After all, how big is the Universe?

The Universe as we know it began sometime around 13.8 billion years ago in the event known as the Big Bang. In its first moments, the Universe was full of particles of matter, antimatter, radiation and existed in an extremely hot and dense state, but which expanded and cooled.

From this initial stage, the Cosmos continues to expand, cool and evolve in accordance with the laws of physics. As this happened, a series of important events led to the structures we observe and inhabit today: an ordinary planet located in the outer arm of an ordinary galaxy on the outskirts of a local supercluster.

The Laniakea supercluster that contains the Milky Way (red dot)Fonte:  Tully, R. B. et al (2014).

As we look at it today, the Universe is a vast and grandiose place, filled with stars, black holes, quasars, galaxies, clusters of galaxies, and huge regions of voids in between. It is quite remarkable how humanity has managed to assemble this entire cosmic history, starting from a “nothing” until reaching these structures, going from the smallest of times to the present day.

We know, for example, that over the ages, gravitational interaction will continue to pull large concentrations of matter onto each other, while the expansion of the Universe will continue to work to pull them apart. Not much more than 20 years ago, we discovered that the universe’s ultimate fate is likely to be a scenario like this: the rate at which it expands (which occurs due to a mysterious component known as dark energy) will eventually completely defeat the exerted force. by gravitational attraction. This means that our Universe may never collapse back on itself at a single point.

Observable Universe TimelineObservable Universe TimelineSource: NASA

As the Universe continues to expand and grow more and more, the question inevitably arises: After all, how big is the Universe?

Well, it’s not possible to answer this question with an exact value, endowed with metric precision. First, because, as you can see, its size is continually increasing. Second, because the best answer we have is an estimate based “only” on what we can observe with our instruments and data analysis.

Talking about this estimate in a relatively simple answer would be: the volume that contains the portion of the Universe that we can see, called the observable universe, has expanded to a radius of about 46.5 billion light years. This is equivalent to saying that the diameter of the universe we can study is approximately 93 billion light years.

However, as we are curious and as such an answer does not usually satisfy us very much, the next one soon comes: but what is beyond? As for the “unobservable” universe, the one that we cannot see or obtain information about?

If answering the first question was no longer easy, trying to answer this one, then…

When we look at ever greater distances in the Cosmos, we see structures not only as they were at that distance, but also as they were in the past: we do a kind of time travel, due to the finite speed of light. The more distant Universe is much less crowded and more uniform, as in the past it has not had enough time to form larger and more complex structures due to the effects of gravity. The distant Universe was also warmer. Its expansion causes all the light that travels through it to be extended in wavelengths. As this happens, photons lose energy and become cooler.

Schematic representation of the evolution of the Universe, from the initial moments until todaySchematic representation of the evolution of the Universe, from the initial moments until todayFonte:  SDSS

Simply put, how we get to the size of the Universe today is by understanding three things: how fast the Universe is expanding today, how hot (or cold) it is currently, and what its composition is. With this information, it is possible to extrapolate back to the first moments of the Big Bang and arrive at a reliable estimate for its age and size.

The map of the Universe with the SDSS astronomical survey.  Each dot is a galaxy and the color bar represents the local densityThe map of the Universe with the SDSS astronomical survey. Each dot is a galaxy and the color bar represents the local densityFonte:  SDSS

About the part of the Universe that is beyond the limits of our observations, we can only make inferences based on the laws of physics. Analyzes of observations from large astronomical surveys carried out in recent decades indicate that the unobservable universe must have at least 250 times the radius of the observable part. This means that the Universe could be at least 23 trillion light-years across, which would correspond to a volume of space more than 15 million times larger than what we can observe it today. All this assuming that ours is the only universe that exists.

Is this a satisfactory answer for you?

Leave a Comment