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Dark Matter

Strikeeagle84015
Posts: 867
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7/18/2010 1:56:39 PM
Posted: 6 years ago
I once came across a post in a forum about dark matter and it said that the primary support for dark matter is because of certain behavior of celestial objects that means
1)That gravity acts differently over long distances
or
2)There is matter that we cannot see but we can observe it's gravitational effects
and it is believed that 2 is the correct answer and this unseen matter is known as dark matter.
But what about the 1st possible answer that gravity behaves different over large distances. We have shown that gravity behaves quite differently on an atomic and subatomic level, so isn't it possible that gravity behaves differently on super large levels?
Discuss
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Puck
Posts: 6,457
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7/18/2010 4:27:27 PM
Posted: 6 years ago
At 7/18/2010 1:56:39 PM, Strikeeagle84015 wrote:
I once came across a post in a forum about dark matter and it said that the primary support for dark matter is because of certain behavior of celestial objects that means
1)That gravity acts differently over long distances
or
2)There is matter that we cannot see but we can observe it's gravitational effects
and it is believed that 2 is the correct answer and this unseen matter is known as dark matter.
But what about the 1st possible answer that gravity behaves different over large distances. We have shown that gravity behaves quite differently on an atomic and subatomic level, so isn't it possible that gravity behaves differently on super large levels?
Discuss

Dark energy/dark matter are place holder terms for effects we can observe but have no concrete way of explaining - in this case the way gravity works on stellar objects on massive scales. The observations don't fit with 'standard' models. Something is going on.

Gravity is a result of interaction between particles. This is why 'new' particles (dark matter) are used to describe the observations. Since our understanding of particles we know don't act in the ways that fit the observations we see, it results in the unknown new particles idea.

Now, for you to say, what if gravity just "acts" differently, you are saying that particles that we know about, act differently, at "long distances" (it's not distance it's size btw - galaxy and above level) - which is slightly nonsensical because those same particles would be present no matter and so would be observed to act "differently" anyway (in short it would be part of the normal information we have about those particles). The concern is why galaxies or clusters of galaxies act they way they do where gravity acts differently on those scales. Gravity is interaction between particles - different observations of gravity means most likely different type(s) of particles.
tvellalott
Posts: 10,864
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7/18/2010 8:28:39 PM
Posted: 6 years ago
At 7/18/2010 4:27:27 PM, Puck wrote:
At 7/18/2010 1:56:39 PM, Strikeeagle84015 wrote:
I once came across a post in a forum about dark matter and it said that the primary support for dark matter is because of certain behavior of celestial objects that means
1)That gravity acts differently over long distances
or
2)There is matter that we cannot see but we can observe it's gravitational effects
and it is believed that 2 is the correct answer and this unseen matter is known as dark matter.
But what about the 1st possible answer that gravity behaves different over large distances. We have shown that gravity behaves quite differently on an atomic and subatomic level, so isn't it possible that gravity behaves differently on super large levels?
Discuss

Dark energy/dark matter are place holder terms for effects we can observe but have no concrete way of explaining - in this case the way gravity works on stellar objects on massive scales. The observations don't fit with 'standard' models. Something is going on.

Gravity is a result of interaction between particles. This is why 'new' particles (dark matter) are used to describe the observations. Since our understanding of particles we know don't act in the ways that fit the observations we see, it results in the unknown new particles idea.

Now, for you to say, what if gravity just "acts" differently, you are saying that particles that we know about, act differently, at "long distances" (it's not distance it's size btw - galaxy and above level) - which is slightly nonsensical because those same particles would be present no matter and so would be observed to act "differently" anyway (in short it would be part of the normal information we have about those particles). The concern is why galaxies or clusters of galaxies act they way they do where gravity acts differently on those scales. Gravity is interaction between particles - different observations of gravity means most likely different type(s) of particles.

Duuuh, yor reel smert Puck.

