09. The Temperature of the Sky — Noise That Would Not Go Away

An antenna that hissed in every direction

In 1964 Arno Penzias and Robert Wilson were preparing a horn antenna at Bell Labs in Holmdel, New Jersey, for radio astronomy. The instrument carried a persistent background hiss that they could not eliminate and could not explain. They checked the electronics, ruled out New York City, ruled out the atmosphere, and ruled out the remains of a recent high-altitude nuclear test. A pair of pigeons had nested in the horn, so the birds were removed and the accumulated droppings cleaned out, and the noise was unchanged. The signal was the same strength in every direction, at every hour, in every season, which no local or galactic source can manage. Their 1965 paper reported it flatly as "A Measurement of Excess Antenna Temperature at 4080 Mc/s," a title that claims nothing at all.

The paper printed next to it

Thirty miles away at Princeton, a group under Robert Dicke had reasoned that a hot early universe should have left a cooled remnant of radiation filling all of space, and were building an instrument to look for it. When the two groups were put in contact, the interpretation was immediate, and the Astrophysical Journal ran the Dicke group's theoretical paper alongside the Penzias and Wilson measurement. The division is unusually clean: one team had the prediction and no data, the other had the data and no idea what it was. Penzias and Wilson shared the 1978 Nobel Prize in Physics for the detection. The temperature has since been measured to extraordinary precision by the COBE satellite's FIRAS instrument at 2.72548 ± 0.00057 kelvin. That spectrum is the closest thing to a perfect blackbody ever observed, in the laboratory or out of it.

What the pigeons are for

The story is told for the birds, but the birds are the least interesting part of it, and the elimination is the point. Penzias and Wilson found the most important signal in twentieth-century cosmology by systematically removing every explanation they could name and refusing to stop when the noise survived. An anomaly that outlives your best efforts to kill it has earned the right to be taken seriously, which is the same move Patterson made with his lead.

Example: A rattle in a car that survives four mechanics is no longer a rattle; it is a finding.

Verdict. The cosmic microwave background was discovered as unexplained noise by observers who did not know what they had, and its temperature is now among the best-measured quantities in astronomy at 2.72548 ± 0.00057 K.

Overturned if: the FIRAS spectrum is shown to deviate measurably from a blackbody in a way that requires a source other than a hot early universe.

Sources. 1965 · A. A. Penzias & R. W. Wilson, "A Measurement of Excess Antenna Temperature at 4080 Mc/s," Astrophysical Journal 142:419 — reference record · 1996 · "Measurement of the cosmic microwave background spectrum by the COBE FIRAS instrument" — NASA NTRS