Recent content by astrolollo

I don't think this is the case.. The high-z quasar has a continuum emission. A cloud at lower z will not interact with the radiation coming from the Ly-a or Ly-beta emission of the quasar, but with photons that were emitted at a higher energy. Since the cloud should be quite cold (~10 K ?)...

Hello everyone! We observe the so-called Lyman-alpha forest in the spectrum of distant quasars and it is said that these multiple absorption lines are due to the presence of intergalactic HI clouds that absorb light at the wavelenght of 1216 A , the Lyman-alpha transition. My question is the...

Thanks for the reply! Is this further contraction due to nuclear burning governed by the virial thm too?

Hello everyone, I know that pre-main sequence stars do heat up because of gravitational contraction, and the increase in internal energy (and so in temperature) comes from this shrinking and is governed by the virial theorem (...

Ok thanks. I guess though that the process is efficient only for very high energetic photons, since they will undergo a gravitational redshift anyway. Is this correct?

Hi everyone, I am a bit confused about the Penrose Process. Let's say a particle with energy E at infinity arrives in the ergoregion of a Kerr black hole and then it decays into two photons. One of them has L_1<0 and E_1<0 and hence it falls towards the singularity, while the other has L_2>0...

@Hornbein Ok, but how does this turbulence restore the polodial field? This is written everywhere but no one explains HOW this happens

Ok, thanks everyone. But I've never seen the geodesic equation written in terms of momentum. On the Internet I can only find something about deriving the geodesic equation from ##\nabla _{\nu}T^{\mu \nu} =0##. Moreover, for massive particles the conserved quantity in S. metric is...

But these conservation laws are not obvious. A mathematical proof is needed. What do you mean by "they're a matter of choosing the best definition"? I mean, it can be proved that if a metric admits a KVF one can define conserved quantities. I know how to prove that ##U_{\mu} V^{\mu}## is...

Ok, Thanks. Where can I find a proof of what you say, namely that the fundamental conserved quantity is ## V^{\mu} P_{\mu}## and not ##V^{\mu}U_{\mu}##?

If a metric admits a Killing vector field ##V ## it is possible to define conserved quantities: ## V^{\mu} u_{\mu}=const## where ## u^{\mu}## is the 4 velocity of a particle. For example, Schwarzschild metric admits a timelike Killing vector field. This means that the quantity ##g_{\mu 0}...

Hello everyone I have a question: I know what alpha/omega dinamo is, and I think I have understood how this model explains Joy's law and Hale's law, but I don't get how it allows the magnetic field to become poloidal again (after having become toroidal due to differential rotation) and with the...

Hello In Newtonian theory Poisson's equation holds: ## \nabla ^{2} U = 4 \pi G \rho ##. So: given a density ##\rho ##, it is possible to find a potential U. On the other hand, I can choose a random function U and give it a gravitational significance if it gives, by Poisson's eq., a density...

I know that from Killing equation the Killing field takes the form (for flat space): ##V_{\alpha}= c_{\alpha} + A_{\alpha \beta} x^{\beta}##. I know this vector field can be expressed as a linear combination of 10 certain vectors. I am wondering how it is possible that those 10 vectors are...

Hello everyone How is it possible that a n-dimensional spacetime admits m> n INDEPENDENT Killing vectors where m=n(n+1)/2 if the space is maximally symmetric?