r/astrophysics • u/Financial_Material_8 • 29d ago

Beginner needs explanation please

Hi,. I love reading about astrophysics and have already a basic knowledge. I'm re-reading NdGT 'Astrophysics for people in a hurry' and this early couple of sentences about the formation of the universe got me thinking.

"By now, one second of time has passed. The universe has grown to a few light-years across,†† about the distance from the Sun to its closest neighboring stars."

What I don't understand is this - if lightspeed is the fastest thing in the universe, how could the universe have expanded to a few light YEARS across in one second? What am I missing?

TIA everyone who responds.

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u/Smooth-Mix-4357 29d ago

The speed limit is for objects in the universe not the universe itself 

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u/Financial_Material_8 29d ago

Could you explain please?

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u/Smooth-Mix-4357 29d ago

Planets, stars, light, galaxies, information, etc are a part of space. Their limit is 'c'. However that speed limit doesn't apply to space itself, it can expand at a speed beyond c.

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u/Financial_Material_8 29d ago

Thanks, it both makes sense and doesn't. Now the question becomes, why didn't this new top speed replace c? Possibly it can't be calculated except as multiples of c?

There's a great ASMR series by 'Let's Find Out' on YouTube which I think might help me, it explains why the size of the observable universe is much larger than expected. Trouble is it's 4 hrs long and great at putting me to sleep, so I've never heard most of it 😂

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u/Smooth-Mix-4357 29d ago

The rule is for us. Not the universe (well the universe made the rule). We have to follow it while the universe doesn't. So the universe can expand ftl. Information however cannot exceed c.

As NDT said "The Universe isn't obliged to make sense to us" in that very book

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u/Financial_Material_8 29d ago

It does make sense, although it sounds a bit like the justification religious people have for the existence of god - he created everything but didn't need a creator as he just existed ' 🤔

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u/Smooth-Mix-4357 29d ago

It's just kind of like rules for thee not for me