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Chapter 1: Astrophysics

Cosmic Background Peak Frequency Change

Dear Dr. SETI:
In 1964, Arno Penzias and Robert Wilson measured cosmic background radiation and determined it to be caused by the big bang. They measured the energy at a certain frequency. My question is:
Would we expect that the frequency (of the peak amplitude) of this cosmic background radiation to change over time as the universe ages? If so, has this ever been measured?
Sincerely, Bernard from Bell Labs

The Doctor Responds:
This is an excellent question, Bernard, because you specifically asked about the peak radiation frequency. All thermal blackbody radiation (and the Cosmic Microwave Background is no exception) covers a wide range of frequencies. You recall correctly that, at Bell Laboratories, Holmdel, NJ, Penzias and Wilson first detected the cosmic microwave background radiation (research which won them the Nobel Prize). They were receiving at a frequency around 4 GHz -- well below the amplitude peak, but still detectable with their huge horn antenna and cryogenically cooled maser amplifier.

Over the nearly half a century since, their measurements have been confirmed over a wide range of frequencies. From these multiple measurements, points on a curve were established, and the peak frequency was determined (it is variously reported as being 279.5 GHz or 160.4 GHz, depending upon how you define 'peak'.) From these observations, the corresponding blackbody temperature has been determined to be on the order of 2.7 Kelvin. From space, the Wilkinson Microwave Anistoropy Probe has further refined this figure to 2.715 Kelvin.

Now, when I went to school, I was taught that the cosmic background temperature was 3 Kelvin. Does this mean that the universe has cooled almost three tenths of a degree in less than half a century? Hardly! We are just getting more precise in our measurements.

On the other hand, the universe is cooling! That's because it is expanding. From the time of the Big Bang some 13.7 Billion years ago, up until now, that background temperature has dropped from millions of degrees to pretty near absolute zero. So, over time, the background radiation's frequency peak should indeed be dropping. Only, that change is occurring over galactic timescales, and would not be measurable over human lifetimes. In fact, instrumentation error will impart variability greater than the changes we are trying to detect. So, no, during the eyeblink of history during which we've had radio astronomy, we have not been able to measure this change.

Still, that was a great question!

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