Chandra Observations Confirm Existence of Dark Energy

New x-ray observations seem to have sealed the case of the universe's
elusive dark energy. Yesterday NASA announced results from the Chandra
telescope that offer independent confirmation that three quarters of the
universe is made up of dark energy. "Dark energy is perhaps the biggest
mystery in physics," says team leader Steve Allen of the University of
Cambridge in England. "As such, it is extremely important to make an
independent test of its existence and properties."

Dark energy was first proposed six years ago when observations of distant
supernova explosions hinted that the expansion of the universe is
accelerating, rather than slowing down as expected. Data from the Wilkinson
Microwave Anisotropy Probe (WMAP) on the cosmic microwave background
radiation also supported the existence of this unseen force. In the new
work, an international team of astronomers used the Chandra X-ray
Observatory to study 26 galaxy clusters located between one billion and
eight billion light-years away. The researchers measured the distance to the
galaxy clusters and determined the amount of hot gas in each one. Plotting
the results over cosmic time, the scientists determined that the universe's
expansion started speeding up about six billion years ago, driven by dark
energy. "Our Chandra method has nothing to do with other techniques,"
remarks study co-author Robert Schmidt of the University of Potsdam in
Germany, "so they're definitely not comparing notes, so to speak."

Read the rest at Scientific American
http://cl.extm.us/?fe9013707262077e7...7360067c711779

Comment:
How does GR cope with the existence of dark energy?

--
Posted by
Robert Karl Stonjek.

"Robert Karl Stonjek" wrote in message
...
Chandra Observations Confirm Existence of Dark Energy

New x-ray observations seem to have sealed the case of the universe's
elusive dark energy. Yesterday NASA announced results from the Chandra
telescope that offer independent confirmation that three quarters of the
universe is made up of dark energy. "Dark energy is perhaps the biggest
mystery in physics," says team leader Steve Allen of the University of
Cambridge in England. "As such, it is extremely important to make an
independent test of its existence and properties."

Dark energy was first proposed six years ago when observations of distant
supernova explosions hinted that the expansion of the universe is
accelerating, rather than slowing down as expected. Data from the
Wilkinson
Microwave Anisotropy Probe (WMAP) on the cosmic microwave background
radiation also supported the existence of this unseen force. In the new
work, an international team of astronomers used the Chandra X-ray
Observatory to study 26 galaxy clusters located between one billion and
eight billion light-years away. The researchers measured the distance to
the
galaxy clusters and determined the amount of hot gas in each one. Plotting
the results over cosmic time, the scientists determined that the
universe's
expansion started speeding up about six billion years ago, driven by dark
energy. "Our Chandra method has nothing to do with other techniques,"
remarks study co-author Robert Schmidt of the University of Potsdam in
Germany, "so they're definitely not comparing notes, so to speak."

Read the rest at Scientific American
http://cl.extm.us/?fe9013707262077e7...7360067c711779

Comment:
How does GR cope with the existence of dark energy?


I will pose this as an answer but would like to be corrected because
I am still trying to understand this and do not think I have it all
straight.

If it is evenly distributed then it slightly counteracts redshift and
gravitational attraction. It would act as a constant repulsive force
that would overcome the diminishing attraction between an expanding
universe. It would be expressed as a density of the vacuum that
counteracts the density that would be caused by the presence of
mass. It would have an effect that is the opposite of dark matter.

What if dark energy is not evenly distributed? If there were concentrations
of it in the voids between galaxies could we measure or observe it?
It would disperse light. Would this cause a noticeable visible effect?

What would the effect on mass that crosses or is created in such a
region be? Cosmic background radiation would be redshifted further
than normal and this would decrease the amount of interaction with
cosmic rays and increase the potential speed of particles. Particles
created in such a region would be accelerated by the increase in the
gravitational potential between such a region and a region without
dark energy. Could high energy cosmic rays be evidence of such
an effect?

My intuitive concept of GR which is often wrong and I appreciate
what few replies I have gotten to these kind of questions.

