely restricts the type of interactions possible. Also the theory has to explain the consistency between redshifts within our own galaxy, to that of far-away galaxies.
    - pigpang 3y ago
      My theory is that redshifting caused by gravitational background noise. I did calculation somewhere on HN or Youtube few months ago and numbers are of same magnitude.
    - deleted 3y ago
      [deleted]
    - thehappypm 3y ago
      Tired light is tricky to be possible because light doesn’t experience time or distance since it travels at c. Light itself from its frame of reference is emitted and received simultaneously. That’s not to say other perspectives can have different views though!
javajosh 3y ago
>It's impossible to triangulate those huge distances Galaxies are BIG. Andromeda is faint, but the same angular size as the moon. It's 2.5Mly away, but it's also 150kly across. Over a long enough time line you could do triangulation on it. In fact it's moving toward our galaxy, but very, very slowly compared with its diameter at 110kps. But yeah, in theory you could do triangulation on it over a very long period of time.
- mr_toad 3y ago
  For triangulation to work you need to move, not your target. Triangulation is only used for objects within about 1000 parsecs, where we can triangulate using the movement of the Earth along its orbit.
__turbobrew__ 3y ago
Degrading light sounds like an interesting idea to me, you could call it the “cosmological damper” if you will. Thought experiment: imagine you have light particles in a perfectly circular orbit around a black hole. Does the light ever fall into the black hole or does it orbit for eternity?
- oneshtein 3y ago
  There a whole set of theories, called "Tired light" theories, which tries to explain Cosmological Red Shift by degradation of photons with time. Buy they require whole set of different cosmological principles: a medium for light propagation is required to dump lost energy into, no Big Bang. But even with a tired light theory, galaxies are accelerating toward attractors, see https://arxiv.org/pdf/1702.02483v1.pdf https://arxiv.org/pdf/1702.02483v1.pdf https://www.youtube.com/watch?v=NpV0GQo3P0c https://www.youtube.com/watch?v=NpV0GQo3P0c
- thehappypm 3y ago
  Can photons collide? My understanding is they can’t. If they could, Hawking radiation would interfere with the eternal orbit.
  - raattgift 3y ago
    > Can photons collide? Yes, at high energies and with electrons or atomic nuclei nearby. https://en.wikipedia.org/wiki/Two-photon_physics https://en.wikipedia.org/wiki/Two-photon_physics > Hawking radiation would interfere with the eternal orbit For tractability Hawking radiation tends to be studied using a non-interacting scalar field (often massless), rather than the fields of the Standard Model (which are very much interacting, and have a variety of masses). Hawking started that in his "Black hole explosions?" (1974) which (if you're not allergic to sci-hub) you can read at <https://sci-hub.se/https://www.nature.com/articles/248030a0 https://sci-hub.se/https://www.nature.com/articles/248030a0>: "consider (for simplicity) a massless Hermitean scalar field \phi which obeys the covariant wave equation ... in an asymptotically flat space time containing a star which collapses to produce a black hole.". In the short paper, he's very much not considering photons or pions, and really doesn't need to in order to make his point. The thermal spectrum of a real black hole will be in all the Standard Model fields, and except very late in the eventual evaporation will be dominated by very low energy photons originating at a fair distance outside the event horizon. What do you mean by "eternal orbit"? Real black holes do not extend to the infinite past. If a black hole ultimately Hawking-evaporates, in finite time there's nothing left to orbit. Late in the evaporation, there will also be a lot of very high energy gamma rays and heavier particles excited in their respective quantum fields of the Standard Model. These will all tend to participate in high-energy multi-particle interactions rather more complicatedly than the end result of Hawking's 1974 description.
garrettgarcia 3y ago
The American astronomer, Halton Arp, had a theory he called "intrinsic redshift". My limited understanding is that he saw evidence of "very close" and "very far away" structures that are connected to each other in space, which makes no sense. He theorized that redshift may also be indicative of the age of a galaxy, rather than only indicating velocity. The interpretation of redshift as velocity is also the primary reason cosmologists think the universe is expanding.
- the-mitr 3y ago
  His book Seeing Red which raises several interesting and troublesome questions for the standard big bang model is worth a read.
at_a_remove 3y ago
You're not going to get a simple answer because the answer is quite complex. Astronomy is the paleontology of photons. You should take an astronomy course if you really want to know, but essentially a "ladder" was built of distances, starting with the very near and slowly building outward using various techniques and discoveries of physics as they became available. This is called the cosmic distance ladder. You start with stellar parallax, then after that you go farther with "standard candles" (particular types of variable stars). But then you have to get even further out, where you can no longer see an individual star, and then you rely on specific breeds of supernovae. Only then do you get to redshift, and compounding tons of data from step three seems to verify the redshift estimates. By the time you get to the Hubble constant, it was a huge rift between two communities over what was still a factor of two difference. It's quite fascinating, but I can't really dump out an entire book into a comment.
- deleted 3y ago
  [deleted]
- codethief 3y ago
  > It's quite fascinating, but I can't really dump out an entire book into a comment. Speaking of which, do you happen to know a good book which illuminates this whole story step by step? I have yet to find one which doesn't start with "Let us assume an FLRW metric…"
ganzuul 3y ago
> "we don't know any other reason for the light to red shift" https://en.wikipedia.org/wiki/G%C3%B6del_metric https://en.wikipedia.org/wiki/G%C3%B6del_metric There are versions of this which do provide another reason for red shift. Personally I keep this in mind as a means to free my thinking from a single narrative.
mr_toad 3y ago
> It's impossible to triangulate those huge distances https://en.wikipedia.org/wiki/Cosmic_distance_ladder https://en.wikipedia.org/wiki/Cosmic_distance_ladder
- narag 3y ago
  Funny, because that article says exactly the same. "Short" distances are calculated by parallax, taking Earth's orbit diameter as the base. From there, it's a extrapolation of the spectrum of known types of objects. Not triangulation. Moreover, IIRC individual stars can't be discerned in other galaxies, even the nearest ones, except maybe supernovas. So at the distances that red shift manifests, no triangulation, only red shift.

