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Different Hubble constants



 
 
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  #1  
Old January 28th 17, 01:10 PM posted to sci.astro.research
Richard D. Saam
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Posts: 240
Default Different Hubble constants

What are the astrophysical implications
of different Hubble constants by
Planck
https://arxiv.org/abs/1502.01589
Hubble constant 67.8 +/- .9 km/s/Mpc
and by
H0LiCOW collaboration
http://shsuyu.github.io/H0LiCOW/site/
Hubble constant 71.9 +2.4 -3.0 km/s/Mpc
?
Richard D Saam

  #2  
Old January 28th 17, 06:44 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 273
Default Different Hubble constants

In article , "Richard D.
Saam" writes:

What are the astrophysical implications
of different Hubble constants by
Planck
https://arxiv.org/abs/1502.01589
Hubble constant 67.8 +/- .9 km/s/Mpc
and by
H0LiCOW collaboration
http://shsuyu.github.io/H0LiCOW/site/
Hubble constant 71.9 +2.4 -3.0 km/s/Mpc
?


First, progress. For decades, people worried about a factor of two or
more between different measurements of the Hubble constant. Now, we are
talking about 5 per cent. Also, any "tension" assumes that the error
bars are correct. We used to have 100+/-10 and 50+/-7 or whatever. (In
these cases, not only were the error bars too large, but the
measurements themselves were wrong.)

If you have one measurement, you know the value. If you have two
different measurements, you don't. :-) So we need a third,
independent, accurate measurement of comparable precision.

Much has been written about this. Just google "tension Hubble constant
Planck" (without the quotes).

Assuming that the tension is real, there are various scenarios which
could account for it, most of which don't call the Big Bang into
question. While it is good to explore these, especially given the
history of the Hubble constant (first estimates were around 600), it is
probably more productive to wait until the tension has been confirmed or
ruled out.


[[Mod. note -- An old mariner's saying (which supports Phillip Helbig's
point): "Never go to sea with two chronometers: take either one or three".
-- jt]]
  #3  
Old January 29th 17, 05:53 AM posted to sci.astro.research
Richard D. Saam
external usenet poster
 
Posts: 240
Default Different Hubble constants

On 1/28/17 11:44 AM, Phillip Helbig (undress to reply) wrote:
In article , "Richard D.
Saam" writes:

What are the astrophysical implications
of different Hubble constants by
Planck
https://arxiv.org/abs/1502.01589
Hubble constant 67.8 +/- .9 km/s/Mpc
and by
H0LiCOW collaboration
http://shsuyu.github.io/H0LiCOW/site/
Hubble constant 71.9 +2.4 -3.0 km/s/Mpc
?


First, progress. For decades, people worried about a factor of two or
more between different measurements of the Hubble constant. Now, we are
talking about 5 per cent. Also, any "tension" assumes that the error
bars are correct. We used to have 100+/-10 and 50+/-7 or whatever. (In
these cases, not only were the error bars too large, but the
measurements themselves were wrong.)

If you have one measurement, you know the value. If you have two
different measurements, you don't. So we need a third,
independent, accurate measurement of comparable precision.

Cepheids and Supernovae data provide
fourth and fifth independent measurements.

I did not properly present the H0LiCOW collaboration data:
"Since the value measured from the SH0ES project using Cepheids
and Supernovae is completely independent of the H0LiCOW value,
both can be combined into a single measurement
of the Hubble Constant in the Local Universe.
This new value of H0 = 72.5 +/- 1.4 km/s/Mpc
is 3.1 sigma higher than the most recent measurement of Planck,
where H0 = 67.8 +/- 0.9 km/s/Mpc."

So the question should be framed
as much more significant H0 difference (3.1 sigma):
What are the astrophysical implications
of different Hubble constants by
Planck
https://arxiv.org/abs/1502.01589
Hubble constant 67.8 +/- .9 km/s/Mpc
and by
H0LiCOW collaboration and SH0ES project
http://shsuyu.github.io/H0LiCOW/site/
Hubble constant 72.5 +/- 1.4 km/s/Mpc
?

Much has been written about this. Just google "tension Hubble constant
Planck" (without the quotes).

Yes, much discussion about dark energy dark matter densities.
Assuming that the tension is real, there are various scenarios which
could account for it

For discussion, point out one.
most of which don't call the Big Bang into
question.

Agreed
While it is good to explore these, especially given the
history of the Hubble constant (first estimates were around 600),

The measured value has been around 75 +/- 10 km/s/Mpc
for the last 20 years.
it is
probably more productive to wait until the tension has been confirmed or
ruled out.

It would appear timing for discussion is here.

