Hubble volume

[ensonik]

I've heard this term, but am unclear on it's exact definition. I think it implies that the entire universe as we know it is just one "volume" inside of a larger universe. Is that an even remotely accurate definition?

[J.C.MacSwell]

I've heard this term, but am unclear on it's exact definition. I think it implies that the entire universe as we know it is just one "volume" inside of a larger universe. Is that an even remotely accurate definition?

 

Not sure but a thought came to mind when I read the title:

 

If the Hubble Constant was really constant then the Hubble "increase in volume rate" would be increasing with time.

 

Seems obvious now that I've typed it.

[Martin]

I've heard this term, but am unclear on it's exact definition. I think it implies that the entire universe as we know it is just one "volume" inside of a larger universe. Is that an even remotely accurate definition?

 

 

Hubble volume is not the same as the volume of the observable universe.

We are currently observing galaxies which are outside the Hubble volume.

 

the Hubble volume is just a convenient volume (which is handy for astronomers to use) based on the HUBBLE LENGTH.

 

do you know what the Hubble parameter is?

 

the currently measured value is H = 71 km/s per Megaparsec

 

it is not constant, it changes with time. often people write H_o for it to make clear that they mean the present value. that means

H_present

 

the Hubble LENGTH or Hubble RADIUS (same thing) is just a length that is easy to calculate from the H_o

you just take the speed of light and divide by H_o

 

R_hubble = c/H_o = 13.8 billion LY

 

it happens that it looks confusingly like the age of the universe but that is mostly coincidence. the age of the universe is estimated more like 13.7 billion years.

the Hubble radius is NOT the outer bound of what we can see. It is routine to observe and study light or microwave from things that are farther than that.

 

but there is a popular misconception that the hubble radius is like "radius of observable universe", watch out for confused people who think that.

 

Now you asked what is Hubble VOLUME, it can be the volume of a sphere with R_hubble as its radius. Or it can be the volume of a cube with R_hubble as side. I have seen it both ways.

 

If you want an exact number think of a cube and think

(13.8 billion)³ cubic light years.

 

the actual observable universe is bigger but that is a convenient chunk and people do statistics studies, galaxy counting studies, surveys etc, estimating what that big a chunk contains.

 

a lot of cubic lightyears though, isnt it?

[ensonik]

Martin, thanks for clearing that up. I guess I held a misconception about the term. So a Hubble volume is essentially a measured distance, and we have the capability to look beyond it. So how far past one H-volume can we see? Into a second or third, or farther?

[Martin]

Martin, thanks for clearing that up. I guess I held a misconception about the term. So a Hubble volume is essentially a measured distance, and we have the capability to look beyond it. So how far past one H-volume can we see? Into a second or third, or farther?

 

the Cosmic Microwave Background is at redshift z = 1100

 

when we observe the CMB we are looking at the so-called "surface of last scattering" which is several Hubble radiuses distant from us.

there is a calculator online to convert z=1100 into a distance in lightyears.

Let's use it Go here:

http://www.scienceforums.net/forums/showthread.php?t=4133

 

Scroll down to post #5 and you will see a couple of calculators that convert redshift into distance.

for example let's use the Morgan one:

http://www.scienceforums.net/forums/showthread.php?t=4133

 

put in Omega = .27 and Lambda = 0.73 and H = 71 (standard parameters)

and put in z = 1100 and press calculate and you get 45.5 billion lightyears.

 

that is the distance of the "surface of last scattering" which is where the CMB photons are coming to us from.

 

Mapping the CMB is routine, it is constantly being observed by satellites such as WMAP. So we can say that it is routine to observe radiation from stuff which is currently 45.5 billion lightyears distant from us.

 

this is over 3 times the hubble radius. (that was what you were asking, how many times hubble radius can we observe)

 

further is theoretically possible with neutrino astronomy but not yet accomplished

[Guest samvado]

When googleing "hubble radius" definition I came across this brilliant thread-

As a none-astronomer (biologist actually) I do have a question: with an estimated age of 13.8 BY how can we observe anything farther than 13.8 LY?

[swansont]

When googleing "hubble radius" definition I came across this brilliant thread-

As a none-astronomer (biologist actually) I do have a question: with an estimated age of 13.8 BY how can we observe anything farther than 13.8 LY?

 

Because the space has expanded in the interim, and the light we see is 13.8 BY old. The expansion of space is not constrained to obey SR.

[Guest samvado]

SR = special relativity? it is not? wonder over wonders .. , I would have bet !

[swansont]

SR = special relativity? it is not? wonder over wonders .. , I would have bet !

 

Yes, special relativity. The equations of SR relate to objects in space, under specific conditions, not space itself.