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

Hubble redshift and calculation of galactic rotation curves

  1. Jun 6, 2015 #1
    I am interested in whether it is necessary to account for the effects of the Hubble Redshift in determining the rotation velocities of galaxies exhibiting keplerian motion and, in particular, whether the associated spatial expansion of the Universe is known to result in spectral shifts that produce an observed asymmetry in the apparent LOS velocity between material in these systems that is moving toward and material that is moving away from the observer. I would also appreciate any references in the literature where this specific subject is discussed.
  2. jcsd
  3. Jun 6, 2015 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I don't know of any asymmetry in the effect of Hubble expansion, on the way towards and away speeds of material are detected. The rotation speeds are not relativistic. Say there is some stuff on one side coming towards us at 300 km/s and on the other side going away at 300 km/s. So wavelengths are either contracted or expanded by a tenth of a percent. Multiplied by 999/1000, or by 1001/1000, compared with the average for the galaxy.

    Then suppose that the galaxy is at 1+z = 3. So everything is coming in larger by a factor of 3. You would expect the Doppler effects to be proportional and the same percentagewise as before. The effect of Hubble expansion would be symmetric: still one tenth of one percent expanded or contracted. either way.

    It seems intuitive to me, not in need of analysis. So I could be missing something--there may be something I can learn from you, if you would like to teach me---IOW how an asymmetry might arise.
    Last edited: Jun 6, 2015
  4. Jun 6, 2015 #3


    User Avatar
    Science Advisor
    Gold Member

    Picture the blades of a helicopter. Does the RPM of the blades appear different when it is receeding as opposed to when approaching, or hovering in place?
  5. Jun 6, 2015 #4
    Thank you Marcus and Chronos. I get the idea. With this in mind, the only asymmetric phenomena one could associate with the redshift that I can conceive of (though I wouldnt necessarily exclude the possibility of others), is where the recession velocity of the system is just below the speed of light so that the rotational velocity of the material moving away from the observer exceeds the speed of light relative to the observer. That material would, presumably be unobservable while the remainder of the system could, at least theoretically, be determined.
    Last edited: Jun 6, 2015
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook