In this blog post, we explore the secrets of the expanding universe, from Hubble’s discovery of redshift to the Big Bang theory, inflationary cosmology, and the nature of dark energy.

How is the universe expanding?

— From Hubble’s discovery to dark energy
In the early 20th century, American astronomer Edwin Hubble made one of the most important discoveries in the history of observation. While analyzing the spectra of various galaxies, he discovered that the spectral lines were shifting toward longer wavelengths, or toward the red end of the spectrum. This phenomenon is called “redshift,” which means that energy is shifting toward lower wavelengths. Hubble drew a very important conclusion from this: the farther away a galaxy is from us, the faster it is moving away from us. This is similar to the phenomenon of blowing up a balloon: the more you inflate it, the faster the distance between the dots on the surface of the balloon increases.
This discovery, together with Einstein’s general theory of relativity, played a decisive role in the birth of the Standard Big Bang Theory, which explains the origin and evolution of the universe. According to this theory, the reason why galaxies are moving away from each other is not simply because they are moving on their own, but because space itself is expanding. This concept of “spatial expansion” was a revolutionary shift that completely overturned the existing static model of the universe.
Furthermore, the Standard Big Bang Theory explains that the early universe began in a state of extreme heat and density, and that over time, through expansion and cooling, normal matter was formed, giving rise to celestial bodies such as stars, galaxies, and planets. During this process, energy was released evenly throughout the universe, which is what we observe today as cosmic microwave background radiation (CMB) at an absolute temperature of approximately 2.7 K (Kelvin). This is the most direct evidence of the early universe.

Mysteries that the standard theory cannot explain

However, the standard Big Bang theory has several significant weaknesses. One of the most representative problems is the “isotropy problem.” The cosmic microwave background radiation mentioned earlier appears to have almost exactly the same temperature and intensity regardless of the direction from which it is observed. However, this phenomenon is difficult to explain theoretically. The maximum speed at which information can travel from one point to another is the speed of light, and therefore, the range that light can reach is limited to the “horizon distance.” In other words, two points that are so far apart that light cannot reach them cannot interact with each other. Nevertheless, standard theory alone cannot explain why these points have the same temperature.
Another problem is the “flatness problem.” The current average density of the universe is measured to be very close to the critical density, which is the level at which the expansion of the universe can barely be restrained by gravity. This critical density is a key factor in determining the shape and fate of the universe. If the average density of the universe is less than this, the universe will become an “open universe” and continue to expand forever. Conversely, if it is greater, the universe will eventually stop expanding and contract into a “closed universe.” If the average density is exactly equal to the critical density, the universe will remain flat and expand slowly, becoming a “flat universe.” However, there was no explanation as to why the average density of the universe observed today is so precisely close to the critical density.

The emergence of inflation theory and a new interpretation

To resolve these issues, Alan Guth proposed the “inflation theory” in the early 1980s. He argued that immediately after the creation of the universe, its size expanded rapidly by a factor of 10^50 in an extremely short period of time, approximately 10^-35 seconds.
According to this theory, all regions of the early universe were initially within the horizon distance and could interact with each other, thus achieving “equilibrium” at the same temperature. The subsequent rapid expansion caused them to spread out over vast distances in an instant, and the isotropy of the universe we observe today is a remnant of this initial equilibrium state.
Furthermore, this rapid expansion made the curvature of the universe extremely flat, which explains why the average density of the universe observed today is almost identical to the critical density. This resolved the two major problems of the standard Big Bang theory.

However, a new mystery—dark matter and dark energy

Even though the inflation theory solved several problems, another mystery began to emerge.
When the average density of the universe is calculated accurately, it is found that the mass of observable matter, such as galaxies, stars, and planets, accounts for only about 5% of the total density. The remaining 95% is estimated to be occupied by unobservable matter, known as “dark matter” and “dark energy.”
Dark matter cannot be observed directly because it does not emit or reflect light, but its existence can be inferred indirectly through its gravitational effects. For example, the rotational speed of galaxies and the movement of celestial bodies within galaxy clusters cannot be explained by the mass we can observe, and mathematically, it must be assumed that much more mass exists.
Currently, neutrinos, hypothetical particles, and other unknown particles are being proposed as candidates, but nothing has been confirmed yet. Furthermore, a 1998 study of supernova observations showed the shocking result that the expansion of the universe is not slowing down, but rather accelerating. This phenomenon cannot be predicted by existing theories.
If the expansion is accelerating, there must be a new form of energy in the universe that pushes everything away from each other. This unknown energy has been named dark energy, and is believed to account for about 70% of the total energy in the universe. On the other hand, dark matter accounts for about 25%, and normal matter as we know it accounts for only 5%.

Will the universe of the future be isolated like islands?

If so, what will the night sky look like in the distant future if the structure and expansion of the universe continue? A research team in the United States recently attempted to answer this question through computer simulations. The results are surprising and somewhat sad. According to the simulation, when the universe reaches twice its current age, our galaxy and the neighboring Andromeda galaxy will collide and merge due to their gravitational pull. As a result, the number of stars visible in the night sky could nearly double.
However, at the same time, more and more galaxies will accelerate away from us due to dark energy, eventually moving away at speeds faster than the speed of light. This means that we will no longer be able to observe them. When that happens, future observers may exist like isolated islands in space, unable to observe any information other than their own galaxy.

Conclusion: The universe we see is not “everything.”

Today, we may think that we know a lot about the beginning, structure, and evolution of the universe. However, that knowledge is only a tiny fraction of the entire universe. We can only observe 5% of the universe, and the remaining 95% is still unknown. Nevertheless, humans continue to pursue the intellectual challenge of tracing the birth and future of the universe beyond the physical limits of our existence.
The science of the future will probably be the science of illuminating the “darkness.” Understanding the nature of dark matter and dark energy may go beyond mere astronomical curiosity and redefine the existence of humankind and the fate of the universe.
When we look up at the night sky, we are not just looking at the stars, but asking questions about the ever-expanding universe and beyond.