5 ms·

> It would be incredibly exciting if there was new physics It would indeed be incredibly exciting if it would lead to a less silly explanation of the redshift

> It would be incredibly exciting if there was new physics It would indeed be incredibly exciting if it would lead to a less silly explanation of the redshift than "universe is expanding".

jaredmosley 6y ago
Why is that explanation silly?
pdonis 6y ago
> It would indeed be incredibly exciting if it would lead to a less silly explanation of the redshift than "universe is expanding". It won't. There is no question about whether the universe is expanding. The issue is over how fast it is expanding; we have two different theoretical models that are giving different answers, and we don't understand (yet) how to reconcile them.
- mpoteat 6y ago
  Not a physicist, but why isn't the simple explanation that light loses energy over cosmological distances not also a valid explanation of redshift? We already know that energy isn't conserved at those scales.
  - pdonis 6y ago
    > why isn't the simple explanation that light loses energy over cosmological distances not also a valid explanation of redshift? It depends on what you mean by "light loses energy over cosmological distances". See below. > We already know that energy isn't conserved at those scales. This suggests that what you mean by "light loses energy over cosmological distances" is that the universe is expanding while the light travels, and in an expanding universe, total energy is not conserved. That is true, and that is the redshift due to the universe expanding--it's not something different. If, however, you are working from the hypothesis that the universe is not expanding, then energy is conserved even over cosmological distances, so I don't understand what you mean by invoking "energy isn't conserved" as an explanation for the redshift. If you're referring to the "tired light" hypothesis, that's been known to be unviable for decades.
  - gpderetta 6y ago
    Energy is not conserved at those scales because of general relativity, which also provides the mechanism for an expannding universe. Einstein himself didn't like this result of GE and tried to hack around it with the cosmological constant. Turns out the math was right.
    - pdonis 6y ago
      > Turns out the math was right. Actually, turns out the math was right with a cosmological constant: that's how the accelerating expansion of our universe is explained. Einstein actually blundered twice: first by putting in the constant for the wrong reason (because he wanted a static universe), but then taking it out again when it turned out that reason wasn't the case (when the expansion of the universe was discovered). If you just look at how to derive the Einstein Field Equation of General Relativity from first principles, the constant should be there; it isn't an add-on to General Relativity at all, it's part of it. It's just that there's no way to know from those first principles what its value is. That we had to figure out from observations.
      - gpderetta 6y ago
        Re: accellerating expansion, I was only commenting on how the expansion itself is a consequence of GE. Thanks for your point about the constant being actually required. I do not understand the math, but is this similar on how integrals always have a constant as a free parameter that need to be determined by other means? I assume that Einstein originally set the constant to exactly balance the expansion, but later set it to zero. In bot cases you are picking arbitrar values but the actual value need to determined by empirical observeations.
        pdonis 6y ago
