The effect is most clearly seen with a light that emits photons at all frequencies this kind of light is known as black-body radiation. The exact lines that appear and their positions in the spectrum (the "pattern" of "absorption lines") depends on the elements present in the gas and the gas's environment. When this light is then shone through a prism the blocked frequencies will appear as black lines in the spectrum (see illustration below). When a light shines through a cloud of gas specific frequencies of light get absorbed.
For those not versed in the topic allow me to explain. In my answer I refer to a "pattern" of "absorption lines". Inspired by a conversation with uhoh in the comments: What is the relationship between red-shift and absorption lines? Since a hotter temperature implies a higher density, this cooling of the CMBR over time is direct evidence for expansion of the universe. Note that the temperature was measured from the specific pattern of lines seen and not from how much the lines had been red-shifted this measurement would yield the same temperature even if there were no red-shifting. This temperature is consistent with the expected temperature for that galaxy's redshift (9 K).
The researchers observed absorption lines in a gas cloud located in a distant galaxy and found that the pattern of lines seen could only be explained if the CMBR temperature at time of absorption was between 6 K and 14 K (it's 3 K now). Here are the details:Īccording to this paper, the CMBR was measurably hotter in the past ( less-technical synopsis here). Its reduction in temperature over time is direct evidence of expansion. The past temperature of the Cosmic Microwave Background Radiation (CMBR) has been directly measured and found to be substantially higher than it is today. Yes, there is direct, non-red-shift evidence of expansion.
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