Theories of the Universe (Edexcel GCSE Physics)

The Big Bang Theory

• Around 14 billion years ago, the Universe began from a very small region that was extremely hot and dense
• Then there was a giant explosion, which is known as the Big Bang
• This caused the universe to expand from a single point, cooling as it does so, to form the universe today
• Each point expands away from the others
  • This is seen from galaxies moving away from each other, and the further away they are the faster they move

• As a result of the initial explosion, the Universe continues to expand

All galaxies are moving away from each other, indicating that the universe is expanding

• An analogy of this is points drawn on a balloon where the balloon represents space and the points as galaxies
• When the balloon is deflated, all the points are close together and an equal distance apart
• As the balloon expands, all the points become further apart by the same amount
• This is because the space itself has expanded between the galaxies
  • Therefore, the density of galaxies falls as the Universe expands

A balloon inflating is similar to the stretching of the space between galaxies

Steady State Theory

• The Steady State theory was the main rival to the Big Bang theory up until the mid-1960s
• The main idea behind the Steady State theory was that the Universe has always existed more or less as it is seen now
• The theory accounted for the expansion of the Universe by suggesting that as galaxies move apart, new ones will be formed in the gaps in between them
  • This implies the density of galaxies remains the same as the universe expands

• Whilst the Steady State theory could explain the expansion of the Universe it could not explain some of the other evidence that emerged in favour of the Big Bang and hence is not an accepted theory today

The Steady State Theory vs the Big Bang Theory

• Since there is more evidence supporting the Big Bang theory than the Steady State theory, it is the currently accepted model for the origin of the Universe
• The main pieces of evidence for the Big Bang are
  • Galactic red-shift
  • Cosmic Microwave Background (CMB) radiation

Evidence from Galactic Red-Shift

• By observing the light spectrums from supernovae in other galaxies there is evidence to suggest that distant galaxies are receding (moving further apart) even faster than nearby galaxies
  • These observations were first made in 1998

• The light spectrums show that light from distant galaxies is redshifted, which is evidence that the universe is expanding
• As a result, astronomers have concluded that:
  • All galaxies are moving away from the Earth
  • Galaxies are moving away from each other

• This is what is expected after an explosion
  • Matter is first densely packed and as it explodes it, it moves out in all directions getting further and further from the source of the explosion
  • Some matter will be lighter and travel at a greater speed, further from the source of the explosion
  • Some matter will be heavier and travel at a slower speed, closer to the source of the explosion

• If someone were to travel back in time and compare the separation distance of the galaxies:
  • It would be seen that galaxies would become closer and closer together until the entire universe was a single point

• If the galaxies were originally all grouped together at a single point and were then exploded a similar effect would be observed
  • The galaxies that are the furthest are moving the fastest - their distance is proportional to their speed
  • The galaxies that are closer are moving slower

Tracing the expansion of the universe back to the beginning of time leads to the idea the universe began with a “big bang”

Evidence from CMB Radiation

• According to the Big Bang theory, the early Universe was extremely hot and extremely dense
  • As a result of this, it must have emitted thermal radiation

• Astronomers discovered radiation in the microwave region of the electromagnetic spectrum which comes from all directions and has a temperature of 2.73 K
  • This is known as the Cosmic Microwave Background (CMB) radiation

• This radiation is thought to be the remains of the thermal energy from the Big Bang, spread across the whole Universe

• The radiation is in the microwave region because over the past 14 billion years or so, that radiation initially from the Big Bang has become redshifted as the Universe has expanded
• The CMB radiation is very uniform and has the exact profile expected to be emitted from a hot body that has cooled down over a very long time
  • This phenomenon is something that other theories (such as the Steady State Theory) cannot explain