Pilar M.ª Ruiz Lapuente (Barcelona, 1964), researcher at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and professor at the Department of Astronomy and Meteorology of the UB, is one of the co-authors of the article on the accelerating expansion of the Universe published in 1998 by the research group led by Saul Perlmutter, head of the Supernova Cosmology Project (SCP) and awarded, together with Brian P. Schmidt and Adam G. Riess, members of the High-Z Supernova Search Team, 2011 Nobel Prize in Physics for this discovery.
After graduating in Physics at the UB, Ruiz Lapuente carried out her doctoral studies at the UB, the Max Planck Institute for Astrophysics and the European Southern Observatory, in Germany. She was a postdoctoral research fellow at the Harvard-Smithsonian Centre for Astrophysics (United States) and she has authored over 130 articles published in international journals, such as Science and Nature. She has recently published the book El enigma de la realidad. Las entidades de la física de Aristóteles a Einstein (Gedisa, 2011).
What did the announcement of the Nobel Prize in Physics in 2011 mean to the SCP team?
It was around 5 November 2011 when we got the news. We knew that in the future a Nobel Prize could be awarded for this discovery. In fact, at the Lawrence Berkeley National Laboratory, in California, where Saul Perlmutter works, there’s a person responsible for updating data on the potential candidates for the Nobel Prize, as this laboratory had several ones, including Perlmutter. We didn’t expect to be awarded so soon, though.
This means that now your work has already been accepted. But, at that time, how was it received?
The result is now confirmed by other studies, but at first they didn’t believe it. They were looking for other excuses, stating that the Universe was not homogeneous, that there was a kind of dust that left no trace but that made the most distant objects become weaker. Different ideas were spread, but little by little they’ve seen that they didn’t match the data and they’ve been gradually rejected.
What was your contribution in the work that was published in 1998?
I worked in La Palma with the telescopes of Roque de los Muchachos Observatory (Canary Islands). I monitored the objects that were discovered from which I obtained their spectra and light curves. On the other hand, I also helped to perform the team’s joint analyses.
What kind of telescopes did you use at the Roque de los Muchachos Observatory?
The work was done before having the Great Canary Telescope, so we used William Herschel and Isaac Newton telescopes. In fact, many of the supernovae studied for this work were monitored from here. So these telescopes made a major contribution.
How is the observation of supernovae carried out, bearing in mind that they are objects that appear suddenly?
We carry out observations every fifteen days. First, we take reference images and then we go back to them after around fifteen days, when we expect to find some new ones. Hence, we can compare images, detect variable objects and identify them as supernovae. Once they’ve been detected, we monitor them.
On average how many supernovae can be detected between one observation and the following one?
It depends on the fraction of sky that you observe, but in a week you can roughly detect around ten supernovae. In our case in La Palma, for example, using Isaac Newton telescope, allowing a 10 x 10 arc minutes field of view, a dozen supernovae could be discovered in two weeks.
Therefore, the discovery that the expansion of the Universe is accelerating was based on the analysis of 42 supernovae.
Data fell far short of reliable data for a Universe that was thought to be decelerating. It showed that it was not decelerating, but accelerating. In fact, it’s been observed that the Universe was slowing down until redshift (shift towards the red) z = 0.5, in other words, when the Universe was about six billion years old, less than half as old as it is today. That was when the acceleration began.
Does it mean that at one point a force different from gravity starts dominating?
To explain this shift, the existence of dark energy was hypothesised in your work. But, what does it refer to?
Dark energy consists in different hypotheses about what may have led the Universe to accelerate. For example, a possibility, the easiest one, is the cosmological constant —proposed by Einstein’s theory of gravitation. Other theories of gravitation would give rise to different possibilities.
Some of the other theories of gravitation do not only have a scalar value, such as in the case of Einstein’s theory, which has an associated particle (the graviton), but also a tensor component —depending on space and time—, which would give rise to a different type of gravity with different results from those of general relativity.
There has also been much speculation as to whether it is due to an effect caused by the fact that we live in a four-dimensional universe called brane universes. This hypothesis implies that there would be a new dimension from where gravity would escape. In this case, the cosmological constant wouldn’t be necessary, but so far data doesn’t match this hypothesis.
Therefore, we can consider that Einstein’s cosmological constant is the most accepted theory, although he once stated that it had been his biggest mistake.
Einstein introduced it to describe a static universe, but when he realised, following the work of Edwin Hubble and the theoreticians of the 1920s and 30s, that the Universe was expanding; he then said that it had been his biggest mistake. But now it could be necessary, because we are seeing that dark energy is constant in the relationship between pressure and density and this corresponds to a constant.
If at some point gravity dominated, slowing down the Universe, could Universe return to a decelerating phase in the future?
Yes, but if dark energy is a cosmological constant it won’t happen; if it was another form of energy, it certainly could. Universe could eventually contract again, but certain conditions should be met which, according to the equation of state, are not so far met.
Does it imply that the Universe could keep growing indefinitely?
Although there are currently many theoretical proposals, every day new ones keep appearing, which means that there are many hypotheses about dark energy. However, current data points to the cosmological constant; so, with the current data, the Universe will keep growing indefinitely.
Scientists increasingly work together in large collaborative projects such as the SCP. How do these groups work?
It’s different from working individually in the sense that many researchers are looking at the same thing, trying to see mistakes, analysing the results, conducting statistics and detecting whether there is any object that doesn’t match the patterns.
But there is always a main researcher, the first author of the paper, who has to supervise everything, taking into account all the observations he or she receives, and this means a lot of pressure. I’ve met researchers who have ended up abandoning astronomy.
Is it easy to get into this kind of international research groups?
To access them you need to contribute something new, although, in fact, these groups, such as ours or the High-Z team, are fairly closed groups and they no longer accept new researchers.