Evolution Of The Universe

This lesson covers: 

  1. Estimating the universe's age using the Hubble constant (H0)
  2. The concept of the observable universe and its estimated size
  3. Overview of significant events in the first moments post-Big Bang
  4. The discovery of dark matter based on galaxy cluster velocities and galaxy rotation speeds
  5. Estimates of dark matter abundance in the universe
  6. The unexpected accelerating expansion of the universe
  7. Current fractions of ordinary matter, dark matter and dark energy in the universe

Estimating the age of the Universe

The universe's age can be inferred by examining its expansion rate, represented by the Hubble constant (H0).

The formula given below provides a simplified method to estimate the age of the universe:


t = H0​1​


Where:

  • t = age of the universe (s)
  • H0 = Hubble constant (s-1)


This method assumes a constant expansion rate, which is a simplification. The actual expansion rate has varied over time, and precise knowledge of H0 is elusive, making the universe's age subject to estimation.

Worked Example - Calculating the age of the universe

Given that H0 = 65 km s-1 MPc-1, estimate the age of the universe.


Step 1: Formula

age = H0​1​


Step 2: Convert km s-1 MPc-1 to s-1

1 parsec = 3.08 x 1016 m

65 km s -1 MPc-1 = 2.11 x 10-18 s-1


Step 3: Substitution and correct evaluation

t = 2.11×10−18 s−11​=4.74×1017 s

The observable universe

The observable universe encompasses all matter from which light has reached us since the universe began. It forms a sphere with Earth at its centre, limited by the distance light has travelled in the universe's lifetime.

The radius of the observable universe is essentially the furthest distance light could have travelled since the universe's birth:


Rmax​=c T


Where:

R = radius (m)

c = speed of light (3 x 108 m s-1)

T = age of universe (s)

Evolution of the early universe

Much of what happened in the moments after the big bang is speculative however the table below summarises the key changes that are likely to have occurred in the early universe.

Time after the big bang (s)Description
10−43^{-43} to 10−36^{-36} sThe universe's earliest moments remain largely speculative, with theories suggesting quantum fluctuations in a rapidly inflating field laid down the groundwork for space-time's emergence.
10−36^{-36} to 10−34^{-34}sThis period saw the universe expand and cool dramatically, leading to the separation of unified forces into gravity, strong and weak nuclear, and electromagnetic forces. Dominated by quarks, leptons, and photons, this era also marked a slight preference for matter over antimatter.
10−34^{-34}sThe confinement of quarks resulted in the formation of protons, neutrons, and other hadrons. The annihilation of remaining antimatter with matter left a slight excess of the latter.
10 sWith temperatures around 1010^{10} K, the universe resembled the interior of a star, populated by nuclei and electrons.
300 sAt about 3,000 L, nuclei began to form atoms, making the universe transparent.

Evidence for dark matter

In the 1930s, astronomer Fritz Zwicky analysed the velocities (v) of galaxies at the edge of the Coma galaxy cluster. Using the formula for centripetal force:

Fc​=rmv2​

He calculated a mass much greater than expected from visible matter.

Similarly in the 1970s, Vera Rubin observed faster than expected orbital velocities in spiral galaxies. Both indicate extra invisible "dark" matter.

Current models suggest dark matter makes up ~25% of the universe, outnumbering ordinary matter by 5:1.

Theories on the composition of dark matter

No consensus exists on what constitutes dark matter. More observations are needed. Two theories are below:

  • MACHOs - Massive compact halo objects like black holes and brown dwarfs, made of ordinary matter in dense form. Unlikely to account for all dark matter.
  • WIMPs - Weakly interacting massive particles, exotic particles like axions and neutralinos that barely interact with normal matter. No confirmed detections so far.

The unexpected acceleration of universal expansion

The galaxies in the universe are all attracted by gravity. So theoretically, the expansion rate should be slowing. However, in the 1990s, the unexpected accelerating expansion was discovered.

To explain this, astronomers proposed "dark energy" which counteracts gravity and fills the cosmos. But its form remains a mystery like dark matter.

Current cosmic inventory

ComponentFraction of universe
Ordinary matter5%
Dark matter25%
Dark energy70%

 Much remains unknown about 95% of the universe! Ongoing studies aim to uncover the secrets of these "dark" components.