‘What do we know about the Universe? We know it’s expanding,’ says Nobel Laureate in Physics
October 12, 2022
By José Tadeu Arantes | Agência FAPESP – The matter we know, or think we know, corresponds to only 5% of the observable Universe. The rest is dark matter (25%), about which we know almost nothing, and dark energy (70%), about which we know even less.
All conventional science from Antiquity until a few decades ago focused on the 5%. Its achievements were noteworthy, in terms of both the understanding of material reality gleaned by basic science and its transformation by technological applications. An unprecedented number of scientific mega-projects in astronomy and astrophysics are on the agenda now, aiming at a deeper understanding of the 5% and making inroads as far as possible into the other 95%.
This was the gist of the 14th FAPESP 60 Years Conference, entitled “Astronomy and Astrophysics”. The speakers were Brian Schmidt, Vice Chancellor and President of Australian National University (ANU), and winner of the 2011 Nobel Prize in Physics for his discovery of dark energy; Angela Olinto, Dean of the Physical Sciences Division of the Department of Astronomy and Astrophysics at the University of Chicago; and Rob Adam, Managing Director of the South African Radio Astronomy Observatory (SARAO), which leads his country’s participation in the Square Kilometer Array Observatory (SKAO), the world’s largest radio telescope.
Schmidt delivered a didactic and fascinating presentation on the latest advances in astronomy and astrophysics, including the observations he led in 1994, which were later to result in the discovery of dark energy. Olinto spoke about ongoing research lines in astronomy and astrophysics, especially the search for ultra-high energy astroparticles, her specialty. Adam spoke mainly about the challenges and opportunities of doing top-tier science in a developing country.
The event was opened by Ronaldo Aloise Pilli, Vice President of FAPESP, and moderated by Beatriz Barbuy, a professor at the University of São Paulo’s Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG-USP).
“In 2022, what do we know about the Universe?” Schmidt began his talk with this question and gave a highly summarized answer: “We know it’s expanding, we know it’s between 13 and 14 billion years old, it’s very close to what we will describe as geometrically flat, and it’s composed mainly of three things: dark energy [70%], dark matter [25%] and atoms [5%].”
In saying “geometrically flat”, he was not referring to the pseudo-scientific “flat Earth” fantasies that have resurfaced in recent years. He meant that the four-dimensional Universe can largely be described by Euclidean geometry in the sense that all the internal angles in a triangle add up to 180 degrees, no less and no more.
Schmidt proposed to explain “how we know these things” and embarked on a brief account of “the story of the Universe”. Measuring the distance to astronomic objects, for example, entails measuring the intensity of the light that comes to us from them. Moreover, we know they are moving away from us because the wavelength of the light they emit “is stretched redward by the Doppler effect” (redshift).
“Edwin Hubble [1889-1953] put this all together in 1929, when he measured distances and redshifts, and discovered that the greater the distance, the larger the redshift and the faster objects are moving away from us,” he said.
Hubble’s conclusion was that the Universe as a whole is expanding. This was a key discovery that led to the theory of the Big Bang. If in a thought experiment “we run the Universe in reverse, things get closer and closer, until there’s a time whenever everything is on top of everything else: the Big Bang. What was the Big Bang? I honestly don’t know. All I know is that about 13 or 14 billion years ago, something put the Universe in motion,” Schmidt said.
The most distant thing we can see is the cosmic microwave background, he explained, the electromagnetic radiation that is a remnant from an early stage of the Universe, 300,000 to 400,000 years after the Big Bang.
“This radiation comes from the fact that the sky then had a temperature of 3,000 degrees [kelvin] and it glowed like the Sun glows. The afterglow of the Big Bang has been traveling all that time through space until it reaches us here on Earth and we can measure it [in the form of microwaves],” he said.
In his PhD research at Harvard University in the United States, supervised by Robert Kirshner, Schmidt used data from observations of type II supernovae to measure the value of Hubble’s constant, which describes how fast the Universe is expanding at different distances from a particular point in space. It is calculated as the recessional velocity of the celestial object divided by the distance from Earth. “The number we’ve all settled on is 70,” he said, meaning 70 km per second per megaparsec. A parsec is equal to 3.26 light-years. So, the velocity with which galaxies are moving away increases by 70 km/sec with every megaparsec of distance from Earth. Based on this value, he was able to estimate the age of the Universe as about 14 billion years.
