How is the universe (at least) 46 billion light years across, when it has only existed for 13.8 billion years?

Emily Rodriguez Emily Rodriguez Dec 10, 2019 · 2 mins read
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The size of the universe at first glance appears to be puzzling. How could a universe that has only existed for 13.8 billion years be at least 46 billion light years across? This question has been a source of confusion for many people over the years. However, there is a simple explanation that answers this question.

First, we should understand that the universe is constantly expanding. The concept of the expanding universe was first proposed by astronomer Edwin Hubble in the 1920s, and later confirmed by observations of cosmic microwave background radiation. This expansion implies that the distance between galaxies is constantly increasing, and the farther apart the galaxies are, the faster they are moving away from each other.

Given that the universe is expanding, the question remains: how can it be larger than its age? The answer lies in the fact that the universe’s expansion has not been constant throughout its existence. In the early moments after the Big Bang, the universe underwent a brief period of exponential expansion known as inflation, which greatly stretched the fabric of space. During this period, the universe expanded faster than the speed of light, which means the universe that we observe today was once small enough to fit inside a single point.

The rapid expansion during inflation is what caused the universe to become so large. Even though it has only existed for 13.8 billion years, it has been expanding for much longer. We cannot see beyond the observable horizon, which is the distance light has traveled since the beginning of the universe, around 13.8 billion years ago. However, there are parts of the universe that are much farther away from us, which we cannot observe because light has not had enough time to reach us since the beginning of the universe. These regions are known as the cosmic microwave background, which gives us a picture of the universe when it was only 380,000 years old.

The universe’s size is not an arbitrary number. It is the result of the universe’s expansion over time, which is described by a parameter called the Hubble constant. The Hubble constant is the rate at which the universe is expanding today, which can be used to estimate the size and age of the universe. Current estimates put the Hubble constant at around 70 kilometers per second per megaparsec, which means that for every 1 million parsecs (about 3.26 million light-years), galaxies are moving apart at a rate of 70 kilometers per second.

In summary, the universe is only 13.8 billion years old, but it is at least 46 billion light-years across because of its expansion. During the brief period of inflation shortly after the Big Bang, the universe expanded much faster than the speed of light, stretching the fabric of space and setting the stage for its present size. The universe’s size is not arbitrary; it is a consequence of its constant expansion over time. While the concept of the universe’s size can seem puzzling at first, a deeper understanding reveals that it is a fundamental result of the universe’s history.

Emily Rodriguez
Written by Emily Rodriguez
Making waves wherever I go