Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

If G is constant, why didn't the early universe collapse?

  1. Mar 24, 2014 #1
    Hope this isn't too simplistic, but;

    • Type 1a supernovae tell us gravity is constant for as far as we can observe
      Type 1a also provide evidence that the rate of cosmic expansion is accelerating

    Wouldn't the early universe have at some point been below the density required by the Tollman-Oppenhiemer-Volkoft limit to form a black hole?

    I'm lead to believe that the early universe was able to expand because it was mostly composed of energy in the first place.

    So my ultimate question here is if you take a certain amount of matter and convert it directly to energy in the same volume of space (e=mc^2 - thanks einstein!) how can the gravity disappear?

    I know photons have no rest mass, but instead a relativistic mass proportional to the amount of energy they possess (and therefore, a sufficiently high energy gamma photon would become a black hole, right?).

    So, if the early universe was able to expand simply because it was made out of energy, why are photons affected by gravity in the universe we currently observe?

    Or if energy *does have mass, and G *is constant why did the universe expand in the first place?


  2. jcsd
  3. Mar 25, 2014 #2
    Good questions. Also not the easiest to answer. So let me ask you this. "what interaction gives a particle mass" The answer is the higgs field. The Higgs field dropped out of thermal equilibrium during the earlier moments of the electroweak epoch. The electroweak epoch has two forces. The electroweak force and gravity. Prior to that everything else is in thermal equilibrium. Google early universe particle physics for more info... Also Grand unification theory. However the previous search method leads to better articles.
    At around this same time inflation occured. This process caused a rapid expansion. In essence the process is a tunnelling effect between a higher vacuum state (true vacuum ) and a lowest possible vacuum state (false vacuum) see false vacuum (earliest inflationary model by Allen Guth) later this model was replaced by chaotic inflation and the process is described by the use of the inflaton.
    The sudden increase in volume allowed temperatures to cool sufficiently for other stable reactions to occur. Such as the photon neutrino interactions. Baryonic and non baryonic matter. As well as the bosons that allow the four forces to interact. Strong.weak and electromagnetic.

    However GUT has yet to be able to unify gravity into the above processes. See the complication with your question?
  4. Mar 25, 2014 #3


    User Avatar

    Staff: Mentor

    Consider a non-rotating, electrically neutral black hole. There is a solution to the Einstein field equations in General Relativity known as the Swarzschild solution that describes spacetime exactly under these conditions and gives us what's known as the Swarzschild radius, which is the radius an object of a certain mass would require in order to become a black hole. But there's a caveat to this. The Swarzchild solution, and other solutions for other types of black holes, require that the black hole exist in non-expanding space.

    In our universe today, all black holes are able to counteract the very weak effect of expansion due to their extreme gravity. However, in the very early universe the rate of expansion was MUCH greater than it is now and under the effect of expansion the normal solutions in GR don't apply. You could say that the expansion counteracted the gravitational pull until the average density of the universe had fallen below what was required to form a black hole.

    There are two different types of expansion. The first, known as the metric expansion of space, is what we are currently undergoing. My understanding is that the universe is expanding today because it was expanding in the past, but I don't know much more than that.

    The 2nd is called inflation and happens for a very different reason. I wish I could explain more, but my knowledge of that particular subject is very limited.

    It cannot. A 1kg box full of photons has the exact same effect on space as a 1kg box of matter.

    No, relativistic mass does not result in a black hole. Consider that somewhere out in space there is a proton or some other small particle moving so fast that it sees the Earth as having enough relativistic mass to collapse into a black hole. Obviously this doesn't happen. Instead, it is the rest mass, or invariant mass that determines whether an object will collapse in on itself and form a black hole.
  5. Mar 25, 2014 #4
    Thanks Mordred

    Is there a simple reason why all four forces were united into one primordial force before the plank era? From what I read it's supposed to be temperature related. So I guess if you get enough matter/energy in the one place the fundamental forces stop working? But *why?

    Also, not sure I understand the tunneling effect you mention. In a quantum sense it refers to 'particles overcoming a barrier they would not classically be able to surmount" (wikipedia). I get that version of it. Is the true/false tunneling effect basically saying that the void energy may be different in different regions of spacetime and particles may suddenly tunnel between them to a lower state, thereby changing all the physical laws that allow our universe to exist? If so what of thermodynamics? Do they radiate a particle when they do so? If so how does this differ from normal photon emission?


  6. Mar 25, 2014 #5


    User Avatar
    Science Advisor

    Sorry, don't mean to be nit-picky, but to be clear: the tunnelling proceeded from the false vacuum to the true vacuum.

    Inflation arising from a 1st-order phase transition furnishes a universe too inhomogeneous to match observations. Modifications to this approach, which include 2nd-order "slow rollover" transitions, address the problems with Guth's original model. In fact, many of the first versions of these so-called "new inflation" models (to contrast with Guth's "old inflation"), including the one initially proposed by Linde, were not truly 2nd-order, but also involved tunnelling from false to true vacuum, but with a sufficiently flat region of the potential just beyond the barrier to support slow roll inflation.

    So it's more correct to say that Guth's model was replaced by more successful "new inflation" models, themselves still making use of transitions from false to true vacuum not unique to Guth's original.
    Last edited: Mar 25, 2014
  7. Mar 25, 2014 #6
    The key term is thermal equilibrium.

    Essentially the interactions are occuring. However due to the high temp, density and small space. Any particle interaction that occurs. Occurs in both directions so quickly that they become thermally indistinquishable.

