I watched this excellent video by Eugene Khutoryansky - https://www.youtube.com/watch?v=GOrWy_yNBvY a while back explaining why life in the universe must end. I don't want to believe this but it seems inevitable since life needs energy and once you fuse matter all the way to Iron, its very hard to get more of that. Lets say there is a master civilization that has made extraordinary strides in technology and can easily perform tasks like creating stars, transporting matter quickly between two points, etc. Is there a plausible scenario where such a civilization could be immortal? Could they have some sort of cycle going that allows them to subsist not for a very long time, but indefinitely?
I'm no scientist, but it seems to me that any investigation of how to survive after we've lost our current supply of usable energy, should at least glance at the related question, "where did the energy come from in the first place?".
Your extremely advanced technical society could probably find the answer to that question and apply that answer to creating more energy from where-ever our current supply came from.
The short answer is that all processes will produce heat losses which are unrecoverable. There would be no way to fuel a civilization indefinitely, because only part of the energy input comes back as useful energy output; the rest radiates off into space.
Almost ... maybe ... for a very limited definition of forever.
Frank Tipler wrote a book titled The Physics of Immortality: Modern Cosmology, God and the Resurrection of the Dead.
I read the book when it was first printed. Long book, lots of equations and some pretty wild assumptions.
The basic idea is that computers get fast, AI becomes real, and we simulate all of the possible universes in a computer. Since the computer is so fast, reality inside the simulation passes very quickly, as in trillions times faster than out meat based existence (maybe even faster as the book never gave a limit that I recall). Also, the computer uses less and less energy to the point it can run all of the situations with consuming too much energy.
Tipler then also assumes a big crunch and the energy levels will rise. With the higher energy levels computers compute faster and faster with the higher energy levels kick in, allow more and more simulation.
Tipler also talks abound surviving the big crunch and setting the initial conditions for the next cycle. And oh yeah, we eventually become God, not just any God, but an infinite, omnipotent, omniscient being very similar to the Christian God.
Book is a serious snoozer. Generally physicists were pretty brutal in their reviews. But if you are ok with a simulated civilization, you can approximate infinity.
Of course, others might prefer the Heavenly civilization with the real triune godhead running the show eternally. Not some 2nd rate simulation you know.
If you're talking real civilizations, I'd say there's still a lot of questions whether a civilization can last forever. It's actually a pretty long standing philosophy question, with some colorful commentaries such as Nietzsche's treatment of The Last Man in Thus Said Zarathustra. There, the Last Man was a pocket of civilization which strived for nothing but immortality. Nothing mattered except things which could extend their society forever.
Nietzsche had some strong words to say about that mentality.
Regardless, since this is world building, we can talk about some rather interesting contrived civilizations, and leave it to the physicists to fill in the details. I am aware of at least one interesting approach to lasting forever which is thermodynamically consistent. It doesn't reach up into the stars, however, trying desperately to find more and more energy. Instead, it reaches inward. Thermodynamics requires entropy to always increase in a closed system, but it does not specify how much it has to increase. If you can find a way to create a system which sustains itself using an amount of energy proportional to the remaining energy in the system, then you can continue to progress using exponentially less and less energy. You effectively need to concentrate the usable energy into smaller or more subtle structures as time progresses.
If you're looking at the ability to compute things, Laudauer's principle indicates that the cost of changing a single bit is proportional to temperature, so in theory, if the temperature of the universe were to cool towards absolute zero, you may be able to stretch out your computation forever.
If you try to develop structures which are capable of this, you start to find "self awareness" to be very important. Without it, it is remarkably hard to ration the amount of energy you spend properly. Every fiber of your being needs to act in concert with every other fiber.
A tall task, but hey. Welcome to world building!
Aparently ancient spiritual quantum knowledge says that what is always was. Meaning that if there is life, Life will always be and always was.
The Human mind is a bit limited in its scope and overstanding of existencial concepts based on misconceptions about life and death and therefore finds it perplexing to accept the eternalness of life and energy.
The answer is Yes. But as always we must define "Subsist" to be able to answer the question in detail. :)