2.2. Modern models of the Universe

Unit 1: Our place in the Universe 1 2 3 4

2.2. Modern models of the Universe

Astronomy is the science which studies the components of the Universe from the information obtained from the electromagnetic radiation that comes from them: visible light, infrared radiation, ultraviolet radiation, etc.

Cosmology is the part of the Astronomy that studies the structure, origin and development of the wholeness of the universe (Cosmos).

Astrophysics is the part of the Astronomy that applies the laws of the Physics to study the composition, structure and evolution of the celestial bodies.

Cosmology describes the universe in an idealisation way applying mathematics models. Models are simplified versions of objects and phenomena that are made to make easier their understanding and study. Mathematics equations allow that cosmologists study in theoretical way the nature of the Universe before that experimental observation confirm or deny their suppositions.

a) Model of the infinite and static Universe

At the beginning of the 20^th century, the accepted cosmological model was the model of the Static Universe. According to this conception, the Universe is eternal and infinite and has always existed and will always exist. It has neither end nor beginning.

In 1917, Albert Einstein (Germany, 1879-1955) exposed his General relativity theory, which provided the most complete description of the Universe until this moment.

In his theory, Einstein:

- Establishes the equivalence between mass (m) and energy (E).

That is to say that mass can change into energy and vice versa.

(E = m.c², where c is the speed of light)

- Unifies the concepts of space and time. So that, gravity is not more

than the deformation provoked by the mass of celestial bodies

in the continuous space-time.

His equations predict a model of a Universe in expansion, but his own deep convictions about an endless and motionless cosmos were so strong that drove him to introduce a factor (the cosmological constant) to “correct” the “mistake” and force his model to stay static.

Video: Albert Einstein

Animation: Gravity in Einstein's universe

b) Model of the dynamic and finite Universe: the Big Bang

In 1929, Edwin Hubble (United States, 1889-1953) demonstrated experimentally that galaxies are separating from each other and as a result the Universe is in expansion.

Basing in this idea, Georges Henry Lamaître (Belgium, 1894-1966) proposed that if the universe is expanding, it had to exist an initial moment when all galaxies were together in a single point. That means that the Universe had a beginning.

The cosmological model of the Big Bang, widely accepted today, proposes a dynamic and finite Universe which was created in a huge explosion from an immaterial and extremely hot and dense point, 13,700 million years ago, and that since that moment does not stop to expand impulsed by an enigmatic and invisible energy (dark energy)

Video: George Lamaître

c) Model of the dynamic and infinite Universe: the steady state.

The term “Big Bang” was used for the first time in a despective way by Fred Hoyle (England, 1915-2001) to refer to Lamaître’s hypothesis.

Hoyle proposed in 1948, with Thomas Gold y Herman Bondi, other alternative model: the cosmological model of the steady state. This model admits the expansion of the universe, but it considers that this universe is infinite (eternal), without a definite beginning and in which matter is generated continually through unknown mechanisms. This universe would show a homogeneous and stable shape and characteristics in any place and time it would be measured (Perfect cosmological principle).

The new matter that is continuously formed, push the pre-existent matter and provokes the expansion of the Universe. So, the density of the Universe remains constant.