According to our current scientific knowledge, how certain is it that heat death shall be the ultimate fate of our universe, and why? Are there any serious hypotheses competing with heat death, and if such hypotheses exist, what are they based on?
Thermodynamics – How Certain is the Heat Death of the Universe?
energy-conservationentropythermodynamicsuniverse
Related Solutions
Yes, and it would be very cold. The paper "finite temperature in a deSitter universe" explains that the cosmological constant (if it is really a constant) creates a "horizon" that acts somewhat like an inside-out event horizon: objects that get too far away from you are unreachable. This Horizon will radiate Hawking radiation at an extremely low temperature of 10^(-30)K; the wavelength range of light this corresponds too is the same order of magnitude as the horizon's radius (also the case for black holes). It's conceivable that some relatively compact system could have an excited quantum state so close to it's ground state that it is thermally accessible even at this extremely cold temperature. Thus, there is still some form of heat swimming around. However, there is no reservoir colder than this temperature to dump heat into so this heat can't be converted into useful energy.
From a thermodynamical point of view, living beings are able to reduce their entropy by exporting entropy to the external world. This does not contradict the 2nd principle, since living beings are open systems. For this reason, in a thermodynamically homogeneous universe (heat death), no change in the entropy can occur, and consequently no living beings (nor sentient beings) can survive.
But you asked about a non-living "robot", or whatever. In this general case, observing the universe means that the robot has to acquire information from the external world. It has to modify its internal state in order to store this information (for example, a picture of the universe, a measure of some physical quantity). This change of the information stored in the internal state corresponds to a decrease of the internal entropy of the robot. But again in this case, one cannot produce a gradient of entropy in a thermodynamically homogeneous universe. This would contradict the 2nd principle of thermodynamics.
One can say that the act of "observing" (which does imply a change in the information stored) the heat death is not compatible with the 2nd principle of thermodynamics.
Edit: Ok, let us suppose that the universe is thermodynamically homogeneous except for a very small region around the "robot", for a period of time $\epsilon$ before the heat death $t_0$, at which time the whole universe (including the "robot") is dead. It is a rather unprobable situation but let us assume so. At this point, what does the universe look like? All thermodynamical quantities are homogeneous, including temperature, density, and so on. Hence, there will be no recognizable structures (no stars no galaxies no anything) and all you could measure is a uniform and isotropic radiation at a temperature $T$ in all directions, similar to the cosmic microwave background. With the difference that the CMB that we measure today does reveal large-scale structures and it is not uniform in all directions. At the end of the day, the robot could describe the entire universe with a bunch of thermodynamical quantities, e.g., temperature and density. What I've written is highly speculative anyway, it is not taken for granted that the universe will undergo heat death in the first place. There are many other scenarios, I think.
Best Answer
we know that we don't know what the 70% of the energy of the Universe is. Also, a comprehensive description of the thermodynamics of the Universe is impossible with the current standard Cosmological model and Einstein's General Relativity. In particular it's very complicated, and incomplete, as I said, to talk about what is now the energy of the Universe. Hence, its thermodynamical evolution has very low sense, with the current knowledge. It's only a speculative subject.
For a more satisfactory answer, I can give you a brief naive explanation of why one can't talk about the energy of an evolving Universe. As you know and as we currently believe, we live in an expanding Universe. This is not a steady state, there is a precise time-line and a temporal asymmetry in its evolution, hence you can't define a globally a well defined conserved charge called energy, in the sense of the Noether's theorem. You can define energy only locally, according to the equivalence principle. Also, there is another problem called quantum vacuum whose energy is non-null and may even increase with the expansion of the Universe.