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Hubble Constant



 
 
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  #1  
Old June 21st 04, 02:20 PM
beavith
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Default Hubble Constant

in a recent thread, the discussion of what was faster than light
popped up.

i seem to recall a fellow back in the 1960's, i wish i could remember
the guys name, but he rapid sketched (for those of you that remember
how to rapid sketch) the lorentz equation and found several asymptotic
regions where useful values could be arrived that were faster than the
speed of light. not that that means anything in the real world...
its a pretty simple exercise anyway.

ceebee mentioned that the Hubble Constant is figured to be 74km/s/Mpc
where km is, of course, kilometers, s is seconds and Mpc is
megaparsecs.

my old HS physics teacher would always admonish us to "watch the
units." our old grade school math teachers would also tell us to
reduce our fractions.

here's the conundrum: if Mpc is roughly 3.25 million parsecs, and km
is another distance, if you reduce the Hubble constant to its basic
terms, won't you get a number with a unit of km^2/s?

if so, why do we keep the Hubble Constant in such a confusing batch of
units? it'd be like measuring an expanding balloon in X
in/sec/mile...

does this mean that the surface of our universe is growing by this
area every second?


  #2  
Old June 21st 04, 05:46 PM
EvolBob
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The Hubble Constant could just as easily be expressed as.

For each Megaparsec distant, the expansion is 74 kilometres per second.
So a galaxy at 10 Mpc is receding at 740 km/ps.

Pasted from the Internet:
http://csep10.phys.utk.edu/astr162/l...istscales.html
Parsec (pc): 3.26 light years (or 3.086 x 10^18 cm).; also kiloparsec (kpc) = 1000 parsecs and megaparsec (Mpc) = 1,000,000 parsecs.

So one could instead say a galaxy 32,600,000 light years distant is moving away due to the Universes expansion at 740 kilometres per
second!

does this mean that the surface of our universe is growing by this
area every second?


We are at this surface here as are all.
The further we see the faster the expansion. We can see nearly 10 billion light years, so that gives a recession of 226,993.86 kps.
This is a fairly large fraction of light speed.

What is so disturbing for some is this is not the furtherest distance.
Which means a small but growing percentage of the visible Universe will be disappearing at an increasing rate over time.


Regards
Robert

"beavith" wrote in message ...
in a recent thread, the discussion of what was faster than light
popped up.

i seem to recall a fellow back in the 1960's, i wish i could remember
the guys name, but he rapid sketched (for those of you that remember
how to rapid sketch) the lorentz equation and found several asymptotic
regions where useful values could be arrived that were faster than the
speed of light. not that that means anything in the real world...
its a pretty simple exercise anyway.

ceebee mentioned that the Hubble Constant is figured to be 74km/s/Mpc
where km is, of course, kilometers, s is seconds and Mpc is
megaparsecs.

my old HS physics teacher would always admonish us to "watch the
units." our old grade school math teachers would also tell us to
reduce our fractions.

here's the conundrum: if Mpc is roughly 3.25 million parsecs, and km
is another distance, if you reduce the Hubble constant to its basic
terms, won't you get a number with a unit of km^2/s?

if so, why do we keep the Hubble Constant in such a confusing batch of
units? it'd be like measuring an expanding balloon in X
in/sec/mile...

does this mean that the surface of our universe is growing by this
area every second?




---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
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  #3  
Old June 21st 04, 05:46 PM
EvolBob
external usenet poster
 
Posts: n/a
Default

The Hubble Constant could just as easily be expressed as.

For each Megaparsec distant, the expansion is 74 kilometres per second.
So a galaxy at 10 Mpc is receding at 740 km/ps.

Pasted from the Internet:
http://csep10.phys.utk.edu/astr162/l...istscales.html
Parsec (pc): 3.26 light years (or 3.086 x 10^18 cm).; also kiloparsec (kpc) = 1000 parsecs and megaparsec (Mpc) = 1,000,000 parsecs.

So one could instead say a galaxy 32,600,000 light years distant is moving away due to the Universes expansion at 740 kilometres per
second!

does this mean that the surface of our universe is growing by this
area every second?


We are at this surface here as are all.
The further we see the faster the expansion. We can see nearly 10 billion light years, so that gives a recession of 226,993.86 kps.
This is a fairly large fraction of light speed.

What is so disturbing for some is this is not the furtherest distance.
Which means a small but growing percentage of the visible Universe will be disappearing at an increasing rate over time.


