The universe is one of the places that we, as human beings, cannot conquer and yet is going through a difficulty understanding it. Humans, or at least scientists, are craving to know about our mysterious universe, even though the reality is that our technology does not allow us for free exploration. Scientists have been discovering a lot about the universe, and one of the most interesting phenomena is the expansion of the universe.

Surprisingly, the first person who came up with a theory that the universe is expanding is not a scientist but instead an American astronomer named Edwin Hubble who stated in 1925 that a direct relationship exists between the speeds of distant galaxies and their distances from Earth. This observation—that the galaxies are moving away from the Earth at speeds proportional to their distance—is known as the Hubble’s Law, even though it has been voted to amend its name the Hubble-Lemaître law since both Hubble and the Belgian astronomer Georges Lemaître contributed to this discovery.

Then, the question is: how is the universe expanding, and why? The answer is we do not know. Physicists today can explain how the universe is expanding but do not fully understand why it is expanding. However, many attribute it to a mysterious force called dark energy, which is also not fully understood by physicists. To briefly explain, dark energy could be thought of as “anti-gravity” that provides negative pressure and causes cosmic objects to move apart from each other at an increasing rate.

As I mentioned previously, even though we do not know the exact reason why the universe is expanding, we can thoroughly explain how it is expanding. This discovery, the expansion of the universe, is based on the measurement of a phenomenon that is known as redshift. Simply put, redshift occurs when the wavelength of light is stretched, making it appear shifted towards the red end of the electromagnetic spectrum, where the wavelengths are the longest. This phenomenon is similar to how the pitch of an ambulance siren changes as it moves farther away from us. The boundary of the redshift is directly related to the distance of the object and its rate of recession, and astronomers can determine the speed the galaxies are moving away from us by measuring the object’s redshift. Also, Hubble’s observation that used a powerful telescope at the Mount Wilson Observatory has revealed that there is a linear relationship between a galaxy’s distance and its recession velocity.

Scientists could not understand the Hubble-Lemaître law right away. They needed more explanation. So, they turned to Albert Einstein’s theory of general relativity. According to the theory of general relativity, the fabric of space itself is capable of expansion or contraction under the influence of matter and energy, and the motion of objects within it is influenced by this expansion. This provided a framework for understanding the expansion of the universe.

To easily explain the expansion of the universe, let’s use a balloon as an example. Imagine a balloon with a dot drawn on its surface. As the balloon inflates, the dots will move away from each other, which can represent the expansion of the universe. We can think of the galaxies, or any objects in space, as the dots drawn on the balloon’s surface that are moving away from each other as the universe expands. To summarize, Hubble-Lemaître law implies that space itself is expanding which causes the galaxies to move away from each other. This discovery was a fundamental piece of evidence for the future discovery of the expansion of the universe.

There is even more evidence that proves the expansion of the universe. The cosmic microwave background (CMB) radiation is the faint radiation that is the part of the entire universe, which is the remainder from the early stages of the Big Bang. Some detailed measurements of the CMB by satellite-based observatories such as the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite have provided us with strong evidence that the universe expands. The measurements have shown tiny temperature fluctuations in the CMB across the sky which has the same temperature fluctuation as that of matter in the early universe.

The universe is much larger than we can inspect and expands every moment. The limitation of technology makes our discovery difficult, but more and more unexpected and interesting facts will be discovered as time passes by, considering that the discovery of the expansion of the universe is relatively modern. The universe is ginormous, and we constantly try hard to discover new facts daily.