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Ordinary Energy - Is it Chopped Liver?

  1. May 13, 2008 #1
    From Scientific American:

    "Data gathered so far suggests that just 5 percent of the universe is made up of ordinary matter; the rest is dark matter (23 percent) and the negative gravity force called dark energy (72 percent)."

    So the universe is made up of 0% "ordinary energy"?
  2. jcsd
  3. May 13, 2008 #2


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    I would think ordinary matter and ordinary energy are basically the same thing.
  4. May 13, 2008 #3
    But dark matter and dark energy are NOT basically the same thing?
  5. May 13, 2008 #4


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    Yes, dark matter and dark energy are not the same thing. The 5% figure quoted is roughly the energy density wrapped up in the mass of ordinary matter. I'm not sure what you mean by 'ordinary energy'? The kinetic energy of the mass in the Universe is relatively small compared to the rest mass energy, so doesn't contribute very much to the energy budget of the Universe. The only other energy term of note is radiation, which I guess could be called 'ordinary energy' since we are very familiar with it. It turns out that the total energy in radiation is also very small, so it usually neglected when giving the the 5/25/70 % breakup of the Universe. In reality radiation contributes a small amount to that total as well, but it is something much smaller than a percent.

    But to re-iterate, dark matter and dark energy are totally separate things, at least in the standard theory although since we really have very little idea what either of them are anything is possible at this point.
  6. May 13, 2008 #5
    Well, with all due respect, what I mean by "ordinary energy" is any energy that is not "dark energy". I thought that would be self-evident.
  7. May 13, 2008 #6


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    But clearly it's not self-evident right? Dark matter is a form of energy that is not dark energy nor what you might call 'ordinary energy'. I'm really not sure what the attitude is for, I just answered your question?
  8. May 13, 2008 #7
    I can assure you that any attitude does not originate with me.

    Clarifying the terminology, then the universe is made up of:

    "dark matter" energy, "dark energy" energy, and "ordinary matter" energy.

    Have I got that right?
  9. May 13, 2008 #8


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    Really? Odd because I can only see polite questions and instructive answers up until post #5.

    Yes, they are the three dominate energy components. As I said before there is also radiation, which contributes much less to the total than those three.
  10. May 13, 2008 #9


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    Here's the thing, when people say "Dark Energy" it's not neccessarily the same concept as energy when we think of "Ordinary" matter. It's merely a term that was coined to help us describe whatever it is that's accelerating the universe. Dark Matter, on the other hand, is simply some form of matter we cannot see (observe, by any means other than gravitationally so far). Some theories do include exotic particles as the source for this unseen mass, which, if you like, can be considered extraordinary matter. However, a good deal of theories believe that dark matter is ordinary particles that are simply undetectable by present means. Possibilities include such prospects as MACHOs and WIMPs. I think it's an issue of terminology here, because dark energy and dark matter are in NO way connected to each other in the fashion ordinary matter and energy are.
  11. May 14, 2008 #10
    Thanks for a very clear, logical response and understanding that "it's an issue of terminology". IMHO, there is obfuscation which should be avoided and/or eliminated in many of these "cosmological" terms. Just because we don't fully understand doesn't mean that we have carte blanche to be imprecise and unclear.

    It seems to me that what people are calling "dark energy" would be more accurately characterized as a "force" rather than energy.

    I guess I don't understand why it seems difficult to pin down so many of these cosmological terms and definitions when in most science, engineering, and math it is paramount to define terms, axioms, operations, etc.
  12. May 14, 2008 #11
    OK as I understand the BIG BANG idea
    matter and anti-matter were created as everything cooled
    and the matter and anti matter almost canceled each other out
    leaving a 3 to 5% matter result with 90+% of everything in the dark stuff class

    SO did that some how create the dark stuff
    or just create light and heat
    and the dark stuff form out of the big bang directly
    if so what happened to the energy created by the matter, antimatter
    as the microwave background at a few degs about zero doesnot look to be enuff
    for all that energy

    side question
    if we create anti matter and let it hit matter does the energy balance
    or is there any missing bits that could turn in to the dark stuff
  13. May 17, 2008 #12


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    Hello Fluxman,
    are you still having problems with that sentence you quoted? Or has it been taken care of.

    Here is a way you could help in future, if you want. give a link to the place where you read something that seemed puzzling, like whatever page in the online SciAm

    or at least give an author date and page reference if it was in print, so one can find it online.

    people writing for a general audience will often cut corners, for journalistic reasons.
    then, especially if you take it out of the context where some terms may be explained, it can be pretty confusing

    one thing that is done at PF sometimes is to dig a little deeper and get closer to the actual science writing---that you find in professional journals. there the concepts are usually a bit more carefully defined

    in the professional literature they wouldn't give the impression that they KNOW what "dark energy" is. that is a goal of research.
    there is an equation called the Einstein equation and you can write it two ways with a term called "cosmological constant" on the left (and no dark energy)
    or alternatively you can write it with a dark energy density term on the right.
    either way comes to the same and either way fits the data.

    and there is a simplification of the einstein equation called the Friedmann equation and the same thing happens.
    you can either put in a cosmo constant which is a tiny constant spacetime curvature, or you can put in an energy density
    (of about 0.6 joules per cubic kilometer)
    and it works. it fits the data beautifully. tons of data.

    so something is there, an extra unexplained curvature (on one side of the equation) or an unexplained evenly distributed constant energy density (on the other side) and people can have various ideas they speculate and stories they tell themselves about but no scientist, I think, ever claims to KNOW what underlies this.

    Wallace or some of the others can correct me if I am wrong about this.

    Anyway it is a great time to be a cosmologist because new orbital instruments are going up and new types of groundbase telescopes are being built and a big effort is going into investigating the cosmological constant and trying to understand where it comes from
    (is there really a very dilute energy field of 0.6 joules per cubic km? or is there some reason that spacetime should have this tiny extra curvature? or is there some other explanation? a small correction to the Einstein law of gravity?)

    the one thing you don't want to do however is start scolding a popular science journalist because he gives you an imprecise idea. he has to give you an imprecise idea because he is writing for the general public. what he says is not representative of the concepts or state of knowledge.

    that's just my point of view, for whatever it's worth to you :smile:
    Last edited: May 17, 2008
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