Regards
Robert

"beavith" wrote in message ...
in a recent thread, the discussion of what was faster than light
popped up.

i seem to recall a fellow back in the 1960's, i wish i could remember
the guys name, but he rapid sketched (for those of you that remember
how to rapid sketch) the lorentz equation and found several asymptotic
regions where useful values could be arrived that were faster than the
speed of light. not that that means anything in the real world...
its a pretty simple exercise anyway.

ceebee mentioned that the Hubble Constant is figured to be 74km/s/Mpc
where km is, of course, kilometers, s is seconds and Mpc is
megaparsecs.

my old HS physics teacher would always admonish us to "watch the
units." our old grade school math teachers would also tell us to
reduce our fractions.

here's the conundrum: if Mpc is roughly 3.25 million parsecs, and km
is another distance, if you reduce the Hubble constant to its basic
terms, won't you get a number with a unit of km^2/s?

if so, why do we keep the Hubble Constant in such a confusing batch of
units? it'd be like measuring an expanding balloon in X
in/sec/mile...

does this mean that the surface of our universe is growing by this
area every second?




---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.705 / Virus Database: 461 - Release Date: 12/06/2004


  #4  
Old June 22nd 04, 07:32 AM
Odysseus
external usenet poster
 
Posts: n/a
Default

beavith wrote:

[snip]

ceebee mentioned that the Hubble Constant is figured to be 74km/s/Mpc
where km is, of course, kilometers, s is seconds and Mpc is
megaparsecs.

my old HS physics teacher would always admonish us to "watch the
units." our old grade school math teachers would also tell us to
reduce our fractions.

here's the conundrum: if Mpc is roughly 3.25 million parsecs, and km


Light-years, not parsecs.

is another distance, if you reduce the Hubble constant to its basic
terms, won't you get a number with a unit of km^2/s?

Umm, no. Speed, distance-per-time, divided by distance is just
inverse time, here s^-1. Unless I've fouled up my arithmetic H = 74
km/s / Mpc = 2.4 * 10^-18 /s. This is a proportional rate, the
quantity that changes over time being dimensionless -- very like an
interest rate (percentages being dimensionless quantities in disguise).

if so, why do we keep the Hubble Constant in such a confusing batch of
units? it'd be like measuring an expanding balloon in X
in/sec/mile...

Because these units are appropriate to the observations on which the
constant is based: a typical galactic recession velocity can be
written in km/s without using a cumbersome exponential notation, and
likewise for most intergalactic distances when given in Mpc. If you
were working on an inflatable dome covering a large American city
(supposing there were such a thing) you might find measuring its
expansion in (in/sec)/mi to be very convenient! At least it might be
more intuitive for some than the 'cancelled-out' version, 0.0000158/s.

There are plenty of circumstances where 'unreduced' units are used
for practical applications; a few such are quite well established.
Take for example the kilowatt-hour: although megajoules would be a
more 'proper' measure for energy -- 1 kW·h = 3600 kW·s = 3.6 MJ -- it
seems that power companies find the former units more convenient.

does this mean that the surface of our universe is growing by this
area every second?


No, although I think one might say that all sufficiently large
distances grow proportionally by that amount, somewhat under one part
in ten billion per year UIFUMA. The surface area of a given region of
intergalactic space would then increase in a square proportion, and
its volume in a cubic.

--
Odysseus
  #5  
Old June 22nd 04, 07:32 AM
Odysseus
external usenet poster
 
Posts: n/a
Default

beavith wrote:

[snip]

ceebee mentioned that the Hubble Constant is figured to be 74km/s/Mpc
where km is, of course, kilometers, s is seconds and Mpc is
megaparsecs.

my old HS physics teacher would always admonish us to "watch the
units." our old grade school math teachers would also tell us to
reduce our fractions.

here's the conundrum: if Mpc is roughly 3.25 million parsecs, and km


Light-years, not parsecs.

is another distance, if you reduce the Hubble constant to its basic
terms, won't you get a number with a unit of km^2/s?

Umm, no. Speed, distance-per-time, divided by distance is just
inverse time, here s^-1. Unless I've fouled up my arithmetic H = 74
km/s / Mpc = 2.4 * 10^-18 /s. This is a proportional rate, the
quantity that changes over time being dimensionless -- very like an
interest rate (percentages being dimensionless quantities in disguise).

if so, why do we keep the Hubble Constant in such a confusing batch of
units? it'd be like measuring an expanding balloon in X
in/sec/mile...

Because these units are appropriate to the observations on which the
constant is based: a typical galactic recession velocity can be
written in km/s without using a cumbersome exponential notation, and
likewise for most intergalactic distances when given in Mpc. If you
were working on an inflatable dome covering a large American city
(supposing there were such a thing) you might find measuring its
expansion in (in/sec)/mi to be very convenient! At least it might be
more intuitive for some than the 'cancelled-out' version, 0.0000158/s.

