Dark Matter and Its Relation to Energy

[PhanthomJay]

If ordinary matter is related to energy per E=mc^2, then can dark matter also be similarly related, and if so, is there a name for this energy? I understand that "dark energy" (the cosmological constant in the expanding universe) and "dark matter" bear no relationship, so I'm wondering that if dark matter = E/c^2, has a name been coined for "E" to distinguish it from the existing definition of dark energy.

 

[Chalnoth]

If ordinary matter is related to energy per E=mc^2, then can dark matter also be similarly related, and if so, is there a name for this energy? I understand that "dark energy" (the cosmological constant in the expanding universe) and "dark matter" bear no relationship, so I'm wondering that if dark matter = E/c^2, has a name been coined for "E" to distinguish it from the existing definition of dark energy.

The equation E = mc^2 is a statement of what we mean when we talk about mass: the equation states that mass is the energy in the internal degrees of freedom of an object. For example, if I have a potato, and I heat it up (add energy internal to the potato), then I increase its mass.

This is one of the basic conclusions of relativity, that mass is just another word for "energy inside something". Therefore dark matter must also have this relationship.

However, this has nothing to do with dark energy. What distinguishes dark matter and dark energy is pressure. Specifically, dark matter experiences effectively zero pressure on cosmological scales, while dark energy has a very large negative pressure. The zero pressure of dark matter makes it act, in many respects, just like normal matter. The negative pressure of dark energy, on the other hand, prevents almost any clumping of dark energy and causes an accelerated expansion of our universe.

 

[Phrak]

I think you're asking of the energy component of dark matter, and if this component has a name, right?

 

[AleLucca]

Dark matter differs from ordinary matter only in the way it interacts with stuff. Dark matter is now believed to be made of massive particles that interact only weakly. The point is: dark matter particles are believed to have a mass, so, yes, you can define their rest energy as E = mc^2, just like you do with ordinary matter.
However there is no particular name for the energy of dark matter.

 

[bapowell]

Dark matter differs from ordinary matter only in the way it interacts with stuff. Dark matter is now believed to be made of massive particles that interact only weakly.

There is no fundamental difference between dark matter and 'ordinary matter'. Ordinary matter can also interact weakly. I'm not pointing this out to be critical, but only because the confusion about dark matter is deep and rampant in these forums. We need to be careful!

 

[Chalnoth]

There is no fundamental difference between dark matter and 'ordinary matter'. Ordinary matter can also interact weakly. I'm not pointing this out to be critical, but only because the confusion about dark matter is deep and rampant in these forums. We need to be careful!

I think the point is that like neutrinos, dark matter does not interact with either the strong nuclear force or electromagnetism. (If it interacted electromagnetically, it would behave like normal matter on cosmological scales. If it interacted with the strong nuclear force, we would have detected it long before now.)

 

[AleLucca]

I think the point is that like neutrinos, dark matter does not interact with either the strong nuclear force or electromagnetism. (If it interacted electromagnetically, it would behave like normal matter on cosmological scales. If it interacted with the strong nuclear force, we would have detected it long before now.)

You are both right. Dark matter particles are believed to interact only through the weak and gravitational force, so they are not different from neutrinos in this aspect. If so, there is no fundamental difference between dark and ordinary matter

 

[Chalnoth]

You are both right. Dark matter particles are believed to interact only through the weak and gravitational force, so they are not different from neutrinos in this aspect. If so, there is no fundamental difference between dark and ordinary matter

Bear in mind that they are not necessarily posited to interact through the weak nuclear force. They may, but it isn't clear at this time. Obviously we hope that they do, so that direct detection is more likely.

 

[AleLucca]

I think the point is that like neutrinos, dark matter does not interact with either the strong nuclear force or electromagnetism. (If it interacted electromagnetically, it would behave like normal matter on cosmological scales. If it interacted with the strong nuclear force, we would have detected it long before now.)

Bear in mind that they are not necessarily posited to interact through the weak nuclear force. They may, but it isn't clear at this time. Obviously we hope that they do, so that direct detection is more likely.

and that's why I repeatedly used the word "believed" :)