All scientific inquiry involves some kind of uncertainty, but the uncertainties in cosmology are particularly daunting. As well as being complex and difficult to describe, the systems cosmologists investigate are almost by definition out of reach. In these conditions of uncertainty, how confident can we be in the models we’ve built? How can we come to know how galaxies form, how the universe began or how much dark matter it contains?

In The Random Universe: How Models and Probability Help us Make Sense of the Cosmos, cosmologist and astrophysicist Andrew Jaffe offers a masterful and refreshing take on these questions. As the director of the Centre for Inference and Cosmology at Imperial College London, UK, and a past co-investigator on ESA’s Planck satellite that provided us with so much new information about the cosmos, Jaffe is well-equipped to walk readers through the tools that allow us to learn about the world despite its uncertainties. Anchoring his insights in beautifully narrated episodes from the history of physics, astrophysics and cosmology, he sheds light on how our theories of the universe evolved into our current best cosmological model – which nonetheless still has limitations.

Jaffe is particularly good at recounting how we refined, revised and even replaced some of our assumptions in light of incoming evidence such as the first measurements of stellar distance; the discovery of galaxies beyond our own; and precise measurements of cosmological parameters. Throughout this journey, he engages thoughtfully with a rich philosophical tradition that runs from David Hume in the 18th century through Willard Van Orman Quine, Imre Lakatos and Paul Feyerabend in the 20th.

Bayes’ theorem is important because it tells us how to update our degree of belief in light of new evidence

The book’s main protagonist, though, is the mathematician Thomas Bayes. For Jaffe, Bayesian probability is key to dealing with uncertainty, and even to solving Hume’s notorious problem of induction: how do we justify inferences from a finite set of observations to generalizations that go beyond them? Bayes’ theorem is important, Jaffe argues, because it tells us how to update our degree of belief in light of new evidence. It shows us why and how, even in an uncertain world, “the more observations we make, the surer we become”.

Of course, before we can use probability to deal with uncertainty, we first need a model. As Jaffe puts it, “Absent a model, I can’t assign a probability, and absent a probability, I can’t quantify my uncertainty about the world.” So, what exactly is a model? I found Jaffe’s answer to be one of the most captivating elements of the book. As the title of a subsection of chapter five puts it, “a model is a story about the world”. Without models, Jaffe tells us, our world would be an unexplained series of unrelated events. Models are also what allow us to take a finite set of data and use it to create something new. Without this leap, Jaffe suggests, we cannot learn anything about the world, because “you can’t make sense of an experimental result unless you have some model to interpret it”.

Of course, if the way we interpret data depends on our model, then different people may well obtain different interpretations depending on which model and which prior probability they choose. This charge of subjectivity is a longstanding accusation against Bayesians, but Jaffe offers at least a partial solution: “If you and I agree on the model, we should assign the same probability.” This, Jaffe argues, is where objectivity emerges, writing that “we come to the same conclusions if we start from the same premises”.

Models allow us to understand the world by bridging subjectivity with objectivity, and the personal with the collective

Many contemporary philosophers are at odds with the idealized view of science as an objective and linear enterprise that produces theories from a God’s-eye view. Instead, they strive to portray it as something that is social and human, but reliable nonetheless. Jaffe’s own views seem to align with this endeavour; as he puts it, “the scientific coupling of models and observations is a living, breathing, subjective endeavour, simultaneously collective and personal”. Through their interplay with data, he argues, models allow us to understand the world by bridging subjectivity with objectivity, and the personal with the collective.

Not everyone will agree with Jaffe that Bayesian probability offers a solution to Hume’s problem of induction. Personally, I (or perhaps the hardcore rationalist in me) was not convinced. Nevertheless, I found the book’s thesis – that uncertainty is not a hindrance to knowledge, but an opportunity to learn (and learn more) about the world – simultaneously liberating, humbling and empowering. I recommend The Random Universe to scientists, philosophers and anyone interested in better understanding the role of probability in science; the philosophical problems with how we handle uncertainty; and how our standard cosmological model came to be. I also hope it will encourage more collaborations between philosophers and scientists. There are many questions we can explore further if we work together.

• 2025 Yale University Press, £25hb, 288pp

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NASA astronaut Mike Fincke is departing the agency on Wednesday after 30 years of service. Throughout his career, he flew four missions, spent 549 days in space, and completed nine spacewalks in support of the International Space Station.

“Few people have had the opportunity to shape as many chapters of NASA’s history as Mike Fincke,” said NASA Administrator Jared Isaacman. “Over a remarkable career, Mike served our nation as a pilot, engineer, astronaut, and mentor. From long-duration missions aboard the International Space Station to helping prepare the Artemis generation, his contributions have helped position NASA for what comes next. The success we’re building on today is possible because of people like Mike, who dedicated their careers to moving our space program forward and preparing the next generation to carry the mission even further. I’d like to congratulate Mike on an incredible career and thank him for his decades of service to NASA, our nation, and the countless people who had the opportunity to learn from and fly alongside him.”

