light and dark matter

At the risk of uttering complete nonsense, I'd like to describe something
I've been wondering about for a while, namely whether the density of light
in the universe might be related to the problem of dark matter and dark energy.

If one takes all of the light in the universe, it has a certain mass and
therefore it exerts a certain gravitational force on the matter in the
universe. I don't know how much that force is taken into account in
computations of the rate of expansion of the universe. For example,
what is the total gravitational attraction exerted by the light from the
cosmic background radiation?

I realize that dark matter is only supposed to interact gravitationally
with the other stuff we know about in the universe, whereas light
interacts electromagnetically and maybe in other ways, but if the light
is of sufficiently low frequency, I think it wouldn't interact in any way
with matter except gravitationally. Conceivably, there is a lot of light
of much lower frequency than the cosmic background radiation and we can't
detect it except gravitationally and, since it is uniformly distributed
throughout the universe, its primary manifestation has been through its
effects as dark matter. For example, if the light had a wavelength larger than
the solar system, we would have no direct way to sample it with any detector
we could construct.

Since it would not interact electromagnetically, strongly, weakly, etc. with
matter, we could naturally ask what process could produce light of such low
frequency. One way would be for it to be doppler shifted from light of greater
frequency as the universe expands.

The notion of dark energy has been offered to explain recent evidence for
the accelerated expansion of the universe. One other property of light,
at least in Gaussian beams, is its tendency to spread out. So, maybe that
tendency to spread out when it is already densely distributed also
contributes to the accelerated expansion of the universe.

I guess I am saying something like the following: light's tendency to spread
out contributes to the accelerated expansion of the universe; that is what
people have attributed to dark energy. As the expansion proceeds, the light
gets doppler shifted so far down that it can't interact with matter other
than gravitationally. The manifestation of that gravitational interaction
is what people have attributed to dark matter.

This is pure speculation and not well informed. I don't expect any of
it to be right. If someone can explain why it is all wrong, that will
help me forget about it and move on to something else. Maybe a few
relevant numbers are all it takes. I don't intend to argue about it:
I'd just like to hear people's opinions and try to learn from them.

If someone already came up with this crackpot idea and published it,
perhaps someone can provide a reference to it and its refutation.

Ignorantly,
Allan Adler


************************************************** **************************
* *
* Disclaimer: I am a guest and *not* a member of the MIT Artificial *
* Intelligence Lab. My actions and comments do not reflect *
* in any way on MIT. Moreover, I am nowhere near the Boston *
* metropolitan area. *
* *
************************************************** **************************

"Allan Adler" wrote in message
...

At the risk of uttering complete nonsense, I'd like to describe something
I've been wondering about for a while, namely whether the density of light
in the universe might be related to the problem of dark matter and dark
energy.

If one takes all of the light in the universe, it has a certain mass


Stop......

No Mass... Light does not have mass....



and
therefore it exerts a certain gravitational force on the matter in the
universe. I don't know how much that force is taken into account in
computations of the rate of expansion of the universe. For example,
what is the total gravitational attraction exerted by the light from the
cosmic background radiation?

I realize that dark matter is only supposed to interact gravitationally
with the other stuff we know about in the universe, whereas light
interacts electromagnetically and maybe in other ways, but if the light
is of sufficiently low frequency, I think it wouldn't interact in any way
with matter except gravitationally. Conceivably, there is a lot of light
of much lower frequency than the cosmic background radiation and we can't
detect it except gravitationally and, since it is uniformly distributed
throughout the universe, its primary manifestation has been through its
effects as dark matter. For example, if the light had a wavelength larger
than
the solar system, we would have no direct way to sample it with any
detector
we could construct.

Since it would not interact electromagnetically, strongly, weakly, etc.
with
matter, we could naturally ask what process could produce light of such
low
frequency. One way would be for it to be doppler shifted from light of
greater
frequency as the universe expands.

The notion of dark energy has been offered to explain recent evidence for
the accelerated expansion of the universe. One other property of light,
at least in Gaussian beams, is its tendency to spread out. So, maybe that
tendency to spread out when it is already densely distributed also
contributes to the accelerated expansion of the universe.

I guess I am saying something like the following: light's tendency to
spread
out contributes to the accelerated expansion of the universe; that is what
people have attributed to dark energy. As the expansion proceeds, the
light
gets doppler shifted so far down that it can't interact with matter other
than gravitationally. The manifestation of that gravitational interaction
is what people have attributed to dark matter.

This is pure speculation and not well informed. I don't expect any of
it to be right. If someone can explain why it is all wrong, that will
help me forget about it and move on to something else. Maybe a few
relevant numbers are all it takes. I don't intend to argue about it:
I'd just like to hear people's opinions and try to learn from them.

If someone already came up with this crackpot idea and published it,
perhaps someone can provide a reference to it and its refutation.