by narag 3y ago

k7sune 3y ago

“Degrading” sounds very intuitive to me. Can the frequency of the light waves simply slow down over a very long distance/time? Or maybe the speed of light simply slows down over an unimaginably long distance? We don’t have any model to describe such behavior, but everyday objects around us all slow down one way or another, what makes light so different?

Sprocklem 3y ago
IIRC, this was one of the explanations proposed when the existence of a red shift was first noted: that the light is somehow slowly losing its energy over very long distances, becoming “redder” as it did so. It ultimately lost out to the dark energy / space-time expansion theory, although I do not recall why. Presumably there was some observation that precluded “degrading” light from being the sole explanation.
- hnfong 3y ago
  https://en.wikipedia.org/wiki/Tired_light https://en.wikipedia.org/wiki/Tired_light
- semi-extrinsic 3y ago
  There are several challenges for "tired light", or indeed any theory that's an alternative to expansion of the universe. The theory has to explain why the light gets redshifted, but does not get blurred, and the spectral lines do not get broadened. This severely restricts the type of interactions possible. Also the theory has to explain the consistency between redshifts within our own galaxy, to that of far-away galaxies.
  - pigpang 3y ago
    My theory is that redshifting caused by gravitational background noise. I did calculation somewhere on HN or Youtube few months ago and numbers are of same magnitude.
  - deleted 3y ago
    [deleted]
  - thehappypm 3y ago
    Tired light is tricky to be possible because light doesn’t experience time or distance since it travels at c. Light itself from its frame of reference is emitted and received simultaneously. That’s not to say other perspectives can have different views though!

javajosh 3y ago

>It's impossible to triangulate those huge distances Galaxies are BIG. Andromeda is faint, but the same angular size as the moon. It's 2.5Mly away, but it's also 150kly across. Over a long enough time line you could do triangulation on it. In fact it's moving toward our galaxy, but very, very slowly compared with its diameter at 110kps. But yeah, in theory you could do triangulation on it over a very long period of time.

mr_toad 3y ago
For triangulation to work you need to move, not your target. Triangulation is only used for objects within about 1000 parsecs, where we can triangulate using the movement of the Earth along its orbit.

