2.3. Evidences of the Big Bang

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

2.3. Evidences of the Big Bang

a) The red-shift

During a lot of time, astronomers had believed that our galaxy, the Milky Way, was the whole universe. In 1924, Edwin Hubble discovered that what were supposed distant nebulae, were actually galaxies similar to ours.

In 1929, he made other genial finding: he measured the distances from the Earth to several galaxies and he proved that they are separating from each other, that is to say that the universe is expanding.

The Hubble’s law establishes that the speed at which a galaxy is moving away is directly proportional to its distance.

v = H_o . D

v is the speed of the galaxy (in km/s)

D is the distance between the Galaxy and the Earth (in megaparsec: Mpc)

H_o is the Hubble’s constant (in 2006 it was estimatedin 70 Km/s/Mpc).

The light that reaches us from stars contain a mixture of different wavelength radiations that can be separated using a spectroscope and result in a spectrum.

Each spectrum is formed by the seven colours of the rainbow (the red is the longest wavelength and the violet the shortest one) and over them a series of black absorption lines which correspond to determinate chemical elements present in the interstellar gas of the galaxy, that absorb part of the radiation.

Animation: Absorption and emission spectra

These spectrums are similar to bar codes. Each chemical element has its own spectrum: the positions (wavelength) in which absorption lines appear is a characteristic constant of each chemical element.

Edwin Hubble measured the position of spectral bars of some elements present in several galaxies located at different distances from the Earth.

Then he compared these spectrums with those obtained from these elements in the laboratory.

He discovered that the lines displaced towards longer wavelength (towards the red). The further the galaxy was, the bigger the displacement.

This phenomena, known as red-shift of the spectral lines is due to the Doppler effect and means that the galaxies are separating from each other.

The german physicist Johann Christian Doppler discovered that when a wave is emitted by an object in movement, the wavelength perceived by the observer is different than that emitted by the object.

The perceived wave length is bigger if the object is shifting away and smaller if the object is approaching.

This phenomena is easy to observe in the sound waves emitted by a whistle of a train in motion: it is more high-pitched when the train is approaching (piiiiiii….) (a) and more low-pitched (paaaaa…) when the train is shifting away (b)

The explanation is that the approaching train compresses the sound waves ahead it. This provokes the shortening of their wavelength (high-pitched sound). In contrast when the train is shifting away produces a contrary effect and the wavelength lengthens (low-pitched sound).

Animation: The Doppler effect and the red shift of galaxies

Animation: Doppler effect sound

b) Cosmic background radiation

The definitive confirmation of the Big Bang theory was the discovery of the cosmic background radiation or microwaves background.

Working with a new type of commercial antenna Arno Penzias and Robert Wilson discovered in 1964 a very weak radiation that seemed to come from every place of the Universe. The interpretation of this discovery was that this radiation was the echo of the Big Bang.

The Universe expansion provoked the photons’ cooling down (light radiation)

until -270ºC. This had as a

consequence the decrement of the intensity of the radiation and, as a result,

the increment of its wavelength until

the frequency of the microwaves.

These observations coincided with the theoretical calculations of other physicists (Gamow, Dicke and Peebes). If the Big Bang theory were correct, there ought to exist a residual radiation of the moment of the huge explosion. This remainder should be very weak, should be located in the microwaves region and should have a temperature around -300ºC.

The cosmic background radiation existence was confirmed in 1992 by the COBE (Cosmic Background Explorer) an artificial satellite designed to measure it. The analysis of the obtained data coincided with the predictions. The temperature of the microwaves radiation is 2.73 K (-271.27 ºC) and it extends in equal way in every direction of the space. Lastly, these deductions were confirmed by the space probe VMAP (Wilkinson Microwave Anisotropy Probe).

In the same way that a thunder rumbles before extinguishes at the end, the echo of the explosion which gives birth to our universe had arrived at today.