Volume of Universe

Kevster

Is this a fair estimate of the volume of the Universe (assuming the Universe is a sphere):

Current width (diameter)
= ~150 billion trillion miles
= ~241.4 billion trillion kilometers
= ~2.414 x 10^23 km

Radius
= diameter / 2
= ~1.207 x 10^23 km

Volume
= 4/3 π r^3
= 7.362 x 10^69 km^3

???

I did say 'rough' estimate, remember.

Dr Pepper

In the words of Ultravox, "It means nothing to me..". I can't even get my head around the size of the Earth most of the time. And I often wonder if anyone can meaningfully understand/visualise the distance even to our nearest neighbouring star (approx 4.4 light years), never mind to distant galaxies.

I mean, I can say it and I know what a "light year" is and 4.4 isn't all that big of a number, but I can't picture it in the same way that I know how far away the local shops are!

/edit - It's actually only 4.2 ly away (39,900,000,000,000 km)

Kevster

Dr Pepper wrote:

It's actually only 4.2 ly away (39,900,000,000,000 km)

Haha! ... ...just at trip 'round the corner to the local butcher's eh! The figure I arrived at is most likely well-off but I love dealing in huge numbers that are impossible to comprehend. You're right; it's even difficult to comprehend the size of the Earth at times; and yet we are - astronoimically-speaking - puny.

MooseJam

Kevster wrote:

Volume
= 4/3 π r^3
= 7.362 x 10^69 km^3

now all you have to do is work out how many smarties it would take to fill

Kevster

Would they be melted or left as they are? If they are left as they are, then a lot of space would be left unfilled.

The Hill Billy

Kevster wrote:

If they are left as they are, then a lot of space would be left unfilled.

Sounds like just what we have at the moment.

I can see the headlines now - "Universe made of Sugar-Coated Chocolate!" Articles to include lots of gags about other confectionary products (Galaxy, Mars, Milky Way, etc).

im_invisible

Kevster wrote:

Is this a fair estimate of the volume of the Universe

im not sure, at all, about this, but

theres 9.461 *10^12 km in a light year,
universe is 13.75 billion years old,
that gives a radius of 1.3 (to your 1.2) *10^23 km,

this is where i get confused, a bit
because we have to take time into account, we cant just treat it as a 3 dimensional sphere, but as a 4 dimensional spacetime thingy,

the easiest way to understand is to take it down a dimension, so say we were living in a 2 dimensional space (say, the surface of a ball), with a third dimension, time.
the universe started out as a point, a fully deflated ball, as time goes on, the universe 'inflates', the surface of the ball expands, but we're still only in 2 dimensional space, but no matter which way we look, into our 2 dimensiopnal universe, we are looking back in time, into the center of the ball. (3D spacetime)(see attached pic, the bit on the left)
but the area of our 2D universe is only the surface area of the ball

now take it up a dimension, we calculated the radius in 4D spacetime, so we should calculate the 'surface volume'*, or 'hyper-area' of the 4D sphere.
(check 3-sphere on wikipedia)

*not a technical term, just something i made up, till i found that it was called 'hyperarea' on wikipedia, still not sure if the two are the same??
but anyway, the hyperarea, or three dimensional volume of a four dimensional sphere, is 2 pi^2 r^3, which is bigger than the volume of a 3D ball by a factor of 3/2 pi. (or, about 4.7 times bigger)

and, that would give a volume of
4.337 *10^70 km^3

now, heres where i get confused
i cant even get it out in words

we calculated the radius, looking back to the begining of the universe, but we presumed we looked straight back, but we arnt looking straight back in time, we are looking straight ahead in space, and the fact that light has a finite speed, means we are looking back in time aswell.
i suppose just take a look at the attached pic, if you look straight ahead, in any direction, you will eventually see the start of the universe, but is there any way to know what path you took?

Son Goku

Whoa!, you're quick! No there is no way to know what path we took and that's a very important concept in modern cosmology. Fair play for figuring it out yourself.

Able

How did you come up with this estimation?? Hubble's law maybe?

im_invisible

Able wrote:

How did you come up with this estimation?? Hubble's law maybe?

funny, im only reading about that on wiki now.

if youre asking about kevs estimation of the diameter, i presume it was worked out the way i worked out the radius, if you mean the age of the universe, got it off the internet

still a bit confused about the whole thing, the further away the galaxies are, the faster they are travelling? so they were moving faster the further back in time you go? so that means they are slowing down, right? untill someons discovers that they are actually speeding up...
id love to understand the whole thing, im actually doing astrophysics, but seems i failed most of my first year exams, so...
(damn calculus, trigonometery, cant even spell them, nevermind anything else)

Seanie M

im_invisible wrote:

im not sure, at all, about this, but

theres 9.461 *10^12 km in a light year,
universe is 13.75 billion years old,
that gives a radius of 1.3 (to your 1.2) *10^23 km,

That radius is assuming Earth is the centre of the Universe, which we probably are not.

Kevster

I got those figures from Astronomy magazine.