__turbobrew__ 3y ago

Degrading light sounds like an interesting idea to me, you could call it the “cosmological damper” if you will. Thought experiment: imagine you have light particles in a perfectly circular orbit around a black hole. Does the light ever fall into the black hole or does it orbit for eternity?

oneshtein 3y ago
There a whole set of theories, called "Tired light" theories, which tries to explain Cosmological Red Shift by degradation of photons with time. Buy they require whole set of different cosmological principles: a medium for light propagation is required to dump lost energy into, no Big Bang. But even with a tired light theory, galaxies are accelerating toward attractors, see https://arxiv.org/pdf/1702.02483v1.pdf https://arxiv.org/pdf/1702.02483v1.pdf https://www.youtube.com/watch?v=NpV0GQo3P0c https://www.youtube.com/watch?v=NpV0GQo3P0c
thehappypm 3y ago
Can photons collide? My understanding is they can’t. If they could, Hawking radiation would interfere with the eternal orbit.
- raattgift 3y ago
  > Can photons collide? Yes, at high energies and with electrons or atomic nuclei nearby. https://en.wikipedia.org/wiki/Two-photon_physics https://en.wikipedia.org/wiki/Two-photon_physics > Hawking radiation would interfere with the eternal orbit For tractability Hawking radiation tends to be studied using a non-interacting scalar field (often massless), rather than the fields of the Standard Model (which are very much interacting, and have a variety of masses). Hawking started that in his "Black hole explosions?" (1974) which (if you're not allergic to sci-hub) you can read at <https://sci-hub.se/https://www.nature.com/articles/248030a0 https://sci-hub.se/https://www.nature.com/articles/248030a0>: "consider (for simplicity) a massless Hermitean scalar field \phi which obeys the covariant wave equation ... in an asymptotically flat space time containing a star which collapses to produce a black hole.". In the short paper, he's very much not considering photons or pions, and really doesn't need to in order to make his point. The thermal spectrum of a real black hole will be in all the Standard Model fields, and except very late in the eventual evaporation will be dominated by very low energy photons originating at a fair distance outside the event horizon. What do you mean by "eternal orbit"? Real black holes do not extend to the infinite past. If a black hole ultimately Hawking-evaporates, in finite time there's nothing left to orbit. Late in the evaporation, there will also be a lot of very high energy gamma rays and heavier particles excited in their respective quantum fields of the Standard Model. These will all tend to participate in high-energy multi-particle interactions rather more complicatedly than the end result of Hawking's 1974 description.

garrettgarcia 3y ago

The American astronomer, Halton Arp, had a theory he called "intrinsic redshift". My limited understanding is that he saw evidence of "very close" and "very far away" structures that are connected to each other in space, which makes no sense. He theorized that redshift may also be indicative of the age of a galaxy, rather than only indicating velocity. The interpretation of redshift as velocity is also the primary reason cosmologists think the universe is expanding.

the-mitr 3y ago
His book Seeing Red which raises several interesting and troublesome questions for the standard big bang model is worth a read.

at_a_remove 3y ago

You're not going to get a simple answer because the answer is quite complex. Astronomy is the paleontology of photons. You should take an astronomy course if you really want to know, but essentially a "ladder" was built of distances, starting with the very near and slowly building outward using various techniques and discoveries of physics as they became available. This is called the cosmic distance ladder. You start with stellar parallax, then after that you go farther with "standard candles" (particular types of variable stars). But then you have to get even further out, where you can no longer see an individual star, and then you rely on specific breeds of supernovae. Only then do you get to redshift, and compounding tons of data from step three seems to verify the redshift estimates. By the time you get to the Hubble constant, it was a huge rift between two communities over what was still a factor of two difference. It's quite fascinating, but I can't really dump out an entire book into a comment.

deleted 3y ago
[deleted]
codethief 3y ago
> It's quite fascinating, but I can't really dump out an entire book into a comment. Speaking of which, do you happen to know a good book which illuminates this whole story step by step? I have yet to find one which doesn't start with "Let us assume an FLRW metric…"

ganzuul 3y ago

> "we don't know any other reason for the light to red shift" https://en.wikipedia.org/wiki/G%C3%B6del_metric https://en.wikipedia.org/wiki/G%C3%B6del_metric There are versions of this which do provide another reason for red shift. Personally I keep this in mind as a means to free my thinking from a single narrative.

mr_toad 3y ago

> It's impossible to triangulate those huge distances https://en.wikipedia.org/wiki/Cosmic_distance_ladder https://en.wikipedia.org/wiki/Cosmic_distance_ladder

narag 3y ago
Funny, because that article says exactly the same. "Short" distances are calculated by parallax, taking Earth's orbit diameter as the base. From there, it's a extrapolation of the spectrum of known types of objects. Not triangulation. Moreover, IIRC individual stars can't be discerned in other galaxies, even the nearest ones, except maybe supernovas. So at the distances that red shift manifests, no triangulation, only red shift.