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How does the expanding universe affect matter/energy

  1. Sep 14, 2014 #1
    Rephrase from another thread I posted a while back;

    If you believe as masses increase in distance gravitational potential increases relative to the distance (the greater the distance the greater the gravitational potential).

    So as matter increases in distance from each other due to expansion is gravitational potential created?
  2. jcsd
  3. Sep 14, 2014 #2

    Simon Bridge

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    Insofar as the gravitational potential energy between two masses is related to their separation ... yes: as masses move apart for any reason then the gravitational potential energy between them increases.
    Careful though - this is a Newtonian picture.
  4. Sep 17, 2014 #3
    Correct but on any scale this is profound...

    "scientists were able to calculate the expansion rate to 42 miles (68 kilometers) per second per 1 million light-years with greater precision, while looking farther back in time" (http://www.space.com/26279-universe-expansion-measurement-quasars-boss.html).

    Lets say the distance between the two objects is 890,700,000 miles or 0.000151518249 light years which is a very small distance (the distance between Saturn and the sun).

    If you follow that law, then every year due to expansion (no forces), the distance between the sun and saturn increase by 200820.7417384602 miles.

    That is a WHOLE lot of gravitational potential energy if you follow the Newtonian equation mgh.

    So even on the newtonian scale, how is it possible to create energy? ( This isn't philosophy keep in mind I am citing evidence from reputable sources and made a few calculations anyone at home can do, I guess no one just ever thought to http://www.space.com/26279-universe-expansion-measurement-quasars-boss.html ).

    So what is wrong Newtonian mechanics as we know it? Or space.com?

    Please help me get a better understanding
  5. Sep 17, 2014 #4

    Simon Bridge

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    The scale of the solar system is not quite Newtonian. The Newtonian scale is only a few to a few thousand kilometers depending on what sort of measurments are being made and the circumstances. Newtonian mechanics is only an approximation, a good approximation ... it has been replaced by the theory of relativity.

    The law of conservation of energy you are used to does not work in general relativity. Cosmological expansion is from general relativity.
    Last edited: Sep 17, 2014
  6. Sep 17, 2014 #5
    Ok where is the definition for distance that defines an event as Newtonian or GR or SR?

    Also, with your statement that you agree that energy is created, is it possible this will be the counter to a deep freeze model?
  7. Sep 17, 2014 #6

    Simon Bridge

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    There is no such definition.
    An "event" in this context (cosmological expansion) is a point in space-time. Events are not Newtonian.

    We use Newtonian physics in circumstances where it is useful.
    i.e. where the difference between the prediction from Newtonian models and reality is smaller than the uncertainty in our measurements.

    No - I did not agree that energy is created - I said that the conservation of energy that you are used to does not hold for GR. This is why you have to be careful about mixing ideas from different models.

    There are other conservation laws related to the stress-energy tensor. Bottom-line: you need to learn more about general relativity before continuing this line of speculation.

    Note: what counts as "big" depends on the scales used.
    Your own calculation has the cosmological drift of Saturn being about 200,000 miles each year.
    This is about 0.2% of the normal variation in Saturn's orbit due to it's eccentricity.
    So - not all that large. Probably too small to notice - though over the 50 or so years of accurate observations this shift would probably have been noticed if there was one.

    Basically the solar system does not expand with the Universe,
    http://www.astro.ucla.edu/~wright/cosmology_faq.html#SS (the rest of that FAQ should be useful to you as well.)
    ... however, the expansion could have some local effects.

    Basically the cosmological expansion is actually an effect that appears on average over very large distances and is not uniform on all scales.
  8. Sep 17, 2014 #7
    Why do you think that is? Also you can't just negate it's value because of the relatively small amount of distance gain, ANY increase is an increase.
  9. Sep 18, 2014 #8


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    No, because the concept of "gravitational potential" doesn't apply to a non-stationary spacetime. The expanding universe is a non-stationary spacetime.
  10. Sep 18, 2014 #9


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    But it's secular, not periodic, so in 50 years it would have built up to 10% of the variation due to eccentricity, which is quite large enough to have been noticed if it had actually happened.
  11. Sep 18, 2014 #10

    Simon Bridge

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    ... which is what I said ;)
  12. Sep 18, 2014 #11

    Simon Bridge

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    Why do I think what is what?
    The cosmological expansion is an average over large distances because that is the meaning of the words. That is how "cosmological expansion" is measured.

