Help me understand Hubble constant plz

[Cmac22]

so i was going over the Lineweaver and Davis' Scientific American article Misconceptions about the big bang and there were a few parts i could use help understanding. specifically having to do with the hubble constant. i dont understand how it changes.

the article says:

"In particular, the constant is proportional to the rate of

increase in the distance between two galaxies, divided by that

distance."

 

im confused at what this means. in an example, if something beyond the hubble distance sent light towards earth, the hubble constant changes? and so we eventually can see the light?

 

basically how does the constant expand?

 

also later in the article it says:

"If light from galaxies receding faster than light

is to reach us, the Hubble distance has to increase, but in an

accelerating universe, it stops increasing. Distant events may

send out light beams aimed in our direction, but this light is

trapped beyond the Hubble distance by the acceleration of the

expansion."

 

so...now the hubble distance doesnt increase anymore but it used to?

 

one more question from the article:

"This reasoning changes if acceleration is not constant, as

some cosmologists have speculated. If the acceleration itself

increased, it could eventually grow strong enough to tear

apart all structures, leading to a “big rip.” But this rip would

occur not because of expansion or acceleration per se but because

of an accelerating acceleration."

 

is there any newer info about this. is the acceleration accelerating? i remember watchin a show that talked about the big rip as the end of the universe but i dunno how accurate that is

 

comments are always appreciated

[Martin]

this is a good post full of clear questions. I have been away. sorry I missed it. I wish someone else had responded.

...

"In particular, the constant is proportional to the rate of

increase in the distance between two galaxies, divided by that

distance."

 

im confused at what this means. in an example, if something beyond the hubble distance sent light towards earth, the hubble constant changes? and so we eventually can see the light?

 

basically how does the constant expand?

 

In usual cosmology the Hubble constant (sometimes called Hubble parameter) has always been getting smaller and will continue doing so more and more slowly

The Hubble distance is a reciprocal----namely c/H---so it is always getting larger. But more and more slowly so that it approaches an upper limit.

 

The Hubble distance is however long a distance has to be in order for it to be be increasing at the speed c. Distances larger than that increase faster than c. Distances smaller, unless some forces interfere, increase proportionally slower than c.

 

H changes with time according to a simple differential equation, which is the main hangup to understanding. Getting from H to the distance c/H is easy algebra. We should at least be able to work thru the algebra, if you want.

 

 

 

 

also later in the article it says:

"If light from galaxies receding faster than light

is to reach us, the Hubble distance has to increase, but in an

accelerating universe, it stops increasing. Distant events may

send out light beams aimed in our direction, but this light is

trapped beyond the Hubble distance by the acceleration of the

expansion."

 

so...now the hubble distance doesnt increase anymore but it used to?

 

Lineweaver's language is imprecise, it doesn't stop increasing. it has an asymptote, a limit, that it cannot get past. As I recall it is something like 16 billion LY. the distance c/H is now about 14 billion LY and it will keep on increasing but never exceed 16 billion. It maxes out so to speak.

 

It is a simple beautiful curve and frustrating not to be able to explain properly. If you look at the differential equation you see that whatever the fraction of dark energy is now----say 0.75-----the ultimate limit on the Hubble distance is 14 billion LY divided by the square root of that

 

so you can type this into google " 14/sqrt(.75) " and it will tell you

EDIT yeah I typed that into the Google search window and it's built in calculator told me that it is about 16, that is 16 billion LY.

 

one more question from the article:

"This reasoning changes if acceleration is not constant, as

some cosmologists have speculated. If the acceleration itself

increased, it could eventually grow strong enough to tear

apart all structures, leading to a “big rip.” But this rip would

occur not because of expansion or acceleration per se but because

of an accelerating acceleration."

 

is there any newer info about this. is the acceleration accelerating? i remember watchin a show that talked about the big rip as the end of the universe but i dunno how accurate that is

 

back almost 10 years ago people dreamed up that BigRip nightmare which depends on accelerating acceleration. It does not have much currency now. Because for 10 years people have been trying to detect anything not constant about the cosmological constant (aka dark energy) and the more data they get and the more accurately they measure the more it really seems constant.

so fewer and fewer people worry about a changing cosmological constant or dark energy, and they dont talk about BigRip so much.

 

but they keep measuring and trying to pin down the quantities in the model more and more precisely and maybe some day they will find evidence of variation. anything is possible.

 

Hey Cmac you probably know the Hubble parameter is estimated to be 71 km/s/Mpc so why don't you use Google to calculate it's reciprocal!

Just type " 1/(71 km/s/Mpc) " into the search box and press search!

 

the built in calculator knows that km/s means kilometer per second and that Mpc means megaparsec, so it will tell you something like

13.77 billion years. I was using 14 as an approx.

 

So if 1/H is 14 billion years, and you multiply that time by c, you get that c/H is 14 billion light years. so you can calculate for yourself what the Hubble distance is. It is better. You are in control.

[Cmac22]

Thanks a lot Martin. I understand it much better. i appreciate the time you spent to reply and the clarity of your post!