The entropy of the universe

Download a PDF of the paper titled From entropy to gravitational entropy, by Sarbari Guha Abstract: The concept of entropy forms the backbone of the principles of thermodynamics. R.C. Tolman initiated a correlation between gravity and thermodynamics. The development of black hole thermodynamics and the generalized second law of thermodynamics led to Penrose's conjecture that the Weyl tensor should serve as a measure of the entropy of the free gravitational field. This entropy reflects the degrees of freedom associated with the free gravitational field. The proposition of gravitational entropy justifies the initial entropy of the universe. This entropy function had to be associated with the dynamics of the free gravitational field from the time of the big bang, so that a gravity-dominated evolution of the universe preserves the second law of thermodynamics. Moreover, the concept of black hole entropy emerges as a particular case of the entropy of the free gravitational field. However, a self-consistent notion of gravitational entropy in the context of cosmological structure formation has eluded us till today. Various proposals have been put forward, initially based on Penrose's Weyl Curvature Hypothesis, and subsequently modified to fit the needs of specific geometries and matter distributions. Such proposals were basically geometric in nature. A few years back a new definition of gravitational entropy was proposed from the considerations of the relativistic Gibb's equation...

Looking back a variety of distances corresponds to a variety of times since the Big Bang. Entropy ... [+] has always increased from any moment to the next, but that doesn't mean that the Big Bang began with zero entropy. In fact, the entropy was finite and quite large, with the entropy density being even higher than it is today. NASA, ESA, AND A. FEILD (STSCI) One of the most inviolable laws in the Universe is the second law of thermodynamics: that in any physical system, where nothing is exchanged with the outside environment, entropy always increases. This is true not only of a closed system within our Universe, but of the entire Universe itself. If you look at the Universe today and compare it to the Universe at any earlier point in time, you’ll find that the entropy has always risen and continues to rise, with no exceptions, throughout all of our cosmic history. But what if we go all the way back to the earliest times of all: to the very first moments of the Big Bang? If entropy has always increased, does that mean that the Big Bang’s entropy was zero? That’s what Vratislav Houdek wants to know, asking: The answer, perhaps surprisingly, is no. The Universe not only wasn’t maximally organized, but had quite a large entropy even in the earliest stages of the hot Big Bang. Moreover, “organized” isn’t quite a sound way to think about it, even though we use “disorder” as an offhand way to describe entropy. Let’s unpack what it all means. Our Universe, from the hot Big Bang ...

We present a centennial review of the history of the term known as the cosmological constant. First introduced to the general theory of relativity by Einstein in 1917 in order to describe a universe that was assumed to be static, the term fell from favour in the wake of the discovery of the expanding universe, only to make a dramatic return in recent times. We consider historical and philosophical aspects of the cosmological constant over four main epochs; (i) the use of the term in static cosmologies (both Newtonian and relativistic): (ii) the marginal-ization of the term following the discovery of cosmic expansion: (iii) the use of the term to address specific cosmic puzzles such as the timespan of expansion, the formation of galaxies and the redshifts of the quasars: (iv) the re-emergence of the term in today's Λ-CDM cosmology. We find that the cosmological constant was never truly banished from theoretical models of the universe, but was marginalized by astronomers for reasons of convenience. We also find that the return of the term to the forefront of modern cosmology did not occur as an abrupt paradigm shift due to one particular set of observations, but as the result of a number of empirical advances such as the measurement of present cosmic expansion using the Hubble Space Telescope, the measurement of past expansion using type SN Ia supernovae as standard candles, and the measurement of perturbations in the cosmic microwave background by balloon and satellite. We ...

Key Takeaways • The central tenet of any Big Bang cosmological model is that the Universe evolves. • Yet that really should not be the case. It is far likelier the Universe would have been born in a state of high entropy that would have left little room for change. • What would a natural solution to the question of initial cosmic conditions, one without any fine tuning or special pleading, look like? This article is the third in a series exploring contradictions in the standard model of cosmology. We invite you to read the The central feature of all Big Bang cosmological models is a Universe that evolves. The past looked different from the present. The present will look different from the future. While these may seem innocuous statements, why the Universe evolves is actually a big mystery. So much so, in fact, that astrophysicist Fulvio Melia included the question in his recent paper where he listed reasons the standard model of cosmology may need to be replaced. Today, as part of my The Universe in dead equilibrium The problem of the Universe’s past has a long pedigree and is linked to one of the most important ideas in all of physics: entropy and the second law of thermodynamics. Entropy is a physicist’s way of saying disorder. According to the second law, any isolated system must evolve from states of low entropy to states of higher entropy. Disorder always increases. If you start with a bunch of atoms all crowded into one corner of a box, they will naturally evolve to ...

Hugh Ross May 10, 2010 Entropy measures the amount of decay or disorganization in a system as the system moves continually from order to chaos. By that definition I have one of the most entropic offices at Reasons to Believe. More discouraging yet, the entropy in my office is increasing. But I am not alone. No matter if it’s an office or something much grander like the Moon, Sun, and even our own galaxy, we observe an increasing buildup of decay or entropy. Astronomers observe that the entropy measure of the whole universe is increasing at an astonishing rate. But astronomers were not the first to take note of the universe’s headlong rush into increasing disorder and decay. Some 1,800 years before astronomers’ discovery of entropy, the apostle Paul under the inspiration of the Holy Spirit wrote, “The whole creation groans…subject to its bondage to decay.” 1 The entropy measure of the universe is important for several other reasons. • It determines which features of the universe are reversible and which are not. • It constrains the evolutionary history of the universe. • In so doing, it can establish which cosmic creation models are valid and which are not. Thus, measuring the entropy level of the universe sheds light on both the beginning and the end of cosmic history. In 2003, 2008, and 2009, different teams of astronomers 2 In February 2010, two Australian astronomers, Chas Egan and Charles Lineweaver, 3 (All medium- and large-sized galaxies contain at their centers supe...

Universe means the system along with its surroundings. I have always got this statement while studying the second law; be it a thermodynamics book (Sears, Salinger) , physical chemistry book (Atkins, Paula) or any site. But really how does this happen? Take for example, the isothermal expansion of gas; the increase in entropy of the gas(which is our system) is given as $$\Delta S_$$ Therefore the entropy if the universe is what? $0$; Then how can the entropy of the universe increase? It remains the same! Then why is the statement telling otherwise? $\begingroup$ By universe I mean what everybody else means: the entirety of everything that exists. That includes the impossibility of boundaries. That, of course, is not a problem from a thermodynamics point of view. The problem here is that the universe is not homogeneous, which already precludes any naive application of thermodynamics models that are homogeneous, like isothermal gas. $\endgroup$ Really how does the entropy of the universe increase? The statement "entropy of the universe increases" is a doubtful extrapolation of thermodynamics. The problem with that statement, as CuriousOne has written in the comments, is that universe is not a closed system amenable to thermodynamic description. It has no volume, no temperature and we have no means to experiment with it the way we do with thermodynamic system such as gas in a piston. For example, heat exchange between the Universe and environment has no meaning because there ...