It is one of the most baffling questions that scientists can ask: how big is the universe that we live in?

lan Davies attempts to answer the proverbial question: how long is a piece of string? But what appears to be a simple task soon turns into a mind-bending voyage of discovery where nothing is as it seems.

Horizon plunges down the biggest rabbit-hole in history in search of the smallest thing in the universe.

Time travel is not forbidden by the laws of nature, but to build a time machine, we would need to understand more about those laws and how to subvert them than we do now. And every day, science does learn more.

Brian Cox explores the ingredients needed for an intelligent civilisation to evolve in the universe - the need for a benign star, for a habitable planet, for life to spontaneously arise on such a planet and the time required for intelligent life to evolve and build a civilisation.

With exclusive behind-the-scenes access, Horizon follows the highs and lows of an extraordinary story in particle physics. In June 2015, teams at CERN started running the large hadron collider at the highest energy ever. Rumours quickly emerged that they were on the brink of a huge discovery.

Dark flow is the latest in a long line of phenomena that have threatened to re-write the textbooks. Does it herald a new era of understanding, or does it simply mean that everything we know about the universe is wrong?

The oceans define the earth. They are crucial to life and we used to think that they were unique to our blue planet. But we were wrong.

This is the story of an extraordinary scientific adventure - the attempt to control gravity. For centuries, the precise workings of gravity have confounded the greatest scientific minds - from Newton to Faraday and Einstein - and the idea of controlling gravity has been seen as little more than a fanciful dream. Yet in the mid 1990s, UK defence manufacturer BAE Systems began a ground-breaking project code-named Greenglow, which set about turning science fiction into reality.

For centuries we have dreamt of reaching the centre of the Earth. Now scientists are uncovering a bizarre and alien world that lies 4,000 miles beneath our feet, unlike anything we know on the surface

Examining Stephen Hawking\'s most controversial theory and possibly his greatest mistake.

Horizon goes behind the scenes at CERN to follow one of the most epic and expensive scientific quests of all time: the search for the Higgs particle, believed to give mass to everything in our universe.

Horizon looks at dark energy - the mysterious force that is unexpectedly causing the universe's expansion to speed up.

Mathematicians have discovered there are infinitely many infinities, each one infinitely bigger than the last. And if the universe goes on forever, the consequences are even more bizarre. In an infinite universe, there are infinitely many copies of the Earth and infinitely many copies of you. Older than time, bigger than the universe and stranger than fiction. This is the story of infinity.

Horizon takes the ultimate trip into the unknown, to explore a dizzying world of cosmic bounces, rips and multiple universes, and finds out what happened before the big bang.

Comedian Ben Miller returns to his roots as a physicist to try to answer a deceptively simple question: what is one degree of temperature?

There is a strange and mysterious world that surrounds us, a world largely hidden from our senses. The quest to explain the true nature of reality is one of the great scientific detective stories.

Particle physicist and ex D:Ream keyboard player Dr Brian Cox wants to know why the Universe is built the way it is. He believes the answers lie in the force of gravity

Physicist Dr Helen Czerski and meteorologist John Hammond make sense of Britain's recent extreme weather and discover that there is one thing that connects all our recent extreme winters.

Until very recently the whole idea of the multiverse was dismissed as fantasy. Horizon asks whether multiple universes exist and, if so, which one we are actually in.

Black holes are one of the most destructive forces in the universe, capable of tearing a planet apart and swallowing an entire star. Yet scientists now believe they could hold the key to answering the ultimate question - what was there before the Big Bang?

Professor Jim Al Khalili delves into over 50 years of the BBC science archive to tell the story behind the emergence of one of the greatest theories of modern science, the Big Bang.

Professor Iain Stewart explores the story of our turbulent relationship with the wind.

Iain explores man\'s relationship with fire. He begins by embarking on an extraordinary encounter with this terrifying force of nature - a walk right through the heart of a raging fire.

The secret alchemy of material science revealed - how we transform materials, and materials transform the world

Professor Mark Miodownik traces the story of ceramics.

Professor Mark Miodownik travels to Israel to trace the history of our love affair with gleaming, lustrous metal.

Professor Mark Miodownik tells the story of plastics - created in the lab, they have brought luxury to the masses and shaped the modern age.

In this series Professor Iain Stewart tells a stunning new story about our planet. He reveals how the greatest changes to the Earth have been driven, above all, by plants.

In the second episode, Iain discovers how flowers have transformed our planet. He journeys to the remote islands of the South Pacific to track down the earliest flowers.