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B Energy and the Big Bang

  1. Jan 31, 2017 #1
    When, as a layman, I read or hear about the Big Bang, it generally comes to the same issues: massive release of energy, very high temperatures, very fast expansion, questions about what came before the Big Bang and so on.

    Yet, there are some issues that, at least to me, appear to be left aside.

    The first one concerns the energy itself.

    As far as I know, energy has to be somehow produced, by means of some transformation process.

    In other words, there are no "pools" of readily available energy floating around somewhere.

    On top of that, we generally add a word to energy, such as nuclear, or wind, or kinetic, and so on.

    Now, when it comes to the Big Bang, it is only "energy" with no specification of what kind and how it happened to be there.

    The second issue is also about this energy.

    We, humans, experience some difficulties to store energy, which would rather escape and dissipate itself.

    Yet, the Big Bang energy, if it can be called like this, was very disciplined and somehow remained locked in a single point.

    How could such a huge amount of energy accumulate itself without blowing away into what we call the Big Bang at a much earlier stage...or was this energy somehow "created" all at once?

    The last issue is about the temperature in the very very early universe, just a fraction of a second into the Big Bang, when energy was released into newly created space.

    What exactly was so hot in this still empty space?

    In other words, what was reacting with what in order to produce heat?
  2. jcsd
  3. Jan 31, 2017 #2


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    This is wrong. Energy is not conserved in an expanding universe.

    A misconception. The Big Bang was not "everything in a single point". The Big Bang occurred everywhere.

    See above. There was no "empty space". The Big Bang was not the explosion that many laymen seem to think about.
  4. Jan 31, 2017 #3
    Many misunderstandings here.

    I am not implying anything about energy conservation, but merely asking where all this energy was coming from...in order to be released with the Big Bang, this energy had to be produced, or created, or ... before the Big Bang occured...

    I am not implying that the Big Bang happened in a single point, but that BEFORE it happened, the energy was concentrated in a single point...how is that possible?

    I am not implying that the Big Bang was an explosion, but merely asking what was so hot in its very early stage...I don't think energy itself can be hot, or cold by the way...some "thing" else than pure energy is needed to release heat...or am I wrong?
  5. Jan 31, 2017 #4


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    No. Energy non-conservation implies that this is not the case.

    "Before" is not well defined and it was not concentrated in a single point.

    There is no such thing as "pure energy".
  6. Jan 31, 2017 #5
    Sorry, but these comments, while useful, are not answering my questions, so let me try to rephrase these questions ?

    1. How does energy conservation explain the existence of the energy released with the Big Bang? To conserve energy, it has to exist in the first place...

    2. If the energy was not concentrated in single point, what was its status? how could it be not in a single point in the absence of space?

    3. where was the heat coming from when the energy was released? what exactly was hot?
  7. Jan 31, 2017 #6


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    It doesn't, because energy is not conserved.

    The standard Big Bang cosmology does not start at a singularity, it starts with the entire Universe in a hot dense state. If you try to extrapolate further, you run into problems such as the Big Bang singularity and we do not really know what happens. In inflationary models, this state is preceded by an inflationary epoch followed by a reheating period.

    The reheating process in inflationary models occurs due to the decay of the inflaton field that creates a soup of particles. Due to their interactions, these particles quickly come into thermal equilibrium and the resulting soup will contain all particles that are in thermal equilibrium. The temperature usually referred to is the temperature of this "primordial soup".
  8. Jan 31, 2017 #7


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    Many are indeed left aside. The BBT does not address it's own creation and before a certain point theories get very speculative and may not ever be possible to figure out, much less prove. Sorta like what is inside a black hole.
  9. Jan 31, 2017 #8
    If the Big Bang starts with the entire universe in a dense state, but not with a singularity, how big, or small, is the universe at this very early stage? bigger than a point, but smaller than a marble?

    In other words, what is the size of the universe just before the inflation process starts?

    With regards to the temperature, if I understand correctly, the universe was not hot right at its very beginning, but became hot only after the primordial soup of particles was created?
  10. Jan 31, 2017 #9


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    This depends on how far back you are willing to extrapolate - and on your definition of "entire universe".

    Inflation is not part of the standard hor Big Bang theory. It is a theory intended to explain its very homogeneous initial conditions.

