# Hubble volume

What is a hubble volume?

## Answers and Replies

I would assume it's the volume of the part of the universe that could possibly have affected us -- roughly, if the unverse is ~15 billion years old, then only stuff within ~15 billion light-years is relevant to us.

That's just a guess, though.

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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.

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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 rhocrit
which is the energy density required for flatness
And for some decades cosmologists have been making a big
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, rhocrit is simple equal to
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 rhocrit is simple equal to
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?

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

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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 H0

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".

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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 H0 is 71 km/sec per megaparsec then
Hubble time = 1/H0 = 13.7 billion years

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 tH is 13.7 billion years
the length time c tH is 13.7 billion light years.

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!

H0 is DIRECTLY MEASUREABLE FROM OBSERVATIONS

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 H0 parameter has an experimental meaning that allows it to be measured from observations (thank goodness!) so it is a rock solid thing. By the way it is not a constant!!!! People call it "hubble constant" but it is only valid for the present moment, it changes thru time---the terminology is deceptive but that's just how the real world is: technical language often is misleading. But at least the thing is a well defined measurable thing.

And so 1/H0 is also a measureable thing! A time quantity. One can say that it is 13.7 billion years (thanks be to Allah the allmerciful) regardless of what you think the age of the universe is and regardless of what model you use to estimate the age of the universe etc.

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.

I still am wondering where you encountered it.

71 km/sec per megaparsec then
Hubble time = 1/H0 = 13.7 billion years

Last edited:
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 10118 meters away, there should be a Hubble volume identical to ours."
"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 108 Kelvins. One way to do the calculation is to ask how many photons could be packed into a Hubble volume at that temprature. The answer is 10118 protons."

That's just a miniscule excerpt.

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

It's roughly the size of the observable universe, not the whole thing, which is likely (and that SciAm article assumes) infinite. Ignoring various GR and inflationary complications (which I don't understand anyways, maybe marcus does) the universe has only been around for ~14 billion years, so the furthest thing that we could observe, or that could have affected us, is 14 billion light-years away. So the volume of the currently observable universe (from our standpoint) is ~4/3 * pi * (14 billion ly)^3 ~ 10^31 cubic light-years.