Published 12 July 2024 by Benjamin Skuse
Cosmology: Illuminating the Dark Universe
Who better to take on the ambitious task of condensing into just over 15 minutes The Greatest Challenges in Cosmology, and How They Could Be Resolved than John Mather and Brian Schmidt, the former having receiving his Nobel award for confirming our view of how the universe began, and the latter opening our eyes to how it will end?
The pair’s Agora Talk on 3 July did exactly what it said on the tin, with Mather beginning by accelerating through a number of open problems in cosmology. “What are space and time? Some people think they’re not obvious. It even could be that they’re produced by quantum mechanics or something else. Why is there more matter than antimatter? That’s not for cosmologists alone to say, there are other people who also have to work on that.”
Similarly, great mysteries such as the existence of quantum gravity, the multiverse and the potential mathematical nature of the universe were presented and briefly appraised almost before the audience could catch their breath. But the Laureates did make space to expand on two fascinating topics currently puzzling cosmologists.
One was, why are there so many black holes, and why are they so big? “Gravitational wave detection allows us now to see the merger of black holes back in time across the universe, and there are literally hundreds of detections, normally one or two a day,” explained Schmidt. “These black holes are huge, much bigger than even 10 years ago we would have thought existed.”
Schmidt said that some researchers have found evidence that a large fraction of the first generation of stars collapsed directly into black holes. But this goes against current dogma surrounding stellar evolution. Other researchers have alternative hypotheses: “People are getting desperate, thinking, maybe they were created in the Big Bang,” said Schmidt. “I’m yet to be convinced.”
“Two Weird Things”
Another mystery Schmidt and Mather highlighted touches upon both of their Nobel Prize winning work. Mather shared the 2006 Nobel Prize in Physics with George Smoot “for their discovery of the blackbody form and anisotropy of the cosmic microwave background [CMB] radiation.” Fortuitously found in 1965 by Arno Penzias and Robert Wilson (1978 Nobel Prize in Physics) while experimenting with a super sensitive radio antenna, the CMB is a faint background glow of microwave radiation that pervades the universe. That it is blackbody and anisotropic provided compelling evidence of its Big Bang origin.
It is now known that the CMB was released about 380,000 years after the Big Bang, when the opaque primordial plasma that made up the universe became diffuse enough that matter and radiation could go their separate ways, leaving a transparent cosmos. It is therefore the universe’s first light – anything preceding the CMB’s release can never be directly observed (except perhaps neutrinos).
Successively improved observations of the CMB since the Mather-led NASA Cosmic Background Explorer ended operation in 1993 have delivered evermore fine-grained CMB maps featuring seemingly random hot and cold spots. “If you make the correlation function, basically how many big spots and how many small spots there are, those random spots become a nice, beautiful curve with bumps which fit our model of the universe – the Standard Model of cosmology [more often referred to as Lambda-CDM] – very precisely,” said Mather. “But we require two weird things that nobody asked for: dark matter and dark energy.”
This is perhaps the greatest cosmological mystery of our time: what are dark matter and dark energy? All that is currently known is that dark matter is the name given to whatever it is that provides the gravitational heft to stop galaxies and galaxy clusters from falling apart. It makes up a staggering 20% of the universe’s energy budget.
In contrast, dark energy is the moniker for the phenomenon behind what Schmidt, and fellow 2011 Nobel Prize in Physics laureates Adam Riess and Saul Perlmutter discovered – that the universe’s expansion is not slowing down or steady but in fact accelerating. Literally expanding space everywhere, dark energy is responsible for around 65% of the universe’s energy budget. To put this into context, you, the device you are reading this on, our planet, and everything we could conceivably see, or indeed think of, shares a tiny 5% sliver of this energy budget.
Waves and Wrinkles
Given their extraordinarily large contribution to the universe, resolving the nature of dark matter and dark energy is a priority for cosmology. Mather and Schmidt did not have enough time to go into any detail about current efforts to illuminate these two dark pillars of the universe. But in his Agora Talk Cosmology Today and Tomorrow on 4 July, George Smoot did.
Smoot explained that researchers have been attempting to tackle the problem from several different directions: “We have CMB observations, we have supernova observations, and we have clusters of galaxies,” he said. “But I want to teach you about baryon acoustic oscillations.”
He explained that baryon acoustic oscillations are the product of how the chaotic cosmic soup that was the early universe expanded and cooled, generating sound waves. These sound waves froze in place when atoms formed, around 380,000 years after the Big Bang, i.e. the time of the CMB. Soon after, large-scale structures started to form. Imprinted on these structures were bubbles of matter created by the sound waves that had been sloshing about in the primordial cosmic soup. These bubbles are visible today as wrinkles in the density distribution of clusters of galaxies spread across the universe, otherwise known as baryon acoustic oscillations.
The Dark Energy Spectroscopic Instrument (DESI) located at the Kitt Peak National Observatory in Arizona, USA, has been specifically designed to map the large-scale structure of galaxies and measure these wrinkles in their distribution. Just recently, results from DESI’s first year of data collection were released, and have caused a stir in the community.
Delivering figures for the expansion rate of the universe at seven different points in time over the past 11 billion years, researchers have extrapolated values from DESI results for dark energy at each of these time points, and there are hints that there is some deviation, suggesting dark energy has evolved over time. “This would be a really big deal if it turns out to be true in terms of its implications for how the universe works,” said Smoot.
Smoot hopes that researchers will not have to wait too long to resolve this mystery. DESI should soon release the next two years of results. And he says that there will be a huge wave of new data coming through in the coming years to help illuminate dark matter and dark energy, including from DESI but also the upcoming Vera Rubin Observatory, Subaru Prime Focus Spectrograph, Simons Observatory, Euclid Space Mission, SPHEREx, Nancy Grace Roman Space Telescope, LiteBIRD and CMB-S4.
“We’re going to have a lot more data coming in,” said Smoot in his concluding remarks. “That’s great – that’s an opportunity for young scientists to get in and participate in the big effort of building up the Standard Model of cosmology.”