I have actually made an attempt once in my life to read The Road to Reality by Roger Penrose. Nah, there are better popular science writers. Penrose is too hardcore of a mathematician.
Penrose had some interesting idea, regarding the cyclic cosmological model of the Universe. Namely, he argued that if entire matter and defined objects become loose energy, so that any possibility to physically measure time, disappears, the Universe eventually would reset, as if a new tree would grow from the trunk of a decayed one. He also speculated that it could be possible to see back in time, so to say, before the "big bang", to conclude upon the influence of earlier world iterations over the one we have now. Pretty crazy stuff. It is called "conformal cyclic cosmology". One may notice how the shape of a tribar figure and the conformal geometry, hint towards this theory. But it seems too esoteric. Anyway, I do not recommend reading Penrose though. Someone would do a good job writing a book having explained to the layman what does Penrose actually mean.
It is rather high speculation ground. Cyclic cosmology is oft dismissed nowadays, for the second law of thermodynamics. Complicated stuff, but in short, it says a universe is unlikely to go cyclic indefinitely, because the entropy level will never decrease, only increase, as long as the system remains hermetic. Entropy is like a summary of possibilities. The more the possibilities, the lower the entropy. The fewer the possibilities, the higher the entropy. Such as, the longer you live, biologically speaking, the less the adaptability you have, with increasing chance of death. Death, could be when you no longer deal with anything that may affect you. Maximum entropy, is when all energy is equal, thus no exchange ever occurs anymore. Penrose tries to outmaneuver the second law of thermodynamics with his theory. In science and exploration, no word is ever final though and definitely we do not know everything about the Universe, even whether it is a hermetic system. But we know it expands and this quite hints towards the entropic death scenario. The universe will grow too cold and dark to hold any life. But until then, many interesting things may happen. Even the Sun death is quite some time ahead, so I guess the entropic death is pretty superfluous thing to worry about, for now. By the way, Sun is not going to explode. Just in case. It will turn to a 'dwarf'.
Especially when we as a species only have decades of lifespans compared to those of larger celestial bodies. And yeah, there are many theories around, some more well-known than other, some with more evidences than the other some being disputed and some already disproven. And that's how science advances.
One of the things proven and most amusing to think about, is that all biological life emerges from the stars. It boils down to the chemistry. Primordial universe, was made of very few kinds of elements. The diversity, required for life to eventually emerge, has been cooked in the stars. With their death, they released this chemical diversity into shared space, having major physical forces do the rest. However to look at it, life is a crown to the creation and sentient life, a pearl in that crown. The question is, could we have life, not based on chemistry? This is exactly the question about what is 'alien'.