Robert Karl Stonjek wrote:
Chandra Observations Confirm Existence of Dark Energy
[...]
How does GR cope with the existence of dark energy?

At present, the study of dark energy is in its infancy. To date, all
observations are consistent with cosmological models having a VERY small
but non-zero positive cosmological constant. This implies that it has a
uniform distribution throughout spacetime.

Dark matter, on the other hand, is measured to NOT be distributed
uniformly, but is located predominantly near galaxies (but in regions
much larger than the visible galaxies). So dark matter has to be put
into the energy-momentum tensor.

There are numerous searches for dark matter. That is, particle-type
experiments are looking for exotic objects that could account for it.
But I know of no searches for dark energy. Yet. But if it is truly best
modeled by the cosmological constant, it's very doubtful that any such
search could be successful.


Tom Roberts

Thanks Tom.

--
Kind Regards,
Robert Karl Stonjek.

"Tom Roberts" wrote in message
y.com...
Robert Karl Stonjek wrote:
Chandra Observations Confirm Existence of Dark Energy
[...]
How does GR cope with the existence of dark energy?

At present, the study of dark energy is in its infancy. To date, all
observations are consistent with cosmological models having a VERY small
but non-zero positive cosmological constant. This implies that it has a
uniform distribution throughout spacetime.

Dark matter, on the other hand, is measured to NOT be distributed
uniformly, but is located predominantly near galaxies (but in regions
much larger than the visible galaxies). So dark matter has to be put
into the energy-momentum tensor.

There are numerous searches for dark matter. That is, particle-type
experiments are looking for exotic objects that could account for it.
But I know of no searches for dark energy. Yet. But if it is truly best
modeled by the cosmological constant, it's very doubtful that any such
search could be successful.


Tom Roberts

On 5/27/2004 4:01 PM, ueb wrote:
Tom Roberts wrote:
At present, the study of dark energy is in its infancy. To date, all
observations are consistent with cosmological models having a VERY small
but non-zero positive cosmological constant.

What definition do you use ?

The usual one from the Einstein field equation:

G + \Lambda g = T

Here G is the Einstein curvature tensor, g is the metric tensor, T is the
energy-momentum tensor, and \Lambda is the cosmological constant.


Does that mean a time-like or a length-like
curvature radius ? In formulae:
\Lambda = 3 K , or \Lambda = -3 K , with the constant part of the
Riemannian curvature K = 1/R^2 , in which R means that curvature
radius.

See above. None of what you wrote is relevant.


May I remark that any cosmological constant aka constant curvature
has nothing at all to do with any energy ?

The cosmological constant has nothing to do with "constant curvature". But, of
course, cosmological models based on FRW manifolds all have constant curvature.

Move the c.c. term to the RHS and one can see it is related.... But yes, "dark
energy" is not really a very accurate description....


What is with the matter that covers the view to the centre of our
galaxy ? Accumulated matter at this place can well explain the observed
velocity distribution. That is simple mechanics (in accordance with
GR .

The mass distribution for certain distant galaxies that image others can be
deduced from the images; they require the mass distribution (of the lensing
galaxy) to extend far beyond its visible boundary. That's why it's called "dark
matter".....


Tom Roberts

Tom Roberts wrote:
Robert Karl Stonjek wrote:
Chandra Observations Confirm Existence of Dark Energy
[...]
How does GR cope with the existence of dark energy?

At present, the study of dark energy is in its infancy. To date, all
observations are consistent with cosmological models having a VERY small
but non-zero positive cosmological constant.

What definition do you use ? Does that mean a time-like or a length-like
curvature radius ? In formulae:
\Lambda = 3 K , or \Lambda = -3 K , with the constant part of the
Riemannian curvature K = 1/R^2 , in which R means that curvature
radius.

This implies that it has a
uniform distribution throughout spacetime.

... according to Schur's theorem.
May I remark that any cosmological constant aka constant curvature
has nothing at all to do with any energy ?

