3 ms·

I know it makes for a "better" headline, but don't be mislead by it thinking Astronomers actually thought dark energy was a cosmological constant, period. Read

I know it makes for a "better" headline, but don't be mislead by it thinking Astronomers actually thought dark energy was a cosmological constant, period. Read section 1.1 of the Euclid review paper from 2016 to get an impression straight from the horses mouth. > Why then is the cosmological constant not the end of the story as far as cosmic acceleration is concerned? There are at least three reasons. https://arxiv.org/abs/1606.00180 https://arxiv.org/abs/1606.00180

sigmoid10 2y ago
Don't confuse accelerated expansion with the cosmological constant. Accelerated expansion's origin is most likely the cosmological constant aka dark energy, but they are not the same thing. The cosmological constant itself arises naturally (kind of how an integration constant emerges for indefinite integrals) when you derive the field equations of general relativity. It is basically a geometric degree of freedom inherent to the Einstein-Hilbert action. Any astronomer who fit any model of the universe to some dataset in the past 25 years was fitting a constant. And even this new paper acknowledges that it will take some tremendous effort to convince people that this constant really is changing over time (which introduces a slew of theoretical issues that need to be explained). It will be particularly interesting to see how (if at all) this can be reconciled with measurements of things like the CMB, because stellar and galactic astrophysics is much less well understood.
- mr_mitm 2y ago
  I'm not sure where you got that I'm confusing something. For what's it worth, my name is in the author list of that paper.
  - sigmoid10 2y ago
    >where you got that I'm confusing something From the idea that the constant that equates to dark energy in the stress energy tensor is not commonly thought to be a constant. People didn't call it that way as some joke. This isn't even up to astronomers, this is something that mathematicians will tell you. And it could potentially cause some real trouble not just for our models of the universe but for the basic mathematics of relativity if it is changing. Motivating that alone will we hard, not to speak of the consequences down the line.
    - deleted 2y ago
      [deleted]
    - deleted 2y ago
      [deleted]
    - pdonis 2y ago
      > it could potentially cause some real trouble not just for our models of the universe but for the basic mathematics of relativity if it is changing. The Einstein Field Equation can accommodate models in which the density of dark energy changes with time just fine. The paper mr_mitm linked to discusses such models. There is no "real trouble" for the basic math of relativity involved here at all.
      - sigmoid10 2y ago
        You can do all sorts of stuff with the Einstein field equations (or any differential equation for that matter). People do this all the time because it's an easy paper. Many people for example make the Ricci scalar into a function. Aka "what happens if I magically introduce this." That way you can sometimes explain some stuff on one end while breaking stuff at other ends. But whenever you introduce such new degrees of freedom, you need to accommodate them somewhere in the overall picture. f(R) gravity in that sense would be better motivated, because in the infrared it boils down to a dumb extra scalar field. And you could get tons of those easily from string theory. But if you want to use local field theories of gravity to explain horizons, then you need something like holography to provide a consistent picture. Even if you go for metastable degenerate vacua, it will be really hard to explain how this leads to an apparently continuously varying cosmological constant. Is it possible? Maybe. Is it more likely that we misunderstood stellar evolution? Absolutely.
        mr_mitm 2y ago
        No one is saying the cosmological constant can vary. The idea is simply that if dark energy is dynamical, it clearly cannot be explained by a cosmological constant, but perhaps a scalar field instead. And my point was that this idea is nothing new, unlike the headline seems to suggest.
  - deleted 2y ago
    [deleted]
  - gadders 2y ago
    This is nearly as good a comment as cperciva winning the Putnam.
    - mr_mitm 2y ago
      I wouldn't want to overstate it either, since I was just a lowly and IMHO mediocre PhD student at the time and my contribution to the paper was minor. Nevertheless, I believe I understand the relationship between the cosmological constant and dark energy just fine.
    - ahazred8ta 2y ago
      "Did you win the Putnam?" "Yes, I did." https://news.ycombinator.com/item?id=35079 https://news.ycombinator.com/item?id=35079 sanj, cperciva, pg - classic
pdonis 2y ago
Thank you for linking to this paper. It looks like a very comprehensive review of the "theory space", so to speak, that's currently being explored.