There are plenty of circumstances where 'unreduced' units are used
for practical applications; a few such are quite well established.
Take for example the kilowatt-hour: although megajoules would be a
more 'proper' measure for energy -- 1 kW·h = 3600 kW·s = 3.6 MJ -- it
seems that power companies find the former units more convenient.

does this mean that the surface of our universe is growing by this
area every second?


No, although I think one might say that all sufficiently large
distances grow proportionally by that amount, somewhat under one part
in ten billion per year UIFUMA. The surface area of a given region of
intergalactic space would then increase in a square proportion, and
its volume in a cubic.

--
Odysseus
  #6  
Old June 22nd 04, 02:16 PM
beavith
external usenet poster
 
Posts: n/a
Default

On Tue, 22 Jun 2004 06:32:23 GMT, Odysseus
wrote:

beavith wrote:

[snip]

ceebee mentioned that the Hubble Constant is figured to be 74km/s/Mpc
where km is, of course, kilometers, s is seconds and Mpc is
megaparsecs.

my old HS physics teacher would always admonish us to "watch the
units." our old grade school math teachers would also tell us to
reduce our fractions.

here's the conundrum: if Mpc is roughly 3.25 million parsecs, and km


Light-years, not parsecs.


thanks. typo. i was in the groove.

is another distance, if you reduce the Hubble constant to its basic
terms, won't you get a number with a unit of km^2/s?

Umm, no. Speed, distance-per-time, divided by distance is just
inverse time, here s^-1. Unless I've fouled up my arithmetic H = 74
km/s / Mpc = 2.4 * 10^-18 /s. This is a proportional rate, the
quantity that changes over time being dimensionless -- very like an
interest rate (percentages being dimensionless quantities in disguise).


hertz or cycles per sec for example. got it. and what you are saying
is what i had originally thought, too. however, give me a quick
primer in fractions...

does x/yx = x/y/x ?

reducing, i get

1/y and x^2/y



if so, why do we keep the Hubble Constant in such a confusing batch of
units? it'd be like measuring an expanding balloon in X
in/sec/mile...

Because these units are appropriate to the observations on which the
constant is based: a typical galactic recession velocity can be
written in km/s without using a cumbersome exponential notation, and
likewise for most intergalactic distances when given in Mpc. If you
were working on an inflatable dome covering a large American city
(supposing there were such a thing) you might find measuring its
expansion in (in/sec)/mi to be very convenient! At least it might be
more intuitive for some than the 'cancelled-out' version, 0.0000158/s.


i think of an old murphys law book where all units are measured in the
most archaic measures, for example, furlongs per fortnight for
measuring a snail pace.


There are plenty of circumstances where 'unreduced' units are used
for practical applications; a few such are quite well established.
Take for example the kilowatt-hour: although megajoules would be a
more 'proper' measure for energy -- 1 kW·h = 3600 kW·s = 3.6 MJ -- it
seems that power companies find the former units more convenient.


yep. it was easier to proportion a crank back when the meters were
all mechanical. maybe MJ is coming to all our bills eventually.


does this mean that the surface of our universe is growing by this
area every second?


No, although I think one might say that all sufficiently large
distances grow proportionally by that amount, somewhat under one part
in ten billion per year UIFUMA. The surface area of a given region of
intergalactic space would then increase in a square proportion, and
its volume in a cubic.



thanks.
  #7  
Old June 22nd 04, 02:16 PM
beavith
external usenet poster
 
Posts: n/a
Default

On Tue, 22 Jun 2004 06:32:23 GMT, Odysseus
wrote:

beavith wrote:

[snip]

ceebee mentioned that the Hubble Constant is figured to be 74km/s/Mpc
where km is, of course, kilometers, s is seconds and Mpc is
megaparsecs.

my old HS physics teacher would always admonish us to "watch the
units." our old grade school math teachers would also tell us to
reduce our fractions.

here's the conundrum: if Mpc is roughly 3.25 million parsecs, and km


Light-years, not parsecs.


thanks. typo. i was in the groove.

is another distance, if you reduce the Hubble constant to its basic
terms, won't you get a number with a unit of km^2/s?

Umm, no. Speed, distance-per-time, divided by distance is just
inverse time, here s^-1. Unless I've fouled up my arithmetic H = 74
km/s / Mpc = 2.4 * 10^-18 /s. This is a proportional rate, the
quantity that changes over time being dimensionless -- very like an
interest rate (percentages being dimensionless quantities in disguise).


hertz or cycles per sec for example. got it. and what you are saying
is what i had originally thought, too. however, give me a quick
primer in fractions...

does x/yx = x/y/x ?

reducing, i get

1/y and x^2/y



if so, why do we keep the Hubble Constant in such a confusing batch of
units? it'd be like measuring an expanding balloon in X
in/sec/mile...