    On smaller scales the energy densities are too lumpy and the distribution of dark matter etc is not uniform. It's like dirt can be treated as a homogeneous substance when you have lots of it but on the small scale it is grainy, it's got stones in it etc. Averages that work for a whole truckload don't work for a thimbleful. Thus we don't normally think of cosmological expansion being all that important at a sub-galactic scale.

    I didn't "just negate it" though - please review the links about why the cosmological expansion does not count so much for the solar system.

    I was trying to make a point about what counts as "big"... Astrophysics throws large numbers at you all the time, it's easy to think they are big just because they have a lot of zeros in them so the trick is to find a useful comparison.
  13. Sep 18, 2014 #12


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    But there is *not* any increase in the solar system, for example in the distance of Saturn from the Sun; if there were, as Simon Bridge said, we would have seen it by now, and we haven't. That was his point.
  14. Sep 18, 2014 #13

    Simon Bridge

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    ... well it was part of the point - and an important consideration.
    This is what I get for trying to be clever :(
  15. Sep 18, 2014 #14


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    Yes, I know, but it was the only part that TheScienceOrca appeared to respond to. (Well, that and "why do you think that is", but as you noted, it's not clear what that was referring to.)
  16. Sep 18, 2014 #15


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    Even if the solar system expanded like this, your numbers are way off. Wolfram Alpha is your friend when it comes to unit conversions and getting numbers:

    The correct incorrect number would be 0.07 miles/year.
  17. Sep 18, 2014 #16
    But the gravitational force still exists so regardless of whatever you like to call "that forces potential energy" it still exists.

    That increase can now be converted to kinetic energy.
  18. Sep 18, 2014 #17


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    Not really. GR's results for energy are not at all intuitive. For a somewhat historical reference see for instance http://www.physics.ucla.edu/~cwp/articles/noether.asg/noether.html.

    I don't care for their use of "local", but the point is the problem exists, and has been around for a long time.

    http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html is a good summary written at the lay level.

    The FAQ's description of the situation is as follows.

    The point I hope to get across is that this is not a simple problem, it's been around for a long, long time, and that it really requires an actually working knowledge of GR (as distinct from misleading popularized notions of GR based on a Newtonian framework) to appreciate the subtle details.
  19. Sep 18, 2014 #18


    Staff: Mentor

    No, it doesn't. Gravity isn't a force in GR. It's spacetime curvature.

    There are still phenomena in the expanding universe that bear a resemblance to masses "attracting each other by gravity" (for example, the presence of ordinary matter and energy in the universe tends to slow its expansion), but calling that a "gravitational force" is a serious misnomer, because it does not work the same way as Newtonian "gravitational force" does. For example:

    No, it doesn't, because "potential energy" is just a label for a particular concept, and if that concept doesn't apply (which it doesn't in an expanding universe), the label simply doesn't label anything.

    How? Please be specific.
  20. Sep 20, 2014 #19
    I think Orodruin answered your question better than anyone else. Your math is off by a factor of 1 million (Im guessing you used /lightyear instead of /1 million light years). Using the numbers you give in your post, the expansion of saturn relative to the sun would only end up being 0.2 miles/year. If the universe were expanding a million times faster, we probably WOULD notice effects on the solar scale. But as it is there is no measurable effect until you get to very large scales
  21. Sep 20, 2014 #20
    First of all expansion occurs on all scales to our knowledge, please link me to one post stating otherwise for those who made that claim within this thread you know who you are I don't have time to quote.

