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

Effects of Magnetic fiedon the Dark Energy and Cosmic String

  1. Jan 5, 2010 #1
    Recent observations claims that Dark Energy is effecting the expansion of universe and increasing the anisotropy in the cosmic microwave background (CMB). Do you agree??
    what about cosmic magnetic field and its effects on the expansion and CMB ? If it is increasing the anisotropy or not?
    Also the rule of cosmic string is to enhance the anisotropy in CMB?
    Can we say that these three are accelerating the expansion and increasing the anisotropy.?
  2. jcsd
  3. Jan 6, 2010 #2


    User Avatar
    Science Advisor

    Well, yes, more or less. A perhaps better way of putting it is that dark energy is a proposed class of possible explanations (still somewhat tentative) for our observations of the expansion.

    Basically, if we assume that General Relativity is correct on large scales, then we have to have some new form of matter with very strange properties (from quantum mechanics, we tend to expect that GR will be modified on short scales, nor large scales, but it is possible we are mistaken). This form of matter we call dark energy. Bear in mind that dark energy isn't any one thing, but instead includes a wide variety of theoretical ideas. Sometimes it's even used to talk about potential modifications of gravity, even though the name dark energy is very misleading in that case.

    In any event, that small digression aside, some form of dark energy is a potential explanation for the observation that the expansion is accelerating. Dark energy also, it turns out, has another consequence: it affects how structure forms. Basically, if you have no dark energy at all, then gravitational potentials are stable: when a galaxy cluster forms, it will stay there.

    If you have dark energy, though, then gravitational potentials slowly decay over time. So what happens in this case is that a CMB photon enters a gravitational potential (like a galaxy cluster), and becomes blue-shifted (gaining energy) as it falls in. But by the time it has started to climb out of that gravity well, the gravitational potential will have decayed somewhat. It loses some energy, redshifting as it comes out, but not as much as it blueshifted when it came in. So the photon gains a small amount of net energy by going through a gravity well.

    Photons that go through voids (regions of space with little matter) do the precise opposite, losing some small amount of energy.

    This effect, the Integrated Sachs-Wolfe Effect, increases the magnitude of the perturbations of the CMB predominantly at large scales. At smaller scales, the photons go through so many voids and gravity wells that overall they neither gain nor lose energy. But on larger scales there is a measurable effect. Unfortunately, this also turns out to decrease the statistical power of the ISW effect, making it more difficult to confirm. You can think of this effect as a potential way of independently verifying a particular model of dark energy. The main difficulty is that it's really hard to present a statistically-significant detection.

    Magnetic fields tend to be local phenomena that don't have any significant impact on cosmic scales.

    Cosmic strings would present us with a very specific signature in the statistics of the CMB, a signature that we don't see at all. So if they exist at all, we know they must be very rare, far too rare to cause an accelerated expansion.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook