If deep space is shown as it was in the past how can we calculate this time lagging regarding the Hubble parameter which is a variable of time?

  • $\begingroup$ It is (at least approximately) a reciprocal of the age of the universe. $\endgroup$
    – safesphere
    Feb 16, 2020 at 9:03
  • $\begingroup$ I flagged this as a possible duplicate since you asked how it can be calculated. It is not a duplicate if you are asking how it can be measured. $\endgroup$
    – ProfRob
    Feb 18, 2020 at 18:51
  • $\begingroup$ @RobJeffries Please let me time to answer...(1 hour) $\endgroup$ Feb 18, 2020 at 18:53
  • $\begingroup$ @RobJeffries ***measured!****Does Ho being infinite at t(o) mean that the energy for inflation is not changing with time? $\endgroup$ Feb 18, 2020 at 19:33
  • $\begingroup$ If you are not asking about measurements, then your question is a direct duplicate. $\endgroup$
    – ProfRob
    Feb 18, 2020 at 19:57

1 Answer 1


The Hubble parameter does vary with time, in a way that depends on the cosmological parameters or the adopted cosmological model.

Observations of the redshifts of objects at known distances, over a broad range of distances, samples the expansion history of the universe over the time it takes for the light to reach us from the most distant objects in the sample.

It is exactly this type of measurement, using type Ia supernovae, that has led to the conclusion that the expansion of the universe was decelerating, but then began to accelerate from about 6 billion years in the past onwards.

Having established what the cosmological parameters are, then our cosmological model tells us what the Hubble parameter was at any cosmic epoch, but actually you can view the whole process as modelling the variation of the Hubble parameter with time to yield the cosmological parameters.

A more technical discussion is here.

  • $\begingroup$ If we make Earth stationary and a distant galaxy moving away would the photon emitted from that galaxy towards Earth have constant velocity regard Earth if space is inflating? $\endgroup$
    – jbradvi9
    Feb 16, 2020 at 19:52

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