Does Hubble's Constant change with distance.

I just had a thought we always relate the hubble's constant with the
age of the universe.
But if it so then the farther we look we must see it change.
Does it really change? Because I don't remember there being any
mention of it's relation with time. It is always stated as a constant.

Now if it is really a constant then we cannot relate it with the age
of the universe or else some other parameter eg light speed must
change to keep it a constant.

Does this make sense? I am no physicist.

-anandsr

It's just like light, light stays the same in space and so does the H.C.

Answer is NO.

--
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brothers of man, who even now fight to survive, somewhere beyond the
heavens.


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wrote in message
oups.com...
I just had a thought we always relate the hubble's constant with the
age of the universe.
But if it so then the farther we look we must see it change.
Does it really change? Because I don't remember there being any
mention of it's relation with time. It is always stated as a constant.

Now if it is really a constant then we cannot relate it with the age
of the universe or else some other parameter eg light speed must
change to keep it a constant.

Does this make sense? I am no physicist.

-anandsr

Starlord wrote:

It's just like light, light stays the same in space and so does the H.C.


Light would never leave space. Too far from home.



Answer is NO.

--
There are those who believe that life here, began out there, far across the
universe, with tribes of humans, who may have been the forefathers of the
Egyptians, or the Toltecs, or the Mayans. Some believe that they may yet be
brothers of man, who even now fight to survive, somewhere beyond the
heavens.

The Lone Sidewalk Astronomer of Rosamond
Telescope Buyers FAQ
http://home.inreach.com/starlord
Sidewalk Astronomy
www.sidewalkastronomy.info
The Church of Eternity
http://home.inreach.com/starlord/church/Eternity.html
AD World
http://www.adworld.netfirms.com/

wrote in message
oups.com...
I just had a thought we always relate the hubble's constant with the
age of the universe.
But if it so then the farther we look we must see it change.
Does it really change? Because I don't remember there being any
mention of it's relation with time. It is always stated as a constant.

Now if it is really a constant then we cannot relate it with the age
of the universe or else some other parameter eg light speed must
change to keep it a constant.

Does this make sense? I am no physicist.

-anandsr

anandsr wrote:
I just had a thought we always relate the hubble's constant with the
age of the universe.
But if it so then the farther we look we must see it change.
Does it really change? Because I don't remember there being any
mention of it's relation with time. It is always stated as a constant.

Now if it is really a constant then we cannot relate it with the age
of the universe or else some other parameter eg light speed must
change to keep it a constant.

Does this make sense? I am no physicist.

Your question does make sense. The Hubble constant does change with
time, but very slowly. If you have not seen any mention of its relation
with time, you haven't looked in the right places--papers that deal with
cosmic evolution over large time scales, billions of years. On those
time scales, the variability of the Hubble "constant" is as fundamental
to the development of the universe as the curvature of the Earth is to
long plane flights. On shorter times scales--say, tens of millions of
years--the Hubble constant is, for most intents and purposes, constant.

I suspect your intuition is correct. As you may have noticed, the
Hubble constant, now estimated to be about 70 km/s per megaparsec, is
essentially in units of s^-1--hertz, in other words. You can think of
it as the instantaneous doubling frequency--how many times the universe
doubles in scale per second. Obviously, this frequency is very small.
In fact, since there are 3.1 x 10^19 km in each megaparsec, the doubling
rate is about 2.3 x 10^-18 Hz--the Hubble constant. Each second, the
universe doubles in size a minucule 2.3 billionths of a billionth times.

But early in the universe's history, it must have been doubling much
faster. Two objects separated by, say, 100 megaparsec are now moving
apart at a relative speed of 7000 km/s. But there must have been a time
when those objects were much closer together--say, just 7000 km. If
they were moving also at 7000 km/s at that time, the Hubble constant
would have been 7000 km/s per 7000 km, or 1 megaparsec/s per megaparsec.
The doubling frequency would have been 1 Hz.

As it happens, the two objects were probably moving apart even faster
than that, so the Hubble constant would have been correspondingly
higher.

However, with the untrue but convenient fiction that the objects were
always moving apart at the same speed, we can interpret the Hubble
constant in yet another way--we can think of it as the inverse, the
reciprocal, of the time since the Big Bang. If you take the reciprocal
of 2.3 x 10^-18 Hz, you get 4.3 x 10^17 seconds, which is about 13.6
billion years. That is indeed the estimated age of the universe.
Again, it's clear that if this is to remain consistent, the Hubble
constant must increase the further back in time you go. About 12.2
billion years ago, for instance, when the universe was a tenth as old as
it is now, the Hubble constant should have been about 10 times greater.

This analysis is simplistic, based on our notion that expansion is
constant. The prevailing model has a varying expansion rate, for
reasons that are beyond the scope of this post (mostly because I don't
know the details!).