Seriously though, I knew of Dark Matter but I hadn't really read much about it. What you're saying it that we use Dark Matter to describe matter that acts differently in identical situations to regular matter?
I thought Dark Matter was the fuel they used in Futurama.
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belle
Posts: 4,113
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7/18/2010 8:50:16 PM
Posted: 6 years ago
At 7/18/2010 1:56:39 PM, Strikeeagle84015 wrote:
I once came across a post in a forum about dark matter and it said that the primary support for dark matter is because of certain behavior of celestial objects that means
1)That gravity acts differently over long distances
or
2)There is matter that we cannot see but we can observe it's gravitational effects
and it is believed that 2 is the correct answer and this unseen matter is known as dark matter.
But what about the 1st possible answer that gravity behaves different over large distances. We have shown that gravity behaves quite differently on an atomic and subatomic level, so isn't it possible that gravity behaves differently on super large levels?
Discuss

well no, the idea is that there are wrinkles we haven't discovered yet, rather like einstein revised newton's conception of gravity. perhaps our equations are not accurate models of how gravity behaves over especially large scales. however, thats most likely NOT the case, given that "modified gravity" models tend to be less accurate and require excessive assumptions. on the other hand, predictions involving WIMPs (weakly interacting massive particles :D ) explain not only the observed effects of gravity but also help to illuminate new findings regarding small variations in the cosmic microwave background and the ultimate form that the large-scale structure of the universe eventually took. so. 2 fits the data better overall.
evidently i only come to ddo to avoid doing homework...
belle
Posts: 4,113
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7/18/2010 8:54:41 PM
Posted: 6 years ago
At 7/18/2010 8:28:39 PM, tvellalott wrote:
Seriously though, I knew of Dark Matter but I hadn't really read much about it. What you're saying it that we use Dark Matter to describe matter that acts differently in identical situations to regular matter?
I thought Dark Matter was the fuel they used in Futurama.

no. when people use the term "dark matter" they are saying "that stuff we can't find that causes these effects we've detected". thats it. in truth, it is thought to behave differently than normal matter in that it is weakly interacting. and it is well known that weakly interacting particles exist (neutrinos!) though they aren't nearly massive enough. but still, further support for the idea of WIMPs. and as a side note, weakly interacting in this case means not subject to the electromagnetic force. we can't see them because they don't interact with photons. i hope you find my cosmology textbook as illuminating as i did on the subject:

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evidently i only come to ddo to avoid doing homework...
brian_eggleston
Posts: 3,347
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7/19/2010 6:16:47 AM
Posted: 6 years ago
My idea is that the explanation of dark matter can be simply found by looking at photons (light particles).

Photons are traditionally said to have no mass because mass can only be measured at rest, which photons never are, of course.

However, we can see that photons are affected by gravity: their paths are deflected by super-massive objects such as large stars and they can be trapped by the gravitational pull of black holes.

Now, since we know that photons have energy and energy equals mass, in special relativity, we are able to define the concept of energy as E, such that E has simple and well-defined properties just like those it has in Newtonian mechanics.

So, when a particle has been accelerated so that it has some momentum p (the length of the vector p) and relativistic mass mrel, then its energy E turns out to be given by

E2 = p2c2 + m2restc4

This formula can then be used to calculate the mass of all the photons ever emitted since the birth of the universe and it should roughly account for the 'mysterious' dark matter.

Puck, if you get a PhD and a Nobel prize for science on the strength of this, you owe me a pint, okay?
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RoyLatham
Posts: 4,488
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7/21/2010 8:04:48 PM
Posted: 6 years ago
Dark matter is pretty well established at this point as being weakly interactive massive particles (WIMPs). The "weakly interactive" part means they do not interact with photons, so they are very difficult to detect, basically transparent to all tests except gravity. While weakly interactive, there is still the possibility they might be detected.

Dark energy http://en.wikipedia.org... is much less well settled as to what it is and how it works. the main theory, as I understand it, is that it is the energy inherent in vacuum. As the universe expands, new vacuum is created and with it new vacuum energy. The gravitational attraction of the vacuum energy explains while the expansion of the universe is accelerating. Current theory is that 74% of the universe is dark energy, 22% dark matter, 3.6% interstellar gas, and 0.4% all the stuff we can see, like stars.

I have never heard of a problem with gravity not working as expected at atomic distances. The gravitational force is overwhelmed by the other forces (strong, weak, electromagnetic) at short distances.
brian_eggleston
Posts: 3,347
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7/22/2010 8:08:21 AM
Posted: 6 years ago
At 7/21/2010 8:04:48 PM, RoyLatham wrote:

I have never heard of a problem with gravity not working as expected at atomic distances. The gravitational force is overwhelmed by the other forces (strong, weak, electromagnetic) at short distances.

Actually, I never thought of it that way, but it is obvious really.

Previously I assumed that different laws applied to particle physics but gravity does have an influence, it is just that with gravity being such an incredibly weak force, it has no discernible influence at atomic levels.
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