In the 1920s, the discovery of the expansion of the Universe was a crucial debate in establishing the Big Bang model, which would later be confirmed with other evidence. In the 1990s, the discussion about the current rate of expansion, also known as H0 (with H(t) being the expansion parameter of the Universe), was a debate with opposing viewpoints and heated discussions at conferences. These were years whose memory always brings a smile as I recall the lively verbal battles between proponents of one value or another, the fierce attacks among scientific luminaries. For a student entering the field, it was an additional incentive to examine the issue independently, contributing a different method.
Today, it must be said that the topic of the value of H0 is still under discussion. But here we will not talk about H0, but about H(t), the expansion parameter of the Universe. According to Einstein’s equations for a Universe homogeneous in all directions, this parameter should reflect a deceleration due to the action of matter. Just as a stone thrown from Earth falls due to the gravitational attraction exerted by our planet, the Universe should tend to “fall upon itself under its own weight,” meaning the matter it contains should lead to a deceleration of its expansion. (In fact, the expansion parameter is nothing more than the variation of the Universe’s scale factor, in units of the scale factor (H(t) = da(t)/dt/a), where a is the scale factor, a measure that gives us an idea of how the size of a region of the cosmos increases).
In reality, and as it has happened, measuring the variation of H(t), a value that connects various epochs evolving over time, is easier than measuring H0, which requires absolute measurements, that is, without relation to another time. Researchers engaged in determining the value of the evolution of the Universe’s expansion only need good cosmological distance indicators with constant or calibratable luminosity throughout its history.
While the determination of the Universe’s expansion was made possible thanks to Henrietta Leavitt’s discovery of the period-luminosity relationship of Cepheids, determining whether this expansion was slowing down (the latter option was not on our minds in the early 1990s) required using Type Ia supernovae, thermonuclear supernovae (also abbreviated as SNe Ia). Cepheids are too faint as indicators, observable only at distances of a few million light-years in the Universe. For this deceleration measurement, we needed a more powerful indicator capable of measuring distances of billions of light-years. We found it in these supernovae, but only once the relationship between brightness and the rate of decline after the peak was calibrated: the brighter Type Ia supernovae decline in luminosity more slowly than the dimmer ones, which show a faster decline in luminosity.
The studies of supernovae conducted at Calán/Tololo in the mid-1990s allowed the refinement of the peak brightness-decline rate relationship of the light curve. The correlation had been known since the 1970s but had not been quantified. In 1993, Mark Phillips presented his analytical expression between brightness and its decline. This was verified with about thirty supernovae by Mario Hamuy and his collaborators at Calán/Tololo. The study was crucial for addressing the determination of the Universe’s deceleration rate and made it possible.
Fig.1 Peak brightness versus decline rate of SNe Ia, as used by the Supernova Cosmology Project.
Since August 1992, I was researching as a postdoc at Harvard using SNe Ia to determine nucleosynthesis and distances. How did I get there? The stars had aligned for me to do my thesis with Leon Lucy, from the European Southern Observatory, in Garching, Germany. In my thesis, I developed a code that allowed determining cosmic distances to supernovae and had published it in the Astrophysical Journal (1) with very positive feedback from the journal’s referee. With my method, I could calculate H0. After presenting my thesis, I received a handwritten letter of several pages from Gustav Tammann with comments that thrilled me, as well as a dedication sending me greetings from Gerard de Vaucouleurs. Gustav Tammann and Gerard de Vaucouleurs were leading cosmologists with opposing ideas about the value of H0. While Tammann favored a low H0 value, de Vaucouleurs was on the opposite spectrum of values for H0.
Fig.2 My thesis advisor Leon B. Lucy. On May 12, 2000, he was awarded the gold medal of the Royal Astronomical Society for his contributions to Astrophysics. https://academic.oup.com/astrogeo/article/41/4/4.7/196431.
After a few months waiting for a postdoctoral offer, my first job was at the Institut d’Astrophysique in Paris, in the gamma-ray astrophysics group. Undoubtedly, my thesis advisor Leon Lucy had written very good recommendation letters. In Paris, following my line of interest, I had tried another method of distance determination in the realm of supernova gamma radiation. It seemed that my next contract would be in Saclay, but in July 1992, I received an offer from Robert (Bob) Kirshner to join his group at Harvard. There, they also focused on determining cosmic distances but using gravitational collapse supernovae. Without hesitation, I enthusiastically accepted; I only asked if I would have health insurance coverage (I had never traveled to North America, and in Europe, tourists’ complaints about mishaps are well-known). I allowed myself to attend the marathon celebration at the Barcelona Olympic Games and, urged from the other side of the Atlantic, arrived in mid-August in New York and then to Boston.
