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I've asked this question before, but I don't think I received a good answer, so figured I'd try asking again. How do we know that galaxies are accelerating awa

I've asked this question before, but I don't think I received a good answer, so figured I'd try asking again. How do we know that galaxies are accelerating away from us and not moving at constant speed? People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration. However, wouldn't we expect to see the same observation even without any acceleration? Imagine there are some objects in space all moving in random directions and speeds, relative to Earth. After long enough time, all objects will appear to be moving away from Earth, even if they were moving towards it initially. And after long enough time, the objects that move fastest should be farthest away, by the simple definition of speed! In short, even if galaxies weren't accelerating, we would still see that the further away a galaxy is, the faster it recedes.

runeb 3y ago
Their light is more red shifted the farther away they are. I'm no expert on this, but I believe in a constant-speed scenario they would have equal red shift no matter the distance
- willis936 3y ago
  The assumption made here is that relative velocity is the only method that would redshift light. Gravitational redshift is a thing and our model of gravity is incomplete.
  - pdonis 3y ago
    > The assumption made here is that relative velocity is the only method that would redshift light. Not in our actual model of the universe, no. The redshift of light is determined by the spacetime geometry and the worldlines of the emitter and receiver. That is a general formula that works in any spacetime. > Gravitational redshift is a thing Not for the universe as a whole, no. Gravitational redshift is only meaningful in certain kinds of spacetimes, namely stationary spacetimes (which, roughly speaking, describe objects that either don't change with time at all, or which are periodic, like a rotating planet or star). The spacetime that describes our universe as a whole is not stationary and there is no meaningful concept of gravitational redshift. > our model of gravity is incomplete In the sense that we do not have a quantum theory of gravity, yes. But that does not affect anything under discussion here. Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating.
    - willis936 3y ago
      >Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating. Then why are there phantoms in the data that need dark matter and dark energy to make the supposed working model fit them?
      - pdonis 3y ago
        I'm not sure what you mean by "phantoms in the data". The distribution of stress-energy is a free parameter in GR; it has to be inferred from observations. The terms "dark matter" and "dark energy" are just names for, respectively, "stress-energy that acts like the matter we can see, but we can't see it", and "stress-energy that acts like a cosmological constant". Neither of those things poses any problem for GR, since both types of stress-energy are allowed for in the theory. "Dark matter" poses a problem for particle physicists, who have so far been unable to find any fundamental particles that would produce the observed properties. "Dark energy" only poses a problem if for some reason you don't like having a nonzero cosmological constant.
        willis936 3y ago
        >"Dark matter" poses a problem for particle physicists, who have so far been unable to find any fundamental particles that would produce the observed properties. It's clear to me why we haven't made any new discoveries in cosmology in the past two decades. It's this exact attitude of "the model is the truth". All models are wrong. The data can help you improve it, but you have to at least want to improve it.
        sophacles 3y ago
        What do you think they do now? How do you propose they do it differently? What evidence do you have that what they are doing now doesn't work, and does the all the evidence of how they work support your hypothesis? Be detailed, because your comment just has some motivational speaker nonsense but no depth. For example, in the last 20 years cosmology has: + Refined its model of stellar formation based on observational data of the number of planets found observationally, and used this to validate and invalidate several model adjustments. + Observed galaxies that appear not to have dark matter, and by their existence and behavior validate some theories of dark matter, and validated others, which predict such galactic behavior. (e.g. some theories attempting to update gravity). + Run simulations of stellar and glactic formation that predicted structures in the universe that were later observed. Everywhere they look they are finding things the models don't explain well, and refining the models - that is literally using the data to improve the models. If you think you can come up with something better, then do it - all you gotta do is make up some mumbo jumbo and write down any old equation. It probably should: - provide the same results as were observed when the plugging in the experimental parameters of existing experiments. - explain "wierd stuff" in the data that existing models couldn't. - predict future observations of the known phenomena with the same or better accuracy as the old model - predict currently unobserved and unpredicted phenomena Go ahead and take a stab real quick - I'm sure you can do it. I mean Gallieo did it, so did Newton and Einstein. Next up is willis936.
