It is believed by many that there is something called 'absolute reality', and that it is a different matter whether or not we humans can know all of it. Certainly there are limits to what we can know. But even within the knowable domain, reality today may not be the same as reality in the future. I shall come to this time aspect later in this post. But even at a particular instant of time, there is no such thing as absolute reality. As argued convincingly by Hawking & Mlodinow(2010), all that we have is MODEL-DEPENDENT REALITY; any wider or deeper notion of reality is a baseless myth.
Does something or somebody exist when we are not viewing it? There are two opposite models for answering this question. Which model of reality is correct? Naturally the one that is simpler, self-consistent, and most successful in terms of its predicted consequences. This is where materialism wins hands down. The materialistic model is that the entity exists even when nobody is observing it. This model is far more successful in explaining reality than the opposite model. And we can do no better than build models of whatever there is to observe, understand and explain.
Suppose 100 persons are asked to describe an object, including its colour, and all of them say that it is a chair. Further, suppose 98 of them say that it is a red chair, but the other two disagree about the colour seen by the majority. If further investigation shows that these two persons have a colour-blindness problem, the model of reality we humans build is that the object is a red chair.
But suppose it turns out that these two persons are not colour blind, and no matter what tests we carry out, we are unable to explain why they do not see or describe the chair as red. We then go (tentatively) by the majority view, or consensus. Of course, any model of reality can change in the light of new data and insights. This is the approach we adopt in science for building up our knowledge. We build models and theories of reality, and we accept those which are most successful in explaining what we humans observe collectively.
A model is a good model if: It is elegant and self-consistent; it contains no or only a few arbitrary or adjustable parameters; it explains most or all of the existing observations; and it makes detailed and falsifiable (i.e. testable) predictions.
That brings me to the M-theory (cf. Part 14) and the cosmic-inflation model (Part 17) in cosmology. Are they good models of reality? There are eminent scientists who vehemently attack both of them, and have even proposed alternative models. Nothing unusual about that. At the cutting edge of science, the edge is blunt or nebulous, rather than sharp: Experts disagree on many issues, and fight it out. But out of this informed debate consensus emerges gradually, usually when additional ('issue clincher') data become available, or when some genius formulates a great new model. M-theory and the multiverse idea are the most accepted we humans have at present, even though there are many arbitrary-looking parameters, and loose ends. In due course the models would get either established or rejected, but they are the best (i.e., most accepted, even beautiful) models of reality at present.
The cosmic-inflation model of reality ties up so many loose ends in cosmology, and explains so many observations, that some form of it is highly likely to survive in any scientific version of cosmology.
Reality is nothing deeper than the best available scientific model for it. Often a phenomenon or entity is so complex that no sensible model has been formulated yet. In such a case, we have to wait till science makes more progress.
I now come to the time dependence of reality. Let us consider our current understanding of the origin of our universe. Scientists agree, by and large, that our universe began with a Big Bang 13.72 billion years ago. This model stands on three pillars: (i) the observed 'Hubble expansion' of the universe; (ii) the observed cosmic-wave background; and (iii) the fantastic agreement between the predicted and observed relative abundances of the light elements hydrogen, helium, and lithium. Lawrence Krauss has discussed them in detail in his recent (2012) book A Universe from Nothing. I shall consider only the first of them here.
Observations show that the rate of expansion of our universe is increasing (cf. Part 15). What is more, the observable universe is expanding at present at a rate that is not much lower than the speed of light. Galaxies which we can see today will one day recede from us at a speed greater than the speed with which even the fastest possible signals (electromagnetic radiation) from them can reach us. They will then become invisible to us, permanently. This will happen ~2 trillion years from now.
We and our solar system will die out in ~5 billion years. But other civilizations and advanced science can emerge elsewhere in the distant future. Suppose you are one of those astronomers in the very distant future. When you look around the cosmos with your highly advanced telescopes, you would see none of the ~400 billion galaxies we humans see today. What will be your model of reality then? Certainly not the same as ours at present. There is no such thing as absolute, unique, invariant reality.
We are 'lucky' to be living in a period when evidence for the Big Bang is available. Astronomers far enough into the future will have no evidence and no reason to believe that the Big Bang occurred at all, and that there were galaxies other than their own.
We live at a very special time . . . the only time when we can observationally verify that we live at a very special time! (Lawrence Krauss et al.)
You can see a timeline of the far future in the Wikipedia.