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Sam Wormley wrote:
The Hubble Space Telescope's prime mission, was to determine,
once and for all, how fast the universe is expanding. Astronomers
working with the Hubble telescope made lots of pretty pictures of
exotica in space, but more important, they gathered reams and
reams of data on Cepheid variable stars in an attempt to
calculate the Hubble constant. After years of data the answer
from Hubble is 72 km/s/Mpc!

The Wilkinson Microwave Anisotropy Probe (WMAP) team made the
first detailed full-sky map of the oldest light in the universe.
The results from the first year of observing by the WMAP were
announced in February of 2003, including the Hubble constant of
H_o = 71 ±4 km/s/Mpc!

Additional data is showing the the exansion rate was decreasing
in the realy univerese, but is now increasing.

I think you might confuse the Hubble constant with the expansion rate
in the OP's mind. The Hubble constant is sort of like the expansion
rate auto-correlated; that is, expressed as a fraction of the current
size.

By way of analogy, if you have money invested in a fund, the Hubble
constant is like the interest rate, and the expansion rate is like the
annual dollar growth in the fund. Obviously, if the interest rate is
fixed, the annual dollar growth must be going up; on the other hand, if
the annual dollar growth is constant, the interest rate must be going
down.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Thanks for your explanation.

So in effect the Hubble's constant that is quoted is at this point
in time, and is not true at all for very far off objects.

I am also expecting that the difference calculated for the Hubble's
constant
at two points in space would be half of the actual difference in the
two
values of the constant.

My reasoning is that the redshift measured will be the average of the
redshifts
that would be expected at the two epochs.

regards,
-anandsr

On Apr 19, 12:18 pm, (Brian Tung) wrote:
anandsr wrote:
I just had a thought we always relate the hubble's constant with the
age of the universe.
But if it so then the farther we look we must see it change.
Does it really change? Because I don't remember there being any
mention of it's relation with time. It is always stated as a constant.

Now if it is really a constant then we cannot relate it with the age
of the universe or else some other parameter eg light speed must
change to keep it a constant.

Does this make sense? I am no physicist.

Your question does make sense. The Hubble constant does change with
time, but very slowly. If you have not seen any mention of its relation
with time, you haven't looked in the right places--papers that deal with
cosmic evolution over large time scales, billions of years. On those
time scales, the variability of the Hubble "constant" is as fundamental
to the development of the universe as the curvature of the Earth is to
long plane flights. On shorter times scales--say, tens of millions of
years--the Hubble constant is, for most intents and purposes, constant.

I suspect your intuition is correct. As you may have noticed, the
Hubble constant, now estimated to be about 70 km/s per megaparsec, is
essentially in units of s^-1--hertz, in other words. You can think of
it as the instantaneous doubling frequency--how many times the universe
doubles in scale per second. Obviously, this frequency is very small.
In fact, since there are 3.1 x 10^19 km in each megaparsec, the doubling
rate is about 2.3 x 10^-18 Hz--the Hubble constant. Each second, the
universe doubles in size a minucule 2.3 billionths of a billionth times.

But early in the universe's history, it must have been doubling much
faster. Two objects separated by, say, 100 megaparsec are now moving
apart at a relative speed of 7000 km/s. But there must have been a time
when those objects were much closer together--say, just 7000 km. If
they were moving also at 7000 km/s at that time, the Hubble constant
would have been 7000 km/s per 7000 km, or 1 megaparsec/s per megaparsec.
The doubling frequency would have been 1 Hz.

As it happens, the two objects were probably moving apart even faster
than that, so the Hubble constant would have been correspondingly
higher.

However, with the untrue but convenient fiction that the objects were
always moving apart at the same speed, we can interpret the Hubble
constant in yet another way--we can think of it as the inverse, the
reciprocal, of the time since the Big Bang. If you take the reciprocal
of 2.3 x 10^-18 Hz, you get 4.3 x 10^17 seconds, which is about 13.6
billion years. That is indeed the estimated age of the universe.
Again, it's clear that if this is to remain consistent, the Hubble
constant must increase the further back in time you go. About 12.2
billion years ago, for instance, when the universe was a tenth as old as
it is now, the Hubble constant should have been about 10 times greater.

This analysis is simplistic, based on our notion that expansion is
constant. The prevailing model has a varying expansion rate, for
reasons that are beyond the scope of this post (mostly because I don't
know the details!).

--
Brian Tung
The Astronomy Corner athttp://astro.isi.edu/
Unofficial C5+ Home Page athttp://astro.isi.edu/c5plus/
The PleiadAtlas Home Page athttp://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) athttp://astro.isi.edu/reference/faq.html

anandsr wrote:
So in effect the Hubble's constant that is quoted is at this point
in time, and is not true at all for very far off objects.

I am also expecting that the difference calculated for the Hubble's
constant at two points in space would be half of the actual difference
in the two values of the constant.