sigmoid10 2y ago

Don't confuse accelerated expansion with the cosmological constant. Accelerated expansion's origin is most likely the cosmological constant aka dark energy, but they are not the same thing. The cosmological constant itself arises naturally (kind of how an integration constant emerges for indefinite integrals) when you derive the field equations of general relativity. It is basically a geometric degree of freedom inherent to the Einstein-Hilbert action. Any astronomer who fit any model of the universe to some dataset in the past 25 years was fitting a constant. And even this new paper acknowledges that it will take some tremendous effort to convince people that this constant really is changing over time (which introduces a slew of theoretical issues that need to be explained). It will be particularly interesting to see how (if at all) this can be reconciled with measurements of things like the CMB, because stellar and galactic astrophysics is much less well understood.

mr_mitm 2y ago
I'm not sure where you got that I'm confusing something. For what's it worth, my name is in the author list of that paper.
- sigmoid10 2y ago
  >where you got that I'm confusing something From the idea that the constant that equates to dark energy in the stress energy tensor is not commonly thought to be a constant. People didn't call it that way as some joke. This isn't even up to astronomers, this is something that mathematicians will tell you. And it could potentially cause some real trouble not just for our models of the universe but for the basic mathematics of relativity if it is changing. Motivating that alone will we hard, not to speak of the consequences down the line.
  - deleted 2y ago
    [deleted]
  - deleted 2y ago
    [deleted]
  - pdonis 2y ago
    > it could potentially cause some real trouble not just for our models of the universe but for the basic mathematics of relativity if it is changing. The Einstein Field Equation can accommodate models in which the density of dark energy changes with time just fine. The paper mr_mitm linked to discusses such models. There is no "real trouble" for the basic math of relativity involved here at all.
    - sigmoid10 2y ago
      You can do all sorts of stuff with the Einstein field equations (or any differential equation for that matter). People do this all the time because it's an easy paper. Many people for example make the Ricci scalar into a function. Aka "what happens if I magically introduce this." That way you can sometimes explain some stuff on one end while breaking stuff at other ends. But whenever you introduce such new degrees of freedom, you need to accommodate them somewhere in the overall picture. f(R) gravity in that sense would be better motivated, because in the infrared it boils down to a dumb extra scalar field. And you could get tons of those easily from string theory. But if you want to use local field theories of gravity to explain horizons, then you need something like holography to provide a consistent picture. Even if you go for metastable degenerate vacua, it will be really hard to explain how this leads to an apparently continuously varying cosmological constant. Is it possible? Maybe. Is it more likely that we misunderstood stellar evolution? Absolutely.
      - mr_mitm 2y ago
        No one is saying the cosmological constant can vary. The idea is simply that if dark energy is dynamical, it clearly cannot be explained by a cosmological constant, but perhaps a scalar field instead. And my point was that this idea is nothing new, unlike the headline seems to suggest.
- deleted 2y ago
  [deleted]
- gadders 2y ago
  This is nearly as good a comment as cperciva winning the Putnam.
  - mr_mitm 2y ago
    I wouldn't want to overstate it either, since I was just a lowly and IMHO mediocre PhD student at the time and my contribution to the paper was minor. Nevertheless, I believe I understand the relationship between the cosmological constant and dark energy just fine.
  - ahazred8ta 2y ago
    "Did you win the Putnam?" "Yes, I did." https://news.ycombinator.com/item?id=35079 https://news.ycombinator.com/item?id=35079 sanj, cperciva, pg - classic

pdonis 2y ago

Thank you for linking to this paper. It looks like a very comprehensive review of the "theory space", so to speak, that's currently being explored.