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Can dark energy be a different dark matter?

  1. May 29, 2014 #1
    I'm not a physics student (chemistry instead), and know extremely little about relativity, so this might be a dumb question and would appreciate simple answers. I'm just curious about this. :)

    So there is dark matter, which I understand to be the matter that is not detected but needed to account for galaxy motion...and then there is dark energy which is the energy not detected but needed to account for universal expansion.

    But, I thought that energy and matter were in a way interchangeable because E =mc^2 . So could you not say that what is missing to account for universal expansion is also missing matter instead of energy? Could it be that, instead of saying it was dark energy, they called it dark matter2 or something?

    I guess what I'm wondering is, is it valid to think of the "missing component" that would help explain the universe's expansion as matter instead of energy?

    And also, it makes sort of intuitive sense to me that dark matter would be used to explain missing gravitational attractions, since matter has gravity, but it wouldn't make sense in my head for matter to cause expansion. Energy I can picture causing expansion. But again, I feel like what I have heard is that matter and energy are interchangeable, so can someone clarify this?

    Thank you :)
  2. jcsd
  3. May 29, 2014 #2


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    Energy is a property of matter, but matter has other properties as well. Also, there are other forms of energy besides matter. So they are not synonymous.

    Nature does not always make intuitive sense! :wink:

    Dark energy is believed to be symptomatic of the Cosmological Constant. This is what best fits the evidence. There have been other proposals made, such as a time-varying field called quintessence.
  4. May 29, 2014 #3


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    Just to be clear, dark matter IS detected, but by virtue of its gravitational effect rather than as individual particles. It is believed to most likely be a particle, probably a WIMP of some kind, but has not been detected in that form (yet).
  5. May 29, 2014 #4
    Ohhhh! Okay that makes sense!

    So another related question, which might be a dumb one.... So when I see something that says the universe is made up of 70% dark energy and 23% dark matter, I feel like then, these two things are not relatable in the same way. I would think you would talk about the composition of the universe by mass, or the composition of the universe by energy...but not together. Since they are two different things.

    Or are they just using the E=mc^2 approach to convert the amount of energy needed from dark energy into a mass? I guess I'm confused as to what it means when I see a pie chart showing the composition of the universe and theres a percentage of dark matter and a percentage of dark energy, % by what exactly?
  6. May 29, 2014 #5
    % 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.
    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.
    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
    Last edited: May 29, 2014
  7. May 29, 2014 #6


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    Curse the cruelty of astronomical nomenclature. The term 'dark' was handy to describe dark matter because it did not emit light. It was long thought it could just be ordinary matter that was too cold to glow. Over the decades it became clear the odds of that explanation are roughly equal to that of finding the winning lottery ticket in a dumpster. The term dark energy was just damned meanness. People unfamiliar with the twisted humor of physicists are led to deduce they must somehow be related, and others to endlessly explain they are about as closely related as a light sabre and low fat salad dressing.
  8. May 29, 2014 #7


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    Specifically, the twisted humor of Michael Turner, who coined the term Dark Energy in 1998. For a further example of his humor, see here.
    Last edited: May 29, 2014
  9. May 30, 2014 #8


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    The percentages are percentages of the total mass/energy density of the universe. E = mc^2 is used to convert back and forth between mass units and energy units, so everything can be compared on the same scale. Which units you choose to use (mass or energy) depends on who's doing the choosing and what for.
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