At that time, in 1992, Brian Schmidt was finishing his thesis with Bob Kirshner on distance determination with gravitational collapse supernovae. I dedicated myself to modifying my code to include a more realistic description (allowing density to vary along the ejected material) of SNe Ia and applying it to observations. In 1993, at a conference organized in Aspen, Saul Perlmutter, from the Lawrence Berkeley National Laboratory, presented the first results of the discovery of a supernova at a distance capable of addressing the measurement of the Universe’s deceleration. It was the supernova SN 1992bi, discovered with the Isaac Newton telescope at the Roque de los Muchachos Observatory in La Palma. The discovery had been confirmed with a spectrum observed at the William Herschel telescope, at the same observatory. The Isaac Newton telescope had a camera with a mosaic of CCD detectors (Charge-Coupled Device). (In our mobile phones, our cameras use these detectors). Before the introduction of CCDs, astronomical cameras used photographic plates, and transient processes in the night sky could not be automated with the required speed and precision for many projects. Saul Perlmutter had started that project in the late 1980s. When the Nobel Prize was awarded to the principal investigators of the discovery in Stockholm, in the opinion of many (including myself), he gave the most complete and generous speech. He showed the inherent difficulties of the project, how they were overcome, highlighted the team’s work, showing the human factor through our faces projected on the screen (see Fig. 7), and mentioned our work meetings before the discovery presentation. That speech can be seen in the bibliography references (2,3).
Fig.3 A young Saul Perlmutter (34 years old) presenting his first high-z supernova at the cosmological distances conference in Aspen. Photo at the top of the cable car (1993).
And indeed, prior to the 1990s, as early as 1968, Charles Thomas Kowal had attempted to determine the deceleration of the Universe’s expansion by various methods, following what Allan Sandage had anticipated in 1961 (4): “the future of observational cosmology, at least for the next three decades, will be the search for two parameters: the Hubble constant and the deceleration parameter of the Universe.” A prescient forecast. But this was not possible while supernovae (thermonuclear or Type Ia) were not sufficiently understood to be used as distance indicators and while telescopes lacked digital CCD detectors. From the mid-1990s, these detectors would be installed in all professional telescopes worldwide. And in 1997, there was a substantial breakthrough in our ability to observe at increasingly greater distances: we would have the Hubble Space Telescope. We would detect key supernovae to determine the evolution of the Universe’s expansion.
Returning to my personal note, in 1993, the so-called High-Z Supernova Search Team or, abbreviated, HZT, had not yet been formed. But, in our hallway at the Harvard-Smithsonian Center for Astrophysics, a young Adam Riess had started his thesis with Bob Kirshner, very close to Brian’s office and a bit further from mine. His idea was to parameterize the variation in the brightness-decline relationship of the SNe Ia light curve using a least-squares method that took into account the entire light curve of the supernovae and the extinction of their luminosity by dust, mainly in the host galaxy. By 1994, Brian, who had been a postdoc after presenting his thesis, moved to live in Australia with his wife, and I returned to Barcelona to join the University. I missed my time at Harvard, where I learned a lot. I realized the importance of the observational aspect and that something as simple as parameterizing the brightness-decline relationship of supernova luminosity could be more necessary than a sophisticated radiation transport code for supernovae.
Fig.4 Bob Kirshner’s 60th birthday party at the Institute for Theoretical Physics in Santa Barbara (2009). A group of students and postdocs who worked with him at some point. In the center Bob Kirshner, from left to right: Bob Fesen, Bruno Leibundgut, Pilar Ruiz-Lapuente, Pete Challis, Maryam Modjaz, Peter Garnavich, Stéphane Blondin, Armin Rest, Kaisey Mandel, Brian Schmidt, Tom Matheson, Ryan Foley, Saurabh Jha. Adam Riess could not be in the photo. Bob Kirshner received the Wolf Prize in Physics in 2015 for (quote) “creating the group, the environment, and the instructions that enabled his graduate students and postdoctoral fellows to discover the acceleration of the Universe’s expansion.”
In Barcelona, I had the intuition that it was necessary to organize an international meeting on supernovae. And so it was done, in Aiguablava (Begur), a secluded place where participants had to interact, as demanded by the funders of what was the NATO Advanced Study Institute on thermonuclear supernovae. There, for the first time, the members of the Supernova Cosmology Project and the High-Z Supernova Search Team, which had been formed at the end of 1994, came together. The presentations made there are historic and are collected in a volume of 890 pages. Both collaborations discussed together for the first time.
