# If nothing can travel faster than light, how can we say the universe is so big ?

How can we be sure of its size, since any info we have sent could only have traveled

a few hundred light years by now?

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• 1 month ago

>>If nothing can travel faster than light, how can we say the universe is so big ?

We don't actually know how big the *entire* universe is; we can really only say the visible universe appears to be about 90 billion light years in diameter.  That figure is arrived at by viewing red shift, primarily... the edge of the visible universe is about 46 billion light years away - and, there may actually be more *beyond* that which we can see.

>>How can we be sure of its size, since any info we have sent could

>>only have traveled a few hundred light years by now?

It's true that our man-made radio signals from Earth have only traveled about 100 or 120 light years so far;  but, we don't use *our* signals to see with... we look at incoming electromagnetic radiation, and analyze the data.  Red shift tells us how an object is; we know the frequency at which hydrogen absorbs and emits light; when we look for that in far-off objects (like quasars), we see how *fast* they are receding from us - and, using that speed, we can determine approximately how far away they must be.

• jehen
Lv 7
1 month ago

Well the light from the far reaches of the universe have been traveling to us for billions of years.  We know how far away those objects are by inference through the observed universe and the laws of physics.  So no, we did not send out a measuring tape or radar signal to find out how far away those things are.  We calculate the distance based on what we can observe (e.g. red-shift) and what we know about the expansion of space (from measurable effects of dark energy).  Think of it like this, you can look at a person, any stranger and closely approximate their age, without knowing their birthdate or having a calendar at hand.  You can do this because you are very familiar with the passage of time, and how others look, move and dress among the people you do know of different ages.  You can do this with little to no effort or thought.  Now imagine scientists, who have been studying the intricacies of the cosmos with instruments, measurements and experiments for the last 500 years, and how they might be able to use that knowledge to determine the size of the observable universe.

• ?
Lv 7
1 month ago

We dont need to send out infirnation to measurr the size of the universe. We can detect electromagnetic radiation that started its journey billions of years ago, and is only just reaching us

• 1 month ago

I would like to know, how do you get the idea of a few hundred light years? Just right here on Earth, we have written history going back over 5000 years.