Is Dark matter Necessary?

[LS George]

Astronomy isn't one of my big areas, but none the less i find it interesting. One thing I particularly enjoy about it is dark matter, and how we really don't know that much about it.

 

But do we need dark matter? From my understanding, we use the dark matter to help explain why galaxies are able to remain the way they are, as there isn't enough normal matter present to be able to hold the shape that they are seen to hold.

 

But are there alternatives to dark matter that could still predict a stable galaxy in the way dark matter does?

 

All response are appreciated!

[beefpatty]

There are other independent lines of evidence for the existence of dark matter. For example, according to standard cosmology we require a dark matter component in order to observe the Universe we see today. This is completely independent of any observations of galaxy rotation rates. Also, you need dark matter to explain pictures like this:

This is known as the Bullet cluster and shows two colliding clusters of galaxies. The (false color) pink regions denote the visible matter. The interesting thing is that, using the visible matter alone, this cluster cannot look like this. For the visible matter to have this distribution, you need "extra" matter to contribute to the gravitational potential, which many people interpret to be dark matter.

[ACG52]

 

 

But are there alternatives to dark matter that could still predict a stable galaxy in the way dark matter does?

Yes. They would have to be homogeneous in distribution through the galaxy, gravitationally active, and non-reactive with the electromagnetic spectrum.

 

Hmmm, sounds like dark matter.

[EdEarl]

Yes. They would have to be homogeneous in distribution through the galaxy, gravitationally active, and non-reactive with the electromagnetic spectrum.

 

Hmmm, sounds like dark matter.

In other words, dark matter is a reasonable euphimism for whatever is causal.

[harshgoel1975]

let me share one interesting quote from an ancient book ( Vishnu Puran chapter 2 stanza 24 ) written hundred of years ago...it talk about how the universe is created... it talk about two kind of matter "pradhan" and "Purush"... though the book doen't describe in great detail.... but this talk about how these kind of matter separate at the beginning of universe and merge together at the end....and how they maintain a balance in between.. not in detail.. but looks quite modern concept... similar to matter and dark matter...

[ajb]

But are there alternatives to dark matter that could still predict a stable galaxy in the way dark matter does?

There is modified Newtonian dynamics, but this is nothing like as popular as dark matter.

 

 

 

let me share one interesting quote from an ancient book ( Vishnu Puran chapter 2 stanza 24 ) written hundred of years ago...it talk about how the universe is created... it talk about two kind of matter "pradhan" and "Purush"... though the book doen't describe in great detail.... but this talk about how these kind of matter separate at the beginning of universe and merge together at the end....and how they maintain a balance in between.. not in detail.. but looks quite modern concept... similar to matter and dark matter...

But this is just a kind of "matrixing" where you see things that are not really there. It is only is great retrospect and imagination can you think that an ancient book says anything useful about modern astrophysics and cosmology.

Edited August 9, 2014 by ajb

[Strange]

let me share one interesting quote from an ancient book ( Vishnu Puran chapter 2 stanza 24 ) written hundred of years ago...it talk about how the universe is created... it talk about two kind of matter "pradhan" and "Purush"... though the book doen't describe in great detail.... but this talk about how these kind of matter separate at the beginning of universe and merge together at the end....and how they maintain a balance in between.. not in detail.. but looks quite modern concept... similar to matter and dark matter...

 

You can interpret this sort of vague comment to mean anything: matter and antimatter, solids and fluids, electrons and protons, software and hardware, bosons and fermions, acids and bases, matter and energy, bitter and sweet, quarks and gluons, ...

[harshgoel1975]

 

You can interpret this sort of vague comment to mean anything: matter and antimatter, solids and fluids, electrons and protons, software and hardware, bosons and fermions, acids and bases, matter and energy, bitter and sweet, quarks and gluons, ...

Agree... but both here are talking about the cosmos.. creation and existence of cosmos..

[Strange]

Agree... but both here are talking about the cosmos.. creation and existence of cosmos..

