Precision of Temperature of Universe

In Karen’s latest Staff Report "What’s the temperature of space?, she states

So, the Universe is 2.7250(20) and the Milky Way is 2.7251(20)? Notice that the difference is only 5% of the error bars. Can we really measure the temperature difference of the Milky Way vs the Universe that much more precisely than the absolute temperature of either? I guess it’s possible, but I’d like some confirmation.

Oh, and good report. :slight_smile:

In this case, yes. You have to pay attention to what the measurements/calculations actually are. In this case, it’s not a matter of measuring the temperature of the Universe at large, and then measuring the temperature of the Galaxy, and subtracting. Rather, what we’re doing is measuring the temperature of the Galaxy, where we are, after all, stuck for the forseeable future, and then calculating how much ambient starlight from the rest of the Galaxy is contributing to that temperature. Subtract that off from the measured “temperature of space”, and you have the temperature far away from any galaxy.

Umm, can you clarify this? Or do you mean the rest of the Universe?

Why can’t we measure the temperature of the Universe by simply pointing a narrowbeam microwave receiver at a point in the sky with no sources (other than the CMB)?

I thought COBE was on trial for sexual assault. Musta probed too far.

I’d like to see the cite for the 0.0001 K number, that seems a bit small. And it seems a tad misleading to quote 2.725 K as the “temperature of the Universe” when there isn’t a place in the universe where you can leave an object and have it cool down to that temperature. (Well, except in some laboratories.) Even in inter-galactic space, you will have visible light from galaxies as well as high-energy cosmic rays that contribue to heating.

The WMAP fact sheet, which you can find here says:

These guys are not slackers!