By ANNA BOLUDA
“The Cosmic Microwave Background changed of understanding of the Universe”
“The standard cosmological model has just a 1% error”
“The next decade is going to be the golden era of large scale structures”
“Cosmology is the key to determine the mass of neutrinos”
Licia Verde is an astronomer at the Institute of Cosmological Sciences – University of Barcelona (ICC-UB). She is specialized in cosmology, and her research topics include theoretical cosmology, cosmic microwave background, large scale structures, galaxy clusters, statistical applications and data analysis. She has been invited to open the European Week of Astronomy and Space Science – EWASS 2015 in Tenerife with a talk about the 50th anniversary of the publication of the discovery of the Cosmic Microwave Background.
We are celebrating the 50th anniversary of the publication of the discovery of the Cosmic Microwave Background (CMB) radiation. How would you describe the relevance of that discovery in the development of modern cosmology?
It has had profound implications for our understanding, both of cosmology and the Universe for several different reasons. Before the discovery of the Cosmic Microwave Background radiation, there were two competing theories about the origin of the Universe. One was the hot Big Bang model, which is the one that we all know and love today –even from sitcom series– and the other one was the steady state universe, where there is no beginning or singularity. The two theories had completely different predictions, and a cosmic microwave background was expected only in the Big Bang theory. Observing the CMB actually meant the confirmation of this model.
Besides that, it was soon realized that this radiation would have a lot of precious information about the nature of the Universe, not just its origin but also its composition and evolution. The Universe is actually very clumpy; on average it is homogeneous and isotropic, but it is very clumpy. Besides gravity, to explain this clumpiness there must have been some initial perturbation, because something completely homogeneous and isotropic would not, by itself, generate the galaxies, and planets and stars that we see today.
Therefore, for many years, there was a hunt for finding anisotropies in this CMB radiation. At the beginning it looked completely uniform, until these anisotropies were found. This was not just a qualitative statement meaning ‘we have seen the seeds of galaxies’, it was also a very powerful quantitative statement as the amplitude of those primordial perturbations must actually match with the clumpiness of the Universe that we see today. And that brings also the idea of dark matter: if there was only ordinary matter, given the Universe that we see today, we would have expected to find fluctuations in CMB much more easily. But they weren’t there. That is why people had to invoke a component of dark matter. This is the other pillar that started building our current understanding of the Universe.
After these perturbations were detected, the chase was on for measuring their properties, as they are like a fingerprint of who the Universe actually is. If we fast-forward to today, we’ll see that there has been a huge observational effort to map these perturbations. Now we have full studies in high resolution produced by the Planck satellite, for example, and we have been able to converge in what is called the standard cosmological model, which is a little bit like the standard model for particle physics; the same kind of intellectual development. The model was established, precision tests were made and now we are even in the position of making very high precision tests of this standard model to see if there is anything beyond. With the current precision, the parameters of the model have just a 1% error.
Where are we today in Cosmology?
Some see the glass half full and some see the glass half empty. We have this standard cosmological model, with a handful of parameters that describe very well the Universe with just a 1% margin of error. So some might say that there is no point in trying to determinate these parameters with yet one more significant theory. Or you can see the glass half full and say: it’s a model. I always like to remember what George E. P. Box said: ‘Some models are wrong, but some are useful’. The model is an approximation of reality, we know that it is incomplete. It is incomplete because we don’t understand what is that energy and we don’t understand the mechanism that set down the primordial perturbation; and it’s somehow incomplete because in the standard cosmological model the neutrinos don’t have mass. So we do know that at some point we will have to give up the model. For the optimistic ones the name of the game is ‘we could try to push the model, so we can learn something about physics beyond that model’. Which is a lot similar to what happened in particle physics.
So where are we today? We just ended what I call the golden era of the Cosmic Microwave Background observation, in the sense that we have observed with an exclusive precision the primary temperature perturbation, and we have started harvesting the power of the polarization of the CMB. But before we can improve dramatically on this, there is going to be a gap of a few years, at least a decade, I guess.
I think the next era is going to be the golden era of the large scale structures, which is mapping the distribution of galaxies, clusters and dark matter. A lot of the tools and rigor that the community had to develop for doing precision cosmology in CMB studies is percolating into the field of the large scale structures, and there is a huge observational effort also going into that direction. Of course this doesn’t exclude that there will be other experiments going for secondary CMB facts, but that is a different kind of physics. I think the next decade is going to be the golden era of large scale structures, before we are ready to harvest really the polarization information again and try possibly to answer the question of what power was behind the Big Bang, this initial acceleration expansion, which we believed caused the perturbation, the seeds of the galaxies.
Why would it take a decade to harvest new significant information on this polarization? What instrumentation would be needed?
There are several experiments positioning themselves to do that, and that is one approach: to do a ground-based experiment that would cover only a partial part of the sky and try to be lucky, and try to control the foreground so that it won’t affect the research. There is one of these experiments in the Canary Islands, Quijote, which is likely to produce results very soon. These ground-based observatories are one strategy.
The other strategy is to do what Planck did for the temperature and isotropy, to have another satellite for the polarization. That of course would be the final experiment to observe the full sky and measuring which such a small error that repeating it would not make sense. But that experiment doesn’t exist yet. It’s yet at a proposal stage. That is why I say that a golden era for polarization is probably a few years away. Although we could be surprised: some of these ground-based experiments could actually provide some ground breaking results in the next few months, science is like that.
Why is it so important to get an estimation of the mass of the neutrinos in order to fully understand the Universe? And what should we expect to get this estimation?
Particle physics type of experiments have tested properties of the neutrinos but are not very sensitive to the absolut mass scale of the neutrinos. So we know that there are at least three neutrinos species, that they oscillate, that they change from one to another, and we know that the forth neutrino needs to have a non-zero mass, but these oscillation experiments can only measure the mass differences, they cannot measure the absolute mass scale. So there are some limits on the absolute mass scale in particle physics. So really cosmology is the key, because cosmology can do so much better. So that is why it is becoming a very hot subject to study neutrinos by looking up at the cosmos.
So in principle, if we can keep the systematics under control, then there is enough signal in the sky from the future generation of the large scale structures surveys to detect a neutrino mass even at the minimum limit predicted by the oscillation. So this will tell us two things: first, since we will be doing some fitting of parameters within a model, that the model is really consistent, it’s not wrong, it’s really useful; and if it is wrong is wrong in a way that doesn’t impact the understanding of the universe. The other thing is that it will give us a number that you cannot get in any other way. And we know that this has implications for our understanding of fundamental physics, because in the standard model of particle physics, neutrinos mass. So this will be one of these interesting interfaces between the infinite small and the infinite big, where each one helps to understand the other.
When can we expect to see something? Within the next generation of large scale structure surveys, within the next decade for sure. There are many people working on that and I think it’s an exciting field of the search. So relatively soon, we might be surprised.
Video interview here.