- px43 3y ago
  As I understand it, if the expansion was constant, farther away stuff would still be more red shifted. Stuff twice as far away appears to be moving twice as fast. It helps me to imagine the expansion of a metal cookie sheet, where the two edges are moving apart faster relative to each other compared to the speed that they're moving away from the center. The surprising bit is that the far away stuff seems to be even more red shifted than that, so we're not just expanding, but the rate of expansion seems to be accelerating.
Steuard 3y ago
You're right: the "more distant galaxies are moving away faster" point is just Hubble's original observation of an expanding universe. It's not an argument for cosmic acceleration. (If you see people making that claim, they're probably either speaking carelessly or not experts themselves.) The conclusion that the expansion is accelerating was a quite recent result: 1990s, I believe. It's based on careful measurements of supernova explosions of a type with computable intrinsic brightness in increasingly distant galaxies, and the exact pattern seen in their apparent redshifts vs. apparent brightness. It was a shocking discovery when it came out, with two separate teams announcing the result pretty much neck and neck. There's also independent and compatible evidence for acceleration from the exact pattern of variations in the temperature of the cosmic microwave background seen at different points in the sky.
- bitcurious 3y ago
  Could we use gravity lensing to directly measure the difference in redshift of the same star at different points in time?
colechristensen 3y ago
If your idea was the case there would always be new things from very far away heading towards us, this is not the case. If the universe is flat and infinite there would be no end in supply of new galaxies with all velocities and you would always have the same mix as the initial mix of velocities. That’s not what we see.
- NemoNobody 3y ago
  I'm so glad "the infinite universe" as an idea is finally falling off. It works great in the Hitchhikers Guide, I love the floopy mattresses and planets that grow screwdrivers but nothing real is infinite. I didn't even realize how many people held that belief til that article about how the universe isn't as big as we thought
  - colechristensen 3y ago
    Huh? Every evidence points to a flat infinite universe. Nothing but speculation points to anything otherwise.
snowwrestler 3y ago
> After long enough time, all objects will appear to be moving away from Earth, even if they were moving towards it initially. And after long enough time, the objects that move fastest should be farthest away, by the simple definition of speed! Sure, but to argue that this explains what we observe today, you would need to show that as of today it has been “long enough,” which is its own can of worms to open. You might say “obviously it has been long enough for full sorting, because we observe a fully sorted data set of speed correlated with distance.” But that would be begging the question.
digging 3y ago
It's because the movement is ascribed to the expansion of space itself, not the individual galaxies. We don't have any reason to believe galaxies are moving in random directions at random speeds (not at scales that explain the redshifts we call the expansion of the universe). In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us. And there may be some very fast-moving galaxies close to us that just started really far away. Objects would be entering our local universe from outside it, and that simply doesn't happen.
- arcastroe 3y ago
  > In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us Thank you, but I suppose I'm not really questioning the big bang piece. My question was mostly in regards to the continued acceleration piece. Feel free to disregard the "in random directions" part of my original post. I'm picturing more of an explosion in empty space. A firework or granade of sorts. Any individual dust/shard of the explosion still sees all other objects moving away from it and the rest of my question stands. But I suppose this would imply a "center" to the explosion, which I've also heard is not the case. Theres a few other comments offering more clarity to the acceleration piece. Thank you everyone!
  - pdonis 3y ago
    > I'm picturing more of an explosion in empty space. No, that's not what the big bang is. > this would imply a "center" to the explosion, which I've also heard is not the case That's correct. The big bang does not work like anything ordinary that you are used to imagining. The math is straightforward and unambiguous, but there is no good ordinary language description that corresponds to the math.
    - alexb23 3y ago
      I find helpful this analogy of the space-time (4-dimension) expansion from the big bang: the surface (2D) of an expanding bubble. YMMV.
    - 3y ago
AnimalMuppet 3y ago
