Comments about the article in Nature: Dark matter and dark energy

Following is a discussion about this article in Nature Vol 458 2 April 2009 by Robert Caldwell and Marc Kamionkowski
For a copy select: Dark matter and Dark energy For an article with the same authors select:The Physics of Cosmic Acceleration In the last paragraph I explain my own opinion.

What is the composition of the Universe?

... the contents of the Universe is approximate 75% dark energy, 20 % dark matter and 5% normal matter
The article does not give any clue why those numbers. My first reaction is that a number of 5% normal matter is very low.

These measurements rely on the validity of the hot Big Bang model, general relativity and the cosmological principal (That the universe is uniform on the largest scales)
Why using the word measurements? IMO none of those values can be measured. Why not numbers? why not estimates?
Does the cosmological principle imply that the universe is uniform at all epochs? starting from the moment at the Big Bang? How do we know?

What is the evidence for dark matter ?

Newton's laws state that the mass of a body can be determined by the motion of its satelites
This is only true for a number of isolated objects. For example the Sun and its planets.

Thus it has been calculated (refrashed) that the mass of a galaxy is far larger than the combined mass of their constituent stars and interstellar gas.
The method used is based on the galaxy rotation curve, which in many cases is flat. The issue is you have to include all stars and planet size body's in the disc. Current observations reveal that there are many more planet size objects in the disc as original thaught. As such you have include many more small sized objects outside the rotation curve, which makes it easier to simulate the rotation even without any dark matter.

For a document about the recent size of our galaxy See: Milky Way as massive as 3 trillion suns or 3*10^12
This is much higher than 2*10^11 in the book Univerese of 1987 or 1*10^12 in 2001. See: The Milky Way Galaxy A grand design

and that the mass of galaxy clusters is far larger than that of their constituent galaxies.
This is a very difficult (and almost impossible) exercise because you accurately have to know the direction and speed of each of the galaxies involved.
The method used is the viral theorem. Virial theorem From Wikipedia, the free encyclopedia
The viral theorem can be used to calculate the radius of for example 10 identical stars with the same mass in a star cluster being in equilibrium. The problem is the radius is not constant.
To do the reverse and calculate the total mass based on the radius is a very tricky exercise because not all stars are identical.
To calculate the total mass of a galaxy cluster gives you only a very rough estimate.
IMO the only way to calculate the mass of a galaxy cluster can only be done by adding the masses of all the individual galaxies and not by applying Newton''s Law.

Yet there is very good reason to believe that this extra stuff is not normal matter. Such an abundance of normal matter would be difficult to conceal from the prying eyes of astronomers
IMO a lot of normal matter can easily be concealed in our Galaxy. For example all Brown stars and planet sized objects.
See also Reflection item 3.

and would furthermore leave a distinct signature in the cosmic microwave background radiation
I doubt this.

Why can't we conclude that Newton's Law breaks down at the distance scales of galaxies or clusters?

This might have been a reasonable hypothesis a few decades ago.
MOND is such a theory, how ever is not mentioned in this article. The problem with MOND is that it uses some "strange" assumptions, which have no physical back up.
I see no reason to claim why Newton's Law can not be used to describe all objects in the Universe. (With the exception of the movement of the planet Mercury)

How much dark matter is there nearby?

Earth's orbit around the Sun implies that the amount of dark matter incident on the Earth varies by about 10% from summer to winter
How do they know that? IMO this is clear speculation. We are speaking here about dark matter i.e. invisible matter. The same comments about fig 1c.

What experiments or observations can help?

Clearly, the most compelling resolution to the dark-matter problem would be the direct detection of dark-matter particles

How else can we see darkmatter?

The most likely sources of these annihilation products would be the Galatatic Centre,where the dark matter density would be high (Fig 1)
Also at fig 1 is written:
near the Galactic Centre, where the dark matter density is expected to be the higest
This is strange, because in figure 1b there is also a sketch of a rotation curve, which shows no dark matter in the centre.

What about dark energy?

Here they reference fig 2. Near fig 2 we read:
The observation that these supernovae are dimmer than expected at a given recessional velocity, has led to the conclusion that the Universe expansion has been accelerating over approx. the past 5 billion years, before which the expansion was decelerating
How do they know the last. The whole idea behind the Big Bang is that there is a lineair relation between Magtitude, log(Recessional velocity) and z. This relation implies at short distances that the recessional velocity increases lineair with distance and that there is no acceleration involved.
The question is if this relation is true at larger distances or that there is an increase in velocity (implying acceleration) or a decrease (implying deceleration).
The problem is that near fig 2 they write that at short distances there is acceleration and at large distances deceleration, which clearly asks for more explanation.
For more detail read:

The cause of this cosmic acceleration is widely attributed to dark energy
See also reflection.
Whereas the posibility for gravitational repulsion does not exist in Newtonian gravity, it does exist in general relativity

what is the best bet for the nature of dark energy?

The simplest candidate for dark energy is Einstein's cosmological constant, which denotes a perfect uniform fluid with negative presure etc
For more detail read: Cosmological constant in Wikipedia
My impression is that the cosmological constant in general relativity is more a factor to match predictions with observations. The cosmological constant is not an explanation of the physical behaviour after the Big Bang.

How reliable are the known laws of gravitation on cosmological scales

General relativity works. It has been extremely well tested in the Solar System, and it is used to make sense of a vast cataloque of astrophysical and cosmological observations.
In fact two issues are here at stake: Special Relativity and General Relativity. The issue is what is each exactly and to what extend can GR be used in order to describe the evolution of the universe.
For example: What is the amount of dark matter required to simulate our Galaxy by using General Relativity such that our rotation curve matches observation.

Reflection

  1. In the book "The Big Bang" By Joseph Silk at page 102 the Big Bang is explained by means of The Friedman Equation which introduces the concepts of kinetic energy and gravitational potential energy. The sum of these two types of energy must be constant with time. There is no mention what these two types of energy physical mean. The Friedman Equation is used to explain different BB models, which can be either accelerating or decelerating, by introducing the factor k which can be either 1,0 or -1.
  2. In laymans words starting from the moment of the BB potential energy is converted in kinetic energy.
  3. The most difficult part in understanding the BB is its initial state during which period all matter was potential energy.
  4. In this document a new concept of dark energy is introduced. Unfortunate nothing is mentioned what the relation is with the two other types of energy nor why those two are not sufficient.
  5. The physical meaning of this dark energy is a fluid that antigravitates. What makes this so dificult to understand why such a hugh amount is required? i.e. 75% of all energy?
  6. Why "such an abundance" of matter. The question is about how much matter are we speaking in order to explain the galaxy rotation curve? IMO using the newest telescope much more vissible matter will be detected in the disc, which reduces the need for dark matter.

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Created: 11 april 2009
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