Common sense, that great invention to quickly decide what to do or justify what has been done, which seems to appeal to a democratic behavior of reality. If everyone or a majority thinks that something is so, it must be so. Who is going to take away the reason of a (former) president of the Government who says that what he does is common sense? But mathematics is not democratic, neither is physics, and reality, by extension, is far from behaving as we think it should based on our very limited and biased experience. Intuition fails sometimes. We count here a property of the universe that goes against common sense. There are more, but this one is already pretty amazing, a good start to address these issues.
We start the attack on our common sense with very basic concepts and a journey. I am in front of the Sagrada Familia in Barcelona, at the foot of this imposing and beautiful architectural work of art. I get a cramp just thinking about trying to see her completely looking up from the front door. It occupies almost my entire field of vision, only with a wide angle lens could I take a picture of it; not even, with a fisheye. I want to see her better, I move away a little to have a better perspective. In order to take a photo with my cell phone, I move a little further away. The further away I am, the smaller the great basilica seems. Common sense: things seem smaller the farther away they are. Presented in a slightly more mathematical way, the angle formed or, rather, subtended by two segments that start from the ends of an object and meet at a point, my eye, is getting smaller the longer the segments are . It’s what’s called angular size, and the size of the cathedral or anything else decreases as I move away. Is this always true? I’m left without an article if the answer is yes, so, obviously, it’s a lie: things farther away don’t always seem smaller. In fact, from a distance (of astronomical dimensions), the further you go, the larger they seem, the angle they subtend increases with distance. Impossible! Well no, I’ll explain.
Let’s start by introducing the galaxy of Andromedaour sister higher, which is twice the size of ours, about 220,000 light years, more specifically. Today we see it (with telescopes, the eye does not allow us to see it completely) with an angular size in the sky about six times larger than the Sun.
Andromeda is too small for us, let’s move on to something bigger, the biggest thing known! If we want to see the entire universe today, at least the one accessible to us, the so-called observable universe, we have to look in all directions of the sky, it surrounds us. Now, because the universe has been expanding for all the time since the Big Bang, our observable universe, which is about 90 billion light-years across, was smaller in the past. Eleven hundred times smaller when it was 370,000 years old. If we keep going back, our entire observable universe today was 220,000 light-years in size not much later than its first year of age (note to picky eaters: I didn’t say that the entire universe was that size). At that time there was no Andromeda, no galaxies, not even the oxygen we breathe or the carbon in our DNA. There was only, counting only the matter that we like the most, protons, electrons, helium nuclei, a few lithium, and number of photons colliding with everything that moved. At that time, therefore, one year after the Big Bang, the size of our Andromeda was the same as that of the entire universe we know today. If our sister galaxy had existed, it would have filled the entire sky for an observer back then. What we see today with an angular size that we can cover with two fingers would have covered the entire celestial vault. An object of exactly the same physical size that existed one year after the Big Bang would look larger in the sky today even if it was farther away. That object would have grown to the size of the observable universe today, it would no longer be like today’s Andromeda! A bit confusing, but the fact that the universe is expanding, that the distance to Andromeda today is five million kilometers greater tomorrow because space-time changes, is also a bit against common sense.
“You’ve made this up, it has to be a lie, it’s not common sense.” Good thought for a skeptic, and every scientist should be. But in science, even to say that something is a lie, you must provide evidence (ask those who appear in the news these days). Only one that you contribute, and it will be true that it is a lie. Now, the results of several astronomical experiments support this amazing property of an expanding universe.
The key to these experiments is to find a good ruler that has been around for billions of years, something that hasn’t changed in size during the eons that galaxies have seen pass since they came into existence. It is not easy to find that universal rule, never better said; we are not worth bernabéus or Eiffel towers, or the Sella river, as Eva told us. That cosmic rule, tending to be eternal (that has been there since time exists, because time may not exist at some point), must also be very bright to be able to see it at great distances; and of a considerable size, that it is not extremely difficult to distinguish it in a wide range of distances.
It seems incredible, but such a rule is found in something that we cannot see directly because nothing, not even light, escapes from its gravitational field. We are talking about black holes, and not just any, but the largest existing ones. These supermassive black holes, as we call them, sometimes take in surrounding material and, before eating it, heat it up to temperatures of millions of degrees, making it ultra-bright. They are known as quasars. The core of the quasar, the innermost part of these cosmic monsters, is quite compact in size, about 36 light years. That’s about the distance to the star Arthur, but within that area for a supermassive black hole there is a mass equivalent to hundreds of millions of suns, instead of a few tens, as in the case of our solar neighborhood. The precise size of the quasar nucleus is fairly predictable, so they seem to be a good cosmic rule of thumb. Observing these objects, very bright and detectable to the ends of the universe, we have found that the further away they are, the smaller their angular size, as our canons dictate, but only up to about 40,000 million light years. Compact quasar nuclei beyond that distance suddenly begin to appear larger to us, contrary to common sense, but in keeping with the physical laws of the universe.
We conclude that sometimes you have to separate yourself from things to see their greatness, and that there is no such thing as common sense, any event in the universe, or the Earth itself or our country, may seem normal or, instead, impossible. . But just because in large part our experience and understanding of reality is (very?) limited, we need to open our eyes to what seems irrational to us, assimilate it, interpret it and use it appropriately.
Pablo G. Perez Gonzalez He is a researcher at the Center for Astrobiology, dependent on the Higher Council for Scientific Research and the National Institute of Aerospace Technology (CAB/CSIC-INTA).
cosmic void it is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It is intended to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than one atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillion atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up Pablo G. Perez Gonzalezresearcher at the Center for Astrobiology; Patricia Sanchez Blazquez, full professor at the Complutense University of Madrid (UCM); and Eve Villaverresearcher at the Center for Astrobiology.
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