Ignorantly,
Allan Adler



************************************************** **************************
*
*
* Disclaimer: I am a guest and *not* a member of the MIT Artificial
*
* Intelligence Lab. My actions and comments do not reflect
*
* in any way on MIT. Moreover, I am nowhere near the Boston
*
* metropolitan area.
*
*
*

************************************************** **************************

In article , "Paul R. Mays" writes:

"Allan Adler" wrote in message
...

At the risk of uttering complete nonsense, I'd like to describe something
I've been wondering about for a while, namely whether the density of light
in the universe might be related to the problem of dark matter and dark
energy.

If one takes all of the light in the universe, it has a certain mass


Stop......

No Mass... Light does not have mass....

A photon doesn't have a mass. A set of photons, in general, will have
mass (except when they all propagate in the same direction). The mass
of a system is not the sum of the masses of its components, in
general.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

Mati Meron wrote:

A photon doesn't have a mass. A set of photons, in general,
will have mass (except when they all propagate in the same
direction). The mass of a system is not the sum of the masses
of its components, in general.

Hmmm. Clues to the answer to a question George Dishman posed
to me a few days ago. Thank you.

Except when they all propagate in in the same direction?
Are you sure about that? If it is so, the mass equivalent of
a bunch of photons depends on the distribution of directions.
Imagine two bunches of photons bouncing between two mirrors.
Sometimes both bunches are moving in the same direction, and
sometimes in opposite directions. Of course the box containing
the mirrors and photons is accelerated whenever a bunch of
photons hits one of the mirrors, and maybe that accounts for
the apparent change in mass of the system?

-- Jeff, in Minneapolis

..

In article , (Jeff Root) writes:
Mati Meron wrote:

A photon doesn't have a mass. A set of photons, in general,
will have mass (except when they all propagate in the same
direction). The mass of a system is not the sum of the masses
of its components, in general.

Hmmm. Clues to the answer to a question George Dishman posed
to me a few days ago. Thank you.

Except when they all propagate in in the same direction?
Are you sure about that? If it is so, the mass equivalent of
a bunch of photons depends on the distribution of directions.

Yes.

Imagine two bunches of photons bouncing between two mirrors.
Sometimes both bunches are moving in the same direction, and
sometimes in opposite directions. Of course the box containing
the mirrors and photons is accelerated whenever a bunch of
photons hits one of the mirrors, and maybe that accounts for
the apparent change in mass of the system?

Again, yes. You're doing well, keep going.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

AFAIK photons have no REST mass, but do have energy,

E=hf, where h Planck's constant and f is the frequency.

So they must have some reletivistic mass as Einstein told us that there
was an equivalence between mass and energy

m = E/c² (which is non null as neither f nor h are zero).

Though it isn't 'dark' as we can see the little blighters.
I cant see how it could account for rotational speed anomalies seen in
our own planetery system, galaxies etc.

Has anyone taken a slide rule to that??

Paul wrote:

Stop......

No Mass... Light does not have mass....




Maybe you want to say the following:

"Light does not have rest mass...."

you have to distinguish mass from rest mass, they are different.


Just to remember you E=m*C^2

"AA" == Allan Adler writes:

AA At the risk of uttering complete nonsense, I'd like to describe
AA something I've been wondering about for a while, namely whether
AA the density of light in the universe might be related to the
AA problem of dark matter and dark energy.

AA If one takes all of the light in the universe, it has a certain
AA mass and therefore it exerts a certain gravitational force on the
AA matter in the universe. I don't know how much that force is taken
AA into account in computations of the rate of expansion of the
AA universe. For example, what is the total gravitational attraction
AA exerted by the light from the cosmic background radiation?

A better way to state it (so as to avoid the endless discussions about
whether light has mass or not) is to note that light has energy.
Gravitation depends upon the total energy density, which of course
includes the rest-energy (i.e., mass) of an object but is not limited
to that. Thus, the light in the Universe contributes to an energy
density, which will affect the Universe's expansion.

The crucial aspect for the Universe's expansion is the critical
density. This is the density at which gravitation can just balance
the expansion. If the Universe's density is above the critical
density, it recollapses; if it is below, it expands forever. (This
all assumes that the cosmological constant is zero or that there is no
dark energy, but it is still a useful way of characterizing the
problem.)

The critical density of the Universe is about 1E-29 g/cm^-3, or
equating mass and energy, about 1E-8 erg/cm^-3.

The energy density of the cosmic microwave background is
4.2E-13 (1+z)^4 erg/cm^-3 at a redshift z. At the current epoch
(z=0), the ratio of the energy density of light in the Universe to the
critical density is about 1E-5.

We think that the total mass density (from both dark and luminous
matter) is within 30% or so of the critical density. Thus, in the
current epoch, radiation (in the form of the cosmic microwave
background) is about 10,000 times too rarified to explain dark matter.

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