The expanding Universe
A growing Universe
- Look far enough and every galaxy is rushing away from us.
- The whole Universe is expanding — and that tells us how it began.
- The clue is hidden in the colour of starlight.
Redshift
- When a galaxy moves away from us, the light we receive is stretched to a longer wavelength — shifted toward the red end of the spectrum. This is redshift.
- The further away a galaxy is, the bigger its redshift.
Light from a distant galaxy is redshifted. This means the galaxy is:
A receding source stretches the light to a longer wavelength — toward the red end of the spectrum.
How does a galaxy's redshift change with its distance?
More distant galaxies recede faster, so they show a bigger redshift — this is the basis of Hubble's law.
The expanding Universe and the Big Bang
- Distant galaxies are redshifted, and further ones more so — so the galaxies are moving apart: the Universe is expanding.
- Run that backwards and everything was once together at a single point.
- This is the evidence for the Big Bang theory.
The redshift of distant galaxies is evidence that:
Galaxies moving apart means the Universe is expanding; running it backwards points to a single starting point — the Big Bang.
More evidence, and the age of the Universe
- Faint microwave radiation comes from every direction in space — the cosmic microwave background radiation (CMBR). It is early radiation stretched out as the Universe expanded.
- The Hubble constant links a galaxy's recession speed $v$ and distance $d$:
- Turning it around, $\dfrac{1}{H_0}$ estimates the age of the Universe — more support for the Big Bang.
The cosmic microwave background radiation (CMBR) is found coming from every direction in space.
The CMBR fills all of space, coming from every direction — it is early radiation stretched into microwaves as the Universe expanded.
The Hubble constant is about $H_0 = 2.0 \times 10^{-18}$ per second. Estimate the age of the Universe ($1/H_0$), in seconds.
Age $\approx \dfrac{1}{H_0} = \dfrac{1}{2.0 \times 10^{-18}} = 5.0 \times 10^{17}\ \text{s}$ (roughly 16 billion years).
The Hubble constant $H_0$ relates a galaxy's recession speed $v$ and distance $d$ by:
$H_0 = \dfrac{v}{d}$; its reciprocal $\dfrac{1}{H_0}$ estimates the age of the Universe.
You've got it
- redshift: light from a receding galaxy is stretched toward red; further = bigger redshift
- distant galaxies all recede → the Universe is expanding → evidence for the Big Bang
- the CMBR (microwaves from all directions) is more Big Bang evidence
- Hubble constant $H_0 = \dfrac{v}{d}$; age of the Universe $\approx \dfrac{1}{H_0}$