The Cosmic Microwave Background and the Light Elements · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Afterglow and ashes of the hot start

Science · Astronomy & Astrophysics · ages 21-22
Name ______________________   Date ____________

  1. When did the radiation we observe as the CMB begin travelling with much less scattering?

    • When the first stars formed and their light spread
    • When the Sun formed and its radiation began crossing the Milky Way galaxy
    • When neutral atoms formed, about 380,000 years after the Big Bang

  2. What is the approximate temperature of the cosmic microwave background today?

    • 273 degrees above absolute zero
    • 2.7 degrees above absolute zero
    • 2700 degrees above absolute zero

  3. The CMB's nearly blackbody spectrum supports the claim that early radiation was in thermal equilibrium.

    Circle one:   True   False

  4. The model predicts about 25% helium by ordinary matter's mass. Which account links this prediction to conditions in the first minutes?

    • Hot, dense matter allowed reactions; expansion cooled and diluted matter, limiting reactions
    • Cold, thin matter allowed reactions; expansion heated and diluted matter, boosting reactions
    • Hot, dense matter blocked reactions; expansion heated and diluted matter, enabling reactions

  5. Why does radiation from the hot early universe now have most of its energy at microwave wavelengths?

    • Dust filtered out shorter wavelengths while leaving photon energies unchanged
    • Expansion lengthened wavelengths and reduced the energies of photons
    • Stars absorbed the visible radiation and left its original microwaves unchanged

  6. What information do the small CMB temperature variations carry?

    • The present locations of stars in today's galaxies
    • Only microwave emission from nearby stars and dust within the Milky Way galaxy
    • Early density, motion and gravitational effects linked to later structure

  7. A student says all cosmic helium came from later stars. Which observation best supports production before stars formed?

    • Gas with few heavier elements has helium matching early nuclear calculations
    • Stars contain helium and make more through nuclear reactions
    • Some helium-rich stellar layers become exposed after stars lose their outer material

  8. A student proposes ordinary scattered starlight as the source of the CMB. Which comparison best tests that proposal?

    • Whether it matches the brightness and the colour in one given direction
    • Whether it matches the thermal spectrum and the nearly uniform sky glow
    • Whether every source star has the same mass, luminosity and chemical composition
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Afterglow and ashes of the hot start W1-mt_oz0MJXiuVt-s1

  1. When neutral atoms formed, about 380,000 years after the Big Bang · Electrons joining nuclei reduced the number of free electrons available to scatter photons. The background radiation already existed.
  2. 2.7 degrees above absolute zero · The measured temperature is about 2.7 K. Kelvin expresses temperature above absolute zero.
  3. True · Thermal equilibrium produces a blackbody spectrum. The CMB closely matches that spectrum at a much lower temperature today.
  4. Hot, dense matter allowed reactions; expansion cooled and diluted matter, limiting reactions · High temperature and density allowed light nuclei to form. Expansion cooled and diluted matter, limiting reactions. Calculations under standard early conditions predict about 25% helium by ordinary matter's mass.
  5. Expansion lengthened wavelengths and reduced the energies of photons · Expansion redshifts the background radiation. Longer wavelengths mean lower photon energies and a lower thermal temperature.
  6. Early density, motion and gravitational effects linked to later structure · The variations record early matter and gravitational effects. Gravity later amplified density differences into galaxies and clusters.
  7. Gas with few heavier elements has helium matching early nuclear calculations · Gas with few heavier elements helps estimate the primordial helium share. Its agreement with early nuclear calculations supports production before stars formed.
  8. Whether it matches the thermal spectrum and the nearly uniform sky glow · The proposal must account for both the measured spectrum and the sky distribution. Ordinary scattering does not turn starlight into one common blackbody spectrum.
Worksheet · LightMySky