Science

Universe, Why so Dark?

What a question, right? In addition to personifying the Universe, this kind of interrogation exposes a premise that plays a lot with our human intuition. We can’t be blamed for that, given how easy it is to assume that our sensory perceptions are right. But I do admit that cosmic complexity exceeds any common sense.

Having said that, let’s think a bit about why the apparent darkness of the Universe (at least the one seen from Earth’s night sky) is a senseless fact.

Photograph of a star cluster.

Although our position within the Milky Way galaxy only obscures a minimum portion of the entire Cosmos, by living in an infinite Universe we should be able to see a bright spot of light in all directions.

After all, our research suggests that we have an exorbitant amount of galaxies, stars and other bright bodies. Which makes it reasonable to assume that such objects would be able to illuminate the entire space vacuum.

This was also the smart deduction of the nineteenth-century German physicist Heinrich Olbers, who stipulated that:

If the Universe is truly infinite with an infinite number of shining stars, regardless of the direction where we looked at, our eyes should eventually get fixed on the surface of a star.

Assumption best known today as the Olbers paradox.

Gif that exemplifies Olbers paradox.

Therefore, following this deduction we must ask a very important question: is the known universe an infinite space?

Under our current understanding of cosmology, I must say that the answer is no. A denial that has its bases at the very beginning of things. That is, the Big Bang.

It has been thanks to the Big Bang model that we accept that the observable universe has not been here forever (ergo, this is not infinite). Consequently, this indicates that we can only observe spatial objects that are located at a finite distance, receiving an equally finite amount of light, heat and energy from them.

In turn, the light coming from the galaxies and stars we know are limited by the speed of light and the physics of the expanding Universe. So, regardless of the probable presence of other luminous bodies, we lack the time needed for us to be affected by the energy that comes from these objects.

Graphical representation of the extent of the observable universe since the Big Bang (not accounting for the cosmic microwave background radiation).

However, this condition is not the only reason behind this phenomenon. So now we shall take into account the human perspective. Specifically: the sensory response of our eyes.

Our vision of things is a process that is confined to the sensory barriers of our eyes. These are restricted to the spectrum of visible light. And this visual spectrum is just a small portion of the bigger electromagnetic spectrum (the spectrum where all forms of visual waves can be found).

Among these waves are the radial and microwave ones, which we have only been able to “observe” through specialised equipment. But if our eyes were equipped with that ability to see these waves, our perception of the Universe would be very different from the one we have today. We would then be able to see a sky of uniform luminosity, devoid of dark spots.

Graphical representation of the visible light spectrum with the rest of the electromagnetic spectrum.

Thus, in response to the initial question, we can conclude that the universal darkness is the product of two factors:

  1. the finite amount of time the Universe has existed and
  2. our visual restriction to the spectrum of visible light.

But we shouldn’t feel saddened about this conditioning, because it is our limits that push us to continue exploring the Cosmos. Discovering at every step the unknown and ignored and everything else that to us is an act of raw amazement.

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