It wouldn’t take long for me to join one of them: the Supernova Cosmology Project. This seemed natural, as I had already observed for Saul Perlmutter in La Palma, informally, during time dedicated to a program on supernovae at different phases, when he had not yet published his first success with the 1992 supernova. On the other hand, the Supernova Cosmology Project had achieved its first distant supernova at the Isaac Newton Telescope in La Palma, and that was an important node for the collaboration. The High-Z Supernova Search Team (High Z Team), with members from Harvard, Chile, and ESO, preferred to operate primarily in Chile in its early days and later also in Hawaii and, of course, with the Hubble Space Telescope. The year 1995 would be decisive, and 1997 very important and discussed.
In 1997, a series of evidence emerged that did not fit with a Universe slowing its expansion. Who was closer to finding evidence of the Universe’s acceleration? Here I would like to provide testimony of what was happening in the fall of 1997, as I experienced it during a three-month stay in California, where I was at the Institute for Theoretical Physics in Santa Barbara, in a program dedicated to supernova physics, organized by Adam Burrows, and also visiting the Berkeley National Laboratory, the main headquarters of the Supernova Cosmology Project.
In Santa Barbara, there were some members of the High Z Team, particularly Bob Kirshner. With the results they had by that September 1997, which were mentioned informally in the afternoon sessions, for the moment, they found no sense in what was found: the few (four) high-z supernovae pointed to a negative matter density of the Universe (Ωm < 0). Peter Garnavich would lead an article submitted on October 13, 1997, concluding, with that small sample of supernovae, that Ωm was equal to -0.1 ± 0.5 if the cosmological constant was zero (5). That would also be mentioned in the Nobel Prize speech given by Adam Riess in Stockholm. Of course, there was a theoretical prejudice that either the cosmological constant was very large or it was zero. At that time, it was assumed to be zero. However, somewhat further north of Santa Barbara, in Berkeley, Gerson Goldhaber, independently, had found a result similar to Garnavich’s with many distant supernovae and had considered what would happen if the Universe were flat (matter-energy density Ωm + ΩΛ = 1). Taking into account the cosmological constant, the data did make sense, and the matter density was 30% of the total. Gerson recounts in his memoir (6) how he presented to the members of the Supernova Cosmology Project two histograms of the data that seemed to indicate that Λ could have a non-zero value. Saul corroborated the conclusion with a software program he made. On that occasion, I had traveled on a Greyhound bus from Santa Barbara to Berkeley and then had gone to San Diego to greet some friends. If we consider that with these friends I crossed into Tijuana, the round trip instructed me about the landscape and people of California (especially those who travel up and down to the border on the Greyhound). It was many hours, but very interesting.
Gerson’s result needed a more detailed examination, and I did not mention anything in Santa Barbara. According to Gerson, he gave a lecture on it at the Institute for Theoretical Physics in Santa Barbara on December 14, 1997, which I could not attend as I was already back in Europe. Although Bob Kirshner judges that Gerson did not quite show the evidence that Λ was positive, others interpreted it that way in various circles. Saul Perlmutter also gave lectures at the University of California in San Diego and Santa Cruz with the result, and there was great enthusiasm. The question loomed, but it is true that it was important to focus on systematic errors (those due to the method). No one wanted to leak a result like that lightly. By early 1998, Adam Riess had reached the same conclusion, as he recounts in his Nobel Prize lecture. The Supernova Cosmology Project would have an intensive work meeting in Paris before sending the definitive article, which happened in September 1998. But our communication of the findings was in the poster presented at the American Astronomical Society meeting in January 1998. Although the press spoke of the Universe’s final destiny being to expand indefinitely due to low matter density, only journalist James Glanz captured that there was evidence of a repulsive force. Something he tried to confirm with Alex Filippenko. At a conference at the University of California in Los Angeles in February 1998, Saul Perlmutter, Gerson Goldhaber, and Alex Filippenko confirmed the presence of Λ.
The two collaborations, the Supernova Cosmology Project led by Saul (7) and the High-Z Supernova Search Team, whose spokesperson was Brian Schmidt and the first author of the work Adam Riess (8), had therefore given their results in 1998. To everyone’s surprise, the Universe was not slowing its expansion due to its matter-energy content, as would have been expected, but was accelerating its expansion due to a component with an effect opposite to gravity, a kind of anti-gravitational repulsion known today as dark energy.
