# Is the Hubble constant a constant or is it a parameter?

• I
• Angela G
In summary, the Hubble rate varies with time. The nomenclature of the "Hubble constant" comes from the original observations of recession velocity being proportional to distance and the proportionality constant being the "Hubble constant".
Angela G
TL;DR Summary
Hello!
I was solving an exercise about the Hubble constant and the Hubble parameter and got that the Hubble parameter ( at t = 1/2 t_0, where t_0 is the actual age of the universe) is 8/9 smaller than the Hubble constant. So my question is if we should consider this difference and talk about the Hubble parameter instead of the Hubble constant? In other words: Does the "Hubble constant" variate with time?
(P.S! this is an open question, feel free to answer)

Yes. The Hubble rate varies with time. The nomenclature of the "Hubble constant" comes from the original observations of recession velocity being proportional to distance and the proportionality constant being the "Hubble constant".

Angela G
As far as nomenclature goes, Hubble constant is the value of the Hubble parameter at t_0. By definition ##H_0## doesn't vary in time. If you want to talk about the time-dependent variable you refer to the Hubble parameter ##H(t)##.

Angela G
Also, if you ended up calculating the value of H at an earlier time as being lower than ##H_0##, then you need to check your work.

Angela G
Open problem. Two different methods to determine value get different nos. (67 and 74) which are too far apart.

Angela G
Orodruin said:
Yes. The Hubble rate varies with time. The nomenclature of the "Hubble constant" comes from the original observations of recession velocity being proportional to distance and the proportionality constant being the "Hubble constant".

If I'm correct, we in fact do not measure the Hubble constant at a given time, but its average value over the time period for the observed object. So, looking at different objects at different distances from us, we would also in fact measure different values for the Hubble constant., as with increasing distance we also look further into the past in which the value of the Hubble constant would have take on different values.

Angela G
elcaro said:
If I'm correct, we in fact do not measure the Hubble constant at a given time, but its average value over the time period for the observed object. So, looking at different objects at different distances from us, we would also in fact measure different values for the Hubble constant., as with increasing distance we also look further into the past in which the value of the Hubble constant would have take on different values.
For low z, as was the case with early observations, the change in the Hubble parameter is low enough that it approximates a constant. For higher z there is indeed a growing deviation from linearity. However, the recession velocities for individual objects don't change fast enough to be observed, so there's no need to average them.

Angela G and elcaro
Bandersnatch said:
For low z, as was the case with early observations, the change in the Hubble parameter is low enough that it approximates a constant. For higher z there is indeed a growing deviation from linearity. However, the recession velocities for individual objects don't change fast enough to be observed, so there's no need to average them.
Also, the difference from non-linearity is a result of looking into the past. According to the main model currently used, recession velocities are exactly proportional to distance. But that proportionality changes over time, so when we look far away we are observing an older (and higher) value of this proportionality.

## 1. What is the Hubble constant?

The Hubble constant is a measure of the rate at which the universe is expanding. It is denoted by the symbol H0 and has units of kilometers per second per megaparsec (km/s/Mpc).

## 2. Is the Hubble constant really a constant?

No, the Hubble constant is not a true constant. It is actually a parameter that describes the expansion rate of the universe at a specific point in time. The value of the Hubble constant has been found to vary slightly over time and in different regions of the universe.

## 3. How is the Hubble constant measured?

The Hubble constant is measured using a variety of techniques, including observations of the cosmic microwave background radiation, the motions of galaxies, and the brightness of certain types of stars. These measurements are then used to calculate the value of the Hubble constant.

## 4. Why is the value of the Hubble constant important?

The value of the Hubble constant is important because it helps us understand the age, size, and future of the universe. It also provides insights into the nature of dark energy, the mysterious force that is causing the expansion of the universe to accelerate.

## 5. How has our understanding of the Hubble constant changed over time?

Our understanding of the Hubble constant has evolved over time as new and more precise measurements have been made. Early estimates of the Hubble constant were much higher than current values, and it is still a topic of ongoing research and debate in the scientific community.

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