I hope this exact question is not a duplicate. I have looked into some high rated answers/questions, but found no satisfying/understandable answer. This is my current line of throught:
In the inflation model, a strong expansion of the universe is assumed during the period, after which we start counting time today. This inflation has a yet unknown cause and ends spontaneously.
Then a slower expansion happens which decelerates due to the energy content of the universe.
Eventually (due to the cosmological constant), the universe again accelerates to expand.
I do not understand (2). With inflation gone and dark energy not yet strong enough, what fuels this expansion?
It seems to boil down to two ingredients:
a) an initial expansion rate that is given by the termination properties of inflation via an athropic principle approach.
b) an "inertia" of the spacetime expansion, which is necessary to allow the inital value to carry forward into future several billion years.
I want to re-phrase my question: What causes spacetime to have "inertia", i.e. why does its expansion not react instantly on its ingredients?
Koschi made me aware of the Friedmann eqs. which describe what I was looking for. The simple picture drawn by popular news outlets that I knew, does not capture some aspects of these eqs.