l**n
发帖数: 67 | 1 Yesterday, i was talking with some people and they
mentioned that recently some kind of new star
has been observed. It seems to me that they call
it brown dwarf(my english is poor and very likely
misheard what they said:(() which are very very
infrared. The wavelength of light emitted is about
10,000A.(not because of the redshift but simply
because those stars are very cold) I wonder with
more and more colder star being found, do we still
need dark matter(non-baryonic matter)? | i*******n
发帖数: 166 | 2
Brown dwarfs are those stars with mass lower than, say,
0.08 M_sun so that they cannot ignite Hydrogen in their
lifetime. Dark matter cannot be all contributed by those
stuffs. In fact, if BBN is right, the observation has
already limited the baryonic density (in unit of critical
density) to be less than 0.1. And most of the baryonic
matter cannot be seen.
【在 l**n 的大作中提到】
: Yesterday, i was talking with some people and they
: mentioned that recently some kind of new star
: has been observed. It seems to me that they call
: it brown dwarf(my english is poor and very likely
: misheard what they said:(() which are very very
: infrared. The wavelength of light emitted is about
: 10,000A.(not because of the redshift but simply
: because those stars are very cold) I wonder with
: more and more colder star being found, do we still
: need dark matter(non-baryonic matter)?
| l**n
发帖数: 67 | 3
That's actually my question. What is the room for changing
the BBN instead of turning to dark matter.
How can you tell by observation that the baryonic density is
less than 0.1 while you cannot "see" those stars by the
current technology? What i mean is it is possible that
tomorrow morning when you get up you suddenly "see" far more
brown stars then you expect by BBN which you cannot see
with todays technology.
【在 i*******n 的大作中提到】
:
: Brown dwarfs are those stars with mass lower than, say,
: 0.08 M_sun so that they cannot ignite Hydrogen in their
: lifetime. Dark matter cannot be all contributed by those
: stuffs. In fact, if BBN is right, the observation has
: already limited the baryonic density (in unit of critical
: density) to be less than 0.1. And most of the baryonic
: matter cannot be seen.
| i*******n
发帖数: 166 | 4
About 0.1: BBN's prediction of the promordial light elements
abundance depends on only one free parameter - baryon-to-photon ratio
which is proportional to \Omega_B h^2. Observation of the promordial
abundances will give a range of this ratio. The observed promordial
abundances of He, D, Li are different by 9 order of magnitutes, while
surprisingly a very narrow range of \Omega_B h^2 (around 0.01 - 0.1)
can satisfy those which limits the baryonic density.
If you believe BBN and we know that
【在 l**n 的大作中提到】
:
: That's actually my question. What is the room for changing
: the BBN instead of turning to dark matter.
: How can you tell by observation that the baryonic density is
: less than 0.1 while you cannot "see" those stars by the
: current technology? What i mean is it is possible that
: tomorrow morning when you get up you suddenly "see" far more
: brown stars then you expect by BBN which you cannot see
: with todays technology.
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