Hubble's law & the Big Bang theory
- Edwin Hubble investigated the light spectra emitted from a large number of galaxies
- He used redshift data to determine the recession velocities of these galaxies, and standard candles to determine the distances
- From these measurements, he formulated a relationship, now known as Hubble’s law, which states:
The recession speed of galaxies moving away from Earth is proportional to their distance from the Earth
- This can be calculated using:
- Where:
- v = the galaxy's recessional velocity (m s-1)
- d = distance between the galaxy and Earth (m)
- H0 = Hubble's constant, or the rate of expansion of the universe (s-1)
- This equation tells us:
- The further away a galaxy is, the faster its recession velocity
- The gradient of a graph of recession velocity against distance is equal to the Hubble constant
Hubble's law graph
A key aspect of Hubble’s law is that the furthest galaxies appear to move away the fastest
Age of the Universe
- If the galaxies are moving away from each other, then they must’ve started from the same point at some time in the past
- If this is true, the universe likely began in an extremely hot, dense singular point which subsequently began to expand very quickly
- This idea is known as the Big Bang Theory
- The redshift of galaxies and the expansion of the universe are now some of the most prominent pieces of evidence to suggest this theory is true
- The data from Hubble’s law can be extrapolated back to the point that the universe started expanding i.e. the beginning of the Universe
- Therefore, the age of the universe T0 is equal to:
- Current estimates of the age of the universe range from 13 – 14 billion years
- There is still some discussion about the exact age of the universe, therefore, obtaining accurate measurements for the Hubble constant is a top priority for cosmologists
Expansion of the Universe
Tracing the expansion of the universe back to the beginning of time leads to the idea the universe began with a “big bang”
Worked example
A galaxy is found to be moving away with a speed of 2.1 × 107 m s-1. The galaxy is at a distance of 9.5 × 1024 m from Earth.
Assuming the speed has remained constant, what is the age of the universe in years?
Answer:
Step 1: Write down Hubble’s Law
Step 2: Rearrange for the Hubble constant H0, and calculate
Step 3: Write the equation for the age of the universe T0, and calculate
Step 4: Convert from seconds into years
- Therefore, the age of the universe is estimated to be about 14.3 billion years