% of total energy. When I was your age, "entropy" was a concept introduced in hmmm freshman general chemistry and used in free energy and thermodynamics. It was (and still is, to some extent) a pretty abstract concept, having no "intuitive" meaning to me. It was 'just' a variable I needed to use to calculate heat, and various other thermodynamic quantities of interest. Why, you're thinking, is this guy talking about entropy??
After my undergraduate days, I read a book on Temperature and how it isn't true that two bodies in contact will eventually attain the same temperature (which is well known to astrophysicists; think of a column of air (above a gravitationally attracting mass). Anyway, the author pointed out (what should have been obvious to me, but was instead a revelation) that entropy and energy are two EQUIVALENTLY abstract concepts. You can no more point to real "entropy" than you can point to "energy" in the real world. Energy and entropy of properties (characteristics) of things (particles, systems, fields, objects,...) It is a logical mistake to think of them as being physical 'things', they are abstractions from physical things. So, when you say mass and energy are interchangeable, you need to be very, very careful that you do NOT mix up a physical thing (matter) with an abstraction (energy). (IOW, mass is NOT matter, rather mass is another abstract property of some objects, particles, and systems.) I don't know about you, but when I don't allow myself to equate mass with matter, my intuitive understanding of mass takes a major hit.
It is (it really is) a moot point if energy is really a conserved quantity. In cosmology/general relativity you can't just speak about energy being conserved, it doesn't work (meaning its not conserved, unless you really distort the meaning of the term). In special relativity we can think of the (possibly) conserved quantity as energy-mass-momentum. What this means is that the energy of a thing depends on its momentum (velocity) as well as its mass (and other things like potential energies (of, say, the chemical bonds). In General Relativity we also have to include stress or pressure into the equations for the conserved quantity, that is what (might be) conserved is energy-mass-momentum-stress. And yes, in G.R. gravity can be repulsive! (gravity is what separates G.R. from S.R. (mostly), and is why stress needs to be added to the conserved 'thing'). In cosmology, there are four terms in the equation(s) describing the state of the Universe: radiation, mass, the cosmological constant, and the Hubble constant (which isn't a constant!). Mass is often subdivided into baryonic mass (electrons, protons,...) and dark (ie. unknown) matter. I should also mention that what equations you must use depend on the model you want to study, so don't expect consistency from one source to the next. This is one of those bootstrap problems, until you're a bit familiar with the various theories/models, its really difficult to make sense out of what seem to be inconsistencies in what people are saying. Anyway, last thing I should mention is that for chemistry, mass is conserved (except in nuclear chemistry), and for chemistry and physics of local (and non-relativistic) systems, the conservation of energy-mass is almost perfect...(We won't discuss whether the speed of the electron in a hydrogen atom is relativistic or not, nor the fact that gold metal gets its color from relativistic effects...) Energy is conserved, period.
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I forgot to mention that most Big Bang models have the Universe evolving so that each of the four terms dominated for a certain range of time. For instance, the Hubble "constant" is defined as ȧ/a, where ȧ =da/dt.
We can only guess whether H₀ (H zero, which is defined as H as it is "today") is increasing, decreasing or constant. Most models assume it has/is changing... Anyway, the H term dominated initially (since the other three terms have powers of t in the denominator). Interestingly, the powers of t (time after time=0) are all different for each term. This results in the evolution of the Universe - which term "dominates" the equation depends on what the value of t is. Anyway, the Universe went through the following 'evolution': Hubble dominated→radiation(energy) dominated→matter dominated→cosmological constant (dark energy dominated).
The fact that the expansion of the Universe is accelerating (and that seems to have just started not so many billion years ago) indicates that we are "just entering" the dark energy dominated Universe.
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If you are at all interested in this stuff, I recommend Susskind's "Theoretical Minimum" video courses. They are adult/continuing education and require no homework, although if you can get through them without taking notes, then you are a better man than I, gunga din. Each course is 10 lectures each lecture runs between 1h40m and 2 hr. The required background is Freshman Physics and calculus up to
partial differentiation. He solves some differential equations but they're just the basics - like dy/dx = x. The courses start with Classical Physics (dynamics) and then Quantum Mechanics, Relativity and finally Cosmology (also statistical mechanics). Its definitely Physics, not chemistry, but after taking them, I'm better able to understand Physical Chemistry (full disclosure: I actually liked Physical Chemistry as an undergrad - not that that meant that I didn't have to repeat a semester of it, LOL)
http://theoreticalminimum.com/courses
