In this article, we are going to review what cosmology is and why it is necessary to develop a theory of cosmology and understand the historical factors that led to its development. We are also going to study a couple of examples where astronomy and cosmology strongly cooperate.

What is cosmology?

In this section, we are going to focus on the definition of cosmology and some of its main objectives and characteristics.

To begin with the definition, cosmology is the branch of astrophysics dedicated to the study of the evolution of the universe with the aim of determining its past and future. It is mainly a theoretical discipline but, like all scientific branches, relies heavily on observational evidence.

Here are some of the main characteristics of cosmology:

• It relies on measurements made on a small and big scale, but it is mainly used to predict the behaviour of big-scale structures.

• Since cosmology is a branch of physics, it does not allow us to probe the earliest stages of the universe, as modern theories state that physics does not describe those early stages.

• Since cosmology deals with big-scale phenomena, it applies a deeply simplified treatment of the universe. For example, it treats galaxies as a mass entity rather than defining each star, planet, etc. that forms the galaxy.

• Since early cosmology involves the examination of the universe in critical conditions, it needs quantum physics to describe those first stages. This is done by a discipline called quantum cosmology.

• Cosmology describes the evolution of the universe. Along with the fact that cosmology ultimately describes the geometry of the universe, it also relies very strongly on Einstein's theory of general relativity, because the main force at the interplanetary scale is gravity.

Cosmological Redshift: The Doppler effect

The Doppler effect is the change in frequency of a wave perceived by an observer with respect to its actual frequency due to the movement of its source. A good example is the sound of an ambulance: as it gets closer to us, it gradually becomes higher-pitched but, as soon as it passes us, it gradually becomes lower-pitched again.

When considering light radiation, the Doppler effect says that when an object emitting light moves towards us, its spectrum will turn bluer and when it moves away from us, its spectrum will turn redder.

The Doppler effect is measured by a quantity named 'redshift', which is defined by the following formula:

\[z = \frac{\Delta f}{f} = -\frac{\Delta \lambda}{ \lambda}\]

Here z is the redshift, is the frequency, Δf is the change in perceived frequency, λ is the wavelength, and Δλ is the change in the perceived wavelength.

Hubble's law and the cosmological principle

In the 20th century, two relevant sets of measurements were undertaken. The first was the measurement of the density of stars in different regions of the sky. It turned out to be the same in every region except towards the Milky Way. Once it was understood that the Milky Way, composed of many stars that are very close to the earth, constitutes an exception, the conclusion was that the density of stars was the same in every direction. This laid the foundation for the formulation of the cosmological principle: at sufficiently large scales, the universe looks the same in every direction.

The other set of measurements was done by Edwin Hubble who, in 1929, published the observation that, out of 24 extra-galactic nebulae, most exhibited a redshift of the spectrum, meaning that they were moving away from the earth. This suggested that the majority of astronomical bodies moved away from us and, after considering many theories and explanations, including the input of general relativity, the widely accepted hypothesis came to be that the universe is expanding.

Hubble then proposed what is known as Hubble's law, which states that the velocity by which galaxies move away from the earth, due to the expansion of the universe, is proportional to the earth's distance to it. The formula is:

\[v = H_0 \cdot d\]

Here v is the velocity at which a body moves away from the earth, d is its distance from the earth, and H₀ is the Hubble constant whose actual value is close to 70 [km / s · Mpc].

If this level of expansion has been approximately the same throughout (which we think it has), it means that the universe once started as a small entity that has been expanding ever since, which leads to the well-known notion of the Big Bang.

Another interesting consequence of the expansion of the universe is the following: since light is the fastest travelling entity in the universe with a speed that is finite, measuring a body's light radiation is equivalent to receiving information from its past, especially at astronomical scales. A well-known example is the Sun: it takes about eight minutes for light to travel the distance between the sun and the earth, which means that, if the sun suddenly turned green, we would not notice for eight minutes.

This notion of light as information about the past is strongly related to redshift. Since, due to Hubble's law, far-away objects move away faster, they have a bigger redshift, and the time it takes for their radiation to reach us is longer. In other words, the bigger the redshift, the older (in the history of the universe) the information we are receiving from that object. Thus, we find that astrophysicists looking for high-redshift objects is equivalent to astrophysicists trying to probe the first stages of the universe.

Examples relating to cosmology and astronomy

We are now going to consider a couple of examples of astronomical objects, quasars and exoplanets, whose detection and measurements are intimately related to cosmology.

Quasars

Quasars are very bright objects that were discovered in the 20th century. Given their intense brightness, they were believed to be very massive stars but, as soon as we had better measurements, it was discovered that they were different astronomical objects. The name 'quasar' is a shortening of the term 'quasi-stellar', that is, almost a star. We are going to explore their main characteristics and the role they play in cosmology.

• Quasars are very bright objects that have a higher luminosity than entire galaxies. This was the first discovery that suggested that these were not stars.
• On the other hand, quasars are relatively small objects, given their luminosity. Usually, the size of a galaxy is about 100,000 times bigger than the size of a quasar.
• Most interestingly, the spectra of quasars feature an extreme redshift. As we already discussed, this means that they are objects from which we can extract information about the early universe.

Their special properties and other emission characteristics, such as being a huge source of radio emission, have intrigued the scientific community for decades. It is now believed that these objects are nuclei of galaxies that are, most likely, powered by supermassive black holes. Their study provides a lot of information on the conditions of the early universe that allowed for the creation of these strange objects.

Exoplanets

Although not constituting a basic pillar of cosmology, astrophysicists and astronomers are constantly looking for exoplanets as possible places where we might find life forms outside the earth. Apart from investigating the presence of water, the atmosphere, the proximity to a star, etc., a big difficulty lies in the fact that planets are not as bright as stars. This makes the detection of exoplanets extremely difficult.

However, some methods have been developed with which hundreds and thousands of exoplanets are being detected each year. These methods make use of techniques that are similar to those employed by cosmologists, and they again are experimentation labs for probing the early universe. Two of these detection methods are:

• The radial velocity method: this essentially relies on the fact that the spectrum of a star may be distorted by the slight pull of a nearby planet. Again, one needs to analyze the spectrum of emission, take into account the Doppler effect, etc.
• The transit method: this method has a certain bias but has still proven to be quite useful. By analyzing the emission of a star, one may observe some variations in intensity, which may be due to a planet orbiting it and getting between the star and the telescope taking the measurements. By accurately measuring and studying the patterns of variation of the star, we may, therefore, be able to detect the presence of an exoplanet.