 

It could still mean anything at all. Or nothing.

[LS George]

Wow, i wasn't expecting such enlightening responses, I guess dark matter does play an important role in the universe (unless something else comes along, obviously). Learn something new everyday, but how would dark energy come into this? (my current understanding is that dark energy is what causes the expansion of the universe [based on current experimental evidence])

[zapatos]

Dark energy is what is responsible for the acceleration of the expansion of the universe.

[EdEarl]

Dark energy is what is responsible for the acceleration of the expansion of the universe.

Perhaps it is more accurate to say that in the 1990's astronomers working with Type 2a supernova noticed the universe was not slowing down, instead it was accellerating away towards infinity in all directions. The expansion is caused by space-time stretching out, faster and faster. The recession of galaxies, indicated by red shift, is added to the stretching of space-time. Thus, some galaxies are receeding faster than the speed of light. The thing causing space-time to stretch is called dark energy. Note: gravity also stretches space-time.

[swansont]

Astronomy isn't one of my big areas, but none the less i find it interesting. One thing I particularly enjoy about it is dark matter, and how we really don't know that much about it.

 

But do we need dark matter? From my understanding, we use the dark matter to help explain why galaxies are able to remain the way they are, as there isn't enough normal matter present to be able to hold the shape that they are seen to hold.

 

But are there alternatives to dark matter that could still predict a stable galaxy in the way dark matter does?

 

All response are appreciated!

 

We can look at Newtonian gravity: F = GMm/r^2

 

What we observe is that the known mass isn't following that equation, given the equations of motion. So we ask what can change? We can change the mass, or we can change the equation. Changing the equation can occur with a new form (deviation from r²) or by making G vary. Mass can vary either by dark matter or by making gravitational mass and inertial mass not be the same.

 

The problem comes in experimentally verifying these modifications. You can see what size of effect you need for the galactic rotation fix, but does that hold up at other scales? With the alternatives to DM you tend to run into a problem of fixing the physics in one place only to break it in another.

[zapatos]

Perhaps it is more accurate to say that in the 1990's astronomers working with Type 2a supernova noticed the universe was not slowing down, instead it was accellerating away towards infinity in all directions. The expansion is caused by space-time stretching out, faster and faster. The recession of galaxies, indicated by red shift, is added to the stretching of space-time. Thus, some galaxies are receeding faster than the speed of light. The thing causing space-time to stretch is called dark energy. Note: gravity also stretches space-time.

I was trying to make a distinction between the metric expansion of space inherent in the Big Bang theory, which accounts for the expansion of space, and Dark Energy, which accounts for the acceleration of the expansion of space. Space would be stretching without Dark Energy.

[SciChallenger]

Recently on www.IntellectualArchive.com I read the article that states that effects of hidden mass in galaxies can easily be explained if "Dyson Spheres" are real. If "Dyson Spheres" exist - they produce gravity but do not produce radiation. In this case the dark matter is not required at all. Current observations (like analysis of gravitational microlens) also do not reject this hypothesis, because "Dyson Spheres" have big enough size and analysis of gravitational microlens rejects only small objects. I can not put all details here. You can read it on IntellectualArchive.com. Number #1321 in Astronomy. (It is also published in "IntellectualArchive" journal).

 

Even if this hypothesis appears weird, in accordance to Occam razor it is more preferable than introduction of new type of matter.

[Strange]

 

Recently on www.IntellectualArchive.com I read ...

 

As you don't provide a link and that is a really badly organised web site, I can't be bothered to try and find it.

 

 

If "Dyson Spheres" exist - they produce gravity but do not produce radiation.

 

My understanding is that Dyson spheres are built around stars. Where does all that energy go, if no radiation leaves the sphere? The temperature inside would just keep getting higher and higher.