    At the start of the standard hot Big Bang, the universe was already hot. In inflation, the Universe was very very cold until reheating - which would occur before the onset of the standard Big Bang.
  11. Feb 15, 2017 #10
    Non-conservation of energy means that there can be physical processes which "create energy from nothing". IOW: end state after the process has more energy than initial state.

    One example is the decay of metastable vacuum in expanding Universe.
    While it did not decay yet, it is indeed a vacuum, meaning that expansion of vacuum does not change its state: heuristically, one cubic cm of "nothing" can expand to one cubic light year of "nothing" and it will still be the same vacuum, same "nothing".
    But when this false vacuum decays, it turns into a lot of particles (a quite hot plasma) on top of a different, lower-energy vacuum state. This decay process gives the same energy density of this plasma everywhere in the decayed vacuum.
    Thus, it means that the more this false vacuum was able to expand before decaying, the more energy "was created" by its decay. Energy was not conserved.
    (We do not currently have a proof that there are more than one vacuum states possible, but it is strongly suspected to be true).

    There are more examples of energy non-conservation, all depend on the Universe's expansion (more precisely, on its volume being non-constant. Contracting Universe would also have energy non-conservation).
  12. Feb 16, 2017 #11
    I had to read this slowly to try to understand...

    Could you explain in more details what you mean by "vacuum decay", especially the "decay" part?

    As you say, vacuum is "nothing"...I have some trouble understanding how nothing can decay...
  13. Feb 16, 2017 #12
    Vacuum is a state of all quantum fields in which it is impossible to remove a single particle of any kind. Which heuristically means that "it has zero particles of any kind".

    The thing is, quantum field theory says that there can be quantum fields of such form that they have more than one vacuum state. (An analogy is a function which has more than one minimum, see pic). In this case, the vacuum state which has more energy is still "empty", you can't remove any single particle from it - but it can transform to another vacuum state by emitting *a whole bunch* of particles. And I'm not talking about a few dozens of them either - in most cases, the amount of particles generated is enormous, resulting in temperatures such as 10^27 K.

    Attached Files:

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  14. Feb 16, 2017 #13
    But wouldn't this lead to assume that quantum fields, and thus all that makes quantum theory, somehow pre-existed the Big Bang?

    In other words, would this mean that "in the beginnng there was the Big Bang" should be replaced by "in the beginning there was a quantum field"?
  15. Feb 16, 2017 #14
    What you are saying does not make sense to me. Do you understand what "field" is?
  16. Feb 16, 2017 #15
    Let me try to put this another way.

    When you refer to quantum theory in your answer above, it seems to me that you assume quantum theory can be used to explain "things" or "states" (I am not sure what the right word is) that predate the Big Bang.
  17. Feb 16, 2017 #16


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    This still does not answer nikkkom's question and it seems to make it even more apparent that you do not understand what a field is. A field is an assignment of a value to each point in space time. There can be no "predating" the Big Bang for any field if there is no time before the Big Bang.
  18. Feb 16, 2017 #17
    No. QFTs are the theories which explain how "elementary particles" work (not "things" or "states").

    Since it turned out that "elementary particles" can not be explained in terms familiar from macroscopic world ("elementary particles" are not tiny billiard balls), QFTs postulate some mathematical concepts (such as "field"), how they fit together, and how they translate to our ordinary macroscopic intuition. For example, disturbance in the electron field is an electron (or several). Vacuum is one possible state of all the fields in a particular QFT (its defining property I explained earlier).

    Now, there are QFTs which are "toy models" (they do not intend to describe real world, but rather they are used to play with the math to learn how to tackle it better). Some of those definitely have more than one vacuum state, and higher-energy vacuum decay can be relatively easily analyzed in those models.

    And then there are OFTs, usually more complex ones, which do intend to describe reality. Standard Model is one. So far Standard Model's math is too complex for us to establish for sure that it has more than one vacuum state, its properties such as lifetime etc. Also, SM might get replaced by a better theory, and _that theory_ may be shown to have more than one vacuum state.
  19. Feb 22, 2017 #18


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    It seems kind of unsatisfying to simply say that energy is not conserved in GR. Clearly, something is conserved. There must be a way to concisely state what it is.
  20. Feb 22, 2017 #19


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    In a general space-time, it does not even make sense to try to define a conservation law in a global way. Your assertion that "something" is conserved therefore cannot be correct. Local conservation laws are something different.
  21. Feb 22, 2017 #20
    In GR, divergence of the stress–energy tensor is zero. You can see it as "the law of local conservation of energy".
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