# What would be the physics of a hypothetical gluon star?

1. Aug 13, 2015

### kodama

imagine a hypothetical star composed of gluon stars, color neutral on whole, consisting solely of gravitationally bound glueballs. what would be its expected physics ? i.e for the mass of the sun, what would its radius be? would it emit radiation and if so what spectrum? would it be stable or would it explode or collapse into a black hole? gravity compresses the glueball. would the gluons create degenerate pressure to counteract gravity?

the reason for this are suggestions of quantum gravity-qcd squared descriptions.

2. Aug 13, 2015

### mathman

Is there any evidence that gluons can exist by themselves, i.e. not part of hadrons?

3. Aug 13, 2015

### fzero

QCD glueballs are not stable, since they mix with mesons. What this means is that, since the glueball mass is expected to be $\geq 1~\text{GeV}$, there is enough energy available to pair produce quarks to form pions, etc. At low densities, these mesons would decay to leptons and photons and it's not clear that you would produce stars.

I'm not sure what you're talking about here, but it probably has something to do with the Kawai-Lewellen-Tye relations between open (gauge theory) string and closed (gravity) string amplitudes. This certainly doesn't imply that gravity is caused by QCD in any way. If there is a gauge dual to the gravitational description of some theory or object, it is unlikely that the gauge theory could be interpreted as QCD itself.

4. Aug 13, 2015

### kodama

maybe inside a collapsing star or neutron or magnetar star is a gravitationally bound glueball, or that inside black holes, gravitons become gluons.

5. Aug 13, 2015

### mitchell porter

You could have a gluon star (a gravitationally bound gluon plasma) if physics was just pure QCD (no quarks) coupled to gravity.

No comment on the deeper issues (KLT etc).

6. Aug 13, 2015

### kodama

what would be the physical properties of a gluon star with mass of say the sun? what would its diameter be would it radiate particles? would it explode or collapse or be sustained by degenerate pressure? would it have color?

7. Aug 14, 2015

### mitchell porter

I don't know, though there wouldn't be degeneracy pressure since there are no fermions in this scenario.

At the classical level, many papers have been written on "Einstein-Yang-Mills solitons". (Even more papers have been written on "boson stars", but those are always about objects made of spin-0 bosons, and this gluon star is a "gauge boson star" made of spin-1 bosons.) A first approximation to the classical gluon star might be a spherically symmetric soliton in an SU(3) field, plus the gravitational field it generates.

Meanwhile, the pure gluon plasma is apparently a mixture of glueballs and gluons. So there will be a hydrodynamics (viscosity etc) and a thermodynamics (relations between temperature, pressure, and relative abundance of glueball types) arising from this, which would govern the macroscopic behavior of the quantum gluon star. One question is whether it gets turbulent enough to irreversibly eject some of its mass (analogous to solar wind) - that's the only form of radiation I can see. Another is whether it has chromoelectric and chromomagnetic fields that extend far into space, or whether they are confined. Apart from that, I guess the main issue is whether it collapses into a black hole, or whether temperature and the repulsive aspects of QCD forces are enough to keep it extended beyond the Schwarzschild radius.

All this is unlike the real world because in the real world, standard model gluons couple to quarks, which couple to photons, which couple to leptons, so even if you could somehow make a pure gluon plasma, you would get the other particles being produced (as fzero said), and you would end up with some sort of ordinary celestial object. So this might seem to be a purely mathematical or hypothetical investigation.

However, there is one place where pure gluon stars could exist, and that is in the dark matter. If the dark sector of physics contained a new SU(n) field and no fermions, there really could be "dark glueballs" and even "dark gluon stars" if the physics permitted it. But that's just one arbitrary possibility among very many.

8. Aug 14, 2015

### Staff: Mentor

If any of the participants have a valid reference for such stars, please PM me and I can re-open the thread. Otherwise this is going to have to remain closed.

Edit: Mitchell porter will post some references for focusing the discussion. So I will reopen the thread.

Last edited: Aug 14, 2015
9. Aug 14, 2015

### mitchell porter

Thank you Dale.

There are several topics crossing over here.

... The physics of a hypothetical class of astronomical object, a type of so-called "boson star".

"Dynamical Boson Stars" is a big review of boson stars in general. Most of the work is for scalar fields but section 3.9 reviews work on "gauged boson stars".

"Gravitating Non-Abelian Solitons and Black Holes with Yang-Mills Fields" reviews that topic at the classical level.

"Boson stars from a gauged condensate" is a paper developing a quantum model of an SU(3) boson star, specifically.

... Various theoretical trends in quantum gravity, especially the "double copy relation" or "BCJ duality" or "color-kinematics duality" which is sometimes informally expressed as "gravity is gauge theory squared" or "gravitons are gluons squared". "Perturbative Quantum Gravity as a Double Copy of Gauge Theory" might be the fundamental paper here.

Concerning the reference to "gluons"... In some theoretical discussions, "gluon" is used as a shorthand expression for "gauge boson". This is one of them. So the double-copy theorists aren't saying that the strong nuclear force, specifically, has a special relationship to gravity; they're saying that the general formulas of gauge field theory have this relationship to those of gravity.

... Hypothetical quantum-gravitational properties of known astronomical objects

Another huge topic, which includes Hawking radiation, black hole evaporation, and most theories of what's inside the event horizon. In the context of this discussion, I would mention:

Boson stars as models of neutron stars and black holes (see my first link, section 5).

Appearance of boson stars in AdS/CFT duality.

Implications of double copy relation for black holes.

10. Aug 14, 2015

### kodama

is it possible there is a special relationship between gluons and gravitons, perhaps inside black holes are gluon stars? glueballs can be spin-2