# I Cosmological constant and rate of expansion of the universe

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1. Nov 28, 2017

### Ranku

Does the presence of the cosmological constant modify the rate of expansion of the universe even during the earlier deceleratingly expanding phase of the universe?

2. Nov 28, 2017

### Orodruin

Staff Emeritus
The effect of a cosmological constant of the size required to explain the current accelerating phase would be for all practical purposes negligible in the early Universe. Of course, the effect would still be there, but it would be extremely small in comparison to the other energy components of the Universe.

3. Nov 28, 2017

### kimbyd

To expand a little on Orodruin's point, the rate of expansion is a function of the average energy density of the universe. The density of a cosmological constant is, well, constant. When the universe was decelerating, the matter (and earlier, radiation) density was far higher. But the cosmological constant was the same.

For example, at $z=9$, distances in the universe were a factor of 10 smaller, which means that the average matter density was 1000x times higher than today (since density scales with the cube of distance). With the current normal + dark matter density at about 32% of the total density, at $z=9$ it would have been 99.8% of the total density, leaving a paltry 0.2% for dark energy. So, as Orodruin points out, its impact would have been very small indeed.

4. Nov 29, 2017

### Ranku

Thank you for the replies.