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

Another expanding universe question

  1. Aug 30, 2009 #1
    I know there are a lot of these questions, but please bear with me, I couldn't really find these precise scenarios elsewhere.

    First of all, am I correct in saying that if we took a distant galaxy that was highly redshifted, and had it emitting light pulses to us once every second from its frame, then the rate of the light pulses received from our frame would depend on its degree of redshift?

    Now if you took that galaxy. After one second its distance from us would be slightly greater, yes? So according to Hubble's law it should have a slightly greater redshift. So we can assume its velocity away from us is getting greater over time. Doesn't this imply the accelerating expansion of the universe by Hubble's constant? I don't see what I am missing.

  2. jcsd
  3. Aug 30, 2009 #2


    User Avatar
    Science Advisor

    Yes; the time between pulses would increase by the same factor as the redshift.

    Your insight here is very good!

    In fact, you are precisely correct in this sense. If Hubble's constant actually is constant, then expansion is indeed accelerating. In the case of "pure inflation", the scale factor of the universe increases exponentially, and the Hubble constant is a constant over time.

    In general, however, the value for Hubble's constant will decrease over time... even in an accelerating universe, as long as the acceleration is moderated a bit from the pure exponential expansion. The rate at which Hubble's constant is decreasing is very very slow. You'd have to wait hundreds of millions of years to get any appreciable difference. Hubble's "constant" is a constant for the present epoch, but not a fixed value for all time.

    Cheers -- sylas
  4. Aug 30, 2009 #3
    Yeah, the differences would be so minute that I don't think we would be able to measure them yet with any substantial accuracy. There's already huge inaccuracy (I think something in the order of 10% of the value?) in the measurements and value ascribed to Hubble's constant and the distance of distant galaxies with significantly large redshifts.

    I read somewhere recently (i.e. within the last 5 years) that there is evidence that the universe is expanding at a greater rate than earlier thought, though I am unable to recall any more of this including the reasoning or what form the evidence took.
  5. Aug 30, 2009 #4


    User Avatar
    Science Advisor
    Gold Member

    The case for accelerating expansion arose from a study of high redshift supernova. See:
    Measuring Cosmology with Supernovae
    Saul Perlmutter, Brian P. Schmidt
    Over the past decade, supernovae have emerged as some of the most powerful tools for measuring extragalactic distances. A well developed physical understanding of type II supernovae allow them to be used to measure distances independent of the extragalactic distance scale. Type Ia supernovae are empirical tools whose precision and intrinsic brightness make them sensitive probes of the cosmological expansion. Both types of supernovae are consistent with a Hubble Constant within ~10% of H_0 = 70 km/s/Mpc. Two teams have used type Ia supernovae to trace the expansion of the Universe to a look-back time more than 60% of the age of the Universe. These observations show an accelerating Universe which is currently best explained by a cosmological constant or other form of dark energy with an equation of state near w = p/rho = -1. While there are many possible remaining systematic effects, none appears large enough to challenge these current results. Future experiments are planned to better characterize the equation of state of the dark energy leading to the observed acceleration by observing hundreds or even thousands of objects. These experiments will need to carefully control systematic errors to ensure future conclusions are not dominated by effects unrelated to cosmology.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook