It all started with Albert Einstein in 1915, when he proposed the General Theory of Relativity. He, himself didn’t believe it. The equations he made suggested that the Universe was expanding. To Einstein, that was absurd, he was a strong believer in a fixed and unchanging universe. To counteract the supposed expansion, he added another term to the equations of General Relativity — the cosmological constant. Einstein thought that the cosmological constant will allow him to avoid the prediction that the Universe was expanding. Einstein was stubborn, and refused to believe what his theory implied. Just a few years later he was be forced to give up.
In the late 1920’s, Edwin Hubble was studying other galaxies in the Universe when he noticed something interesting about them. The emission spectra of the galaxies was being red-shifted.
When electrons in atoms are excited and are in a higher energy state, they release the energy to return to their lowest energy level. Elements on the periodic table emit a distinctive pattern of electromagnetic wavelengths when returning to a low energy state. This pattern is called an emission spectrum and each element has an unique emission spectrum.
When an object emitting electromagnetic radiaton is moving away from an observer, the observer will see the wavelength of the radiation increase. In other words, the wavelength of electromagnetic radiation gets stretched if the object is moving farther away. Longer wavelengths correspond to the red end of the visible light spectrum, therefore this phenomenon is called “red-shift“.
Galaxies are obviously composed of elements so they also have an emission spectrum. Since the emission spectrum is just electromagnetic waves, it is also subject to the redshift phenomenon. Hubble saw the emission spectra of the galaxies in our Universe being redshifted and announced that the Universe is expanding. This was a revoluntionary discovery. Einstein had to accept that his equations didn’t need to be changed and were right all along. His idea of a static, elegant universe was wrong. But this isn’t the end of the story.
In 1998, 2 teams of scientists discovered something more amazing. The teams of astronomers from Lawrence Berkeley National Laboratory and the High-Z Supernova Search were competiting to measure the expansion rate of the Universe with respect to time. Due to the effects of gravity, both teams expected the expansion to be slowing down. By comparing the brightness of distant supernovae to closer supernovae, they found that the distant supernovae were 25% more faint then they should be. This means that the expansion of the Universe is accelerating!
Scientists expected the expansion of the Universe to slow down because they predicted that force of gravity will start to pull matter back together. But the expansion of the Universe is accelerating, meaning that there is some force that is counteracting the effects of gravity. Scientists called this force “dark energy”. Dark energy is the repulsive force that is causing the acceleation of the expansion. In fact, this force is becoming stronger as the universe expands and gets larger. Other than that, there’s not much scientists know about this force, hence the name “dark energy“.
It doesn’t stop there. Dark matter also exists in our Universe but we don’t know much about it either. Scientists calculated that the gravity of visible matter is not strong enough to form structures like galaxies. There must be some other form of matter that is producing the gravitional effects required to form galaxies and other complex structures in our Universe.
Scientists are pretty sure that dark matter and dark energy exist. They can only measure the effects of dark energy and matter and don’t really know anything else about them such as their properties. Dark matter doesn’t interact with “normal” matter or light, so scientists haven’t directly observed it. Dark matter makes up an astonishing 70 % of the Universe, dark energy makes up 25% while “normal” matter only makes up 5% of the Universe. In fact, I really shouldn’t be referring to the matter we’re made up of as “normal” matter because it’s such a small percentage of the actual Universe! Most of the Universe is still a mystery to us. It’s astonishing that all the matter humanity has every saw only makes up a small percentage of the incredibly vast universe. Despite our advances in science, the Universe is intimadating and eerie and most of it’s dark and unknown. Will we ever be able to completely uncover the mysterious of our Universe?
Doesn’t cosmology give you the chills!?