Dismiss Notice
Join Physics Forums Today!

The expanding universe

  1. May 18, 2012 #1
    How do we know that the universe is more like raisin bread baking and not like all matter and energy leaving a central point?

    I somewhat understand the concept of space increasing rather than matter and energy just leaving the big bang, just wondering what evidence we have of this?
  2. jcsd
  3. May 18, 2012 #2
    To start, the fact that every galaxy appears to be moving away from us is a good one.

    Most importantly, however, is the cosmic microwave background. The CMB is a black-body radiation that was emitted ~380,000 years after the big bang. It has been studied and mapped considerably by the COBE and WMAP satellites, take a look at it here. The inhomogeneities in that image are dramatically increased. The CMB is incredibly homogeneous, varying only by a thousandth of a degree from place to place. This paints a picture of an early universe that was filled by a hot, dense, smooth plasma at every location. Since the universe was homogeneous then, it follows that it still is now. Hence, to maintain this homogeneity, we can deduce that it expands from every point.
  4. May 18, 2012 #3
    If space is expanding then how is their a blue-shift? unless i misunderstood galaxies moving slowly towards the milkyway can be detected by a blue shift while the ones leaving at a faster pace can be detected by the red-shift.

    I'm just having a hard time grasping the concept of space expanding, and would like any links, books, etc.
  5. May 19, 2012 #4
    That would also be true in the alternative model though, galaxies would be moving at a speed proportional to how far they had moved from the origin. How do you separate a universe obeying the Cosmological Principle from say one which is spherically symmetric but over scales much larger than the particle horizon? It's not as easy as it looks because the latter is asymptotic to the former as the diameter increases.
  6. May 19, 2012 #5


    User Avatar
    Gold Member

    Local clusters of galaxies are NOT expanding. There's a more detailed discussion here:

  7. May 19, 2012 #6
    See this article. Also, check out Phinds' page on the balloon analogy that he posted
    The cosmic microwave background shows otherwise, as I explained in Post #2.
  8. May 19, 2012 #7


    User Avatar
    Science Advisor

    If you look at how the galaxies move with respect to the Earth, and then extrapolate how things would look if we were standing on some other, far-away galaxy, you get the exact same picture either way.
  9. May 19, 2012 #8
    Accepting what is being said, can I ask, how does this relate to hyper velocity stars? I understand that the anology (and our normal discussion of the anology) is limited to gravitationally unbound systems, but I assume that it would also not apply to HVS's even though they are also unbound. Am I correct? Also, could you explain (or point me to something that discusses) how / why it does / does not apply?

    Much appreciated,

  10. May 19, 2012 #9


    User Avatar
    Science Advisor

    HVS's tend to have velocities around 1000km/s or so, which is of the same order of magnitude as the fastest galaxies, and isn't really fast enough to have significant relativistic effects. Bear in mind that our own motion with respect to the overall expansion is 627km/s, and having some stars out there in the universe that have double or even triple this velocity really doesn't have a significant impact on the overall picture that is presented.
  11. May 19, 2012 #10
    Thanks Chalnoth, and (kind of) understood. Even though they are so much closer than "the fastest galaxies", it does not impact on effects (averaged over large scales). Is that correct?


  12. May 19, 2012 #11


    User Avatar
    Science Advisor

    Right. And over time they'll slow down anyway as they catch up to other matter that is expanding in the direction they're moving.
  13. May 19, 2012 #12
    Thanks again.


  14. May 20, 2012 #13
    Has anyone studied the book "The First Three Minutes" written by Steven Weinberg? In this book the author stated the early universe conditions as: "At about one hundredth second after the big bang, the temperature of the universe was about a hundred thousand million degrees Centigrade. As the explosion continued, the temperature dropped, reaching thirty thousand million Centigrade after about one-tenth of a second, ten thousand million after about one second and three thousand million degrees after about fourteen seconds".

    Can anyone tell me about a scientific book or paper where these data are written in a reliable format?

Similar Discussions: The expanding universe
  1. Expanding Universe (Replies: 7)

  2. Expanding universe (Replies: 8)

  3. The expanding universe (Replies: 43)