Dark matter, on the other hand, is measured to NOT be distributed
uniformly, but is located predominantly near galaxies (but in regions
much larger than the visible galaxies). So dark matter has to be put
into the energy-momentum tensor.

What is with the matter that covers the view to the centre of our
galaxy ? Accumulated matter at this place can well explain the observed
velocity distribution. That is simple mechanics (in accordance with
GR .

Ulrich

Although the study of Dark Energy is still very much in its infancy as you
have pointed out, I am very much surprised that no one has come up to
suggest these observations of red shifts versus luminosities of these
distant Type Ia supernovae are inherently distorted just like these funny
mirrors in a circus. After all, light can play a lot of trick on the
observer just like these mirrors and among many other things.

----- Original Message -----
From: "Tom Roberts"
Newsgroups: sci.physics,sci.physics.relativity
Sent: Wednesday, May 26, 2004 05:32 PM
Subject: Article: Chandra Observations Confirm Existence of Dark Energy

Robert Karl Stonjek wrote:
Chandra Observations Confirm Existence of Dark Energy
[...]
How does GR cope with the existence of dark energy?

At present, the study of dark energy is in its infancy. To date, all
observations are consistent with cosmological models having a VERY small
but non-zero positive cosmological constant. This implies that it has a
uniform distribution throughout spacetime.

Dark matter, on the other hand, is measured to NOT be distributed
uniformly, but is located predominantly near galaxies (but in regions
much larger than the visible galaxies). So dark matter has to be put
into the energy-momentum tensor.

There are numerous searches for dark matter. That is, particle-type
experiments are looking for exotic objects that could account for it.
But I know of no searches for dark energy. Yet. But if it is truly best
modeled by the cosmological constant, it's very doubtful that any such
search could be successful.


Tom Roberts

Thomas J Roberts wrote:
On 5/27/2004 4:01 PM, ueb wrote:
Tom Roberts wrote:
At present, the study of dark energy is in its infancy. To date, all
observations are consistent with cosmological models having a VERY small
but non-zero positive cosmological constant.

What definition do you use ?

The usual one from the Einstein field equation:

G + \Lambda g = T

Here G is the Einstein curvature tensor, g is the metric tensor, T is the
energy-momentum tensor, and \Lambda is the cosmological constant.

Does that mean a time-like or a length-like
curvature radius ? In formulae:
\Lambda = 3 K , or \Lambda = -3 K , with the constant part of the
Riemannian curvature K = 1/R^2 , in which R means that curvature
radius.

See above. None of what you wrote is relevant.

The equation quoted by you means the case \Lambda = -3K ,
i.e. a time-like curvature radius.

May I remark that any cosmological constant aka constant curvature
has nothing at all to do with any energy ?

The cosmological constant has nothing to do with "constant curvature".

It is even the same for the factor -3 . In which, I spoke of the
constant *part* of the Riemannian curvature, i.e. all terms of the
Riemannian curvature are in general not constant.

Ulrich

ueb wrote:
Thomas J Roberts wrote:
The usual [definition of the cosmological constant]
from the Einstein field equation:
G + \Lambda g = T
The equation quoted by you means the case \Lambda = -3K ,
i.e. a time-like curvature radius.

Not true. \Lambda is an independent parameter that can be set to any
appropriate value (e.g. fitted to observations).


Tom Roberts

Tom Roberts wrote:
ueb wrote:
Thomas J Roberts wrote:
The usual [definition of the cosmological constant]
from the Einstein field equation:
G + \Lambda g = T
The equation quoted by you means the case \Lambda = -3K ,
i.e. a time-like curvature radius.

Not true. \Lambda is an independent parameter that can be set to any
appropriate value (e.g. fitted to observations).

Even. Why should not be true what I said ? -
I have my wisdom about Riemannian curvature (especially constant
curvature) from Eisenhart, Riemannian Geometry. Only if you like,
I could compare the formulae. But here seems rather to be a
misunderstanding.

Ulrich