7 ms·

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

by 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.

by 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.