l**n
发帖数: 67 | 1 Before we embark on more ambitious task, we'd better take a look at
another observational pillar of mordern cosmology, the 2.7K cosmic
microwave background radiation(CMBR). The angular power spectrum of
the CMB contains information on virtually all cosmological parameters
of interest,including the geometry of the universe, the Hubble
constant, \Omega_\Lambda, baryon density, number of neutrinos...
CMBR itself is a seperate branch in cosmology, so i am not going
to review the imprint of each para |
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c**********g
发帖数: 222 | 2
>>> it is proved that the freely expanding (adiabatic expansion)
photon field
will remain the balckbody spectrum and in thermal equilibrium as long
as it is
in equilibrium at the begining. You can try to prove it. it is fun.
>>>> yes, the photon energy is not conserved. The energy density drops
at the
rate of V^(-4/3) or (1+z)^4 or a^(-4). V is the considered volume. a
is the scale
factor. for the ideal fluid with p=w \rho. (w is a constant. \rho is
density and
p is pressure), then \rho drop |
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f*******d
发帖数: 339 | 3
long
The photon field would keep its blackbody spectrum during
expansion of the Universe, but it is NOT in thermal
equilibrium! It has decoupled from matter since recombination. The
self-interaction of the photoes are also neglible.
sense.
photon
drops
no
etc.
I am not sure I understand your later statement. The cosmic virial
theorem
relates potential energy and peculiar velocity, but here he is talking
about uniform motion, there is no potential enengy and peculiar
velocity? |
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i*******n
发帖数: 166 | 4
it is energy density per unit frequency
I would like to say:
\pho(\nu) d\nu is the energy density of the photon with energy
at the range h\nu to h(\nu+d\nu)
To my point of view, some other equations in physics books also
has such kind of inaccurate description which make students uncomfortable
and sometimes it may lead to wrong understanding.
It is still a unresolved problem to find a suitable energy-momentum
Tensor in GR. Currently, energy conservation is regard |
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c**********g
发帖数: 222 | 5
fun.
>>>>Here, I mean that the photon field itself is in thermal
equilibrium. Because
the photon is decoupled from the matter after recombation, it expandes
adaibatically. Its evolution is the same as that of the closed box of
photon
in adaibatic expansion. For this reason I say that it is in thermal
equibrium
with itself though no interaction at all.
a
is
the
not
talking
Yes, you are absolutly right. Because gravity play a cruical role
in cosmology,
I cite the cosmic energy theorem just i |
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l**n
发帖数: 67 | 6 Our discussion of the CMBR is far from completion. And I am still
not 100% convinced where does the energy of the CMB photon go? Let's
take a typical attitude of physicist, find a problem, couldn't solve
it and forget about it:) Commented by the experts, the blackbody
spectrum could be preserved when the photons decoupled from the
matter which acts the wall in the typical black cavity. To us non-
expert we should just be aware of the fact that the space and the
wave-length of the photon are stre |
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c**********g
发帖数: 222 | 7
CMBR.
the
???
distance.
equivalent
I also think that you are wrong here. let's consider the photon's
equation.
c^2 dt^2=a^2 dr^2/(1-kr^2). here, k=-1,0,1. then c dt=a dr/Sqrt[1-k
r^2];
So, we know the meaning of c*(t_now-t_then). it is the PHYSICAL
distance that
the photon travel. But it is not either angular diameter distance,
comoving distance or luminosity distance. The reason is due to the a
or the
expansion of the universe. Because the universe is expanding, this d
i |
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