Not long afterward, in 1994, he obtained the information for his greatest discovery. Contrary to the belief held by the entire community of astronomers at the time that gravitational attraction between components of the Universe was slowing its expansion, observations of type Ia supernovae made by his group at the Cerro Tololo Observatory in Chile, and later confirmed by observations at the Keck Observatory on Maunakea in Hawaii, with the world’s largest optical and infrared telescopes, showed that the Universe is expanding at ever-greater speeds. “It was a complete surprise,” Schmidt said.
If objects in space are accelerating rather than decelerating, something bigger than gravitational attraction must be pushing them away from each other. This led to a return to the hypothesis of a “cosmological constant” proposed and later discarded by Albert Einstein (1879-1955).
Whether it is called a cosmological constant or dark energy, no one knows what it is. Not knowing is a strong driver of scientific development. To transform ignorance into knowledge, an extraordinary set of projects is operating or under construction, as Olinto showed in the event’s second presentation.
Beginning with a spectacular image of the deep Universe produced by the James Webb Space Telescope, Olinto spoke about several of these projects.
“James Webb is a project that took more than 20 years to get results. It’s a great success and makes us very happy. The partnership with FAPESP in Brazil, with the University of Chicago in the US, with Brian in Australia and with others is to build the Giant Magellan Telescope [GMT], which will represent a new generation of ground-based telescopes. James Webb’s technology is 20 years old because you don’t change detectors after you’ve spent 20 years testing them. On the ground, our capacity is much greater and we can have a telescope with a collecting area that’s more than 25 m in diameter, compared with James Webb’s 6 m mirror,” she said.
Besides the GMT, other facilities are operating or will be soon in all parts of the electromagnetic spectrum: radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays and gamma rays. Huge complexes built to detect gravitational waves include LIGO, Virgo and KAGRA, among others. The Pierre Auger Observatory in Chile is studying the physics of cosmic rays, the most energetic and rarest of astroparticles.
Cosmic rays are charged subatomic particles that constantly rain down on Earth from all directions. They are made up of atomic nuclei or electrons that produce particle showers when they interact with Earth’s atmosphere. “High-energy astroparticles are produced by supernovae. The origin of ultra-high energy particles, which are my research interest, is unknown. We’re trying to find out. There are cosmic rays with energies on the order of 1020 electron-volts (eV), far more than can be produced on Earth by accelerators like the Large Hadron Collider (LHC), and for that very reason they can tell us about even earlier stages in the formation of the Universe,” Olinto said.
Neutrinos are another major focus. In fact, they are the simplest dark matter candidates. Giant detectors include the Deep Underground Neutrino Experiment (DUNE) and the IceCube Neutrino Observatory, an international collaboration that observes high-energy cosmic neutrinos using thousands of detectors buried 1.5-2.5 km under the ice sheet at the South Pole.
Should developing countries with limited resources take part in mega-projects like these? The question was addressed by Adam, the third speaker, who began by saying his presentation was “more of a policy talk”. His institution is a key player in the Square Kilometer Array Observatory (SKAO), an international collaboration that will build and operate the world’s largest radio telescope.
The complex will be 50 times more powerful than any existing facility of the kind. Thousands of 15 m mid-frequency dishes will be installed in South Africa, and hundreds of thousands of low-frequency antennas, eventually surpassing a million, will be located in Australia.
Adam situated the initiative in the context of South African history. Despite the difficulties, he argued, a project of this size is a huge incentive to local industrial development and to science and technology in universities. “Bringing the most respected and creative scientists and engineers to the center creates a stimulating environment for local scientists and students,” he said.
Refuting the argument that developing countries should only do science immediately relevant to their socio-economic needs, Adam said: “There’s nothing wrong with focusing on the most appropriate science for local nutrition, health and energy, but the most likely citizens to solve these problems are those who have had their minds stretched by the big global science projects of the age.
“If the big projects are located only in the developed countries, you’ll never allow the developing countries to make significant progress in the hard sciences and technology because you’ll always be draining off the best students, researchers and engineers.”
As Pilli noted in his opening remarks to the event, “At some time in our lives we’ve all felt perplexed by the grandeur and sheer enormity of the sky above.” Advanced research in astronomy and astrophysics does not deprive us of this perplexity. On the contrary, it takes the stimulus to an even high level.
A recording of the 14th FAPESP 60 Anos Years Conference on Astronomy and Astrophysics can be watched at: www.youtube.com/watch?v=WNapRhYeVM8.
The previous events in the series are at: 60anos.fapesp.br/conferencias.
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