- #1

RuroumiKenshin

What is a hubble volume?

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- #1

RuroumiKenshin

What is a hubble volume?

- #2

damgo

That's just a guess, though.

- #3

marcus

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Originally posted by MajinVegeta

What is a hubble volume?

Isnt it true, Majin, that "Hubble time" is simply the

reciprocal of the Hubble parameter?

This is established mainstream terminology. Values like 13.7 billion years are given for the Hubble time, corresponding to

recent figures like 71 km/s per megaparsec measured for

the parameter itself.

The "Hubble length" is simply the length that corresponds to

the Hubble time. If the time is 13.7 billion years then the

length is 13.7 billion light years.

Anytime you have a length there is a corresponding area and a volume.

the real thing you might wish to work thru is the first and most basic, namely the Hubble time.

the dimensions of the Hubble parameter are

dist/time per distance and that boils down to 1/time

so instead of saying 71 km/sec per megaparsec you can just as well say what is absolutely equivalent to it-----1/13.7 billion years.

- #4

marcus

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Originally posted by MajinVegeta

What is a hubble volume?

Darnit Majin, I am curious.

Where did you come across the term "hubble volume"?

I hope somewhere on line that I can go to and check it out.

Also you and damgo should know something very cool about

the hubble area

If you do not know this already please let me have the privilege

of telling you, it is very neat.

There is this energy density called rho

which is the energy density required for flatness

And for some decades cosmologists have been making a big

noise about this rho

asking if the actual density of the universe is less than or equal to or greater than it----or if omega is less than equal to or greater than one, omega being the ratio of the real rho to the critical rho and so on. So this rho is kind of a celebrity.

In natural units, rho

the numerical constant (3/8pi) times the reciprocal of

the hubble area

that makes it sooooo easy to calculate

and if you want it in ordinary units there is a universal force constant F = 12E43 newtons such that rho

the numerical constant (3/8pi) times

F/hubble area

Note that F/hubble area is a pressure and therefore

(since the two are the same type of quantity) an energy density.

and in fact F/hubble area gives you the energy density you want,

once it is multiplied by that numerical constant which is about 1/8.

So that is something good about Hubble area.

What is good about Hubble volume?

- #5

Alexander

May be Hubble volume and area are volume and area of visible universe?

- #6

marcus

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Originally posted by Alexander

May be Hubble volume and area are volume and area of visible universe?

this is unlikely

I assume you know that the "Hubble time" is definitely not the same concept as the age of the universe.

"Hubble time" is a mainstream concept meaning the reciprocal

of the Hubble parameter H

under various assumptions it can be close to the estimated age of the universe, but it does not logically have to be so

I cannot imagine why an astronomer would refer to the volume of the observable universe as "hubble volume".

And what could the "area of the visible universe" possibly mean?

We should do a google search and see how these terms are being used!

Starting with "hubble time".

- #7

marcus

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Originally posted by marcus

We should do a google search and see how these terms are being used!

Starting with "hubble time".

http://www.astro.virginia.edu/~jh8h/glossary/hubbletime.htm

I just did a google search on "Hubble time" and confirmed that indeed it is the "inverse" i.e. the reciprocal of the hubble parameter.

if H

Hubble time = 1/H

I just did a google search on "Hubble length" and confirmed

that it is c multiplied by the "Hubble time"

http://www.astro.virginia.edu/~jh8h/glossary/hubblelength.htm

In other words since the time t

the length time c t

THIS IS CERTAINLY NOT EQUAL TO THE RADIUS OF THE OBSERVABLE UNIVERSE which in comoving distance terms is more like three times that. For goodness sake Alexander check out

the University of Virginia Astro department site and get it straight.

Hubble length is 13.7 billion light years and this is not to be confused with the radius of the observable universe!

Majin! here is something for you to think about!

H

this is terribly important, please forgive the caps Majin

you are a young fellow and maybe dont realize how important this is.

the age of the universe is not a measureable quantity. One can only INFER it using some theoretical model or other

but the H

And so 1/H

And the same with the Hubble length, which is c times the time.

(it is a very useful length, but it is not the radius of the observable universe, Ned Wright's cosmology tutorial, the distance scale in particular, should make that clear enough!)

Have fun.

Good question, asking about Hubble volume.

I still am wondering where you encountered it.

71 km/sec per megaparsec then

Hubble time = 1/H

Last edited:

- #8

RuroumiKenshin

Darnit Majin, I am curious.

Where did you come across the term "hubble volume"?

I hope somewhere on line that I can go to and check it out.

Marcus:

I read about it in the Scientific American article "Parallel Universes".

Here's how its used: "About 10 to the 10

"These are extremely conservative estimates, derived simply by counting all possible quantum states the a Hubble volume can have if it is no hotter than 10

That's just a miniscule excerpt.

So, is one Hubble volume about the size of the universe?

- #9

damgo

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