How many universes are there?

Our quest to understand the universe will be an unending adventure throughout human history. This is a delightful journey of thought, stretching from primitive myths to profound philosophies, and from there to the most complex equations of modern science. We have progressed from a time when we thought the Earth was the center of the universe and the Milky Way was the entire universe to a stage where we can map vast galaxy clusters. One of the most radical, exciting, and perhaps most challenging milestones on this joyous journey is the idea of ​​multiple universes. Far from being a science fiction fantasy, let's briefly examine the foundations of this idea, which arose from the imperatives of existing cosmological models and particle physics theories and pushes the boundaries of science.

According to modern cosmology, everything begins with the Big Bang. Approximately 13.8 billion years ago, our universe emerged from a tiny point of density—a singularity. This wasn't a gigantic explosion in space. The Big Bang was the first moment in which the fabric of space and time we exist in began to form. Numerous scientific observations support this idea. The first of these is the redshift in the spectrum of light from galaxies discovered by Edwin Hubble. This demonstrated that galaxies were moving away from each other, meaning the universe was expanding, and demonstrated that this expansion must have begun at a specific point. Another observation is the amount of light elements in the universe. The total amount of hydrogen (approximately 74%) and helium (approximately 24%) in the universe is quite consistent with the measured age of the universe. If the universe were 100 trillion years old, the amount of hydrogen in the universe would be much less than it is today.

Supporting Evidence

One of the strongest pieces of evidence supporting the Big Bang is the cosmic microwave background radiation. The cosmic microwave background radiation is the universe's oldest light, emerging approximately 380,000 years after the Big Bang as the universe cooled. As the universe expanded, this radiation cooled and became a background that now fills the entire sky, perceptible only at microwave frequencies. In 1965, Arno Penzias and Robert Wilson were calibrating radio telescopes at Bell Laboratories when they noticed a faint but persistent noise coming from all directions. Initially mistaken for bird droppings or equipment malfunction, this signal was later revealed to be an echo of the Big Bang. A small fraction of the faint hiss we see or hear on television or radio is actually the trace of cosmic microwave radiation from the Big Bang 13.8 billion years ago that still reaches us.

While observations strongly support the Big Bang theory, this story isn't perfect. The biggest problem is flatness. The geometry of the universe appears to be almost perfectly flat, and matter density is precisely aligned with the critical value. However, if there had been even the slightest deviation from this at the time of the Big Bang, billions of years of expansion would have either collapsed our universe or made it completely warped. Inflation theory comes into play to address this problem.

INFLATION THEORY

Inflation theory was proposed by Alan Guth in the 1980s. Guth proposed that, immediately after the Big Bang, in an incredibly small fraction of a second (between 10 to the −36 and 10 to the −32 seconds), the universe expanded exponentially at an incredible rate. This period was called cosmic inflation. This brief but striking period solved the problem of the universe's flatness because such rapid expansion flattens its geometry, much like inflating a balloon smooths its surface. It also addressed the horizon problem. For cosmic microwave radiation to be the same temperature in all directions, very distant regions must have touched each other in the past. Thanks to inflation, these regions were initially extremely close together and part of the same hot plasma. The rapid expansion threw them across cosmic distances, but their shared history explains the homogeneity observed today. To give a concrete example, this expansion was so rapid that a piece of space smaller than an atom grew to the size of a galaxy in the blink of an eye.

The most radical consequence of inflation theory is the idea of ​​multiple universes. Inflation predicts that spacetime can expand infinitely, and that new "baby universes" can form during this process due to quantum fluctuations. Like bubbles in a bubble bath, countless universes, each carrying their own unique physical laws and constants, constantly emerge from this cosmic foam. This is a powerful alternative scientists have proposed to the fine-tuning theory. Fine-tuning argues that our universe possesses physical laws and constants so precisely tuned for the existence of life that this cannot be a coincidence. The multiverse theory, in contrast, proposes that our universe is not specially designed; it is simply one lucky habitable universe from an infinite pool of potential universes. In other words, we have come to exist in a universe where life is possible by chance, because life would not be possible in others. This leads us, at the intersection of science and philosophy, to seek the answer to the question, "Why are we here?" not only in the laws of physics but also in the striking mathematics of probability and infinity.

We currently have no direct observations to prove the existence of multiple universes. Yet, the clues offered by the laws of physics suggest that our universe may not be the only one. If there truly are countless universes, ours is merely a single drop in a cosmic sea of ​​possibilities. This demonstrates how fragile yet unique our existence is. Whether we are meticulously crafted, carefully crafted creations or mere coincidences in a sea of ​​infinite possibilities, it makes no difference. The joy of using reason and questioning is unparalleled.