BBC The Sky at Night - The Expanding Universe (2025) / AvaxHome

The team explores one of the greatest discoveries of modern astronomy - that our universe is expanding - and the new questions it raises about how the cosmos works.

In the 1920s, astronomers studying galaxies realised they were moving away from us, with the most distant ones receding the fastest. The conclusion was revolutionary: space itself was stretching, and the universe was expanding in every direction. But once this discovery was made, a new challenge emerged – how fast is the universe expanding, and what could that tell us about its age, size and ultimate fate?

Chris Lintott travels to the University of Oxford to explore one of the most extraordinary tools astronomers use to answer these questions - the cosmic microwave background (CMB), the faint afterglow of the Big Bang. Working with Professor David Alonso, Chris learns how scientists use the CMB as a starting point for measuring cosmic expansion. He discovers how the Simons Observatory, a network of telescopes in Chile, is creating the sharpest map yet of this ancient light. These observations promise to refine our understanding of the expansion rate and may shed light on a long-standing puzzle known as the 'Hubble tension', where different measurements of expansion don't agree.

Meanwhile, Maggie Aderin-Pocock investigates a different cosmic speedometer: supernovae, or exploding stars. Meeting Dr Philip Wiseman at the University of Southampton, she learns how type 1a supernovae, which shine with a predictable brightness, can be used to measure cosmic distances. A discovery in the 1990s revealed something astonishing - not only is the universe expanding, but the expansion is accelerating. The Vera Rubin Observatory, with its enormous mirror and world-record-breaking camera, is about to survey the southern skies, capturing supernovae in unprecedented numbers. With its data, astronomers hope to refine how fast the universe's expansion has accelerated over billions of years.

George Dransfield takes on the most mysterious part of the story - dark energy. Coined to explain the force driving the acceleration, dark energy remains entirely unknown. At the University of Portsmouth, George meets Dr Seshadri Nadathur, who is part of the Dark Energy Spectroscopic Instrument (DESI) project. By mapping tens of millions of galaxies across 11 billion years of cosmic history, DESI is building the largest 3D map of the universe ever made. This map is revealing subtle patterns in how galaxies are distributed, providing new insights into how fast the universe has expanded over time – and whether dark energy itself has changed throughout cosmic history. Together, these projects reveal both the power and the limits of our current models of cosmology. Different methods of measuring expansion don't line up, and far from being a mistake, this mismatch may be a vital clue that new physics is waiting to be discovered.

As ever, Pete Lawrence is on hand with his guide to the skies this month, including how to catch Saturn's rare ring and moon events, as well as highlights of the autumn equinox.

With cosmic clues building from the earliest light of the Big Bang to the faint glow of distant supernovae, the programme takes viewers on a journey through one of the most profound questions in science: how fast is our universe expanding, and what might that reveal about its past, present and future?

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