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The constancy of the energy of the universe?

  1. Jul 29, 2008 #1

    Given that the universe is a closed and isolated system, suppose it should satisfy the following equation:

    However, it appears to be that it is impossible that the energy of the universe can be constant after all. Specific higher dimensional theories (such as String/M-Theory) predict that gravity can leak through higher dimensions and so called p-branes due to its special properties (roughly speaking closed loop strings cannot be attached to the p-branes because there are no loose ends). Thus, energy of the universe is variable because it simply leaks.

    I would also like to point out the fact that QM may have parallel universes. This could have an impact of the total energy of the universe as well, primarily because the gravity may leak from such universes to our one.

    Indeed it appears to be that the universe is not isolated at all and the total energy may be increasing or decreasing to that respect.

    Thanks for all the responses!

    Best regards, Cygni
  2. jcsd
  3. Jul 29, 2008 #2


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    You seem to see it as a foregone conclusion that those theories are true. You're a tad bit premature.
  4. Jul 29, 2008 #3
    Indeed I am slightly premature, I just finished my GCSEs. But i was just wandering whether my statement is correct or not. I am not trying to show off or anything - I am being curious because I enjoy physics and i want to know the answers.

    Best Regards, Cygni.

    P.S. Thanks for your response anyway.
  5. Jul 29, 2008 #4
    Indeed it looks like it. I know the certain implications that arise in string/m-theory which look pretty ugly though (e.g. working on background-dependent theories constrained by a lot of symmetries, 10^500 of vacuums etc.). String/m-theory is not proven, however such assumptions that I have stated do have some sense in that respect.

    Best regards, Cygni.
  6. Jul 29, 2008 #5

    Please correct me if I read your post wrong but first you state that the Universe is a closed system, then you state that gravity "leaks" out of it. That would imply that the universe isn't a closed system wouldn't it?
  7. Jul 29, 2008 #6
    No, I am not wrong, closed system in thermodynamics refers to that matter may not cross the boundary, this does not apply to energy, however, thermodynamics does not fully incorporate with string theory because one can imply that EM or S or W forces would be able to escape the boundary but they cannot due to specific dynamics and properties of such strings.

    Best regards, Cygni.


    I would like to apoligise to the forum readers - I have made a terrible mistake - in my first post I have stated that universe is an isolated system - what i meant was tat universe is partly closed system which allows only gravity to "leak". The reason why is written above.
    Sorry again about the inconsistencies.
    Last edited: Jul 29, 2008
  8. Jul 29, 2008 #7
    Does the universe "leak" Hawking radiation at its event horizon? Do two separate observers perceive differing cosmological boundaries, perhaps differing energies, for the universe? Are fundamental constants, like the Planck energy, truly invariant over all spacetime? Is local vacuum energy indicative of its global manifestation of energy conservation?
  9. Jul 30, 2008 #8

    Unfortunately, the universe itself cannot "leak" excluding special cases of black holes where the matter may escape due to the fact that black holes can have connections to the other universes.

    There may be very small amount of evidence that two separate observers would perceive differing cosmological boundaries i.e. energy. However, such changes in the energy of the universe would be uniform as the universe itself is homogeneous and isotropic. Thus we would expect very uniform energy changes throughout the universe which could be completely unnoticeable due to the fact that gravity is a weak force and approx. 70% of the universe contains dark energy.

    I believe certain constants would change, particularly those constants that describe the properties and quantities of the fields, but again they would probably be minuscule and barely would have any impact on the laws of physics operating in our universe.

    Due to the fact that the universe may be partly isolated (allowing only one type of energy to pass) the energy is not conserved globally. However, we do not know how much of the "energy" we receive from other universes, therefore we can presume that energy is/is not conserved.

    Best regards, Cygni.
  10. Jul 31, 2008 #9
    An important thing to remember Cygni, that you mentioned in your first post, is that

    Total Energy = KE - PE not KE + PE

    If you have plenty of similar questions on Cosmology and Universe Physics, I recommend "The Cosmic Onion" as an excellent book dealing with elementary particle physics, and unified theories in general (Don't worry, there's nothing you won't struggle to understand). Hope this was helpful,

  11. Aug 1, 2008 #10
    I appreciate your response, I will try to buy this book as soon as i can. :smile:
  12. Aug 1, 2008 #11
    Sorry for the double post. I just went on wikipedia and the total energy is defined as EK + EP = Constant.

    Quote: In classical physics, the total energy of an object is the sum of its potential energy and its kinetic energy. Note that since all other forms of energy can be derived from these two types, the total energy is effectively the theoretical maximum amount of energy that could be taken from the object. It also represent the net amount of energy on/of an object.

    In modern physics, the total energy of an object is the sum of its rest energy, its total kinetic energy, and its potential energy.

    Reference: http://en.wikipedia.org/wiki/Total_energy

    Best regards, Cygni
  13. Aug 1, 2008 #12


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    That is certainly NOT true. The total energy, or, more precisely, the "Hamiltonian" of the system is KE+ PE. What you give, KE- PE is the "Lagrangian" which not in any sense the "total energy".
  14. Aug 1, 2008 #13
    Sorry to mislead you Cygni, I've always been used to dealing with Lagrangians and negative PE. Regardless, I recommend the book. Your school/local library may well have it.

  15. Aug 1, 2008 #14
    Iv often pondered the same thing Cygni and have come to believe the total energy in the universe is not necessarily constant, but is infinite. Also I have come to believe that given a certain astronomical unit of volume, T^3*, the average energy (and mass) per this volume throughout the universe is constant.

    Also, M-theory isn't technically a theory, its a postulate. As was first suggested don't believe everything you read.

    *I call this constant "T", Topher's constant. :) Sort of similar to Planc's constant but on a cosmological scale at which the universe can be considered a homogeneous mixture of mass and energy.

    My $0.02 whether you asked for it or not.
  16. Aug 1, 2008 #15


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    This is not correct. You need to refer back to a thermo text and figure out why what you have stated is wrong.

    While we certainly do not mind and welcome questions such as this in PF, at some point, we need to base it on the correct understanding of basic physics. It would be impossible to answer such question AND have to keep on going back to basic physics, because we will be backtracking every few posts.

    Start with the foundation first. THEN to extrapolate out of those foundations. If not, this would be come highly speculative and would be beyond what is allowed in the PF Guidelines.

  17. Aug 1, 2008 #16
    Zapper, according to my thermo text he is correct. Are you sure you are not confusing a closed system with an isolated system?
  18. Aug 1, 2008 #17
    Although I would have agreed with ZZ, a quick browse of wikipedia finds:

    Closed systems are able to exchange energy (heat and work) but not matter with their environment. A greenhouse is an example of a closed system exchanging heat but not work with its environment. Whether a system exchanges heat, work or both is usually thought of as a property of its boundary.
  19. Aug 1, 2008 #18


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    Hum... maybe that's what I meant.

  20. Aug 1, 2008 #19
    Seems to me that if energy could be shown to somehow "leak" out of the universe, we'd simply re-define "universe" to contain the leak :)
  21. Aug 2, 2008 #20
    As a general rule, I would not use concepts from theories that you do not have an complete understanding of mathematically (Not to insult you personally Cygni). This helps prevent confusion, and innacuricies in discussion.

    Energy cannot be created or destroyed, only transferred, thus energy throughout a multiverse system (if that is what our universe "leaks" into) would be constant, surely. Really, the whole question smacks of similarities with "What would happen if one fell into a black hole?". Whilst it may be interesting, there is no exact answer, and not really an exact question either, as "What" has a huge scope of meanings.

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