 

 

"Dyson Spheres" have big enough size and analysis of gravitational microlens rejects only small objects

 

I don't know where you the author gets that idea. These Dyson sphere would cause gravitational lensing and would also occlude other objects. If there were enough invisible stars (which is basically what you the author is proposing) to make up the required mass then they would be readily detectable. Do you Does the author provide a quantitative analysis?

 

Also, how do you does the author account for the fact that dark matter is distributed in a sphere around the galaxy, rather than just in the disk where the majority of the stars are.

 

And the existence of dark matter solves a number of other problems in cosmology. How do you does the author adress those?

 

And ... oh, I give up.

 

 

in accordance to Occam razor it is more preferable than introduction of new type of matter.

 

So, rather than the idea of a new form of matter (which is not an unprecedented event) you the author proposes some sort of advanced technological civilisation that has converted 5/6ths of the stars in the galaxy to Dyson spheres and moved them out of the plane of the galaxy?

 

I think your razor needs sharpening. And as for the author ...

Edited September 3, 2014 by Strange

[Janus]

Recently on www.IntellectualArchive.com I read the article that states that effects of hidden mass in galaxies can easily be explained if "Dyson Spheres" are real. If "Dyson Spheres" exist - they produce gravity but do not produce radiation. In this case the dark matter is not required at all. Current observations (like analysis of gravitational microlens) also do not reject this hypothesis, because "Dyson Spheres" have big enough size and analysis of gravitational microlens rejects only small objects. I can not put all details here. You can read it on IntellectualArchive.com. Number #1321 in Astronomy. (It is also published in "IntellectualArchive" journal).

 

Even if this hypothesis appears weird, in accordance to Occam razor it is more preferable than introduction of new type of matter.

There are a number of problems with this hypothesis.

 

1. To explain the galactic rotation curves we see, the extra mass has to be distributed a lot differently than the visible matter is. just adding extra matter to the galaxy is not enough. Why would a civilization build Dyson spheres around all the stars above and below the galactic disk and not those in the galactic disk?

 

2. Dyson sphere's would still need to radiate away waste heat. This is the only way that a civilization can make use of the energy it would trap from the star it surrounds.(basic thermodynamics) They may be dark at the visible spectrum, but wouldn't be so at other parts of the spectrum.

 

3. The extra mass for the Dyson spheres would be in the from of everyday baryonic matter. There are very strong reasons as to why the universe cannot contain very much more baryonic matter than what we visibly see. (for one thing, the relative ratios of elements would be much different)

 

4. It doesn't explain the Bullet Cluster observations. Here we see dark matter being separated away from visible matter in a colision of galaxy clusters(as evidenced by its gravity lens silhouette). This is possible because DM has different properties from baryonic matter and thus behaves differently. Dyson spheres, being made of the same type of matter as everything else, would not behave differently and would not be separated.

[Strange]

 

in accordance to Occam razor it is more preferable than introduction of new type of matter.

 

So, rather than the idea of a new form of matter (which is not an unprecedented event) you the author proposes some sort of advanced technological civilisation that has converted 5/6ths of the stars in the galaxy to Dyson spheres and moved them out of the plane of the galaxy?

 

And, of course, they have managed to make these spheres perfect insulators, which would probably require ... a new type of matter.

[SciChallenger]

Hi Strange,

Thank you for comments.

1. For me it was not a problem to find this article in IntellectualArchive.com. I even put the reference number #1321 in my initial post. Here is direct link for your convenience: http://www.intellectualarchive.com/?link=item&id=1321
2. Article answers almost all the questions you asked (like excessive heat, microlensing graphs etc) . Obviously I cannot put the entire article here. Always read articles first and criticize later.

Edited September 3, 2014 by SciChallenger

[Strange]

 

Article answers almost all the questions you asked (like excessive heat, microlensing graphs etc) .

 

 

Not really. You just make a few assertions and invoke "magic" (unknown advanced technology) to make the spheres invisible. It is speculation piled on speculation.

 

You also fail to account for the mass and distribution of dark matter.

 

Or address the other problems that dark matter solves.

 

Also, no Dyson spheres have yet been detected.

