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

Inflationary Theory out of thin air?

  1. Sep 4, 2006 #1
    Hey all, especially spacetiger, I have a question about inflationary theory for you.

    Its my understanding that the Period of Inflation proposed to have occured during the first second of the BigBang, was introduced to give a suitable answer to the Horizon Problem. The Horizon problem says that the thermal equillibrium we observe, as in the CMB, could not of been achieved in the early Universe since space expanded faster than C.

    My question is: What evidence is there to support a Period of Inflation? Is the notion merely theoretical, or is there observable grounds for this theory?

    Thanks.
     
  2. jcsd
  3. Sep 5, 2006 #2
    if C is speed of light
    then how can anything expand or grow or be faster than C
    according to einteins theories
     
  4. Sep 5, 2006 #3
    Relativity applies to matter and light, it says nothing about the expansion of the fabric of space itself. In fact, many regions of space are expanding at speeds in excess of C.
     
  5. Sep 5, 2006 #4
    how do you explain that phenomena
    any theories'??? to support
     
  6. Sep 5, 2006 #5
    i don't think they have any proof, they just beleive that that would best explain what they are seeing. Otherwise it would completely blow open the previous theories
     
  7. Sep 5, 2006 #6
    you cant just bend science to your will
     
  8. Sep 5, 2006 #7

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    There is much discussion of the evidence for inflationary theory here:

    WMAP Summary: Our Universe

    Very briefly, inflation predicts that the initial distribution of matter (prior to gravitational collapse) is described by a gaussian random field. There are several lines of evidence for this, including analysis of the CMB and large scale structure. Another prediction of inflation is that the spectral slope should be near, but not exactly equal to, unity. This has been confirmed by recent WMAP results.

    However, both of these predictions are also consistent with Dr. Steinhardt's cyclic universe, so a real smoking gun for inflation would be the discovery of B mode polarization in the CMB. We hope to discover this with the Planck mission.

    I can't give more detail right now, but just know that there is observational evidence for inflation that goes beyond the original motivations for it (as a solution to the horizon, flatness, and monopole problems).
     
  9. Sep 5, 2006 #8

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That objects can recede from one another at faster than the speed of light (due to the expansion of space) is a natural feature of Einstein's theory of gravitation and does not violate any tenets of relativity.
     
  10. Sep 5, 2006 #9

    In flat, static spacetimes, c is a limiting relative velocity. In curved, time-varying geometries things are different.
     
  11. Sep 5, 2006 #10

    Please explain to me what B mode polarization is and why it implies a cyclic universe?

    BTW, I subscribe to the idea that the universe will collapse in the future and then undergo another bigbang.
     
  12. Sep 5, 2006 #11

    I'm pretty sure there have been astonomical observations of galaxies with recessional velocities in excess of C, and in fact in excess of 2C. Is this bending science, or empirical evidence?
     
  13. Sep 5, 2006 #12
    It depends on which model you use to convert the redshift (observed) to a velocity. If you use plain old special relativity (SR) to derive a recessional velocity, you won't get a velocity exceeding c. Please note that cosmological redshift is not due to the SR redshift effect (ie. redshift due to velocity), hence using the SR method is wrong.

    If you use general relativity (specifically, the [tex]\Lambda[/tex]CDM model), you will derive a recessional velocity greater than c at redshifts > 1.6 since the redshift is due to the expansion of space, not some velocity through space.

    So the term recessional velocity is a bit of a misnomer and astrophysicists generally use redshift instead, since this assumes no model.

    PLease see http://arxiv.org/abs/astro-ph/0310808 for further information.
     
  14. Sep 6, 2006 #13
    Which version of recessional velocity does the Friedmann Equation use then?
     
  15. Sep 6, 2006 #14
    The Friedmann Equations are derived from Einstein's Fields equations (General Relativity) under the assumption of isotropy and homogeneity. The first Freidmann equation relates the Hubble parameter to the density parameters and redshift and gives the time evolution of the Hubble parameter, H(z). This is used in determining the cosmological recession velocity in eqn 1 of the Davis and Lineweaver paper I linked above.
     
  16. Sep 6, 2006 #15

    hellfire

    User Avatar
    Science Advisor

    Every theory that explains the origin of structures in quantum fluctuations of a scalar field predicts scalar and tensor modes that correspond to the E and B-mode of polarization respectively. In case of inflation the scalar field is the inflaton and is responsible for the accelerated expansion of space. In case of the cyclic model the dynamics in 3+1 space-time can be described also with an effective scalar field that produces the modes during the contraction phase before the big-bang.
     
    Last edited: Sep 6, 2006
  17. Sep 6, 2006 #16

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I hope I didn't say that anywhere in the WMAP thread. It's the opposite actually, the detection of B mode polarization in the CMB would be evidence for inflation and against the cyclic universe. Most theories of inflation produce gravitational radiation, which induces tensor modes, which in turn induce B-mode polarization in the CMB light. The cyclic universe, on the other hand, generically produces no gravitational radiation, so no B-mode polarization in the CMB.

    An alternative to (or supplement to) looking for B-mode polarization in the CMB is to just look for the gravitational radiation directly, but this will likely be much more difficult. We've yet to detect any gravitational waves directly.
     
  18. Sep 6, 2006 #17
    This is not consistent with:

    Is the expansion factor put in by hand or derived?
     
    Last edited: Sep 6, 2006
  19. Sep 6, 2006 #18
    Does anyone know when the false vacuum of the Higgs mechanism fell to its present low compared to the acceleration/deceleration of the expansion rate? Did this fall to the present stable energy level take place about the time that the universe started to decelerate in it expansion?

    I'm considering a theory where the energy in the Higgs field, at its greater than present vacuum energy level, is what forced the universe to expand exponentially. Then at some point there was no longer enough vacuum energy density to keep the expansion accelerating. But the momentum of expansion put a force on the vacuum energy to cause it to fall to a new level, creating particles, etc, in the process. That would be supported by the fact (if true) that massive particles first appeared at about the same time the universe started to decelerate, and not before. Is this indeed the case? Thanks.

    If so, then what does that mean for the present acceleration of expansion? Are we looking at a new phase of particle creation when the present vacuum energy can no longer support expansion, but the momentum of expansion creates a force instead on the present vacuum energy that causes it to fall again to a new energy level? I wonder what the implications of that would be?
     
    Last edited: Sep 6, 2006
  20. Sep 6, 2006 #19
    That is because Chaos was talking about Special Relativity.
    The expansion factor ([tex]\dot{R}(t)[/tex]) is given by eqn 26 in the Davis and Lineweaver paper. I just put in values for redshift and the cosmological parameters for the [tex]\Lambda CDM[/tex] model (ie., [tex]\Omega_{\Lambda}=0.7[/tex] and [tex]\Omega_{m}=0.3[/tex]).

    What I was trying to get across is that we don't measure recessional velocities, we measure redshifts and convert them using a model dependant method.
     
    Last edited: Sep 6, 2006
  21. Sep 7, 2006 #20

    hellfire

    User Avatar
    Science Advisor

    It is difficult to understand that while it produces a similar spectrum of density perturbations, it difers so much in the tensor perturbations... ?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?