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

If gravity is curvature of spacetime, why unified force @ Planck epoch

  1. Apr 13, 2014 #1

    Bear with my possible ignorant. I am puzzled over this dilemma. If General Relativity states that gravity is the curvature of spacetime, that is, no spacetime no gravity, and the cause of curvature is matter (mass), it means that if no matter, there is no gravity. I understand that scientists say the Superforce existed in Planck epoch (zero to 10e-43s after the Big Bang) at which point matter did not exist even if spacetime existed. If there was no matter (mass), there was no curvature. If no curvature, there was no gravity.

    So, why is it assumed that gravity existed in Planck epoch?

    I might be totally off. I appreciate any help to understand this.
  2. jcsd
  3. Apr 13, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Welcome to PF;
    You seem to have a number of small but important misunderstandings, lets see:

    In General Relativity, it is not matter that curves space-time, but energy-density.
    Matter is a very dense form of energy, so we see lots of curvature where there is matter.
    But you can, in principle, have gravity without matter.

    In the Planck epoch though, the Universe is in a very dense state anyway so intuitions we get off the current, low density, epoch won't work.

    The whole "gravity is curvature" thing is proposed to be a manifestation of a deeper law which will allow the unification of all four fundamental forces.

    What is being assumed to exist in the planck epoch is this new super-unified force, which the four forces we know today emerged from.
  4. Apr 14, 2014 #3
    Thank you very much Simon for both the response and the welcoming. I already feel I understand it more clearly.

    I think I have hard time to understand the force at Planck epoch. What is force after all when 'nothing' exist? does force precedes energy? what causes it to exist if not energy? I don't expect you to respond to these questions but if you could provide me with a source I appreciate that. I need to get my head around it. Thanks again
  5. Apr 14, 2014 #4
    Its easier to understand it in terms of thermal equilibrium, the extremely high temperatures, small volume (observational universe) and high density, any reactions that occur are unstable and the reverse reaction occurs. This essentially means that under these conditions it can be described via its temperature only. Google thermal equilibrium. As the density decreases via an increase in volume stable reactions can start to occur and particles begin to decouple (freeze out). Google ideal gas laws for the relations. If you google ideal gas laws cosmology. you will be able to get the equations of state in regards to cosmology applications. The reason forces are also in thermal equilibrium is that the force carrying bosons are also in equilibrium.

    References depend on articles covering symmetry breaking, so the exact temperatures involved depend on our knowledge of particle physics. As we cannot come close to creating the temperatures involved there is variations in articles depending on which symmetry- super symmetry article you read.

    http://www.wiese.itp.unibe.ch/lectures/universe.pdf this doesn't cover the epochs but rather later on, however it does a good job of covering thermal equilibrium.

    David Griffiths "Introduction to particle physics" textbook does a good job covering the different levels of decoupling for various particle species but again he doesn't cover the epochs. If you google " Early Universe thermodynamics" you can pull up numerous articles however many can be misleading. Googling grand unification theory will also tend to pull up articles however the previous search method tends to pull up more accurate articles.

    By the way the articles breaking down the first second are older articles, Modern cosmology doesn't place as much weight on the epoch stages as there is too much conjecture. At least that's what I can tell judging from the various searches I have done on the subject

    so you probably won't find current literature covering charts as per on this site


    edit : checked my links on the epoch breakdown in pdf form and they had largely disappeared, many replaced with the more modern breakdown. Similar to the first link I provided.
    Last edited: Apr 14, 2014
  6. Apr 14, 2014 #5

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Force is a problematical concept there yes.
    The models you know involving forces get replaced later in your education.

    Energy is more fundamental.

    It takes a while. You've seen the reading material Mordred has linked to...
  7. Apr 14, 2014 #6
    Its definitely challenging, and my current area of study. Could take me a year to fully understand lol. Defining volume, energy-density to pressure and how the classical view of the ideal gas laws are used in Cosmology to define thermal equilibrium was one of my recent hurdles. After all you don't have a container for particles to bounce off of. So I also had to figure out how volume is defined. Thanks to PF help I'm more clear on that.


    challenges I can still foresee is finding the appropriate metrics for quark-gluon thermodynamics. Fermi-Dirac distributions cover fermions, Bose-Einstein distributions cover the bosons.
    - Where does the Higg's field fit in this and what thermodynamic properties does it entail as a phase transition? is another challenge
    -Is inflation a result of a phase transition? what temperature to energy-density ratios are a result of reheating. Think its roughly 1019 K but not positive
    -How does dark matter affect the thermodynamics ?
    -is lepton-genesis a requirement for baryon-genesis to occur ? according to what I understand of conservation of leptons and the eightfold way probably, if so is there a corresponding phase transition ?
    -How baryon-genesis affects thermodynamics?
    -then I need to go through the various particle species of big bang nucleosynthesis, and its correlations to thermodynamics.

    after all that comes last scattering and the temperature correlations as it applies to the anistropies found in the CMB.

    FUN lol, I'm a sucker for punishment, but I love learning
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: If gravity is curvature of spacetime, why unified force @ Planck epoch