The expansion of the Universe maintains an accelerated rhythm, but there is no consensus to explain why.
Coffee or tea? Beatles or Rolling Stones? We are doomed not to agree, even on something as serious as cosmology. At the moment we are witnessing a strong conflict between the predictions of two great schools regarding the rate of expansion of the Universe. New measurements indicate that it is expanding much faster than expected. But what is happening?
Does the Cosmos take us for a joke when talking to us about itself? It is not enough for him just to scare us with the fact that he is expanding, but some data that he offers us suggests a different rate of expansion than others. Worse yet, the statistics support a marked disagreement and turn it into a serious problem. Taking a shortcut, I now clarify that these discrepant values are those provided by the Planck and Hubble observatories .
Let’s start, what does the Hubble Space Telescope measure?
We can remember what it feels like to light a fire. Very close almost burns, but moving away relieves us. That gives a vague idea of how to measure distances to stars. Basically, we would have to identify two stars with the same physical characteristics and compare the light that reaches us from them. And how does this relate to the expansion of the universe?
The expansion of the Universe and the panettone dough
Let’s imagine the raw dough of a panettone, which contains raisins separated from each other (at least in my favorite version). The heat of the oven makes the dough grow, forcing the raisins to separate from each other (and from each other). But, hey! their size does not change. It only remains to think that panettone is space-time and raisins are galaxies.
As our pannetoneverse (or universe represented by a pannetone) expands, the raisins do not move from their initial position in the dough. Those that have changed have been the relative distances between some raisins and others. Dividing the flare by the baking time we get the speed of our raisins, sorry, I mean galaxies (wink wink).
Now, how do we measure in cosmology how fast a star is moving away from us? Well, using a sophisticated version of the Doppler effect: just as a police siren sounds more serious as it moves away, the light of the stars becomes a little redder as it moves away. Almost a hundred years ago, Edwin Hubble surprised the world by showing that the farther away a star is, the faster it moves away. From this it follows, nothing more and nothing less, that the Universe is expanding.
The distance is linked to the speed of departure through the (of course) so -called Hubble constant . According to the very recent estimate of its value by the team led by Nobel Prize winner Adam Riess, a star that is 1 megaparsec away recedes at 73 km/s, and one twice as far away recedes at 146 km/ s. s (a light-year is to a parsec about what a foot is to a meter). That is, looking deeper into the Universe we see the rate of expansion increasing over time. This is why the expansion is also accelerated.
The rainbows produced by the stars
Hubble based his work on spectroscopy: a detailed reading of the rainbows produced by the stars. They are a series of stripes of different colors and widths typical of each star (its fingerprints), which by comparison indicate redshifts, that is, a decrease in the frequency of the light of the object when moving at a higher speed.
This pioneer and his successors had to know the distances to the stars used. But in general, measuring distances in astronomy is hard work. It is very difficult to obtain direct data, and it is usual to resort to physical models, generally built on the basis of variations in luminosity. Thus the cosmic ladder of distances is obtained. This is a concatenation of methods that returns distances to distant objects based on intermediate objects, supported in turn by those of nearby objects.
Planck demands to rethink the ‘certainties’ about the expansion of the Universe
Broadly speaking, this was how Riess’s team obtained the current value of the homonymous constant using the Hubble telescope in a very precise way, specifically based on three steps of nearby stars. But this value cannot be statistically reconciled with the one obtained by the Planck collaboration: 67 km/s/Mpc.
This experiment with its exquisite data speaks of very small alterations in the cosmic microwave radiation background imprinted billions of years ago. And through them it informs us of the proportions and nature of the different ingredients of the cosmic soup, the one with which our Universe has been feeding itself in different stages.
Actually, in this field, Einstein’s equations are used to see how the before influenced the now. That is, we reconstruct the journey of that radiation through billions of years. And in this puzzle, a small mistake in one place spreads to another, like the famous butterfly effect. For this reason, the estimation of the value of the expansion obtained from these data must be treated with caution.
At a theoretical level, there are many of us who rack our brains playing with the equations of that theory to which I have just referred, and which puts us back in touch with the idea that we have to know the composition of the Universe in order to accurately estimate the value of the Hubble constant today.
The effect of energy and dark matter
The main ingredients of that soup that describes our universe are dark matter and energy, rich substances that have made it grow like a girl or a boy. And any nutritionist will tell us that the scarcity or poor quality of food harms that development. It can thus be understood that the variations in the amounts of energy and dark matter in the Universe have determined the separation between its galaxies at different times. And this, let us remember, allows us to estimate how our creature has been changing in size as it has grown.
Of course, the solvency of the most distinguished research groups in cosmology is beyond doubt. They are duels of the titans. Siding with Planck or Hubble is like choosing between Lionel Messi and Cristiano Ronaldo. They both have a lot of highlights and also some shadows.
Red giant stars give an intermediate data
But if new figures are already appearing in football to take over, we also find other options with great potential in cosmology. In this case the alternative is conciliatory. A recent study using red giant stars falls between the two contenders suggesting 69.6 km/s/Mpc. And while the accuracy of the measurement drops a bit, there is room for improvement as the physics of these stars still requires development.
Perhaps there are systematic errors in the numerical treatment, perhaps there is unexplored physics, perhaps some of the theoretical approaches that are made are somewhat crude. There are also those who use other types of completely different astrophysical studies, for example gravitational lenses , to end up offering support to one side or another.
Perhaps the James Webb telescope will help close this debate, but as long as it is open, it will be a joy to follow it live, even better if we have popcorn and a good sofa.