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The beginning and End of the Universe

In this two-part series, Prof Jim Al-Khalili tackles the biggest subject of all: the universe. 

About the programme

In this 2-part series, Prof Jim Al-Khalili tackles the biggest subject of all: the universe.

Through a series of critical observations and experiments that revolutionised our understanding of our world, Jim guides us through the greatest cosmic detective story of all. Taking us from the beginning of the universe to the end of time, Jim answers the question: where did the universe come from and how will it end?

His story is rooted in the 20th century, of the men and women instrumental in uncovering this cosmic mystery. It is a story of maverick triumph with huge leaps of imagination and serendipity. Of human strength and fallibility, determination in the face of adversity, belligerence in the face of evidence. But ultimately it is a tale of the triumph of the scientific method: of experimentation, prediction and observation.

Jim takes us to Atlantic Island mountaintops, home to the largest telescope in the world, where he gazes into the furthest reaches of space. He takes us deep underground where he witnesses an experiment to recreate a moment less than a millionth of a second after the big bang itself. Jim reveals what has been called humankind's greatest achievement: understanding how the universe began and how it might end

Discover the range of qualifications and modules from the OU related to this programme:

Galaxy image

What's space expanding into?

You can try imagining curved four-dimensional spacetime, but it might be a bit easier to start with some ants...

I often get asked this question, but it's hard to answer because our brains aren't well-equipped for imagining more than three dimensions. 

NASA Gravity Probe

Copyright: NASA

Sometimes, people imagine the expanding Universe as the surface of a balloon. Galaxies and stars are stuck to the surface while the balloon expands. The surface of the balloon is curved, and we can tell it's curved because we're looking at it from outside, in our three-dimensional space.

So if our Universe is expanding, does that mean there's some higher dimension it's expanding into? 

This is where things get weird. Imagine a line of army ants on the surface of the balloon. Army ants march in straight lines. They try to do this on the surface of the balloon, but the surface is curved, so the line wraps its way all over the surface.

The balloon could even be crinkly in crazy ways so the line of ants even crosses itself in places. The ants don't understand this because they are so tiny that they don't know the surface is curved - like flat-Earthers they think their land is flat.

It turns out that the only thing you need to completely describe the curviness of space in relativity is these army-ant lines. Sure, you could describe the curvature of the balloon in our three-dimensional space, but the army ants would do a perfectly good job just looking at the lines they made. 

This is handy for us, in our expanding Universe, because we only need measurements inside our own space to measure how it curves. We don't need some gigantic cosmic surveyor living in a higher dimension to do it for us. The mathematical equations that describe the curvature only use measurements we make ourselves.

But then we hit a philosophical problem. If we can measure and define everything we need from inside our curvy space, there is no need for it to live in a higher-dimensional space. Maybe there is a higher dimension out there, maybe there isn't. But there is no need to suppose it's there. 


Now, I've been talking about 'space' this whole time, but in Einstein's relativity, time and space aren't separate. Instead of a three-dimensional space and a single time dimension, we have a four-dimensional composite called spacetime. Personally, I find three dimensions hard enough to imagine, and a curved three-dimensional space is next to impossible. A four-dimensional spacetime is pretty much impossible for me, except in a few limited examples, and as for curved spacetime - no way. Human brains haven't evolved the cognitive machinery for doing this. I genuinely believe that physics makes the hardest demands on a person's imagination of any subject. 

It turns out that the way that curvy space gets described can also be used for spacetime. When you do this though, you find you can also describe the expansion or contraction of the Universe. The expansion or contraction is a sort of 'curvature', but it involves time as well. 

And then we hit the same philosophical problem. Like the army ants on the balloon, we can define and completely describe the expansion of the Universe using only measurements from inside our Universe. We don't need some great cosmic giant standing outside to tell us how much the Universe is expanding. Maybe there is a higher dimension out there that our Universe is expanding into, or maybe there isn't. The point is, we aren't required to suppose it's out there. 

This is very hard for our brains to imagine: a surface can be curved, but not embedded in a higher-dimensional flat space. And the best answer I can give to "what is the Universe expanding into?" is that our Universe isn't necessarily expanding into anything - it's just expanding.


Person holding tape measure

If the Universe is expanding, why aren't I getting taller?

Stephen Serjeant looks at cosmic expansion and explains why other factors mean we don't grow with the Universe.

Let's just say that for me this is... a sensitive issue.

But if the expanding Universe stretched my height, it would stretch everyone else's too, so it's not much help. In fact, we'd never know. We can only tell the entire Universe is expanding because some things inside it aren't. You could even think of the wider Universe staying the same and those things inside shrinking—thanks a bunch, universe!—but because it's much harder to explain why lots of different objects have the same shrinkage, it makes a lot more sense to think of space expanding. 


The expanding Universe sounds big, but, from a human perspective, it's very, very slow. If I got taller as the Universe expanded, it'd only amount to the size of a handful of atoms in my whole life. But that's not why I don't grow as the Universe expands. It's because this effect is so weak it's easily overwhelmed by other things that happen to me. I don't just mean like eating broccoli (if you believe my grandmother), but just the fact that my body is held together by the electrical attractions between atoms and molecules. That on its own is enough to stop the cosmic expansion stretching me. 


And it doesn't have to be electrical forces that can frustrate hopes of a cosmic stretch. Even things floating freely in space, like the Earth and the Sun, can have a gravitational attraction that easily overwhelms the effect of cosmic expansion. 

But some things can still get stretched. A light wave travelling across the Universe will get stretched as the Universe expands.

The wavelength gets longer, which means the light gets redder, and that's why distant galaxies look red-shifted.

These red-shifted galaxies are one way that we know the Universe has had a growth spurt, even if I haven't.


Meet the OU academic

Professor Stephen SerjeantProfessor of AstronomyVIEW FULL PROFILE
Professor Stephen SerjeantProfessor of Astronomy

Professor Stephen Serjeant is Professor of Astronomy, specialising in using ground-based and space telescopes to find warps in space and time (called "gravitational lenses") and to work out how galaxies formed their stars early in the Universe. He works in particular in infrared astronomy, delving into the "dark sides" of galaxies that are hidden to optical telescopes. Stephen explores giant data sets with artificial intelligence, as well as with the help of citizen scientists.

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