The Planck Collaboration has quoted the angular size of the Sound Horizon:$$\Theta_*= 0.0104$$I decided I'd try a rough, back-of-the-envelope calculation to understand how they arrived at this value. The redshift of CMB is estimated to be 1090 based on the Peebles model for recombination. Using Ned Wright's calculator, that gives an Angular Size Distance$$D_A = 12.723\space {\rm Mpc}$$ So now we need the physical sound horizon, which is the distance that sound travels in the plasma from the dawn of time to recombination. According to Ned's same calculator, $t(z_{CMB}) = 372,000$ yr. The distance that sound travels in that time is:$$\int_0^{t_{CMB}}\frac{c}{\sqrt{3\left(1+\frac{3\rho_B(1+z)^3}{4\rho_{\gamma}(1+z)^4} \right)}}dt$$But it can be safely approximated by $\frac{c\space t}{\sqrt{3}}$ (because the density of baryons is well below that of radiation for most of the integral range), so the sound horizon is roughly:$$r_S=0.066\, {\rm Mpc}$$ Which means that the angle is:$$\Theta_*=\frac{r_s}{D_A}=\frac{0.066}{12.723}=0.005$$This is almost half of the value quoted in the Plank Study and, in no way that I can find, gets us to a multipole of $l=220$ ($\Theta_*=0.015)$. So what's missing from this (probably) naive calculation?

  • $\begingroup$ Wonder if you are using the correct value for r_s? Whether you want it measured at matter-radiation equality instead which was at z ~ 3400 according to this: physics.stackexchange.com/a/216033/97766. Don't have a pen and paper on me right now so can't check! $\endgroup$
    – astronat
    Dec 26, 2018 at 23:12
  • $\begingroup$ @astronat - Yes, like I said, if you take the current radiation and current matter densities and do the actual integration with those values, they add very little to the distance that sound travels in 378,000 years. If I've made a mistake, please point it out. $\endgroup$
    – user32023
    Dec 26, 2018 at 23:34

1 Answer 1


I think there is a small problem in your assumption that the universe is radiation dominated up to the point of recombination. This really isn't true.

But that doesn't help because this means that the sound speed is lower and therefore your sound horizon is (a bit) smaller.

I think the main issue is your calculation of the sound horizon distance. This isn't valid in an expanding universe. For example the current (light) horizon distance is about 46 billion light years, not 13.7 billion light years.

As described in the Planck collaboration (2013) paper you cite: "The characteristic angular size of the fluctuations in the CMB is called the acoustic scale. It is determined by the comoving size of the sound horizon at the time of last-scattering, rs(z∗), and the angular diameter distance at which we are observing the fluctuations, DA(z∗)".

The correct calculation of the comoving sound horizon distance at recombination is $$r_{s,z*} = \int^{t_{\rm rec}}_0 \frac{c_s dt}{a(t)} = \int^{a_{\rm rec}}_0 \frac{c_s\ da}{H\, a^{2}} = \frac{c_s}{\Omega_m^{1/2}H_0} \int^{\infty}_{z_{\rm rec}} \frac{dz}{(1+z)^{3/2}}$$ (e.g. see here.) and also see equation 6 from the Planck collaboration paper you cite where the assumption of a matter-dominated universe with $da/dt = aH$ and $H \simeq H_0 \Omega_m^{1/2} a^{-3/2}$ has been made and noting that $a = (1+z)^{-1}$.

For typically assumed parameters and assuming the sound speed is $c/\sqrt{3}$, this yields $$r_{s,z*} = \frac{2c}{\sqrt{3}\Omega_m^{1/2} H_0}(1 + z_{\rm rec})^{-1/2}$$ For $H_0 = 70$ km/s/Mpc, $\Omega_m=0.3$ and $z_{\rm rec} = 1090$, this gives about 270 Mpc, which needs to be divided by $1+z_{\rm rec}$ to put it in the angular diameter distance terms of your calculation.

This gives an angular scale of 0.019 radians.

But if the sound speed is slower then this scale becomes smaller.

The sound speed is actually $$c_s = \frac{c}{\sqrt{3(1+3\rho_b/4\rho_r)}} $$ and the ratio of baryon to radiation density increases with time in proportion the scale factor $a(t)$. At recombination the ratio $3\rho_b/4\rho_r\sim 1$, and $c_s(t_{\rm rec}) \simeq c/\sqrt{6}$.

This provides a downward correction to the angular sound horizon of about the right size.

  • $\begingroup$ @DonaldAirey You say "The distance that sound travels in that time is..." and that is incorrect. The distance must take account of the expansion of space. Which is what I have done. $\endgroup$
    – ProfRob
    Dec 29, 2018 at 16:52
  • $\begingroup$ Thank you, but I don't understand why the waves in Wayne Hu's animation are fixed. If the proper way to think about the Sound Horizon is comoving coordinates, why don't the wavelengths expand with time? Is Wayne's animation misleading? $\endgroup$
    – user32023
    Dec 29, 2018 at 17:03
  • $\begingroup$ @DonaldAirey Possibly you are confused by the use of co-moving distances which are used as a standard measure of freely-expanding separations in the universe. Please follow the links provided to see how comoving distance, proper distance and angular diameter distance are related. $\endgroup$
    – ProfRob
    Dec 29, 2018 at 17:04
  • $\begingroup$ @DonaldAirey Presumably because the animation is done in comoving coordinates! $\endgroup$
    – ProfRob
    Dec 29, 2018 at 17:07
  • $\begingroup$ I can't reproduce your results. For most of the integration range, we're in the Matter-dominated era where $a(t)\propto t^{2/3}$ , so $r_s=\int_0^{t_{CMB}}\frac{c_s}{t^{2/3}}dt$. Over the range of t= 0 to 372,000 yr, this gives $3.8\times 10^{-11}\space Mpc$. This integral results in a much smaller distance, not a greater distance. What am I missing? $\endgroup$
    – user32023
    Dec 29, 2018 at 20:47

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