Why the skies are changing

Today it’s sunny, tomorrow it will be cloudy, and the day after that the weather forecast says it will rain. Every day I look in the mirror and I see myself a little older. My daughter is growing up quickly. Everything changes. Is there anything that doesn’t change in the universe? Something to hold onto, to think that everything will stay the same and nothing will happen to me in the future? Is there something perfect and immutable, that will last forever and ever?

Faced with these questions, Aristotle told us that the universe was a set of celestial spheres, with the Earth in the middle, and that the last sphere, beyond the one that housed the Moon, was where the stars would be found. That sphere was perfect, incorrupt, immutable. It doesn’t seem silly: if we look at the sky night after night, the stars are always in the same position relative to one another. Yes, they move throughout the night, they rise and set like the sun itself, but the next night, and if not, then next year around the same time, everything will be the same. There is a perfect rhythm that repeats itself imperturbably year after year.

Galileo Galilei, looking through his telescope, discovered that the Moon was not a perfect sphere, but was full of craters, and he observed that there were small worlds revolving around Jupiter, which did not have the Earth as their center. The Catholic Church did not like that very much, as it went against doctrine. In science we would say that it went against the current paradigm.

Today we may be tempted to think that this was a long time ago, centuries ago, in a very primitive world. But in our 21st century we resist acknowledging that the climate of the entire planet has changed and that we are largely to blame. Nor can we fathom that a catastrophic event from outer space could ever occur. Our vital and cognitive inertia tells us that it is impossible for a meteorite to impact the Earth and threaten our existence.

Returning to astrophysics (and history), the sky is not at all immutable, although it is true that its variability usually has space-time scales that are not comparable to those of our lives. The Greeks, with their logical thinking, did not conceive their geocentric system and that last sphere of immutable stars without a scientific basis. An erroneous basis, yes, but one based on observations and theories to explain them. Science does not have to be right, only not stuck in what is not demonstrably false. Indeed, the Greeks argued that if the Earth really moved, then the stars, fixed and immutable on their sphere, would surely change their relative position. Today we know that stars change their apparent position slightly when we observe them in one part of the orbit around the Sun compared to when we observe them from the other end of the orbit. It’s like looking at something with two eyes: if you cover one eye, look at a nearby object and cover it with your thumb, then open the second eye, you will see that the thumb no longer covers the object — the relative positions of the thumb and what we covered have changed. For the sky, the change in relative position is simply so small, a result of the stars being so far away, that the Greeks did not conceive that the distances could be so great, and they did not conceive it because they could not measure what is known as the parallax, a feat that was first achieved just under 200 years ago.

The mutability of the sky goes further. Stars have their lives: they are born, grow, and die. And in that process their brightness can vary. Normally, the change is small, in the case of the sun it only varies on the order of 0.1% throughout its 11-year cycle (so no blaming it for climate change), and some studies (much more complicated for me, as they are no longer just about astrophysics) say that this variability was an order of magnitude greater billions of years ago (which can be verified by studying other stars similar to the Sun, a calculation that falls to astrophysics).

The Sun itself, at some point in the next 4.5 billion years or so, as well other stars that are more or less massive than the Sun, or that keep poor company, can vary much more and on more earthly time scales. There are stars that explode, like supernovae, and others that swallow their companions, like some types of novae. There are yet others that vary their brightness slightly with a very constant frequency that has allowed us to calculate the size of the universe.

Here are some last examples of cosmic variability, quite spectacular in my opinion. Black holes, especially the supermassive ones that inhabit the nuclei of all galaxies like ours, sometimes swallow the matter around them, sometimes even entire stars, and in the process a large amount of energy is released, and its brightness (the brightness of what is around the black hole, which is what it emits) varies. That variability can have time scales of years, days, hours, even seconds.

Another variability of the heavens is so extremely subtle and ephemeral, so amazing, that we have only managed to verify it a few years ago. Sometimes the stars literally align, and that doesn’t affect our lives, but it does affect what we see. Alignment is a chance event, and it can happen be between things as different and distant as a star at the beginning and ends of the universe, or even a planet (closer) and a tiny black hole in our own galaxy or one very close by. Although it is impossible to see the two stars under normal conditions, if the alignment occurs, the apparent brightness of the distant object is increased significantly (even thousands of times). Of course, it will only happen for a short period of time that will never again be repeated. This is gravitational lensing, but it is a fortuitous lensing that only happens once in a lifetime (in the lifetime of the universe!).

The concept of variability is closely linked to time. Today it is not easy for us to conceive that something like time changes, is relative, and can be stopped, or not exist at all. In short, we already know that the heavens are not immutable, despite our resistance to the concept of change, of that cognitive inertia that gives us a sense of security; Thinking that things never seen before can happen scares us too much. Which surely makes us pessimists by nature, because who is to say that what we have never seen, these cosmic changes, might not be positive? At least discovering and understanding things never seen before is a beautiful thing.

Pablo G. Pérez González is a researcher at Spain’s Astrobiology Center.

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