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Possibly a dumb question: can the tension between the two rates simply be resolved because one method measures an old historical rate and one method measures th

Possibly a dumb question: can the tension between the two rates simply be resolved because one method measures an old historical rate and one method measures the current rate, and it’s been dropping over time? Or are they both measuring the same time periods? (I assume it’s not that simple, but it’s very hard for me to understand and reason about data that involves looking back in time and space simultaneously)

venusenvy47 3y ago
That idea is mentioned in the article, with the hypothesis that there was more dark energy in the past that has since dissipated. They label that idea as "New Physics" in the chart above that comment.
privong 3y ago
> Possibly a dumb question: can the tension between the two rates simply be resolved because one method measures an old historical rate and one method measures the current rate This is already generally being accounted for (assuming we have the correct cosmological model for the Universe). The CMB measurement is probing the Universe at an early state (when the Universe was ~380,000 years old) and the Hubble "constant", H(t=380,000 yr), was different at that point from what it is now, H_0 = H(t=13.7 billion years). The comparison to the current Hubble constant (e.g., determined from Cepheids) is made by evolving the CMB Hubble "constant" value to today, under the assumption of our best cosmological model model. Of course, if we have the wrong cosmological model, then that could be a reason for the discrepancy, and that might point to "new physics". Alternately there may be some systematic in the Cepheid technique that is causing our H_0 estimate to be somewhat off.
mr_mitm 3y ago
Both methods measure H_0, which is the Hubble parameter today. It's a bit confusing that it's sometimes called the Hubble constant, because it's not really a constant. The expansion rate H changes over time, so we write H(t) and by H_0 we mean H(t_today). It's probably the case that they didn't directly measure H_0 with the early universe probes, so they measured H at that time and then used the standard model to extrapolate this value to H_0, roughly speaking. This means, that if the measurements didn't suffer from some unknown systematic error, that the model is wrong. That's what they call "new physics" and could be the most exciting thing in cosmology since the 90s.
- denton-scratch 3y ago
  > because it's not really a constant. Way back when, we used to joke about "Murphy's Variable Constant", the fiddle factor required to make any equation balance nicely. It's sad that the Hubble Constant is now a variable.
  - DiogenesKynikos 3y ago
    H(t) is not an arbitrary function. Up to an overall scaling factor (H_0), it's fully determined by the relative abundance of baryonic (normal) matter, dark matter, dark energy, radiation, and other constituents of the Universe.
    - denton-scratch 3y ago
      > it's fully determined Seriously? How can it be fully-determined by things we can't detect, let alone measure?
      - DiogenesKynikos 3y ago
        It's fully determined, given the densities of the various constituents I listed above. There are separate constraints on almost all of those densities, from different observations. For example, big bang nucleosynthesis puts constraints on the density of baryonic matter, dark matter and radiation, while the CMB essentially tells us the density of radiation. Cosmology is based on a web of different observations, tied together by well understood physics. It's very difficult to fudge things to match one observation without messing up the results for other observations.
        T-A 3y ago
        > Cosmology is based on a web of different observations, tied together by well understood physics. It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the universe. [1] That doesn't quite live up to my idea of "well understood physics". [1] https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy https://science.nasa.gov/astrophysics/focus-areas/what-is-da...

venusenvy47 3y ago

That idea is mentioned in the article, with the hypothesis that there was more dark energy in the past that has since dissipated. They label that idea as "New Physics" in the chart above that comment.

privong 3y ago

> Possibly a dumb question: can the tension between the two rates simply be resolved because one method measures an old historical rate and one method measures the current rate This is already generally being accounted for (assuming we have the correct cosmological model for the Universe). The CMB measurement is probing the Universe at an early state (when the Universe was ~380,000 years old) and the Hubble "constant", H(t=380,000 yr), was different at that point from what it is now, H_0 = H(t=13.7 billion years). The comparison to the current Hubble constant (e.g., determined from Cepheids) is made by evolving the CMB Hubble "constant" value to today, under the assumption of our best cosmological model model. Of course, if we have the wrong cosmological model, then that could be a reason for the discrepancy, and that might point to "new physics". Alternately there may be some systematic in the Cepheid technique that is causing our H_0 estimate to be somewhat off.

mr_mitm 3y ago

Both methods measure H_0, which is the Hubble parameter today. It's a bit confusing that it's sometimes called the Hubble constant, because it's not really a constant. The expansion rate H changes over time, so we write H(t) and by H_0 we mean H(t_today). It's probably the case that they didn't directly measure H_0 with the early universe probes, so they measured H at that time and then used the standard model to extrapolate this value to H_0, roughly speaking. This means, that if the measurements didn't suffer from some unknown systematic error, that the model is wrong. That's what they call "new physics" and could be the most exciting thing in cosmology since the 90s.

denton-scratch 3y ago
> because it's not really a constant. Way back when, we used to joke about "Murphy's Variable Constant", the fiddle factor required to make any equation balance nicely. It's sad that the Hubble Constant is now a variable.
- DiogenesKynikos 3y ago
  H(t) is not an arbitrary function. Up to an overall scaling factor (H_0), it's fully determined by the relative abundance of baryonic (normal) matter, dark matter, dark energy, radiation, and other constituents of the Universe.
  - denton-scratch 3y ago
    > it's fully determined Seriously? How can it be fully-determined by things we can't detect, let alone measure?
    - DiogenesKynikos 3y ago
      It's fully determined, given the densities of the various constituents I listed above. There are separate constraints on almost all of those densities, from different observations. For example, big bang nucleosynthesis puts constraints on the density of baryonic matter, dark matter and radiation, while the CMB essentially tells us the density of radiation. Cosmology is based on a web of different observations, tied together by well understood physics. It's very difficult to fudge things to match one observation without messing up the results for other observations.
      - T-A 3y ago
        > Cosmology is based on a web of different observations, tied together by well understood physics. It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the universe. [1] That doesn't quite live up to my idea of "well understood physics". [1] https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy https://science.nasa.gov/astrophysics/focus-areas/what-is-da...