He ranks fourth among NASA astronauts in accumulated time in space, and his spacewalks total 48 hours and 37 minutes. Most recently, Fincke piloted NASA’s SpaceX Crew‑11 mission, which launched in August 2025 and returned in January. During the mission, he served as a flight engineer for International Space Station Expedition 73 and commander of Expedition 74.

Fincke joined NASA’s 16th astronaut class in 1996 and first flew to space in 2004 aboard Soyuz TMA‑4 in support of the space station’s Expedition 9. Serving as a science officer and flight engineer, he helped maintain station systems and performed four spacewalks. He returned to space in 2008 on Soyuz TMA‑13 as commander of Expedition 18, preparing the space station for its transition to six‑person crews at the time and completing two more spacewalks.

In 2011, Fincke flew on STS‑134, the final flight of space shuttle Endeavour. As mission specialist and robotic arm operator, he completed three spacewalks and helped deliver and install the Alpha Magnetic Spectrometer.

“Mike’s remarkable career reflects three decades of dedication to NASA’s mission and the advancement of human spaceflight,” said Vanessa Wyche, director of NASA’s Johnson Space Center in Houston. “From his time aboard the International Space Station to his commitment to mentoring the next generation, Mike has made an immense impact across our agency. His legacy of service, mentorship, and dedication to exploration will continue to inspire the generations to come.”

Throughout his career, Fincke bridged spacecraft development, flight testing, and mission operations. Early in the International Space Station Program, he helped test and integrate several of the station’s initial modules before launch. His flight experience spanned multiple generations of human spacecraft, including two missions aboard Soyuz, one aboard the space shuttle, and later piloting the SpaceX Dragon.

Fincke was a foundational contributor to NASA’s Commercial Crew Program. As chief of the Astronaut Office’s Commercial Crew Branch, he worked to ensure astronaut needs, crew safety, and human spaceflight experience informed development of the nation’s next generation of crewed spacecraft. He spent five years supporting Boeing’s Crew Flight Test program training as a crew member and backup pilot, contributing to flight software, systems integration, integrated testing, and spacecraft interfaces.

Fincke also supported station operations from the ground as a crew test support team member in Russia, a capsule communicator, or capcom, and crew procedures team lead. He helped translate complex engineering and operational requirements into clear instructions for crews working in orbit. His continuity across development, integration, mission support, and long‑duration flight gave him an end‑to‑end perspective on space station assembly and operation.

“Mike approached every assignment with experience, humility, and an unwavering focus on the mission,” said Scott Tingle, chief of the Astronaut Office at NASA Johnson. “Whether flying aboard the station, supporting crews from the ground, or helping shape the spacecraft that future crews will rely on, he consistently strengthened our team. His legacy is woven into the way we fly today.”

A native of Emsworth, Pennsylvania, Fincke holds bachelor’s degrees in aeronautics and astronautics and in Earth, atmospheric, and planetary sciences from the Massachusetts Institute of Technology, where he also studied in the Soviet Union through an exchange program with the Moscow Aviation Institute. He earned master’s degrees in aeronautics and astronautics from Stanford University and in planetary geology from the University of Houston, Clear Lake.

Fincke is a retired U.S. Air Force colonel and distinguished graduate of the U.S. Air Force Test Pilot School. He served as a space systems engineer and flight test engineer at Edwards and Eglin Air Force Bases and later as the U.S. flight test liaison to the Japanese‑U.S. XF‑2 fighter program at Gifu Air Base in Japan. He accumulated more than 2,000 flight hours in more than 30 aircraft types.

“After exactly 30 years, I am departing NASA, but I remain deeply committed to the work of exploration.” Fincke said. “NASA gave me the extraordinary privilege of serving alongside remarkable people, flying and helping develop spacecraft, and contributing to the International Space Station from its earliest days through command in orbit. I am profoundly grateful to my crewmates, the teams on the ground, our international partners, and the families who make this work possible. I am excited to carry those lessons forward and help prepare the next generation of engineers, explorers, and leaders. Together, we will return humanity to the Moon, travel to Mars, journey outward to the planets and moons beyond Earth, and someday reach for the stars – all while caring for Earth, the most beautiful planet in our solar system.”

To learn more about NASA’s astronauts and space exploration, visit:

https://www.nasa.gov/astronauts

-end-

Jimi Russell
Headquarters, Washington
202-358-1100
james.j.russell@nasa.gov

Anna Schneider
Johnson Space Center, Houston
281-483-5111
anna.c.schneider@nasa.gov