The universe is expanding , that’s a fact. But how do we know? And what causes this expansion exactly? For more than a century , we have been finding evidence that our universe is expanding, that its scales seem to be growing or stretching. Vesto Slipher , an American astronomer, was the first to observe, back in 1912, that some galaxies showed a redshift in their light .
That is, they showed a more reddish light than we might expect. Let’s temper this. Of course, from a distant galaxy we do not have to know what light we are going to receive , it will depend on what stars, nebulae, supernovae, gas clouds and etc. the galaxy contains. However, there are some processes that always emit the same light , no matter where in the universe they take place.
When electrons orbiting certain atoms jump between energy levels (after absorbing some light, for example), they always emit the same light. This is what is known as the emission spectrum of each of these atoms . These spectra are unique to each atom and are identical no matter where the atom is or what processes it is undergoing. Well, if we now look at an atomic spectrum that is shaped like, say, the spectrum of hydrogen, but has all of its light shifted to lower energies, we know that that hydrogen atom was moving relative to us when it emitted its light. This is due to the Doppler effect , the same effect that changes the pitch of an ambulance or the beep of a car or train as they approach or move away from our position .
Slipher observed the same thing, that some distant galaxies emitted known atomic spectra, but that these spectra were shifted towards lower energies (towards the red, because red is the least energetic of the colors of the rainbow). It was later deduced that this meant that these galaxies were moving away from Earth . In the following decade, astronomers such as the Swedish Knut Lundmark and the American Edwin Hubble , using their own measurements and those of others such as Henrietta Leavitt, observed that the speed with which the galaxies were moving away increased the further away each galaxy was. This indicated that the universe must be expanding as a whole and that was what was pushing all the galaxies away.
To understand this, we usually talk about a balloon or a cake with raisins and how they swell. Imagine a deflated balloon , on which we draw some points, representing different galaxies. Before inflating it we will have a specific distance between each galaxy . If we now inflate the balloon to the maximum, we will see that this distance has increased . Not only that, but the distance to the more distant “galaxies” has increased more than that of the closer ones. In addition, we can also see that no galaxy occupies a privileged place: they all see how the other galaxies move away faster the further away they are . Something like that happens to the universe and that’s how we discovered it.
The reason for this expansion is more complicated. We know, precisely because the universe has been expanding for billions of years, that it all started in an incredibly dense, hot, tiny state . This state is known as a singularity and what came after, the Big Bang . The universe expanded and as it did so it cooled. That expansion was such that even today we continue to feel its effects. It has been proposed that there is a period in the first moments of the universe , in the first ten thousand sextillionths ( 10-32 seconds after the Big Bang) during which the universe underwent an incredible expansion , growing as much as if we were expanding a DNA molecule until it was several light years away. This period is known as inflation .
Despite being a perfect explanation for many issues that the Big Bang model could not explain and despite matching what we observe today, direct evidence of the field necessary to give rise to said inflation has not yet been found .
However, even if the Big Bang and the subsequent period of inflation set the expansion of the universe in motion, it should have slowed down since then, due to the gravity exerted on each other by the different galaxies and objects in the universe. This was true until about 4 billion years ago , when the expansion of the universe began to speed up again. This has been discovered more recently, since the 1990s, with even more precise observations of the redshift of light from supernovae and other phenomena incredibly far away in space and time.
This recent expansion acceleration is explained by resorting to the concept of dark energy . This energy would have, globally, an effect contrary to that of gravity , repelling everything and therefore accelerating the expansion of the universe. However, locally it would not be equal but opposite to gravity, since this energy seems to come from the fundamental state of spacetime itself. At present we do not know exactly what gives rise to this dark energy , whether it is a particle, a field or something else, although we do know for sure that it is one more element of our universe .
Sometimes it happens that, faced with the inability of science to explain some phenomenon in detail, pseudoscience attacks by completely questioning the validity of science or at least of concrete theories. What we know, we know very well . And what we don’t know is not a sign that science is incapable, but rather that it is a complex problem that requires more work. Any new theory that intends to replace the current one must offer a better explanation of everything we know so far and not simply a mystical and unfounded explanation, as is often the case from pseudosciences.
Reference:
I. Steer, 2012, Who discovered Universe expansion?. Nature 490, https://doi.org/10.1038/490176c