[[Mod. note -- An old mariner's saying (which supports Phillip Helbig's
point): "Never go to sea with two chronometers: take either one or three".
-- jt]]

As an old Navy navigator before GPS,
we had the luxury of
NIST WWV Fort Collins, Colorado -Greenwich Mean Time.
A 3 star (with 120 degree Hour Angle separation) fix
was the standard.
Daylight sun and moon 2 LOP fixes were problematic.
The interfering parameters were clouds
and a non stable observing platform for the sextant
due to high sea state for surface ships
and air turbulence for aircraft
resulting in positions a few nautical miles in breadth
which were adequate in the vast ocean expanse.
(present ~cm GPS positions
would have been deemed unnecessarily accurate)

RDS
  #4  
Old February 1st 17, 04:42 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
external usenet poster
 
Posts: 273
Default Different Hubble constants

In article , "Richard D.
Saam" writes:

If you have one measurement, you know the value. If you have two
different measurements, you don't. So we need a third,
independent, accurate measurement of comparable precision.

Cepheids and Supernovae data provide
fourth and fifth independent measurements.

I did not properly present the H0LiCOW collaboration data:
"Since the value measured from the SH0ES project using Cepheids
and Supernovae is completely independent of the H0LiCOW value,
both can be combined into a single measurement
of the Hubble Constant in the Local Universe.
This new value of H0 = 72.5 +/- 1.4 km/s/Mpc
is 3.1 sigma higher than the most recent measurement of Planck,
where H0 = 67.8 +/- 0.9 km/s/Mpc."

So the question should be framed
as much more significant H0 difference (3.1 sigma):
What are the astrophysical implications
of different Hubble constants by
Planck
https://arxiv.org/abs/1502.01589
Hubble constant 67.8 +/- .9 km/s/Mpc
and by
H0LiCOW collaboration and SH0ES project
http://shsuyu.github.io/H0LiCOW/site/
Hubble constant 72.5 +/- 1.4 km/s/Mpc
?


The main point is that the PLANCK measurement involves much higher
redshifts than the other three, which are, by comparison, "local".
Cepheids and supernovae are just different types of standard candles.
H0LiCOW measures the Hubble constant via gravitational-lens time delays,
and as such is independent (not based on the distance-ladder approach),
but still in roughly the same redshift range. The fact that all of the
non-Planck approaches agree is really amazing.

The real question is whether there is any real tension between Planck
and "local" measurements. Of course, all tension goes away if the error
bars have been underestimated. Since the local and PLANCK approaches
don't agree, at least one MUST have error bars which are underestimated,
or there is some systematic effect.

Again, considering the history of the Hubble constant, it probably makes
sense to wait and see if the tension is real before trying to come up
with something which would make the Hubble constant depend SLIGHTLY on
the redshift range in which it is measured.

WMAP was also a CMB satellite and it measured a higher value for the
Hubble constant. We shouldn't just forget that, especially since PLANCK
is more recent and perhaps all systematic effects are not yet understood
(there are other indications that there might be systematic effects in
the PLANCK data).
  #5  
Old February 4th 17, 12:06 AM posted to sci.astro.research
Steve Willner
external usenet poster
 
Posts: 1,172
Default Different Hubble constants

In article ,
"Phillip Helbig (undress to reply)" writes:
The real question is whether there is any real tension between Planck
and "local" measurements.


I agree that the most likely reason is some small systematic error.

it probably makes sense to wait and see if the tension is real
before trying to come up with something which would make the Hubble
constant depend SLIGHTLY on the redshift range in which it is
measured.


While I also agree that coming up with possible explanations isn't
worth a major effort, I think a bit of speculation might not be a bad
idea. It might suggest independent avenues of research to see
whether there might be evidence for some hypothesis or other.

I'm not enough of an expert to have any real idea what might cause
discrepancies between the local and CMB values of the Hubble
constant. I am _guessing_ a "hiccup" in the expansion history might
do it, but I have no idea what might cause such a hiccup nor how one
might test that. Any other ideas?

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #6  
Old February 4th 17, 05:41 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
external usenet poster
 
Posts: 273
Default Different Hubble constants

In article , Steve Willner
writes:

The real question is whether there is any real tension between Planck
and "local" measurements.


I agree that the most likely reason is some small systematic error.


Especially since there are some other indications that there might be
systematic errors in one of the PLANCK frequency bands.

While I also agree that coming up with possible explanations isn't
worth a major effort, I think a bit of speculation might not be a bad
idea. It might suggest independent avenues of research to see
whether there might be evidence for some hypothesis or other.

I'm not enough of an expert to have any real idea what might cause
discrepancies between the local and CMB values of the Hubble
constant. I am _guessing_ a "hiccup" in the expansion history might
do it, but I have no idea what might cause such a hiccup nor how one
might test that. Any other ideas?


