Our knowledge of the universe is still negligible, and we can not confidently assert that the universe is not under the influence of external forces, or may be considered as a thermodynamic system. However, it is the concept of heat death was the first step to realize the possible finiteness of the Universe, although we do not know when and on what scenario will happen of its destruction.
At the present stage of existence (13.72 billion years), the universe radiates as a black body with a temperature of 2,725 K. Its maximum to the frequency 160.4 GHz (microwave radiation), which corresponds to a wavelength of 1.9 mm. It is isotropic up to 0,001% - the standard deviation of temperature is approximately 18 IWC.
The heat death is a possible final thermodynamic state of the universe, in which it has "run down" to a state of no thermodynamic free energy to sustain motion or life. In physical terms, it has reached maximum entropy. The hypothesis of a universal heat death stems from the 1850s ideas of William Thomson, 1st Baron Kelvin who extrapolated the theory of heat views of mechanical energy loss in nature, as embodied in the first two laws of thermodynamics, to universal operation.
The idea of heat death of the universe derives from discussion of the application of the first two laws of thermodynamics to universal processes. Specifically, in 1851 William Thomson outlined the view, as based on recent experiments on the dynamical theory of heat, that "heat is not a substance, but a dynamical form of mechanical effect, we perceive that there must be an equivalence between mechanical work and heat, as between cause and effect." [1]
In 1852, Thomson published his "On a Universal Tendency in Nature to the Dissipation of Mechanical Energy" in which he outlined the rudiments of the second law of thermodynamics summarized by the view that mechanical motion and the energy used to create that motion will tend to dissipate
References: [3] Smith, Crosbie & Wise, Matthew Norton. (1989). Energy and Empire: A Biographical Study of Lord Kelvin. (pg. 500). Cambridge University Press. [4] Thomson, William. (1862). "On the age of the sun’s heat", Macmillan’s Mag., 5, 288-93; PL, 1, 394-68. [5] Physics Timeline (Helmholtz and Heat Death, 1854) [6] http://www.physlink.com/Education/AskExperts/ae181.cfm [7] "An introduction to cosmological inflation". proceedings of ICTP summer school in high-energy physics, 1998. Retrieved 2006-09-09. [8] "Black holes and thermodynamics". Phys. Rev. D 13, 191–197 (1976). Retrieved 2006-09-09. [9] "Thermodynamics of black holes in anti-de Sitter space". Comm. Math. Phys. 87, no. 4 (1982), 577–588. Retrieved 2006-09-09. [11] A dying universe: the long-term fate and evolution of astrophysical objects, Fred C. Adams and Gregory Laughlin, Reviews of Modern Physics 69, #2 (April 1997), pp. 337–372. [12] Particle emission rates from a black hole: Massless particles from an uncharged, nonrotating hole, Don N. Page, Physical Review D 13 (1976), pp. 198–206.