Is the Hubble Constant Truly Constant?

[ea251ah]

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?

 

[phyzguy]

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.

 

[ea251ah]

Thanks for the clarification. So the discrepancy among estimates is still a conundrum?

 

[phyzguy]

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.

 

[kimbyd]

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.