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As we age our immune systems do too, making us less able to fight infections and more prone to chronic inflammation. But a team of scientists has been able to reverse these effects in mice, rejuvenating their immune systems by targeting their stem cells. But there’s a long road to trying the same thing in humans.

Have you seen the incredible new black hole image? Just a couple of years since the Event Horizon Telescope’s first, fuzzy image of Sagittarius A* – the black hole at the centre of our galaxy – a new picture offers a closer look. The stunning image released this week features the spiralling lines of Sgr A*’s magnetic field, which is seeding new questions about how black holes behave.

Millions of tonnes of unexploded ordnance litter the globe from conflicts both ongoing and long past. And as time passes these bombs are not getting any less dangerous – new research finds some are actually becoming more prone to exploding.

Physicists have theorised that there is a particle called the graviton that carries the force of gravity – much like a photon carries light, or a gluon carries the strong nuclear force. But the graviton has so far remained elusive. Now, researchers think they’ve seen one, or at least a particle with the correct properties to be a graviton. How this experiment unfolded, and why even a possible sighting is exciting to theorists.

Plus: How a bad night’s sleep makes you feel older; why therapy horses get stressed when they don’t have a choice; and a robot that can design, build and test paper planes.

Hosts Christie Taylor and Sophie Bushwick discuss with guests Grace Wade, Alex Wilkins, Michael Le Page and Karmela Padavic-Callaghan. To read more about these stories, visit newscientist.com.

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Weekly: Immune system treatment makes old mice seem young again; new black hole image; unexploded bombs are becoming more dangerous

The World, the Universe and Us