Expanding Universe
Section: Space Physics | Syllabus: Cambridge IGCSE Physics (0625)
Cosmic Microwave Background Radiation (CMBR)
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- Cosmic microwave background radiation (CMBR) is microwave radiation of a specific frequency that is observed at all points in space around us, arriving from every direction with nearly equal intensity
- CMBR was produced shortly after the Universe was formed in the Big Bang, when the Universe was extremely hot and dense and released very high-energy radiation
- As the Universe expanded, this radiation was stretched by the same factor, expanding its wavelength into the microwave region of the electromagnetic spectrum
- CMBR is detected at all points in space - it is often described as the "afterglow" of the Big Bang, and its existence is further strong evidence for the Big Bang Theory
Hubble's Law
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Edwin Hubble found that the speed at which a galaxy moves away from us is proportional to its distance from us.
- The speed, v, at which a galaxy is moving away from the Earth can be found from the change in wavelength of the galaxy's starlight due to redshift - the greater the redshift, the faster the galaxy is receding
- The distance, d, of a far galaxy can be determined using the brightness of a supernova in that galaxy - a given type of supernova always has the same intrinsic brightness, so comparing its apparent brightness with its known brightness gives its distance
- The Hubble constant, H_0, is defined as the ratio of the speed at which a galaxy is moving away from the Earth to its distance from the Earth:
- H_0 = (v/d)
- v = speed at which the galaxy is moving away (m/s)
- d = distance of the galaxy from Earth (m)
- the current estimate for H_0 is 2.2 × 10^-18 s^-1
Hubble's Law: recession speed is proportional to distance, v = H₀d
Worked Example: Finding a Galaxy's Speed
Muumbe measures a galaxy at a distance of 1.4 × 10^25 m. Using H_0 = 2.2 × 10^-18 s^-1, find its recession speed.
- Rearrange H_0 = (v/d) to give v = H_0 × d
- v = 2.2 × 10^-18 × 1.4 × 10^25 = 3.1 × 10^7 m/s
Age of the Universe
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Hubble's Law can be used to estimate the age of the Universe. If all matter was once at the same point and has been moving apart ever since the Big Bang, the time since the Big Bang can be estimated from:
- age = (d/v) = (1/H_0)
- Using H_0 = 2.2 × 10^-18 s^-1:
- age ≈ 12.2 × 10^-18 ≈ 4.5 × 10^17 s ≈ 14 billion years
- This equation represents an estimate for the age of the Universe, and is evidence for the idea that all the matter in the Universe was once present at a single point
Summary: Evidence for the Big Bang
| Evidence | Observation | What it tells us |
|---|---|---|
| Redshift | Light from distant galaxies has longer wavelengths than expected | Galaxies are moving away - the Universe is expanding |
| Hubble's Law | More distant galaxies move away faster (v = H_0 d) | Consistent with all matter expanding from a single point |
| CMBR | Uniform microwave radiation observed in all directions | Afterglow of the Big Bang, stretched to microwave wavelengths as the Universe expanded |
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