        > is this similar on how integrals always have a constant as a free parameter that need to be determined by other means? Not really, no. It's a consequence of the assumptions made when the Einstein Field Equation (EFE) is derived from a Lagrangian using the principle of least action. The assumptions are that the Lagrangian should be a Lorentz scalar (which is required of any Lagrangian) and that it should include no more than second derivatives of the metric. The Ricci scalar R meets this requirement and is the Lagrangian that was originally used by David Hilbert to derive the EFE (without a cosmological constant). But a simple constant (the cosmological constant) also meets the requirement, and therefore should be included in the Lagrangian; including it leads to the cosmological constant term in the EFE.
- grishka 6y ago
  > There is no question about whether the universe is expanding. Why? > we have two different theoretical models that are giving different answers, and we don't understand (yet) how to reconcile them. Maybe... just maybe... that's because it isn't actually expanding, but there's some other, unknown effects at play? It's about time we think outside the box. To me, as a regular programmer kind of person, the very idea that the universe is expanding makes about as much sense as dark matter. I get it, both make the calculations work out, but they're basically "we have no clue what this actually is, here's our best guess" kind of variable.
  - pdonis 6y ago
    > Why? Because all of the other theoretical models that attempt to explain the observed redshifts fail to match observations. > Maybe... just maybe... that's because it isn't actually expanding, but there's some other, unknown effects at play? Nope. Already been tried. Doesn't work. > It's about time we think outside the box. Cosmologists have been trying out of the box ideas for decades. All of them failed. That's why we have the models we have now: they're the only ones that survived that process. > I get it, both make the calculations work out, but they're basically "we have no clue what this actually is, here's our best guess" kind of variable. For dark matter, you are correct: we don't know what it is, and the term is basically just another way of saying "whatever we need to add to our model to make it match observations works basically like ordinary matter does in the equations". But that is not true of expansion itself. All of the alternative models that have been proposed to eliminate the need for dark matter still have an expanding universe. The expansion itself is a much more solid conclusion than dark matter is.
  - pdonis 6y ago
    > It's about time we think outside the box Another way of looking at it, to add to my previous post: dark matter and MOND are thinking outside the box. They cover both possibilities for how to expand on our current theories: either (a) there's more "stuff" out there than our current theories of particle physics know about; or (b) there's more aspects to gravity than our current theory of gravity knows about. Consider whatever "unknown effects" you like: they will end up coming down to one of those two possibilities. Thinking "inside the box" would be something like: it's far more likely that either the data is wrong or we haven't calculated the predictions of our current theories correctly than that our current theories, which have tons of experimental confirmation, are wrong. Historically, most of the time in science, when there's been a discrepancy between theories and data, that is how the discrepancy has ended up being resolved: either we've figured out something was wrong with the data, or we've figured out that something was wrong with how we calculated the predictions of our current theories. The reason why cosmologists are driven to consider models like dark matter and MOND is that they have checked and double checked and triple checked both the data and our predictions from our current theories, and the mismatch hasn't gone away. So they are driven to consider "out of the box" ideas, and, as above, dark matter and MOND cover the possibilities.

