Cosmology

Cosmology is a branch of astronomy which investigates the nature of the universe, including its beginning and future.

Doppler Effect

The Doppler effect is the changes in the frequency of a wave produced by a moving source and/or moving observer.

This effect is extensively used in cosmology and astronomy in general to find the velocity of far-away objects. As light is a wave, the Doppler effect applies to it. If the emitter of the wave is moving away from the observer (or if the observer is moving away from the source), then the perceived wavelength is longer. In the opposite case- it is shorter.

Due to the Doppler effect, the objects moving closer to us look bluer, and objects moving away from us look redder.

A quantitative measure describing this shift is the redshift, defined by:

z = λ - λ₀ λ⁰,
where λ is the observed wavelength, λ₀ is the emitted wavelength. If z > 0, then the object looks redder, and if z < 0, then it looks bluer.

The velocity is calculated using this formula, considering that the wave is light:

z = v c, where v is the relative velocity of the emitter/observer, and c is the speed of light.

Expansion of space

Astronomers noticed that most objects have a redshift. This means that most objects are moving away from us. As it turns out, space expands, making the objects move away from us. The further an object is, the faster it moves.

This is described by Hubble's law:

v = H · d,
where v is the velocity of the object in km·s^-1, where H is the Hubble's parameter, which is around 70 km·s^-1·Mpc^-1, d is the distance to the object in Mpc.

The exact value of Hubble's constant is unknown, as different studies measuring it get different results.

The past and the future

The Hubble's law describes the expansion of space. This means that space is dynamic and can change extremely when modelling the future or the past. By modelling the universe on a large scale, scientists found that it is likely to continue expanding in the future, resulting in a heat death of the universe. In the far future, dark energy and dark matter is said to occupy a large and increasing portion of the universe, reducing the relative amount of visible matter, until it is negligible. Other findings show that the universe keeps getting smaller the further we go back in time, leading to the conclusion thet everything began with the Big Bang.

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