I'm not an expert, but I think it's like this: If the universe were expanding uniformly, we would see galaxies moving away from us. The further away they are, the faster they would move. Distance and velocity would have a linear relationship, with the Hubble Constant as the scaling factor. But what we actually see is that, if we measure precisely enough, galaxies further away are moving faster than that. The conclusion is that the expansion is accelerating.
- pdonis 3y ago
  > galaxies further away are moving faster than that No, you have it backwards. Accelerating expansion means, roughly speaking, that we see galaxies further away moving away slower than a "uniform" expansion would predict. Remember that we are seeing galaxies further away as they were a longer period of time ago--so "accelerating expansion" means the universe was expanding slower then, when the light was emitted, than it is now. Actually, though, we don't observe the distance to a galaxy directly. We infer it from other observations. The actual observed quantities are redshift, brightness, and angular size, and the relationship between those three observed quantities is what tells us the expansion history of the universe.
pdonis 3y ago
> How do we know that galaxies are accelerating away from us and not moving at constant speed? More precisely, we see that galaxies started accelerating away from us a few billion years ago; before that they were decelerating (moving away from us but with the "speed" decreasing instead of increasing). > People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration. That observation tells us that the universe is expanding, but by itself it does not tell us whether the expansion is accelerating or decelerating or neither. So you are correct that that observation alone is not sufficient to show that the expansion is accelerating. What we look at to see how the expansion rate changes with time is a comparison of three pieces of observed data, galaxy by galaxy: redshift, brightness, and angular size. The relationship between these three quantities is what cosmologists use to construct a model of the expansion history of the universe, which in turn tells us things like what I said above, that the expansion has been accelerating for the last few billion years but before that it was decelerating.
- AlecSchueler 3y ago
  By few billion are you talking like 3 billion? Why the change?
  - pdonis 3y ago
    Meaning, why did the expansion change from decelerating to accelerating a few billion years ago? Because that was when the density of matter, which had dominated the dynamics until then, became smaller than the density of dark energy, which has dominated the dynamics since then. The dark energy density doe not change with time, but the density of matter decreases as the universe expands.
    - 1d22a 3y ago
      How is is that dark energy density does not change with time? Surely the total amount of dark energy has to be constant (energy can't be created or destroyed, and all that), and then as the universe expands, that's then the same amount of energy over a larger volume, right?
ijustlovemath 3y ago
One thing that is not often mentioned is that this effect only applies outside the local supergroup; within the supergroup gravity overrides the expansion of spacetime and holds us together (for now!)
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
EnterpriseTell 3y ago
>In short, even if galaxies weren't accelerating -Galaxies are not accelerating, space is expanding. > Imagine there are some objects in space all moving in random directions and speeds, relative to Earth... -No, In your scenario then end result would be a most static average distance between all objects in the universe. As an infinite number of objects come from infinite distances, there would ALWAYS be objects in the neighborhood. I think what you're imagining is a bunch of objects in a box, give them random vector and then remove the box. If they maintain course, all will eventually move outside the original box boundaries, and away from each other. (not the way the universe is). They know space is expanding. The primary mechanism we know this is the speed with with we measure an object (moving away) is redshifted. Objects at the same distance from Earth, but opposite regions of space are moving at the ~SAME measured velocity. There simply is no existing theory which can account for what we are seeing besides space expanding. I'm not big on thinking we understand it all, but in this particular measurement, there is basically zero doubt. Space is expanding, which has the affect of accelerating all objects in the universe away from you, with an acceleration relative to the distance. The more space between you, the more opportunity to expand.
- munksbeer 3y ago
  What does "space is expanding" mean? That the distance between objects is increasing? How can you tell the difference between "space expanding" and "objects moving in a non-expanding space"? Is there any way to tell the difference or is it just that all objects are moving away at the exact speeds that satisfies the "space expanding" explanation and nothing else? But then, I'm back to what does "space is expanding" mean? What is doing the expansion?
  - baq 3y ago
    space expanding means what it literally says: there is more space everywhere at once. there was less a moment ago and now there's more. the longer the distance, the more space gets added in between, thus the effect is extreme on universe scale and undetectable on planetary scales.
    - 3y ago

