What is the source of gravitational repulsion in certain cases?

[juan avellaneda]

i was wondering this

if the nuclear force overcomes the electromagnetic force in the nucleus, then what prevents the nuclei for colapse itself

i read in somewhere this
" the nuclear force has a repulsive core which prevents the nuclei from collapsing in on themselves"

what is this "repulsive core"?

 

[xeguy]

The potential energy between two nucleons obeys the Yukawa potential. At small separation distances the potential energy is positive, and there is a strong repulsive force between the two nucleons. At larger distances, the potential energy is negative, and there is an attractive force. You can search for Yukawa potential and find out everything you want to know.

 

[juan avellaneda]

nuclei

many thanks for your reply, it seems that all forces are not always attractive or repulsive, but dependes of quarks spin orientation in the particles
thats what i understood in some lecture i attend

 

[FZ+]

Except gravity. As far as we can tell, gravity is always attractive.

 

[pmb_phy]

Originally posted by FZ+
Except gravity. As far as we can tell, gravity is always attractive.

In some cases gravity is replusive. This happens because pressure is also a source of gravity and because a non-zero cosmological constant gives gravitational repulsion.

When there is a negative pressure present in that source which is so large as to overcome the attractive source due to other sources like mass terms in Einstein's equations, then there can be a resulsive effect.

A vacuum domain wall is a well-known example from cosmology.