 

So, overall, your idea seems to be a worse fit for the available evidence than dark matter.

More reasons why it won't work:

http://physics.stackexchange.com/questions/2872/why-cant-the-missing-mass-evidence-for-dark-matter-be-matrioshka-brains-dyson

And what about the dark matter distribution around galactic clusters? Does this imply that your hypothetical advanced civilisation has not only moved stars out of the plane of the galaxy, but also into intergalactic space?

[SciChallenger]

Hi Janus,

Thank you for your comments.

 

Here are few replies.

>1. Why would a civilization build Dyson spheres around all the stars above and below the galactic disk and not those in the galactic disk?
Why not? How we can tell them where to locate their stars? We equally can ask “Why would a civilization build Dyson spheres in the galactic disk and not above and below the galactic disk?”

 

>2. Dyson sphere's would still need to radiate away waste heat. This is the only way that a civilization can make use of the energy it would trap from the star it surrounds.(basic thermodynamics)
No. It is NOT the only way. The answer presented in article. (Hint: m=E/c^2, remember? )

 

>3. The extra mass for the Dyson spheres would be in the from of everyday baryonic matter. There are very strong reasons as to why the universe cannot contain very much more baryonic matter than what we visibly see. (for one thing, the relative ratios of elements would be much different)

Models that cosmology tested a ‘priory included the dark matter as one of parameter and did not include the Dyson spheres.

Quoting mentioned article
"Another argument in favor of dark matter arrives from the analysis of cosmic microwave background radiation. Without access to this data it is difficult to say if it contradicts, is neutral or supports the “Dyson Spheres” hypothesis. For example, authors of the article Spergel at al. (2003), state from the beginning that they consider the model of “a flat Universe with radiation, baryons, cold dark matter and cosmological constant, and a power-law power spectrum of adiabatic primordial fluctuations”. They ruled-out the warm dark matter, but they never tested the model where “Dyson Spheres” is responsible for the hidden mass."

 

Thank you for comments any way!

Edited September 3, 2014 by SciChallenger

[Strange]

Models that cosmology tested a ‘priory included the dark matter as one of parameter and did not include the Dyson spheres.

 

Please show your analysis that including Dyson spheres in the model produces the amount of deuterium we see.

[EdEarl]

Also, how do you does the author account for the fact that dark matter is distributed in a sphere around the galaxy, rather than just in the disk where the majority of the stars are.

I hope not to hijack this thread, but...

 

Do we know why or are there any hypothesis to explain why matter tends to form a disk around galaxies and dark matter tends to form a sphere? Or, do we know that dark matter sometimes forms disks?

[Strange]

Do we know why or are there any hypothesis to explain why matter tends to form a disk around galaxies and dark matter tends to form a sphere? Or, do we know that dark matter sometimes forms disks?

 

I don't know the details, but it is because the gas and dust in the galaxy interacts with itself (as we expect gas and dust to). This means that energy can be dissipated and this, along with the conservation of angular momentum, means that it becomes a flattened disk.

 

This might explain it: http://physics.stackexchange.com/questions/25950/why-are-some-galaxies-flat

 

Because dark matter does not take part in electromagnetic interactions this doesn't happen. (Remember, it is electromagnetic force that makes things appear solid, allows molecules in gas to exert pressure, etc.)

[SciChallenger]

>Re: Dyson sphere's would still need to radiate away waste heat. This is the only way that a civilization can make use of the energy it would trap from the star it surrounds.(basic thermodynamics)

 

Found nice calculations of DS surface temperature if it would be built around our Sun at Pluto orbit. Posted by Rod Vance from Australia at

http://physics.stackexchange.com/questions/69649/thermodynamically-possible-to-hide-a-dyson-sphere

 

I.e. in accordance to "basic thermodinamics" temperature of DS would be about 60 degrees, or undetectable. (Even if we do not count the m=E/c^2 option).

Edited September 4, 2014 by SciChallenger