385 reputation
16
bio website savannah.nongnu.org/projects/…
location Rio de Janeiro, Brazil
age 34
visits member for 1 year, 11 months
seen yesterday

Theoretical Physicist and Cosmologist. Currently doing research on theoretical, observational and numeric cosmology and quantum field theory in curved space time.


Mar
1
comment Conservation of stress-energy and the fluid equation in cosmology
That's why I wrote the problem in a coordinate free way, I showed what is the exact mean to the action of choosing the value of a free index. You can write the vector $n^\mu$ in a coordinate system in which $n^\mu \doteq (1,0,0,0)$, then, in this case you have T_{\mu\nu}n^\nu = T_{\mu0}, however the presence of a covariant derivative complicates things since it does not commute with the normal vector.
Mar
1
comment Conservation of stress-energy and the fluid equation in cosmology
Did you read the last paragraph?
Mar
1
answered Conservation of stress-energy and the fluid equation in cosmology
Sep
4
awarded  Yearling
Aug
29
comment Why the temperature is getting lower when the universe is expanding
In your comment you give examples of possible energy definitions for the gravitational field, that is why I pointed to the first paragraph. If you are concerned with the definition of energy for the matter content, note that the energy density is defined as $\rho_{(v)} = T^{\mu\nu}v_\mu{}v_\nu$ for some global timelike vector field $v^\mu$. This means that we need to be able to define global spatial sections (Frobenius's Theorem) and that the energy definition is frame dependent as usual. Finally, in the answer I used the FLRW homogeneous and isotropic frame to define energy.
Aug
29
comment Why the temperature is getting lower when the universe is expanding
@BenCrowell did you read the first paragraph of my answer?
Mar
5
answered Why the temperature is getting lower when the universe is expanding
Mar
5
answered Relation between the determinants of metric tensors
Sep
21
awarded  Custodian
Sep
21
reviewed Reviewed High speed and low speed photons
Sep
21
comment High speed and low speed photons
This supernova would come from outside of our observable universe and on top of that from before the big bang? Sorry but this and the rest are just nonsense.
Sep
21
awarded  Critic
Sep
21
reviewed Reviewed de sitter cosmologic limit
Sep
21
comment de sitter cosmologic limit
To throw ideas is easy. The difficult part is to know the literature, the observations and work on the ideas to match everything already known (and more).
Sep
21
comment de sitter cosmologic limit
People sometimes forget that there are several observational support for a hot dense epoch. The point is, ok, you may assume that all CMB radiation comes from other place. But to take your idea seriously you have to be compatible with all other observations. For instance, say that you explain the black body radiation, what about the $10^{-5}$ perturbations around it with an almost Harrison-Zeldovich spectrum? And how can you explain the abundance of light elements with a radiation dominated phase? The baryonic acoustic oscillations which match the CMB perturbations?
Sep
21
awarded  Autobiographer
Sep
20
answered Where does this equation for a perturbed metric come from?
Sep
19
answered Cosmological relativistic effects : misunderstanding between cosmological and relativistic communities?
Sep
19
comment Could Dark Matter be a manifestation of tachyons tunnelling between black holes?
@JohnRRamsden As a general note, all these mechanisms like inflation, dark matter and dark energy are very much constrained by the inner works of the standard cosmological model and some parts of it by observations. It is necessary to understand very well all these mechanisms before trying to propose a new one. Only after you know all the constraints and all things that dark matter has to explain you will be able to suggest something without bumping into these constraints.
Sep
19
comment Could Dark Matter be a manifestation of tachyons tunnelling between black holes?
@JohnRRamsden Actually, in the final phase of inflation the spatial curvature is almost zero (for most of the inflationary models), also the perturbations are of order $~10^{-5}$, not enough to generate any kind of structure. On top of that, in general, is believed that all matter is created (or heated) after the end of inflation, so we don't usually want dark matter in the end of inflation.