This section explores how the Big Bang model accounts for the Universe’s expansion and how the cosmic microwave background provides observational evidence for this origin.

Big Bang Theory: Core Ideas

The Big Bang theory is the leading cosmological model describing the origin and early development of the Universe. It proposes that the Universe began as a hot, dense state and has been expanding over time. Crucially, the model does not describe an explosion into existing space; instead, it describes the expansion of space itself. This distinction helps students understand why distant galaxies appear to be moving away even though they are not travelling through space in the everyday sense.

The model’s success rests on multiple observational pillars, but for this subsubtopic the focus is the cosmic microwave background (CMB), the relic radiation that remains from the early Universe. Before introducing the CMB, it is essential to understand that the early Universe was filled with extremely energetic photons interacting constantly with charged particles.

Conditions in the Early Universe

In the earliest moments, the Universe was a plasma of photons, electrons, and protons. In this state, photons were repeatedly scattered by free electrons, preventing the formation of stable atoms and making the Universe opaque. As expansion progressed, temperatures and densities fell.

Around 380,000 years after the Big Bang, the Universe cooled sufficiently for electrons and protons to combine and form neutral hydrogen. This transition is commonly called recombination, though it was the first time electrons and nuclei combined.

Following recombination, photons decoupled from matter and began travelling unimpeded through the cosmos. These photons have been reaching Earth ever since.

The Origin of the Cosmic Microwave Background

The cosmic microwave background is the cooled, stretched remnant of the radiation released when photons decoupled from matter. At the moment of decoupling, the radiation featured a temperature of roughly 3000 K. Due to the continuing expansion of the Universe, the photons’ wavelengths have stretched dramatically, placing them in the microwave region of the electromagnetic spectrum.

This background radiation now has a characteristic temperature of approximately 2.7 K, exactly as required by the OCR specification. The extremely low temperature reflects both the cooling effect of cosmic expansion and the redshifting of the original high-energy photons.

Observations from satellites such as COBE, WMAP, and Planck reveal that the radiation has an almost perfect blackbody spectrum, providing a powerful indicator that the early Universe was in thermal equilibrium.

This plot compares the measured intensity of the cosmic microwave background with the theoretical blackbody curve at 2.7 K. The COBE/FIRAS data closely follows the curve, confirming the thermal nature of the CMB. This level of agreement provides decisive evidence for the hot Big Bang model. Source.

Why the CMB Supports the Big Bang Model

The CMB is one of the strongest pieces of evidence for the Big Bang model. Several key features confirm theoretical predictions:

Uniformity of the CMB

The CMB is remarkably isotropic, appearing almost the same in every direction. Such uniformity indicates that the early Universe was extremely homogeneous. Small variations exist, but they are tiny — on the order of one part in 100,000. These small temperature fluctuations reveal the seeds of later cosmic structures such as galaxies and clusters.

This all-sky Planck map shows tiny temperature variations in the cosmic microwave background. The nearly uniform field demonstrates large-scale homogeneity, while small fluctuations mark initial density differences. These variations later grew into galaxies and galaxy clusters under gravitational attraction. Source.

Blackbody Spectrum

Measurements show that the CMB follows an almost perfect blackbody curve. This supports the idea that the early Universe was a hot, dense environment in thermal equilibrium.

Temperature of 2.7 K

The predicted cooling of radiation as the Universe expands agrees with the observed temperature of the CMB. This matches the expectation that photon wavelengths lengthen as space stretches.

Redshifted Origin

Because the photons we detect today were emitted when the Universe was much smaller, they have been greatly redshifted. This aligns strongly with the Big Bang’s central claim that space itself is expanding.

Key Processes and Observational Evidence

To deepen understanding, students should connect the following processes with the supporting observational data:

• Expansion of the Universe

  • Seen in the redshift of light from distant galaxies.

  • Sets the context for interpreting the CMB as stretched early-Universe radiation.

• Photon decoupling

  • Allowed photons to travel freely through space.

  • Directly produced the CMB.

• Cooling due to expansion

  • Explains the shift from a 3000 K plasma to a present-day temperature of 2.7 K.

• Anisotropies in the CMB

  • Tiny temperature differences indicate early density variations.

  • These fluctuations grew under gravity to form the large-scale structure of the Universe.

After considering these points, it becomes clear why the CMB is often described as a “snapshot” of the early Universe. It captures conditions from a time far earlier than any other observable signal.

Relationship Between Big Bang Theory and CMB Evidence

The Big Bang model makes precise predictions about both the existence and properties of the CMB. The remarkable match between prediction and measurement is why the CMB stands as a cornerstone of modern cosmology. It confirms the Universe had a hot, dense origin, has been expanding ever since, and retains a measurable thermal relic from its earliest observable moment.