runeb 3y ago

Their light is more red shifted the farther away they are. I'm no expert on this, but I believe in a constant-speed scenario they would have equal red shift no matter the distance

willis936 3y ago
The assumption made here is that relative velocity is the only method that would redshift light. Gravitational redshift is a thing and our model of gravity is incomplete.
- pdonis 3y ago
  > The assumption made here is that relative velocity is the only method that would redshift light. Not in our actual model of the universe, no. The redshift of light is determined by the spacetime geometry and the worldlines of the emitter and receiver. That is a general formula that works in any spacetime. > Gravitational redshift is a thing Not for the universe as a whole, no. Gravitational redshift is only meaningful in certain kinds of spacetimes, namely stationary spacetimes (which, roughly speaking, describe objects that either don't change with time at all, or which are periodic, like a rotating planet or star). The spacetime that describes our universe as a whole is not stationary and there is no meaningful concept of gravitational redshift. > our model of gravity is incomplete In the sense that we do not have a quantum theory of gravity, yes. But that does not affect anything under discussion here. Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating.
  - willis936 3y ago
    >Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating. Then why are there phantoms in the data that need dark matter and dark energy to make the supposed working model fit them?
    - pdonis 3y ago
      I'm not sure what you mean by "phantoms in the data". The distribution of stress-energy is a free parameter in GR; it has to be inferred from observations. The terms "dark matter" and "dark energy" are just names for, respectively, "stress-energy that acts like the matter we can see, but we can't see it", and "stress-energy that acts like a cosmological constant". Neither of those things poses any problem for GR, since both types of stress-energy are allowed for in the theory. "Dark matter" poses a problem for particle physicists, who have so far been unable to find any fundamental particles that would produce the observed properties. "Dark energy" only poses a problem if for some reason you don't like having a nonzero cosmological constant.
      - willis936 3y ago
        >"Dark matter" poses a problem for particle physicists, who have so far been unable to find any fundamental particles that would produce the observed properties. It's clear to me why we haven't made any new discoveries in cosmology in the past two decades. It's this exact attitude of "the model is the truth". All models are wrong. The data can help you improve it, but you have to at least want to improve it.
        sophacles 3y ago
        What do you think they do now? How do you propose they do it differently? What evidence do you have that what they are doing now doesn't work, and does the all the evidence of how they work support your hypothesis? Be detailed, because your comment just has some motivational speaker nonsense but no depth. For example, in the last 20 years cosmology has: + Refined its model of stellar formation based on observational data of the number of planets found observationally, and used this to validate and invalidate several model adjustments. + Observed galaxies that appear not to have dark matter, and by their existence and behavior validate some theories of dark matter, and validated others, which predict such galactic behavior. (e.g. some theories attempting to update gravity). + Run simulations of stellar and glactic formation that predicted structures in the universe that were later observed. Everywhere they look they are finding things the models don't explain well, and refining the models - that is literally using the data to improve the models. If you think you can come up with something better, then do it - all you gotta do is make up some mumbo jumbo and write down any old equation. It probably should: - provide the same results as were observed when the plugging in the experimental parameters of existing experiments. - explain "wierd stuff" in the data that existing models couldn't. - predict future observations of the known phenomena with the same or better accuracy as the old model - predict currently unobserved and unpredicted phenomena Go ahead and take a stab real quick - I'm sure you can do it. I mean Gallieo did it, so did Newton and Einstein. Next up is willis936.
px43 3y ago
As I understand it, if the expansion was constant, farther away stuff would still be more red shifted. Stuff twice as far away appears to be moving twice as fast. It helps me to imagine the expansion of a metal cookie sheet, where the two edges are moving apart faster relative to each other compared to the speed that they're moving away from the center. The surprising bit is that the far away stuff seems to be even more red shifted than that, so we're not just expanding, but the rate of expansion seems to be accelerating.

