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When we look into space, we can see many objects. In order to see these objects, we must receive the light they emit. With the help of special relativity and modern electrodynamics, we have a deep understanding of light. In modern cosmology, we also use general relativity as one of the primary tools for understanding and modeling the universe. Cosmology, of course, is more than just relativity; it uses all aspects of physics, including Newtonian mechanics, thermodynamics, quantum mechanics, and quantum field theory. So, to solve cosmological problems, we use more or less the entire toolbox of physics to try to understand the universe in which we live.

Unsplash Theory Development

Cosmology is one of the oldest subjects in physics and became a rigorous science with the introduction of the Copernican principle in the 17th century. The Copernican principle, which suggests that all heavenly bodies are governed by the same physics, seems obvious today, but it was not clear whether the laws of physics would be different on Mars.

With Newtonian mechanics, we have laws that allow us to understand things like planetary orbits. But we didn't know much about things outside our solar system. The field of cosmology did not really begin to develop until 1915 with Einstein and his general theory of relativity. We finally have a theory that corrects the errors of Newton's mechanics and which we can trust to describe the universe beyond our solar system. General relativity has provided us with many new things, such as black holes and gravitational lenses.

After 1915, in the years that followed, many interesting discoveries and observations were made, and through Friedman's equations we obtained models of the entire universe. Soon after, Irving Hubble discovered galaxies beyond our own. All these developments of the 20th century form the heart of modern cosmology, but we are still discovering new things and learning more about our universe.

Source Pixabay observations

Our understanding of cosmology comes first from observations, so let's talk about some key observations. One of the most important discoveries of the last century was undoubtedly the discovery of the cosmic microwave background, which is essentially light from when our universe was very young. By observing the CMB, we can learn a lot about the universe, such as the matter and energy content of the universe.

We are also able to observe very distant supernovae, and by observing the redshift of such objects, we have a better understanding of the expansion of the universe. The fact that the universe is expanding was first understood in the 1920s, but only recently did we realize that the universe is expanding at a faster rate, which led to the Nobel Prize in 2011.

All of the things mentioned so far have helped scientists improve our understanding of the universe, leading to the λCDM model we have discussed, which is mathematically based on Friedman's equations.

However, some big mysteries remain to be solved. One problem was that in the 1930s, galaxies were found to be spinning too fast. Quick calculations show that gravity in these galaxies is not enough to keep galaxies together, which leads to the proposal of dark matter. This is a new type of matter that interacts only with gravity and is therefore almost impossible to detect. Recently, evidence for dark matter has been mounting, and it is crucial to our understanding of the universe. To make sure this hypothesis is correct, physicists have tried to figure out what new particles might be behind this dark matter, but so far without success.

Another mystery is the gravitational constant or dark energy. This energy acts as some kind of antigravity, causing the universe to expand at an accelerated rate. What this dark energy is remains a big mystery.

Pixabay interesting research areas

An interesting area of study in cosmology is gravitational waves. Gravitational waves are ripples in the fabric of spacetime, and we only managed to observe the first gravitational waves in 2016. The theory behind these waves comes from general relativity, which has been theoretically predicted for decades, but the problem is that these ripples in spacetime are so small that we need to use the extremely precise tools of modern physics to detect them. As our technology advances, we can detect even smaller waves, opening new avenues for discovering new physics and testing current theories. There is expected to be a gravitational wave background, just like the CMB, which could tell us something new about the early universe because these signals could have been generated earlier than the CMB signals.

Another interesting aspect of cosmology is the formation of structure, that is, how galaxies, stars, and solar systems form. This is an extremely complex topic, but one can go a long way by considering Newtonian mechanics. However, the universe is extremely large and complex, so we don't have particularly accurate models to understand how large-scale structures form. One way to learn more is to look out in space and make a map of the universe. Then we can also look at places where objects such as stars are still forming, so we can get a good idea of star formation.

Cosmology has come a long way in the last century, and we have come to realize that we live in a very large universe. Interestingly, we still don't know if the universe is infinite, and that's because we can't see any edges of the universe. Similarly, we still have a lot to learn about the early universe, because we will eventually reach a point beyond known physics. So while there is still a lot we don't know, progress has been made, and with the help of new experiments and new physics, we can expect to learn and understand more about cosmology.

This article comes from Weixin Official Accounts: Vientiane Experience (ID: UR4351), by Eugene Wang

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