> It would be incredibly exciting if there was new physics It would indeed be incredibly exciting if it would lead to a less silly explanation of the redshift than "universe is expanding".

jaredmosley 6y ago

Why is that explanation silly?

pdonis 6y ago

> It would indeed be incredibly exciting if it would lead to a less silly explanation of the redshift than "universe is expanding". It won't. There is no question about whether the universe is expanding. The issue is over how fast it is expanding; we have two different theoretical models that are giving different answers, and we don't understand (yet) how to reconcile them.

mpoteat 6y ago
Not a physicist, but why isn't the simple explanation that light loses energy over cosmological distances not also a valid explanation of redshift? We already know that energy isn't conserved at those scales.
- pdonis 6y ago
  > why isn't the simple explanation that light loses energy over cosmological distances not also a valid explanation of redshift? It depends on what you mean by "light loses energy over cosmological distances". See below. > We already know that energy isn't conserved at those scales. This suggests that what you mean by "light loses energy over cosmological distances" is that the universe is expanding while the light travels, and in an expanding universe, total energy is not conserved. That is true, and that is the redshift due to the universe expanding--it's not something different. If, however, you are working from the hypothesis that the universe is not expanding, then energy is conserved even over cosmological distances, so I don't understand what you mean by invoking "energy isn't conserved" as an explanation for the redshift. If you're referring to the "tired light" hypothesis, that's been known to be unviable for decades.
- gpderetta 6y ago
  Energy is not conserved at those scales because of general relativity, which also provides the mechanism for an expannding universe. Einstein himself didn't like this result of GE and tried to hack around it with the cosmological constant. Turns out the math was right.
  - pdonis 6y ago
    > Turns out the math was right. Actually, turns out the math was right with a cosmological constant: that's how the accelerating expansion of our universe is explained. Einstein actually blundered twice: first by putting in the constant for the wrong reason (because he wanted a static universe), but then taking it out again when it turned out that reason wasn't the case (when the expansion of the universe was discovered). If you just look at how to derive the Einstein Field Equation of General Relativity from first principles, the constant should be there; it isn't an add-on to General Relativity at all, it's part of it. It's just that there's no way to know from those first principles what its value is. That we had to figure out from observations.
    - gpderetta 6y ago
      Re: accellerating expansion, I was only commenting on how the expansion itself is a consequence of GE. Thanks for your point about the constant being actually required. I do not understand the math, but is this similar on how integrals always have a constant as a free parameter that need to be determined by other means? I assume that Einstein originally set the constant to exactly balance the expansion, but later set it to zero. In bot cases you are picking arbitrar values but the actual value need to determined by empirical observeations.
      - pdonis 6y ago
        > is this similar on how integrals always have a constant as a free parameter that need to be determined by other means? Not really, no. It's a consequence of the assumptions made when the Einstein Field Equation (EFE) is derived from a Lagrangian using the principle of least action. The assumptions are that the Lagrangian should be a Lorentz scalar (which is required of any Lagrangian) and that it should include no more than second derivatives of the metric. The Ricci scalar R meets this requirement and is the Lagrangian that was originally used by David Hilbert to derive the EFE (without a cosmological constant). But a simple constant (the cosmological constant) also meets the requirement, and therefore should be included in the Lagrangian; including it leads to the cosmological constant term in the EFE.
grishka 6y ago
> There is no question about whether the universe is expanding. Why? > we have two different theoretical models that are giving different answers, and we don't understand (yet) how to reconcile them. Maybe... just maybe... that's because it isn't actually expanding, but there's some other, unknown effects at play? It's about time we think outside the box. To me, as a regular programmer kind of person, the very idea that the universe is expanding makes about as much sense as dark matter. I get it, both make the calculations work out, but they're basically "we have no clue what this actually is, here's our best guess" kind of variable.
- pdonis 6y ago
  > Why? Because all of the other theoretical models that attempt to explain the observed redshifts fail to match observations. > Maybe... just maybe... that's because it isn't actually expanding, but there's some other, unknown effects at play? Nope. Already been tried. Doesn't work. > It's about time we think outside the box. Cosmologists have been trying out of the box ideas for decades. All of them failed. That's why we have the models we have now: they're the only ones that survived that process. > I get it, both make the calculations work out, but they're basically "we have no clue what this actually is, here's our best guess" kind of variable. For dark matter, you are correct: we don't know what it is, and the term is basically just another way of saying "whatever we need to add to our model to make it match observations works basically like ordinary matter does in the equations". But that is not true of expansion itself. All of the alternative models that have been proposed to eliminate the need for dark matter still have an expanding universe. The expansion itself is a much more solid conclusion than dark matter is.
- pdonis 6y ago
  > It's about time we think outside the box Another way of looking at it, to add to my previous post: dark matter and MOND are thinking outside the box. They cover both possibilities for how to expand on our current theories: either (a) there's more "stuff" out there than our current theories of particle physics know about; or (b) there's more aspects to gravity than our current theory of gravity knows about. Consider whatever "unknown effects" you like: they will end up coming down to one of those two possibilities. Thinking "inside the box" would be something like: it's far more likely that either the data is wrong or we haven't calculated the predictions of our current theories correctly than that our current theories, which have tons of experimental confirmation, are wrong. Historically, most of the time in science, when there's been a discrepancy between theories and data, that is how the discrepancy has ended up being resolved: either we've figured out something was wrong with the data, or we've figured out that something was wrong with how we calculated the predictions of our current theories. The reason why cosmologists are driven to consider models like dark matter and MOND is that they have checked and double checked and triple checked both the data and our predictions from our current theories, and the mismatch hasn't gone away. So they are driven to consider "out of the box" ideas, and, as above, dark matter and MOND cover the possibilities.