Steuard 3y ago

You're right: the "more distant galaxies are moving away faster" point is just Hubble's original observation of an expanding universe. It's not an argument for cosmic acceleration. (If you see people making that claim, they're probably either speaking carelessly or not experts themselves.) The conclusion that the expansion is accelerating was a quite recent result: 1990s, I believe. It's based on careful measurements of supernova explosions of a type with computable intrinsic brightness in increasingly distant galaxies, and the exact pattern seen in their apparent redshifts vs. apparent brightness. It was a shocking discovery when it came out, with two separate teams announcing the result pretty much neck and neck. There's also independent and compatible evidence for acceleration from the exact pattern of variations in the temperature of the cosmic microwave background seen at different points in the sky.

bitcurious 3y ago
Could we use gravity lensing to directly measure the difference in redshift of the same star at different points in time?

colechristensen 3y ago

If your idea was the case there would always be new things from very far away heading towards us, this is not the case. If the universe is flat and infinite there would be no end in supply of new galaxies with all velocities and you would always have the same mix as the initial mix of velocities. That’s not what we see.

NemoNobody 3y ago
I'm so glad "the infinite universe" as an idea is finally falling off. It works great in the Hitchhikers Guide, I love the floopy mattresses and planets that grow screwdrivers but nothing real is infinite. I didn't even realize how many people held that belief til that article about how the universe isn't as big as we thought
- colechristensen 3y ago
  Huh? Every evidence points to a flat infinite universe. Nothing but speculation points to anything otherwise.

snowwrestler 3y ago

> After long enough time, all objects will appear to be moving away from Earth, even if they were moving towards it initially. And after long enough time, the objects that move fastest should be farthest away, by the simple definition of speed! Sure, but to argue that this explains what we observe today, you would need to show that as of today it has been “long enough,” which is its own can of worms to open. You might say “obviously it has been long enough for full sorting, because we observe a fully sorted data set of speed correlated with distance.” But that would be begging the question.

digging 3y ago

It's because the movement is ascribed to the expansion of space itself, not the individual galaxies. We don't have any reason to believe galaxies are moving in random directions at random speeds (not at scales that explain the redshifts we call the expansion of the universe). In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us. And there may be some very fast-moving galaxies close to us that just started really far away. Objects would be entering our local universe from outside it, and that simply doesn't happen.

arcastroe 3y ago
> In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us Thank you, but I suppose I'm not really questioning the big bang piece. My question was mostly in regards to the continued acceleration piece. Feel free to disregard the "in random directions" part of my original post. I'm picturing more of an explosion in empty space. A firework or granade of sorts. Any individual dust/shard of the explosion still sees all other objects moving away from it and the rest of my question stands. But I suppose this would imply a "center" to the explosion, which I've also heard is not the case. Theres a few other comments offering more clarity to the acceleration piece. Thank you everyone!
- pdonis 3y ago
  > I'm picturing more of an explosion in empty space. No, that's not what the big bang is. > this would imply a "center" to the explosion, which I've also heard is not the case That's correct. The big bang does not work like anything ordinary that you are used to imagining. The math is straightforward and unambiguous, but there is no good ordinary language description that corresponds to the math.
  - alexb23 3y ago
    I find helpful this analogy of the space-time (4-dimension) expansion from the big bang: the surface (2D) of an expanding bubble. YMMV.
  - 3y ago

AnimalMuppet 3y ago

I'm not an expert, but I think it's like this: If the universe were expanding uniformly, we would see galaxies moving away from us. The further away they are, the faster they would move. Distance and velocity would have a linear relationship, with the Hubble Constant as the scaling factor. But what we actually see is that, if we measure precisely enough, galaxies further away are moving faster than that. The conclusion is that the expansion is accelerating.