The most common idea is an appreciable departure from homogeneity. Of
course, the universe is not homogeneous, but it is to a good
approximation. However, if we were near (otherwise there would be a
dependence on direction, which is not observed) the centre of a region
with a different density than the universal average, it could affect the
Hubble constant measured within that region, compared to the value
measured on a larger scale. On the one hand over- or underdensities can
affect various distance measures. On the other hand, the density
contrast might be large enough that it affects the rate of expansion
(so-called backreaction). In the first case, the Hubble constant is not
really different; rather, the model to calculate the observed brightness
knowing the absolute brightness and the redshift is too simple. In the
second case, it is really different.
  #7  
Old February 8th 17, 04:53 PM posted to sci.astro.research
Martin Brown
external usenet poster
 
Posts: 1,707
Default Different Hubble constants

On 03/02/2017 23:06, Steve Willner wrote:
In article ,
"Phillip Helbig (undress to reply)" writes:
The real question is whether there is any real tension between Planck
and "local" measurements.


I agree that the most likely reason is some small systematic error.


Indeed. The speed of light in vacuum determination as a function of time
with error bars is a salutary lesson in this regard. I remember the
graph from an introductory relativity textbook in our library.

The gist of the problem was that a very famous experimentalist made an
error in the sign of the correction for imperfect vacuum and everyone
afterwards made exactly the same mistake. It was only when a new method
with even greater precision and radically different systematic errors
came onstream that the problem was uncovered.

BTW Anyone recognise the book title from this description?

it probably makes sense to wait and see if the tension is real
before trying to come up with something which would make the Hubble
constant depend SLIGHTLY on the redshift range in which it is
measured.


While I also agree that coming up with possible explanations isn't
worth a major effort, I think a bit of speculation might not be a bad
idea. It might suggest independent avenues of research to see
whether there might be evidence for some hypothesis or other.

I'm not enough of an expert to have any real idea what might cause
discrepancies between the local and CMB values of the Hubble
constant. I am _guessing_ a "hiccup" in the expansion history might
do it, but I have no idea what might cause such a hiccup nor how one
might test that. Any other ideas?


If the universe is accelerating with time due to dark energy then the
Hubble constant might be expected to vary slightly with increasing Z.
But I suspect by amounts much smaller than the experimental error bars.

--
Regards,
Martin Brown
  #8  
Old February 9th 17, 06:20 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
external usenet poster
 
Posts: 273
Default Different Hubble constants

In article , Martin Brown
writes:

On 03/02/2017 23:06, Steve Willner wrote:
In article ,
"Phillip Helbig (undress to reply)" =

writes:
The real question is whether there is any real tension between Planck
and "local" measurements.


I agree that the most likely reason is some small systematic error.


Indeed. The speed of light in vacuum determination as a function of time=


with error bars is a salutary lesson in this regard. I remember the
graph from an introductory relativity textbook in our library.

The gist of the problem was that a very famous experimentalist made an
error in the sign of the correction for imperfect vacuum and everyone
afterwards made exactly the same mistake. It was only when a new method
with even greater precision and radically different systematic errors
came onstream that the problem was uncovered.


OK. I only parsed it correctly on second reading. This is an example
where similar "tension" was resolved when a systematic error was
corrected for. (You don't mean that a variable speed of light in vacuum
could be the cause of the current tension with regard to the Hubble
constant.)

it probably makes sense to wait and see if the tension is real
before trying to come up with something which would make the Hubble
constant depend SLIGHTLY on the redshift range in which it is
measured.


While I also agree that coming up with possible explanations isn't
worth a major effort, I think a bit of speculation might not be a bad
idea. It might suggest independent avenues of research to see
whether there might be evidence for some hypothesis or other.

I'm not enough of an expert to have any real idea what might cause
discrepancies between the local and CMB values of the Hubble
constant. I am _guessing_ a "hiccup" in the expansion history might
do it, but I have no idea what might cause such a hiccup nor how one
might test that. Any other ideas?


If the universe is accelerating with time due to dark energy then the
Hubble constant might be expected to vary slightly with increasing Z.
But I suspect by amounts much smaller than the experimental error bars.


This is a red herring. In general, the Hubble constant is not constant
in time. (It is called the Hubble constant because it is a constant
like "a" in the equation y = ax +b, not because it is constant in time.)
It varies quite dramatically. At the big bang, it is infinite. It
decreases at first, then can increase again, in some universes (such as
hours) asymptotically approaching a constant value. This is well known
and is always taken into account. The tension in measurements we are
talking about is tension in measurements of H_0, which is the current
value. Even if the objects involved are at high redshift, it is still
the current value which is measured. (Even at low redshift, the Hubble
constant is not "directly" measured.)
 




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