Dark energy is today one of the most relevant research topics in cosmology. It comprises 69% of what the cosmos contains. Its nature is yet to be determined. Within the Supernova Cosmology Project, we continue to investigate it with projects that increasingly obtain with greater precision the so-called w or coefficient of the dark energy equation of state, which seems very close to -1. This coefficient w is the ratio between its pressure p and density ρ (p = wρ). If it were exactly -1, it would be the cosmological constant, a term Einstein introduced and then removed from his General Relativity equations.
The discovery of the Universe’s acceleration and, therefore, dark energy, is considered fundamental to our understanding of the cosmos. While today the High-Z Supernova Search Team and new scientists have integrated into various collaborations with different names that attempt to determine the nature of dark energy, see Bob Kirshner’s article (9), the Supernova Cosmology Project remains active as such, expanded and operating from different points. Now, from La Palma, we no longer use the Isaac Newton Telescope or the William Hershell Telescope but the Gran Telescopio de Canarias, with a diameter of 10.4 meters. From it operates the program of the La Palma node, which I direct and to which much support is given, especially from the telescope’s operations management. This task is coordinated with colleagues from the Supernova Cosmology Project, who mainly observe at the Keck telescope, the Subaru, the Gemini in Hawaii, the Very Large Telescope in Chile, and smaller diameter telescopes like the Anglo Australian Telescope.
More than twenty years after the discovery of the Universe’s acceleration, the nature of dark energy remains under discussion. Thousands of Type Ia supernovae at different redshifts (corresponding to different ages in the Universe’s expansion) have reduced uncertainties about the value of the dark energy equation of state. We are at a point where if this value is -1, equivalent to the cosmological constant or vacuum energy, it will soon be confirmed.
Within the Supernova Cosmology Project, we are about to publish the results of the analysis of thousands of Type Ia supernovae, the so-called Union 3 sample, as it is the third sample (actually the fourth because there has been a first Union sample, a second, and a 2.1), since the initial results were published that led to the discovery of the Universe’s acceleration and subsequent reaffirming conclusions. As projects that will contribute to this Union 3 sample, the See Change project stands out, which has been observing Type Ia supernovae in very distant galaxy clusters.
Since November 2016, the program called SUSHI (SUbaru using the Hyper-Suprime Camera (HSC) and the H**ST for infrared follow-up) has been operating at the Subaru telescope, providing hundreds of supernovae at high distances to the Hubble diagram. This diagram shows how the brightness of supernovae evolves against the epoch of the Universe from which we receive their light: the epoch of the Universe is quantified by z, the “redshift,” a measure of how much the scale factor has changed between the supernova’s explosion and the current moment when we receive its light.
Fig.5 Left: examples of SNe Ia discovered with the Hyper Suprime Camera at the Subaru telescope, within the mentioned SUSHI program. Each line contains: reference image (left), new image (middle), and difference (right). The supernovae thus found are at very high redshift. Right: Hubble diagram of the Subaru Suprime Project with the Hyper Suprime Camera (SUSHI program) with SNe Ia superimposed on the Union 2.1 database of the SCP (blue points). Our sample will effectively fill the z > 1 range.
On the other hand, the Nancy Grace Roman Space Telescope mission is preparing to make the final assault on determining what dark energy is. From space, it will be possible to reach, in the infrared, supernovae at very long distances coming from a time when the Universe’s scale was much smaller than it is now. Its data collection will be on the order of tens of thousands of supernovae, but much more distant. It will try to discriminate with very high precision whether we are in a Universe where dark energy is the cosmological constant or not. In the latter case, it may glimpse what it is.
It must be said that not only Type Ia supernovae are proposed to discover what lies behind dark energy. There are also other methods such as the use of gravitational lenses and baryon acoustic oscillations. Due to space limitations, we will not explain here how they work, but we provide bibliography for consultation (10). These currently give results consistent with those of supernovae. However, as Bob Kirshner literally says (11): “the concordance of these various methods does not mean they should support each other like a trio of drunks. On the contrary, those using each proposal need to evaluate their present weaknesses and work to remedy them.”
Today we can say that dark energy constitutes 69% of the matter-energy of the Universe. It is now the dominant component in the evolution of the cosmos’s energy-matter density. The Universe has gone through different stages where the dominance of each component has varied. We began in an era dominated by radiation, continued with a period of matter dominance, which occurred about 60,000 years after the Big Bang, and 5 billion years ago, we entered the era of dark energy dominance. This can be seen in the attached graph, see also (10).
Fig.6 The three epochs of the Universe’s evolution: the radiation-dominated era, the matter-dominated era, and the current dark energy-dominated era.
If dark energy is the cosmological constant, our Universe will be infinite in time and will keep expanding indefinitely while becoming colder and with very tenuous density. There will be no stars forming to illuminate the cosmos, no light shining for others. In this case, we will face a thermal death.
