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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
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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). Version: 6.0.705 / Virus Database: 461 - Release Date: 12/06/2004 |
#3
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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). Version: 6.0.705 / Virus Database: 461 - Release Date: 12/06/2004 |
#4
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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
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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
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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. |
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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. |
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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
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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
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"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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