    Take for example a photon that forms a neutrrino and an electron reaction. In reverse the neutrino and electron combine back to the photon. This will happen at high enough of a temperature in a space small enough for the reverse reactions to occur.

    Its not that the laws of physics have changed but rather the small volume and high temperatures will not allow stable interactions to remain long enough to be measurable.

    Quantum tunnelling itself is better answered by others. my quantum theory understanding isn't up to a proper definition. However its essentially a method that particles can move from one region to another through a barrier. In the false vacuum example two regions are two different pressure regions with the higgs field as the barrier.

    Edit just saw your post Bapowell. Lol I always get the direction of tunnelling incorrect lol
  8. Mar 25, 2014 #7
    Thanks Drakkith!

    Why don't the normal Solutions of GR apply in the early universe? What I mean is that the assumptions of the Schwarzild solution are already partially incorrect as spacetime is not flat, but accelerating & expanding, right? - yet GR still holds.

    How much expansion is required before GR no longer holds? And with accelerating expansion of the universe, will we not reach that limit again sometime in the future? Is it a gradual thing? If so, shouldn't there be some minute difference between the current state of our curved universe and that predicted by Swarzild/GR? Can we measure the difference between prediction and actuality?

    Okay, got it, because the local frame is the one that matters for local gravitational collapse. But here again, I see an issue; if a 1KG box of matter has the same effect as a 1KG box of photons, , what if the matter contained within that box was 30 solar masses of matter, or photons. Presuming the matter is at zero K, the matter has only rest mass, and the photons have only relativistic mass. Yet if the dimensions of the box are smaller than the schwarzild radius, either situation results in a black hole, right?

    Is it a matter of differentiating between the relativistic mass created by different reference frames, and total energy in one area of spacetime? So a photon with enough energy would form a black hole, but flying away from a particle at close to C would not?
  9. Mar 25, 2014 #8
    Right, so It's not true to say that all the four forces were united into one force. More that the temperature was so great that even particles could not form for long enough to even exhibit the strong or weak force. Once things had cooled enough to form particles, baryons, etc, there was matter that could exhibit the strong and weak forces.

    So were the gauge bosons always there, or did they too emerge from the primordial soup at some time of another? If yes, I guess all our physical laws break given high enough pressure and temperature. Which is I guess what they've been saying all along.
  10. Mar 25, 2014 #9


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Wasn't the threshold density above which a black hole forms calculated in the context of non-expanding space? What if space is already expanding very rapidly (as in the Loop Quantum Cosmology rebound picture of the start of expansion--the so called "Big Bounce"--aren't the bets off, then, about the formation of black holes right around then?

    Maybe not "all" physical laws :smile:
    When GR is quantized as proposed in LQC you get equations with quantum correction terms which become important when the density gets up around 1% of Planck density. Gravity in effect becomes REPELLENT due to quantum effects at high density. Abhay Ashtekar and his group run computer simulations of the early universe using the quantum GR equations and they typically get a bounce occurring at around 40% of Planck density.

    In LQC the bounce is followed by a period of faster-than-exponential expansion. Because conventional inflation is only exponential and slower-than-exponential, this LQC faster growth is called super-inflation. There is no question of black hole collapse in conditions predicted to occur around a cosmological rebound. (After all, according to the quantized law, gravity is repellent for a very brief interval of extreme density, after which expansion continues for a while at very high rate.)

    So "high enough density to break all physical laws" may actually not be reached. the density may, in reality, only GET so and so high. Maybe it's 40%, or something else, to be determined.

    And maybe not ALL physical laws break at those high levels of density. Maybe whether it breaks or not depends on which law. This stuff has to be checked observationally by examining the ancient CMB light for traces of a bounce.
  11. Mar 25, 2014 #10


    User Avatar
    Science Advisor

    They do apply, of course. For black hole vs the early universe, the boundary conditions are totally different, hence different solutions.

    Black hole is spherically symmetric, center of symmetry, vacuum exterior region. At any point there's a radial direction. Solution is Schwarzschild.

    Early universe is non-vacuum everywhere, uniform in all directions. Solution is Friedmann universe. It can expand or contract, depending on initial conditions, but it remains uniform.
  12. Mar 25, 2014 #11

    You have the general idea. As you can see from Marcus post there are other theories involved. Keep in mind the time period we are discussing cannot be observed as of yet. So much of what we are discussing is based on indirect observation of later signs. Much of the GUT for example is theretical.. We cannot come close to producing the temperatures involved. So the theoretical temperatures where Particles reach thermal equilibrium is based on the particles mass.

    A good textbook covering this is by Griffith.

    "Introduction to particle physics"

    His methodology of explaining the process is done in an easy to read and straight forward manner.

    The metrics of the ideal gas laws during the universes earliest moments. Ie the planch epoch. Get quickly complex.

    For example the simplest case is when only photons drop out of equilibrium. The photon being its own anti particle has an entropy value due to its spin. That value is S=2. Whose distribution is a bose-Einstein condensate with a Fermi-dirac distribution..
    As more particles such a guage bosons drop out of equilibrium it gets far worse.

    Now wasn't that a mouthful.

    That was an example from another textbook.
    "Particle physics of the Early Universe" can't recall the authors name.

    Griffith however avoids those complexites by simply detailing GUT processes with less math.
    Key thing to keep in mind much of what we understand of this time period is based on the metrics being used. LQC would describe it in another manner as compared to symmetry or super-symmetry.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook