Based on structure formation and the lifetime of the universe why is there an upper bound on the cosmological constant? I understand that significantly greater values than the cosmological constant would result in difficulty for the formation of large gravitationally bound structures within the lifetime of the universe.
Could anyone add to this?
Thanks!
 A: Based on the current best values
(published in
https://www.cosmos.esa.int/documents/387566/38765/Planck_2018_results_L06.pdf)
for the five parameters parameters
(H_0, Omega_r, Omega_m, Omega_k, and Omega_Lambda)
used in the Friedmann equation,
https://en.wikipedia.org/wiki/Friedmann_equations#Detailed_derivation,
the cosmological constant is the last of these five parameters. I think  you are misunderstanding the variability over time of Omega_Lambda.
The equation appears to show that the value of Omega-Lambda is a constant over time, but this is wrong. The sum of the four Omegas is set to "1". The values of the four Omegas change as the variable "a" changes, getting smaller or larger as time progresses. As "a" get larger, the first three Omega get smaller terms. Each of the these three terms need to be divided by the sum of the revised four Omegas including the denominators of the corresponding "a" powers. Then the new value for Omega_Lambda is calculated by subtracting the values of the three revised Omega values (r, m,and, k) from "1". When you do this you will find that Omega_Lambda value gets larger, and approaches "1" as "a" approaches infinity.
This "1" value for the sum of the four Omegas represents the total relative energy density in the universe dynamics.
