PH207 topics

Matter era

The matter era is defined as the time period when matter began to dominate the universe, rather than light. The formation of atoms marks the early threshold of the matter era.


We define atoms as  nuclei having bound electrons. The particles that make up atoms were formed in the lepton epoch (electrons) and in the nuclear epoch, but atoms were not formed until the universe further cooled to temperatures where electrons could stay bound to atoms.


An important aspect of the matter era is that when atoms formed, with bound electrons, most photons of light were freed up to travel freely, since only photons of certain energies would interact with atoms by raising wlwctrons to excited states.

Recombination (decoupling of light)

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When the electrons became bound to nuclei, only photons with certain energies interacted with matter. Photons with energies that corresponded to differences between excited electron states could interact with electrons. Other photons were free to flow. When atoms were formed, the universe became transparent to most wavelengths of light. This phenomenon is called recombination, or the decoupling of light from matter.

  • Atomic epoch
    • Matter begins to dominate
    • Atoms form
      • Temperature drops low enough

                   for electrons to become

                   bound to atoms

    • EM radiation decouples
      • Universe becomes transparent
      • Only photons with particular

                   wavelengths interact with

                   bound electrons

    • CMBR released

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When the universe became transparent to photons, light was free to move. We can calculate the distance that the light has traveled since its release, at somewhat under 14 billion parsecs, by comparing the peak wavelength that the light would have at the temperature where atoms could form to the redshifted light that we see in the cosmic microwave background radiation.

Horizon problem

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The cosmic microwave background radiation is very uniform across the sky. In every direction, we measure the same temperature, with very tiny fluctuations. The problem is that when the light was released, the universe was already so large that light would not have enough time to traverse the distance to carry the signal from one horizon to the other. In other words, light could not have a causal effect across distant regions of the universe. The regions marked "A" and "B" in the diagram above look practically identical to us, yet they were separated by millions of parsecs when they emitted the light that is reaching us now. It is possible, but unlikely, that the  reason these regions look the same now is that they started out with the same characteristics.


One way of solving the horizon problem is to consider the possibility that they universe went through a short burst of very rapid expansion at the end of the quark era, called "inflation." We believe that the universe underwent something like a phase change when the fundamental forces reorganized as they were being frozen out. It entered a false vacuum state with very high energy, acquiring enormous pressure, resulting in an ultrafast readjustment.


A very rapid expansion of the universe could be seen as analogous to a rapid phase change in normal matter, like what is seen when supercooled water freezes.

Very pure water can be cooled to a temperature somewhat below the freezing point without actually freezing until it is perturbed, then it freezes very rapidly. Cosmologists believe that the same thing could have happened in the early universe. It may have been undergoing transitions like phase changes, and delayed the change, staying in a metastable phase for a bit too long, then rapidly dropping into the lower energy state. Some cosmologists think this could have happened several times.

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An inflationary period solves the horizon problem by recognizing that the inflation took two points that were causally connected could be rapidly dragged apart if the expansion was faster than the speed of light. The laws of physics say that nothing can travel faster than the speed of light. To be more precise, no signal can travel faster than light speed. This doesn't mean that the whole universe could not expand faster than light speed, since it is space that is expanding, not a signal propagating across space. In the diagram above, points A and B start out close enough together that they can be causally related - they can affect each other because light can pass between them. After inflation rapidly expands space, A and B are no longer causally connected, but as the universe further expands they are both within sight again. They look similar because they were causally connected before inflation happened.


Flatness problem

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Another cosmological issue that inflation addresses is the flatness problem. When we add up all of the normal matter, dark matter and dark energy of the universe, the density of the universe turns out to be extremely close to the critical density of the universe. We appear to have a flat universe. Simply stated, the flatness problem asks, of all possible densities for the universe, why we should happen to have this special value?

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Inflation solves this problem by looking at the whole picture. At early times before inflation, there could have been strong curvature to the universe. After the universe was blown up by inflation, the curvature looks locally flat because we are only seeing a very small part it.

Image source:


After recombination, the universe entered what we call the "Dark Ages." There would be nor more light produced until fusion took place in the first stars. The galactic epoch was marked by the formation of the large scale structure of the universe.

  • Galactic epoch
    • Large scale structure forms
    • First stars and quasars shine
    • Galaxies form and grow
  • Stellar epoch
    • Galaxies merge and evolve
    • Star formation peaks
    • Dark energy begins to dominate
  • Dark energy epoch
    • Dark energy dominates the universe

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It is believed that cold dark matter played a very important role in the formation of the large scale structure of the universe. Quantum mechanical fluctuations are very similar in character to large scale fluctuations we see today. Matter densities, however, vary much more than the CMBR temperatures. Hot dark matter does not have a tendency to clump, since the particles are moving so fast. Cold dark matter is attracted gravitationally but not repelled electromagnetically, and moves at slow enough speed that it could form clumps early in the history of the universe. It could provide the scaffold for normal matter to gravitationally contract into structures like early galaxies.

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Computer simulations show good agreement between structure seen in quantum mechanical fluctuations and large scale structure seen in the universe today.

Image credit NASA/ESA/Massey


Three dimensional mapping of dark matter using gravitational lensing slices indicates that there is good agreement between dark matter haloes and large scale structure involving galaxy clusters.

Animation source


Analysis of data from NASA's Fermi gamma ray telescope was used to understand the fossil radiation field of light from stars even after they cease to shine. This animation tracks several gamma ray photons along their journey through the universe, encountering optical and UV photons from starlight, ultimately arriving at the Fermi telescope.