Is the Hubble Constant Truly Constant?

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Discussion Overview

The discussion revolves around the Hubble constant (HC) and whether it is truly constant over time. Participants explore various estimates of the Hubble constant derived from different sources, including the Cosmic Microwave Background (CMB), Cepheid variables, and Type 1 supernovae, and the implications of potential variations in the Hubble constant on these estimates.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Exploratory

Main Points Raised

  • Some participants note that Hubble constants have been estimated from various sources, but not all estimates agree within their error bounds, leading to questions about whether this discrepancy represents a conundrum.
  • One participant suggests that if the Hubble constant varies with time, it could reconcile the different estimates based on the average age of the universe for each emission source type.
  • Another participant argues that the variation of the Hubble constant with time is already accounted for in the estimates, which are extrapolated to the current time using the Lambda-CDM model.
  • There is a claim that the actual Hubble constant value at the time of the emission of the CMB was significantly greater than the current value, indicating that the rate of expansion has changed over time.
  • Some participants express that the discrepancy among estimates could be due to larger error bars in measurements or a need for changes in the cosmological model, but which of these is the source of the discrepancy remains debated.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the discrepancies in Hubble constant estimates represent a true conundrum or if they can be explained by existing models and error margins. Multiple competing views remain regarding the implications of the Hubble constant's potential variability.

Contextual Notes

The discussion highlights limitations in the current understanding of the Hubble constant, including the dependence on the Lambda-CDM model and the assumptions made in extrapolating estimates to the present time.

ea251ah
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Hubble constants (HCs) have been estimated based on the CMB, on Cepheid variables, gravitationally lensed quasars, Type 1 supernovae, and red giant luminosity. Not all agree within their estimated error bounds. This has been represented by some as a conundrum. Is it? The mean age of the universe when each type of source emitted its electromagnetic radiation differs among the source types. E.g., the CMB was emitted within a few hundred thousand years of the Big Bang, according to the prevailing theory, and the value estimated by the ESA Planck Mission based on its analysis of the CMB, 67.8 km/sec/MPc, is the lowest of current estimates. If the HC varies with time, could not the various HC estimates be reconciled based on the average age of the universe for each emission source type?
 
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The variation of the Hubble constant with time is already taken into account in comparing these estimates. All of the estimates are extrapolated to the current time using the accepted Lambda-CDM model of the universe. The actual Hubble constant value at the time of the emission of the CMB radiation was approximately 10^6 times greater than what we see today.
 
Thanks for the clarification. So the discrepancy among estimates is still a conundrum?
 
ea251ah said:
Thanks for the clarification. So the discrepancy among estimates is still a conundrum?
Yes, either the measurements have larger error bars than the given estimates, or our cosmological model needs to change. Which of these two is the source of the discrepancy is still debated. You might read through this thread.
 
phyzguy said:
The variation of the Hubble constant with time is already taken into account in comparing these estimates. All of the estimates are extrapolated to the current time using the accepted Lambda-CDM model of the universe. The actual Hubble constant value at the time of the emission of the CMB radiation was approximately 10^6 times greater than what we see today.
To clarify a bit, the Hubble constant is the rate of expansion now. The rate of expansion was very different in the past. The Hubble constant itself doesn't change, as it just sets the overall scale of the rate of expansion, with the change in expansion over time determined by the mass/energy content of our universe.

The rate of expansion at the time the CMB was emitted, for instance, was tens of thousands of times greater than the rate of expansion today. But we still use the same constant to describe both because you can calculate the rate in the past given the current rate and the contents of our universe.
 

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