Well, now you're on somewhat shakier grounds. The problem is that there
are multiple ways in which to measure the distance to different objects,
One is the distance between us and the object now, one is the distance
between us and the object at the time the light we now see from the
object was emitted, and one is the distance traversed by the light.
These are all different, with the "distance now" being the greatest, and
the "distance then" being the least. They are related by the red shift,
and provided you use the right one (distance now, I think), the Hubble
constant is valid for all objects at this epoch.

In practice, astronomers do not deal directly with distances. They deal
directly with red shifts, because they are the measurable quantity here.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

On 18 Apr 2007 22:48:54 -0700, wrote:

I just had a thought we always relate the hubble's constant with the
age of the universe.
But if it so then the farther we look we must see it change.
Does it really change? Because I don't remember there being any
mention of it's relation with time. It is always stated as a constant.

The word constant is unfortunate I guess, and is a holdover from
previous decades. Indeed the current models of the evolution of the
Universe show variability of the Hubble expansion rate with time.

A good explanation can be found at:

http://map.gsfc.nasa.gov/m_uni/uni_101fate.html

Note the graphic on that page:

http://map.gsfc.nasa.gov/m_uni/101bb2_1.html

The current data (WMAP, Type 1a supernovae, etc.) supports the idea
that our Universe is following the red curve. The slope of the red
curve is the Hubble expansion rate at any give point in time. As you
can see, it is not constant with time.

Now if it is really a constant then we cannot relate it with the age
of the universe or else some other parameter eg light speed must
change to keep it a constant.

The parameters that affect the value of the Hubble expansion rate
include the amount of baryonic matter (stuff we are made of), dark
matter, and dark energy. However, it turns out that according the
present model, baryonic matter has almost zero influence over the
long-term behavior of our Universe.

---
Michael McCulloch

On Thu, 19 Apr 2007 07:50:47 -0700 (PDT), (Brian Tung)
wrote:

anandsr wrote:
So in effect the Hubble's constant that is quoted is at this point
in time, and is not true at all for very far off objects.

I am also expecting that the difference calculated for the Hubble's
constant at two points in space would be half of the actual difference
in the two values of the constant.

Well, now you're on somewhat shakier grounds. The problem is that there
are multiple ways in which to measure the distance to different objects,
One is the distance between us and the object now, one is the distance
between us and the object at the time the light we now see from the
object was emitted, and one is the distance traversed by the light.
These are all different, with the "distance now" being the greatest, and
the "distance then" being the least. They are related by the red shift,
and provided you use the right one (distance now, I think), the Hubble
constant is valid for all objects at this epoch.

Or, in other words (correct me if I'm wrong), the Hubble expansion
rate is a constant across the entire Universe at any given point in
time.

However, we cannot directly observe the distance *now* of the Universe
since light requires time to travel to us. The objects we observe with
high-redshifts have long since evolved to something possibly entirely
different and far away from the location at which the light was
emitted in our direction.

The quasar we photograph today may be the progenitor of a Milky Way to
some alien life far removed from that light image we record. Those
aliens might also be presently photographing our own galaxy in its
infant stages. But, they too are measuring the same present Hubble
expansion rate as us.

---
Michael McCulloch

Michael McCulloch wrote:
However, we cannot directly observe the distance *now* of the Universe
since light requires time to travel to us. The objects we observe with
high-redshifts have long since evolved to something possibly entirely
different and far away from the location at which the light was
emitted in our direction.

That's a good start, but it's even more perverse than that. When we
talk about "now" with respect to how we are looking back in time with
distance, we are still in an important sense fooling ourselves.

Time is relative. What that really means is that there is no "now" that
can be shared universally. One way to look at that is to realize that
each observer has his/her own perspective in time, just as we each have
our own perspective in space. Imagine a large room with a statue in the
middle and people scattered all around it. None of these people can
move about the room. Each person will see the statue from a different
perspective; one may see only the front, and another may see only the
back, etc. That's a sort of spacial relativity. It turns out that the
way the universe works each observer has his/her own perspective in
*time* as well. Each observer is wearing a watch and they read
differently. If the statue were to move an arm, no two would agree that
it happened at the same moment. So there really is no such thing as
"now", at least not one shared by everyone.

Our lives on earth depend on the simplification that we are all
relatively close to one another and moving at about the same speed. The
result is the illusion of a universally shared "now" because our
perspectives in both time and space are so similar. But the moment you
move significantly away from the earth into the solar system, "now"
becomes artificial, and when you move to greater distances still, it
eventually becomes rather meaningless.

If we think about the consequences of living in such a universe we are
led to the conclusion that this idea of looking back into time with
distance is somewhat illusionary. We can never know what those galaxies
look like "now", in the sense that time has elapsed there since the
light was emitted, any more than we can look into our own future. Since
our "now" is as good as any, in a very real sense those galaxies are
exactly as they appear to us to be "now."

Greg

--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)

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