We know that due to entropy, as outlined in the second law of thermodynamics, the present universe has a finite life span and yet we are promised eternal life.
Eternity, by definition, is a long time that is not in any way represented by a colossal number of years. It is not 10 to the power of 40 years (with the 40 being the number of noughts after the ten) and neither is it 10 to the power of 6000 years. In fact, it would be 10 to the power of infinity years which brings us right back to where we were before we started the calculation. Finite creatures are totally unable to understand that which lasts forever.
When it was discovered that the universe was dying as an outcome of the uncompromising effect of the second law, many scientists, philosophers, and Christians became despondent at the prospect. Bertrand Russell wrote in his book, 'Why I am not a Christian:' "All the effort of the ages, all the devotion, all the inspiration, all the noonday brightness of human genius, are destined to extinction in the vast death of the solar system and the whole temple of man's achievement must inevitably be buried beneath the debris of a universe in ruins."
Infinity is representative of a time-scale that is qualitatively different in the sense that it never comes to an end so how can this be for us unless there is either a dimension that we will eventually inhabit that does not obey the known laws of physics, or our Lord is a liar or simply mistaken? As both of the latter propositions are, to me, totally inconceivable, I therefore believe Him absolutely when He tells me that life for eternity awaits those that take Him into themselves, and that eternity means just that.
I have written elsewhere about the non-existence of either the past or the future and the fact that whilst time does exist, it is only relevant to a finite, memory/hope-possessing consciousness such as ours. So it is conceivable that eternity is simply a place where linear time does not exist and therefore does not possess meaning and those who inhabit that place will, therefore, simply 'be'.
Each moment of eternity will be a fresh and joyful experience. Presumably, as we are promised, we will be perfected and constantly fulfilled with the joys of a continuous Holy present. There will be no reason to examine either possibilities or likelihoods as we will be too busy taking joy from the source from which they all come.
Bertrand Russell appeared to believe the afterlife that has been promised us does not inhabit a different dimension and therefore eternity depends for its existence on the continuation of the existing physical universe. The universe, as we see it, was created that we might come into existence and develop. If it had not been so gigantic it would not have lasted a sufficiently long time (12-20 billion years) to enable humankind to emerge and develop to its present state.
In Scripture we are told the world as it is currently formed will not last forever, unlike those who are Resurrected.