Lesson 4 of 6 · Last scattering, spectra & tiny variations
Measure a warm past in a cold sky
10 minute lesson · 3 checks along the way
Your goal for this lessonExplain the microwave background’s origin and interpret temperature contrasts without confusing a processed map, a foreground, and the underlying measurement.
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The photons were already there
The , or CMB, is relic radiation from the hot early universe. Before neutral atoms became abundant, free electrons frequently scattered photons. Around 380,000 years into the standard cosmic history, cooling allowed many electrons to bind to nuclei and greatly reduced that scattering. Photons could then travel much farther. This era is different from saying that all the background photons were first created when atoms formed.
The background has an almost perfect with a present temperature about 2.725 kelvin. Expansion stretched its wavelengths from a much hotter radiation field into the microwave range. Its temperature is a property inferred from the radiation spectrum, not the temperature of every object in today’s universe. Hot stars and cold dust coexist with that pervasive radiation because local processes can establish very different conditions.
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A colorful map represents small differences
CMB maps often use contrasting display colors to reveal departures from the mean temperature, after removing major contaminants and the strong pattern caused by our motion. Their bright colors are not visible-light photographs of primordial gas clouds. Typical cosmological temperature variations are tiny compared with the mean. Their pattern across angular scales encodes information about conditions and processes before last scattering.
Planck measured the sky through multiple frequency bands. This helped distinguish the CMB from , including emission from our own galaxy. Its detectors also required careful cooling, calibration, and noise control. In the cleanroom photograph the spacecraft is under protective covering: no colored sky map is visible. The hardware image documents preparation for a measurement whose meaning depends on subsequent observations and analysis.
Planck in a cleanroom at Europe’s Spaceport in Kourou, French Guiana, on March 13, 2009. The payload is covered during this preparation view. The photograph does not show a microwave map or reveal detector temperatures.
Look closely
What does the protective covering prevent you from inspecting here, and what further evidence would establish a tiny CMB temperature contrast?
See what to notice
The covering hides much of the payload’s detailed appearance. A sky contrast requires later calibrated detector measurements, uncertainty estimates, and foreground separation; neither the covering nor the photograph’s visible colors supplies that result.
Work through a decision
Calculate the contrast before choosing a display
For an illustrative map, take a rounded reference temperature of 2.700000 kelvin and a pixel 27 . A microkelvin is one millionth of a kelvin, so the pixel temperature is 2.700027 kelvin. The fractional departure is 0.000027/2.7 = 0.00001, or one part in 100,000. These invented values explain the scale and are not a particular Planck pixel.
A display might assign very different colors to the reference and the warmer value so the contrast is readable. That does not turn the latter into a region thousands of degrees hotter. An uncorrected offset of 30 microkelvin would exceed the entire stated 27-microkelvin signal. Before interpreting the pixel cosmologically, compare calibration uncertainty, instrument noise, frequency behavior, and processing choices with the size of the effect.
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Use the pattern to test a history
Models predict statistical relationships among temperature and polarization patterns, including how much variation appears at different angular scales. Comparing these predictions with measured maps constrains combinations of geometry, matter content, and early conditions. A fitted parameter is an inference within that model, with uncertainties and correlations. The map is not a list of directly photographed dark-matter particles or a unique answer to every cosmological question.
Last scattering limits ordinary light-based imaging of the earlier opaque , but it does not make earlier history untestable. Light-element abundances and other relics carry different information. A useful cross-check asks whether one expansion and matter history accounts for these independent records together. Disagreement could expose foreground treatment, calibration, or a model assumption; identifying which requires a further discriminating test, not a more dramatic color palette.
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Put it together
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