Cosmology · Cosmic contents and background radiation

Ordinary matter, plasma and radiation

Follow four hydrogen atoms through ionization, then distinguish the charged matter from the light that transfers energy to it.

Preparation: ratios, powers of ten and ordinary arithmetic. Physical quantities and notation are introduced as they are needed.

Back to the chapter

After reading, you can

Identify nuclei and electrons as ordinary matter whether they are bound or free.
Distinguish a plasma from the radiation interacting with it.
Explain the separate roles of particle counts, photon energy and energy density.

Begin with four hydrogen atoms

Imagine a small teaching sample containing four neutral hydrogen atoms. Each consists of one positively charged proton and one negatively charged electron in a bound quantum state. The charges cancel in each atom. A drawing can mark the two constituents, but the electron is not a little planet following a definite circular orbit.

Supply enough ionizing energy to free one electron from every atom. The sample now contains four positive hydrogen ions, which are protons, and four free electrons. No proton or electron has been replaced by an unfamiliar kind of matter. There are still four of each, and the total electric charge is still zero.

Ionization changes the binding state. It takes energy; recombination, in which an electron becomes bound again, can release energy as radiation. The particles can also gain or lose kinetic energy. Conservation of charge is an exact counting statement here, whereas an energy account must include the energy supplied and any light that enters or leaves.

This four-atom example supplies no cosmic abundance or spatial scale. Its purpose is to separate what a constituent is from whether it is bound into a neutral atom. The same distinction becomes useful when cosmology describes ordinary matter before and after neutral atoms became common.

A chosen four-hydrogen sample has four protons and four electrons before and after ionization. Before, each dashed box groups one proton with one electron in a neutral bound atom; the boxes are not electron orbits. After ionizing energy is supplied, four protons and four free electrons are drawn separately. Positive circles and negative diamonds distinguish constituents without relying on colour. Charge is measured in elementary-charge units: +4 − 4 = 0 in both states. The diagram represents neither spatial scales, cosmic abundances nor photon counts; radiation and supplied energy must be included in a full energy account. Original Ark teaching model.

A plasma is ordinary matter with mobile charges

A plasma contains mobile charged particles whose collective electric and magnetic effects matter. An ionized gas with free electrons and positive ions is an important example. Overall neutrality does not make each constituent neutral: an electric field can push positive and negative charges in opposite directions.

Ordinary cosmic matter includes nuclei and electrons in neutral gas, ionized plasma, stars and other forms. Ordinary does not mean bright, cold or familiar in appearance. A cloud can contain ordinary matter even when it emits little visible light. Visibility and physical composition answer different questions.

Cosmologists often call the nuclei-bearing component baryonic matter. Protons and neutrons are baryons; electrons are not. In a cosmological matter budget, the associated electrons are conventionally included with this ordinary component. The label does not mean that electrons cease to matter or that every ordinary particle is a baryon.

Ionizing hydrogen does not turn it into dark matter. Ionized material still has charged constituents that interact electromagnetically, and it can absorb, emit or scatter radiation under the appropriate conditions. Later observations can reveal plasma through light outside the visible band.

Light carries energy without being a gas of atoms

Electromagnetic radiation includes radio waves, microwaves, visible light and X-rays. Its quantum of energy is a photon. For a photon of frequency ν, the relation E = hν connects energy E to frequency: h is Planck’s constant. Frequency counts cycles per second, measured in hertz, while photon energy can be measured in joules.

At twice the frequency, a photon has twice the energy. This statement concerns an individual photon. It does not say that a beam is twice as energetic overall: that also depends on how many photons cross the detector during the measurement.

Likewise, an energy density describes energy per volume, for example joules per cubic metre. It is neither a particle count nor a temperature by definition. To turn a photon count into a radiation energy density, one must know the photon energies and the volume being described.

Matter and radiation exchange energy through physical interactions. An absorbed photon whose energy meets or exceeds the atom’s ionization threshold can free an electron; an electron can scatter an existing photon. These are different processes. A scattering event is not automatically a photon creation event, and an arbitrary radio photon cannot ionize hydrogen just because it is radiation.

Expansion changes radiation in two ways

Consider a region following a smooth expanding background, with all its lengths doubled. Its volume increases by 2 × 2 × 2 = 8. If a fixed population of slow-moving particles remains inside this expanding region, its number density becomes one eighth of the original value. The particles have been spread through more volume.

For freely propagating background photons, the same expansion also doubles each wavelength and halves each frequency and photon energy. If their number is conserved in this comparison, there is one eighth as many photons per volume and half as much energy per photon. The radiation energy density becomes one sixteenth.

These scalings describe a separately conserved ideal component over an interval without injection, absorption or transfer of energy between components. The matter comparison treats particles as nonrelativistic, meaning that their speeds are small compared with light speed and their rest energy dominates. It does not describe every hot particle species at every epoch.

One need not calculate a whole cosmic history to use the comparison. First ask what is counted, what volume is used, and whether each particle’s energy changes. The extra change in photon energy explains why radiation and nonrelativistic matter need separate entries in cosmology.

A chosen expansion by a length factor of 2, with no creation, absorption or energy exchange during the comparison
Quantity	Value divided by its initial value
Following volume	8
Conserved nonrelativistic particle number per volume	1/8
Freely propagating photon energy	1/2
Conserved photon number per volume	1/8
Radiation energy per volume	1/16

Two components can share a history without being the same thing

In the hot early Universe, light and charged matter interacted frequently. Electrons scatter photons; electromagnetic interactions couple the charged matter. This connection helps explain why the radiation and matter could influence one another before neutral atoms became common.

The connection does not merge the categories. A proton remains ordinary matter whether or not it is surrounded by photons. A photon remains radiation when it scatters from an electron. We will use this distinction when asking how the microwave background acquired its spectrum and why many of its photons later travelled with little further scattering.

For a first reading, retain the four-atom charge account and the doubled-volume comparison. Detailed plasma physics and thermal distributions are later return paths. They refine these mechanisms; they are not needed to recognize that binding state, particle identity and radiation energy are different questions.

Worked reasoning

Account for the constituents

All four neutral hydrogen atoms in the teaching sample are ionized once. How many protons and free electrons are present, and what is the total charge?

Each hydrogen atom starts with one proton and one bound electron. Four atoms therefore contain four of each.
Ionization frees the electrons; it does not remove the protons or convert the electrons into photons.
Four proton charges and four electron charges cancel. The ionizing energy belongs in a separate energy account.

There are four protons and four free electrons. Their combined electric charge is zero.

Try the reasoning

Does a neutral gas become dark matter when it is ionized?
Show the answer and reasoning
No. Ionization changes the binding of its ordinary constituents.
The free charges retain electromagnetic interactions; a lack of visible glow would not by itself identify dark matter.
Photon A has three times photon B’s frequency. Compare their individual energies.
Show the answer and reasoning
Photon A has three times photon B’s energy.
E = hν uses the same constant h for both. The comparison supplies no beam brightness without a photon count and measurement interval.
Lengths double while a nonrelativistic particle population is conserved. Does its number density become one half or one eighth?
Show the answer and reasoning
One eighth.
Volume changes in three dimensions: 2³ = 8. The same number divided by eight times the volume gives one eighth the density.

What to carry forward

Neutral atoms and ionized plasma can contain the same ordinary constituents.
Photons carry energy E = hν; a photon energy, a count and an energy density are different quantities.
For the stated conserved-component model, a length doubling reduces matter number density by 8 and radiation energy density by 16.

References

Sources supporting the explanations, physical distinctions and examples in this lesson.