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

Viewing size of Galaxies 7 Billion light years away?

  1. Apr 25, 2008 #1
    Would a galaxy 7 billion LY away appear to be twice the expected size (angular size) as a consequence of the expansion of the universe? (Assuming the universe is 14 Billion years old). My math says yes. But of course, I am just using trig and geometry. Just curious.
  2. jcsd
  3. Apr 25, 2008 #2


    User Avatar

    Staff: Mentor

    I'm not sure how you are using that math, but the galaxy isn't expanding, just the distance between it and here. It would appear a size dictated by the actual size and distance to it.
  4. Apr 25, 2008 #3
    Update and correction

    Sorry, I messed up the math. It wouldn't be double. I know the Galaxies are not expanding. But the Universe is. Space is.

    The question is how does the expansion of universe affect the preceived angular size of the galaxy? For example, if the galaxy were 100,000 ly across, you would expect it to occupy an angle of 0.0008 degrees. Does it occupy this angle, a smaller angle or a larger angle.
  5. Apr 25, 2008 #4
    The space between galaxies expands so does the space within a galaxy, so probably you will not see the difference in angular size of that distance galaxy.
  6. Apr 26, 2008 #5
    I think Remo Aviron is talking about the 'turn over' in the angular size vs redshift relation which occurs at about a redshift of ~1.65. If he/she is, then if we consider a standard ruler which has physical size 1kpc at a comoving distance of 1Mpc (0.003 billion LY), it will have an apparent angular size = 200 arcsecond. Moving this object to 7 billion LY (redshift =0.6), it will have apparent angular size = 0.15 arcsec, but moving it to reshift 6 (27.468 billion LY) it will actually have a larger apparent angular size when compared to the object at 7 billion LY, ie. apparent angular size = 0.17.

    See http://www.atlasoftheuniverse.com/redshift.html" [Broken] link for more information.
    Last edited by a moderator: May 3, 2017
  7. Apr 26, 2008 #6


    User Avatar
    Science Advisor
    Gold Member

    Well done, matt, I had a less delicate reply in mind. Scientists excel at discerning facts, politicians excel at manipulating them.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook