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Dark energy isn't really a theory, it's more of a placeholder for the observation that the universe's expansion is accelerating. There are a variety of ways yo

Dark energy isn't really a theory, it's more of a placeholder for the observation that the universe's expansion is accelerating. There are a variety of ways you can potentially explain this observation. The simplest theory is the standard lambda-CDM model for dark energy where it is taken to be a cosmological constant. This model has worked well for about 20 years or so, but it is starting to come in tension with newer high precision observations.

SoftTalker 2y ago
That seems reasonable; I guess I'd rather hear "for reasons we don't understand the universe appears to be expanding and the expansion is accelerating by a constant factor we're calling 'lambda'." But I guess "dark energy" is fewer words, if that's what it means.
- dwaltrip 2y ago
  That’s what it means. The field is pretty transparent about it. It’s often poorly conveyed in pop science, but that’s how it goes.
tengwar2 2y ago
As I remember it (from 40+ years ago, and well before dark energy was discussed), lambda naturally arises as a constant of integration in GR rather than being "invented" as in the usual story. Assuming this to be true, I'd suggest that any model should either allow for a non-zero lambda, or give a reason for it to be zero. I.e. even if an alternative for dark energy is found, we still have to deal with lambda.
- antognini 2y ago
  Yes, given the equations of GR, there's no particular reason to assume that the cosmological constant should be zero. (Though one of the major problems in theoretical physics is that back-of-the-envelope estimates as to what it "should" be give numbers that are ~120 orders of magnitude larger than its observed value. So if dark energy is a cosmological constant, it's not really understood why it is non-zero, but extremely small.)
  - SoftTalker 2y ago
    Isn't that why they came up with "dark matter" i.e. invisible, undetectable mass.
    - antognini 2y ago
      No, dark matter is quite different. There are a number of different lines of evidence for dark matter. One of these is sort of similar to dark energy, in that it is needed to explain the slower-than-expected expansion of the universe at early times. But the most compelling line of evidence for dark matter (IMO) is baryon acoustic oscillations in the CMB.
- floxy 2y ago
  I thought that lambda was directly in the differential equation form? Rμν − (1/2)*gμν*R + Λ*gμν = (8*π*G / c^4) * Tμν
cryptonector 2y ago
Lambda-CDM is a model, yes, but DE sure is a theory, or hypothesis if you prefer. Look at https://en.wikipedia.org/wiki/Dark_energy#Nature https://en.wikipedia.org/wiki/Dark_energy#Nature and reference [23] and note that there's a proposed mechanism for the acceleration of the expansion of the universe. [23] http://philsci-archive.pitt.edu/398/ http://philsci-archive.pitt.edu/398/

SoftTalker 2y ago

That seems reasonable; I guess I'd rather hear "for reasons we don't understand the universe appears to be expanding and the expansion is accelerating by a constant factor we're calling 'lambda'." But I guess "dark energy" is fewer words, if that's what it means.

dwaltrip 2y ago
That’s what it means. The field is pretty transparent about it. It’s often poorly conveyed in pop science, but that’s how it goes.

tengwar2 2y ago

As I remember it (from 40+ years ago, and well before dark energy was discussed), lambda naturally arises as a constant of integration in GR rather than being "invented" as in the usual story. Assuming this to be true, I'd suggest that any model should either allow for a non-zero lambda, or give a reason for it to be zero. I.e. even if an alternative for dark energy is found, we still have to deal with lambda.

antognini 2y ago
Yes, given the equations of GR, there's no particular reason to assume that the cosmological constant should be zero. (Though one of the major problems in theoretical physics is that back-of-the-envelope estimates as to what it "should" be give numbers that are ~120 orders of magnitude larger than its observed value. So if dark energy is a cosmological constant, it's not really understood why it is non-zero, but extremely small.)
- SoftTalker 2y ago
  Isn't that why they came up with "dark matter" i.e. invisible, undetectable mass.
  - antognini 2y ago
    No, dark matter is quite different. There are a number of different lines of evidence for dark matter. One of these is sort of similar to dark energy, in that it is needed to explain the slower-than-expected expansion of the universe at early times. But the most compelling line of evidence for dark matter (IMO) is baryon acoustic oscillations in the CMB.
floxy 2y ago
I thought that lambda was directly in the differential equation form? Rμν − (1/2)*gμν*R + Λ*gμν = (8*π*G / c^4) * Tμν

cryptonector 2y ago

Lambda-CDM is a model, yes, but DE sure is a theory, or hypothesis if you prefer. Look at https://en.wikipedia.org/wiki/Dark_energy#Nature https://en.wikipedia.org/wiki/Dark_energy#Nature and reference [23] and note that there's a proposed mechanism for the acceleration of the expansion of the universe. [23] http://philsci-archive.pitt.edu/398/ http://philsci-archive.pitt.edu/398/