But if dark energy is different from the cosmological constant, it could be a manifestation of gravity that differs cosmologically from Einsteinian gravity, known as “modified gravity,” or it could be another unidentified element in the cosmos’s composition, perhaps leading to a different end.
Of course, it is a fascinating investigation. And the amount of instrumentation dedicated to determining the nature of dark energy is enormous, offering many opportunities to young cosmologists.
Barcelona, January 20, 2022.
Acknowledgments
At this stage of my life, nothing would be the same without having participated in this extraordinary discovery. First of all, I owe my recognition to my thesis advisor, Leon B. Lucy, to whom I will always be immensely grateful, as without his support, my research would not have led to determining the Hubble parameter over time. It was also an extraordinary opportunity that Bob Kirshner invited me to Harvard. Observing the research priorities he led, I learned a lot. And what can I say about the exciting path that led to dark energy alongside Saul Perlmutter and the Supernova Cosmology Project? They have been and continue to be brilliant collaborators in this fascinating study. I also greatly appreciate Nao Suzuki for involving me in the SUSHI project. My thanks go to the University of Barcelona, the Institute of Fundamental Physics of CSIC, and the Institute of Cosmos Sciences for their great support and backing of the project. And undoubtedly, to the committees that allocate time for this project in La Palma and the operations director of Grantecan, Antonio Cabrera Lavers. Finally, I want to congratulate my students who examine what dark energy might be by contrasting theoretical ideas with observations. They are the future, and there is nothing better than seeing their brilliant careers take off, as they are the new generations who will definitively illuminate us about the nature of dark energy causing the cosmos’s acceleration.
Fig.7 Nobel Lecture by Saul Perlmutter, December 8, 2011, in the Aula Magna of Stockholm (2,3). Recognition for teamwork, our faces projected on the screen.
Bibliography
(1) Ruiz-Lapuente, P. & Lucy, L.B. (1992). Nebular spectra of Type Ia Supernovae as probes for extragalactic distances, reddening and nucleosynthesis. Astrophysical Journal, 400, 127 (thesis article, see also (12)).
(2) Perlmutter, S. (2011): https://www.nobelprize.org/prizes/physics/2011/perlmutter/lecture/
(3) Perlmutter, S. (2012). Nobel Lecture: Measuring the acceleration of the cosmic expansion using supernovae. Rev. Mod. Phys. 84, 1127-1148: https://journals.aps.org/rmp/pdf/10.1103/RevModPhys.84.1127
(4) Sandage, A. (1961). The ability of the 200-inch telescope to discriminate between selected world models, Astrophysical Journal, 133, 355.
(5) Garnavich, P.M., Kirshner, R. P., et al. (1998). Constraints on cosmological models from Hubble Space Telescope Observations of high-z supernovae. Astrophysical Journal, 439, L53-L57.
(6) Goldhaber G. (2009). The acceleration of the expansion of the Universe: A brief early history of the Supernova Cosmology Project (SCP) in Sources and detection of dark matter and dark energy in the Universe: Proceedings of the 8th UCLA Symposium. AIP Conference Proceedings, 1166, 53-72.
(7) Perlmutter, S., et al. (the Supernova Cosmology Project). (1999). Measurements of Omega and Lambda from 42 high-redshift supernovae. Astrophysical Journal, 517, 565-586.
(8) Riess, A.G. (the High-Z Supernova Search Team). (1998). Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant. Astronomical Journal, 116, 3, 1009-1038.
(9) Kirshner, R.P. 2010. Foundations of supernova cosmology, in Dark energy: Observational and Theoretical Approaches, ed. Pilar Ruiz-Lapuente (Cambridge: Cambridge University Press), 151-176.
(10) Ruiz-Lapuente, Pilar (2019). The Acceleration of the Universe. ed. La Catarata.
(11) Kirshner, R.P. 2010. Foundations of supernova cosmology, in Dark energy: Observational and Theoretical Approaches, ed. Pilar Ruiz-Lapuente (Cambridge: Cambridge University Press), 164.
(12) Ruiz-Lapuente, P. (1996). The Hubble constant from 56Co powered nebular candles, Astrophysical Journal, 465, L83-86.
Pilar Ruiz Lapuente
Doctor in Astrophysics.
Research Professor at the Institute of Fundamental Physics (IFF-CSIC),
Visiting Professor at ICCUB.
SCIENCE, and the “relative chance”
This article belongs to the project [https://cienciayelazarrelativo.blogspot.com](SCIENCE, and the “relative chance”).
