More than 50 years has passed since the discovery of the Cosmic Microwave Background Radiation. Since then, the universe has presumably gotten older and larger.
Have we noticed any change in the CMB since we first discovered it?
Note that CMB radiation has the form of a black-body curve (characteristic of a body with a temperature of just over 2.7 K). IOW, it consists of radiation at a range of frequencies.
I’d guess that the change over 50 years is well below our ability to measure.
Doing a back-of-the-envelope calculation, and using a Hubble constant of 70 km/s*Mpc, I estimate that the volume of the observable universe has increased by about 1 part in 10[sup]8[/sup] over the past 50 years. (I got dV/V = 3 H[sub]0[/sub] dt, if anyone wants to check me.)
Yes, that’s correct, for timespans small compared to the Hubble time (which is certainly the case here). But the relevant factor for redshift is just the length change, not the volume change, so you can drop the factor of 3.
The time since its discovery isn’t really the relevant question here, though, since the first measurements weren’t nearly precise enough in their frequency measurement. A better comparison would be the COBE data, from 1990 or so. And even that, while incredibly precise, was only good to within about 1 part in 10[sup]3[/sup]. So we’ve got about five orders of magnitude to go, between some combination of longer observation time or better instruments, before we’ll be able to detect this.
I’m off by only a factor of 3. Woo hoo!
So, making a clock/calendar device that works by reading the CMB would be overly ambitious at this point — is what I hear you saying.