pdonis 3y ago
> galaxies further away are moving faster than that No, you have it backwards. Accelerating expansion means, roughly speaking, that we see galaxies further away moving away slower than a "uniform" expansion would predict. Remember that we are seeing galaxies further away as they were a longer period of time ago--so "accelerating expansion" means the universe was expanding slower then, when the light was emitted, than it is now. Actually, though, we don't observe the distance to a galaxy directly. We infer it from other observations. The actual observed quantities are redshift, brightness, and angular size, and the relationship between those three observed quantities is what tells us the expansion history of the universe.

pdonis 3y ago

> How do we know that galaxies are accelerating away from us and not moving at constant speed? More precisely, we see that galaxies started accelerating away from us a few billion years ago; before that they were decelerating (moving away from us but with the "speed" decreasing instead of increasing). > People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration. That observation tells us that the universe is expanding, but by itself it does not tell us whether the expansion is accelerating or decelerating or neither. So you are correct that that observation alone is not sufficient to show that the expansion is accelerating. What we look at to see how the expansion rate changes with time is a comparison of three pieces of observed data, galaxy by galaxy: redshift, brightness, and angular size. The relationship between these three quantities is what cosmologists use to construct a model of the expansion history of the universe, which in turn tells us things like what I said above, that the expansion has been accelerating for the last few billion years but before that it was decelerating.

AlecSchueler 3y ago
By few billion are you talking like 3 billion? Why the change?
- pdonis 3y ago
  Meaning, why did the expansion change from decelerating to accelerating a few billion years ago? Because that was when the density of matter, which had dominated the dynamics until then, became smaller than the density of dark energy, which has dominated the dynamics since then. The dark energy density doe not change with time, but the density of matter decreases as the universe expands.
  - 1d22a 3y ago
    How is is that dark energy density does not change with time? Surely the total amount of dark energy has to be constant (energy can't be created or destroyed, and all that), and then as the universe expands, that's then the same amount of energy over a larger volume, right?

ijustlovemath 3y ago

One thing that is not often mentioned is that this effect only applies outside the local supergroup; within the supergroup gravity overrides the expansion of spacetime and holds us together (for now!)

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

EnterpriseTell 3y ago

>In short, even if galaxies weren't accelerating -Galaxies are not accelerating, space is expanding. > Imagine there are some objects in space all moving in random directions and speeds, relative to Earth... -No, In your scenario then end result would be a most static average distance between all objects in the universe. As an infinite number of objects come from infinite distances, there would ALWAYS be objects in the neighborhood. I think what you're imagining is a bunch of objects in a box, give them random vector and then remove the box. If they maintain course, all will eventually move outside the original box boundaries, and away from each other. (not the way the universe is). They know space is expanding. The primary mechanism we know this is the speed with with we measure an object (moving away) is redshifted. Objects at the same distance from Earth, but opposite regions of space are moving at the ~SAME measured velocity. There simply is no existing theory which can account for what we are seeing besides space expanding. I'm not big on thinking we understand it all, but in this particular measurement, there is basically zero doubt. Space is expanding, which has the affect of accelerating all objects in the universe away from you, with an acceleration relative to the distance. The more space between you, the more opportunity to expand.

munksbeer 3y ago
What does "space is expanding" mean? That the distance between objects is increasing? How can you tell the difference between "space expanding" and "objects moving in a non-expanding space"? Is there any way to tell the difference or is it just that all objects are moving away at the exact speeds that satisfies the "space expanding" explanation and nothing else? But then, I'm back to what does "space is expanding" mean? What is doing the expansion?
- baq 3y ago
  space expanding means what it literally says: there is more space everywhere at once. there was less a moment ago and now there's more. the longer the distance, the more space gets added in between, thus the effect is extreme on universe scale and undetectable on planetary scales.
  - 3y ago