    I haven't double checked the math but I am willing to take your word for my mistake as it doesn't change the outcome of the question.

    "No, it doesn't, because "potential energy" is just a label for a particular concept, and if that concept doesn't apply (which it doesn't in an expanding universe), the label simply doesn't label anything."

    Great so lets use the proper terminology for the same concept;

    Let me help explain this to you, it doesn't matter what you call the term or the idea at the end of the day energy is being created but since you would like to be vocab let me explain this concept to you in an easier manner; If you are still having troubles understanding let me know and I would love to clarify and correct my vocabulary if it is indeed wrong.

    Imagine a universe with 2 masses separated by 1 million light years.
    Imagine the objects are also not moving relative to each from both frames of reference.

    The distance between them is increasing due to expansion even though they are not travelling or accelerating.

    This is an important fundamental point to the question.

    Lets imagine this scenario again, but now with no forces on the masses.

    Due to space curvature these masses will now fall towards each other increasing in kinetic energy exponentially.

    Upon contact this kinetic energy would likely be turned into much heat and other usable forms of energy, although that is irrelevant to the concept.

    So regardless of what you like to call it a force, or space time curvature, and the end of the day energy is still created and that can't be avoided by trying to say the whole concept is wrong due to a misuse of vocabulary that is just absurd.
  22. Sep 20, 2014 #21
    Also since gravity is something most scientists agree exists, when any number of masses greater than 1 are separated the space time curvature have potential to accelerate towards each other thus gaining energy.

    So I think it would be fair to say this is POTENTIAL ENERGY without referring to the term potential energy which is commonly used in newtonian mechanics.

    I am using potential energy in a literal sense which is exactly what it means.


    Those two masses have gravitational potential energy.

    You could think of it as this; Those two masses have the potential to create kinetic energy due to the space time curvature.

    Both statements are true.

    Hopefully this explanation helps you.
  23. Sep 20, 2014 #22

    Drop an apple on your head and you can feel it yourself.
  24. Sep 20, 2014 #23
    "which it doesn't in an expanding universe), the label simply doesn't label anything."

    Again it is a literal explanation not a label.
  25. Sep 20, 2014 #24


    Staff: Mentor

    I'm not sure we would, because the solar system would still be gravitationally bound, and that effect would still be much stronger than any inertia involved with the expansion.

    This does assume, though, that the solar system (and gravitationally bound systems in general) would still exist if our universe were expanding a million times faster. That might not actually be the case: if the universe were expanding a million times faster, and had been doing so right back to the Big Bang, gravitational clumping might not have been able to take place at all. But then we wouldn't be here.
  26. Sep 20, 2014 #25


    Staff: Mentor

    Um, what? Either they're moving relative to each other or they're not.

    Huh? There weren't any forces on them before. They were both moving inertially.

    You do understand that in our expanding universe, all the galaxies, which are moving apart because of the expansion, are moving inertially, feeling zero force, right? The expansion is not a force pushing on the galaxies to move them apart. It just is the galaxies moving apart.

    That depends on the spacetime. See below.

    According to GR, I am not dropping an apple on my head; I am releasing it and letting it float freely, while the Earth pushes me upward into it. I am the one feeling a force, not the apple, and the force I feel is not "gravity", it's the Earth pushing up on me.

    When I say the concept of "potential energy" doesn't apply to the expanding universe, I'm not talking about terminology or labels; I'm talking about physics. I'm saying that the physics that's going on in an expanding universe is not the same as the physics going on when an apple falls. I'm saying that the physics you are trying to apply the the expanding universe is the wrong physics; you are trying to apply the physics that works in a stationary spacetime, where the concept of "potential energy" is meaningful, to a non-stationary spacetime (the expanding universe), where it isn't. The same underlying theory (GR) can be used to describe both, but the specific models used--the specific solutions to the Einstein Field Equation--are different, and have different properties. The solution that applies to the apple falling has "potential energy" as one of its properties; the solution that applies to the expanding universe does not.
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