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Cambridge IGCSE Physics · 0625

Chapter 6: Space physics — Part 5

Topic 6.2.3 · The Universe

Galaxies, redshift and the Big Bang

The Milky Way is one of many billions of galaxies in the Universe. Its diameter is about 100 000 light-years.

Redshift is an increase in the observed wavelength of EM radiation from receding stars and galaxies. Light from distant galaxies appears redshifted compared with light emitted on Earth.

Redshift of distant galaxies is evidence that the Universe is expanding and supports the Big Bang theory.

Redshift: spectral lines from a distant galaxy shifted to longer wavelength compared with the same lines on Earth, evidence of recession and an expanding Universe.
Diagram 1: Redshift: spectral lines from a distant galaxy shifted to longer wavelength compared with the same lines on Earth, evidence of recession and an expanding Universe.

CMBR and the Hubble constant

Cosmic microwave background radiation (CMBR) is microwave radiation of a specific frequency observed in all directions. It was produced shortly after the Universe formed and has been stretched into the microwave region as the Universe expanded.

The speed v at which a galaxy recedes can be found from the change in wavelength of its starlight (redshift). Distance d of a far galaxy can be found using the brightness of a supernova in that galaxy.

The Hubble constant H0 = v / d. The current estimate is 2.2 × 10−18 s−1.

1 / H0 is an estimate of the age of the Universe. That supports the idea that all the matter in the Universe was once at a single point.

A galaxy recedes at 6.6 × 105 m/s. Its distance is 3.0 × 1023 m. Calculate the Hubble constant from these data.

Worked example: Hubble constant is velocity over distance equals 2.2 times 10 to the minus 18 per second
H0 = v / d = 2.2 × 10−18 s−1.

Take H0 = 2.2 × 10−18 s−1. Estimate the age of the Universe.

Worked example: age of the Universe is 1 over H0 equals 4.5 times 10 to the 17 seconds
age ≈ 1 / H0 = 4.5 × 1017 s.
CMBR filling space, and Hubble’s relation v = H0 d with H0 = 2.2 × 10⁻¹⁸ s⁻¹ and an age estimate of about 1 / H0.
Diagram 2: CMBR filling space, and Hubble’s relation v = H0 d with H0 = 2.2 × 10⁻¹⁸ s⁻¹ and an age estimate of about 1 / H0.

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