Because these units are appropriate to the observations on which the
constant is based: a typical galactic recession velocity can be
written in km/s without using a cumbersome exponential notation, and
likewise for most intergalactic distances when given in Mpc. If you
were working on an inflatable dome covering a large American city
(supposing there were such a thing) you might find measuring its
expansion in (in/sec)/mi to be very convenient! At least it might be
more intuitive for some than the 'cancelled-out' version, 0.0000158/s.


i think of an old murphys law book where all units are measured in the
most archaic measures, for example, furlongs per fortnight for
measuring a snail pace.


There are plenty of circumstances where 'unreduced' units are used
for practical applications; a few such are quite well established.
Take for example the kilowatt-hour: although megajoules would be a
more 'proper' measure for energy -- 1 kW·h = 3600 kW·s = 3.6 MJ -- it
seems that power companies find the former units more convenient.


yep. it was easier to proportion a crank back when the meters were
all mechanical. maybe MJ is coming to all our bills eventually.


does this mean that the surface of our universe is growing by this
area every second?


No, although I think one might say that all sufficiently large
distances grow proportionally by that amount, somewhat under one part
in ten billion per year UIFUMA. The surface area of a given region of
intergalactic space would then increase in a square proportion, and
its volume in a cubic.



thanks.
  #8  
Old June 22nd 04, 02:28 PM
Mark Hansen
external usenet poster
 
Posts: n/a
Default

On 6/22/2004 06:16, beavith wrote:

On Tue, 22 Jun 2004 06:32:23 GMT, Odysseus
wrote:

beavith wrote:

[snip]

ceebee mentioned that the Hubble Constant is figured to be 74km/s/Mpc
where km is, of course, kilometers, s is seconds and Mpc is
megaparsecs.

my old HS physics teacher would always admonish us to "watch the
units." our old grade school math teachers would also tell us to
reduce our fractions.

here's the conundrum: if Mpc is roughly 3.25 million parsecs, and km


Light-years, not parsecs.


thanks. typo. i was in the groove.

is another distance, if you reduce the Hubble constant to its basic
terms, won't you get a number with a unit of km^2/s?

Umm, no. Speed, distance-per-time, divided by distance is just
inverse time, here s^-1. Unless I've fouled up my arithmetic H = 74
km/s / Mpc = 2.4 * 10^-18 /s. This is a proportional rate, the
quantity that changes over time being dimensionless -- very like an
interest rate (percentages being dimensionless quantities in disguise).


hertz or cycles per sec for example. got it. and what you are saying
is what i had originally thought, too. however, give me a quick
primer in fractions...

does x/yx = x/y/x ?


x divided by y times x gives you 1/y, as x and 1/x factor each other out.

  #9  
Old June 22nd 04, 02:28 PM
Mark Hansen
external usenet poster
 
Posts: n/a
Default

On 6/22/2004 06:16, beavith wrote:

On Tue, 22 Jun 2004 06:32:23 GMT, Odysseus
wrote:

beavith wrote:

[snip]

ceebee mentioned that the Hubble Constant is figured to be 74km/s/Mpc
where km is, of course, kilometers, s is seconds and Mpc is
megaparsecs.

my old HS physics teacher would always admonish us to "watch the
units." our old grade school math teachers would also tell us to
reduce our fractions.

here's the conundrum: if Mpc is roughly 3.25 million parsecs, and km


Light-years, not parsecs.


thanks. typo. i was in the groove.

is another distance, if you reduce the Hubble constant to its basic
terms, won't you get a number with a unit of km^2/s?

Umm, no. Speed, distance-per-time, divided by distance is just
inverse time, here s^-1. Unless I've fouled up my arithmetic H = 74
km/s / Mpc = 2.4 * 10^-18 /s. This is a proportional rate, the
quantity that changes over time being dimensionless -- very like an
interest rate (percentages being dimensionless quantities in disguise).


hertz or cycles per sec for example. got it. and what you are saying
is what i had originally thought, too. however, give me a quick
primer in fractions...

does x/yx = x/y/x ?


x divided by y times x gives you 1/y, as x and 1/x factor each other out.

  #10  
Old June 22nd 04, 02:33 PM
John Zinni
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Posts: n/a
Default

"beavith" wrote in message
...

however, give me a quick
primer in fractions...

does x/yx = x/y/x ?

reducing, i get

1/y and x^2/y


you should not be getting x^2/y

If we take x/y/x and arrange it like this ...

(x/y)/x

division by x is the same as multiplication by its inverse, so ...

(x/y)/x = x/y * 1/x = 1/y

 




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