Any of the class of the heaviest → subatomic particles that
includes → protons, → neutrons,
as well as a number of short-lived particles
whose decay products include protons. Baryons obey the
→ Fermi-Dirac statistics. They form a subclass
of the → hadrons and
are further subdivided into → nucleons
and → hyperons.

In cosmology, one of a series of peaks and troughs that are present in the power spectrum
of matter fluctuations after the → recombination era, and on large
scales. At the time of the Big Bang, and for about 380,000 years
afterwards, Universe was ionized and photons and baryons were tightly
coupled. Acoustic oscillations arose from perturbations in the
primordial plasma due to the competition between gravitational
attraction and gas+photons pressure. After the epoch of recombination, these
oscillations froze and imprinted their signatures in both the
→ CMB and matter distribution. In the case of the photons,
the acoustic mode history is manifested as the high-contrast Doppler peaks in the
temperature anisotropies. As for baryons, they were in a similar
state, and when mixed with the non-oscillating → cold dark matter
perturbations, they left a small residual imprint in the clustering of
matter on very large scales, ~100 h^{-1}Mpc (h being the
→ Hubble constant
in units of 100 km s^{-1} Mpc^{-1}).
The phenomenon of BAOs, recently discovered
using the Sloan Digital Sky Survey data, is a confirmation of the current model of cosmology.
Like → Type Ia supernovae, BAOs
provide a → standard candle for determining cosmic distances.
The measurement of BAOs is therefore a powerful new technique for probing how
→ dark energy has affected the expansion of the Universe
(see, e.g., Eisenstein 2005, New Astronomy Reviews 49, 360; Percival et al. 2010, MNRAS 401,
2148).

The observation that in the present → Universe
there is → matter but not much
→ antimatter. Observations do not show the presence of
galaxies made of antimatter, nor gamma rays are observed
that would be produced if large entities of
antimatter would undergo → annihilation
with matter. However, the → early Universe
could have been baryon symmetric, and for some reason the matter excess has
been generated, through some process called → baryogenesis.
→ Sakharov conditions.

1) The difference between the total number of → baryons and
the total number of → antibaryons in a system of
→ subatomic particles.
It is a measure of → baryon asymmetry and is
defined by the quantity
η = (n_{b} - n_{b-})/n_{γ},
called the → baryon-photon ratio,
where n_{b} is the → comoving number
density of baryons, n_{b-} is the number of
antibaryons, and n_{γ} is that of photons. The value of η for
the → cosmic microwave background radiation (CMBR)
has been very well determined by the → WMAP satellite to be
η = (6.14 ± 0.25) x 10^{-10}. The baryon number is assumed to be
constant. The photons created in
stars amount to only a small fraction, less than 1%, of those in the CMBR.
2) A property of an → elementary particle represented by a
→ quantum number. It is
equal to +1 for a baryon and -1 for an antibaryon.
→ Bosons, → leptons, and
→ mesons have a baryon number B = 0.
→ Quarks and → antiquarks
have baryon numbers of B = +1/3 and -1/3, respectively.
The baryon number is → conserved in all observed types of
particle-particle interaction.

The → baryon number compared with the number of photons in the
→ Universe. The baryon-photon ratio can be estimated in a
simple way. The
→ energy density associated with
→ blackbody radiation of → temperatureT is aT^{4}, and the mean energy per photon is
~kT. Therefore, the number density of blackbody photons for T = 2.7 K is:
n_{γ} = aT^{4}/kT = 3.7 x 10^{2}
photons cm^{-3}, where a = 7.6 x 10^{-15}
erg cm^{-3} K^{-4} (→ radiation density constant)
and k = 1.38 x 10^{-16} erg K^{-1}
(→ Boltzmann's constant). The number density of baryons can be
expressed by ρ_{m}/m_{p},
where ρ_{m} is the mass density of the Universe and
m_{p} is the mass of the → proton
(1.66 x 10^{-24} g). → CMB
measurements show that the baryonic mean density is ρ_{m} =
4.2 x 10^{-31} g cm^{-3} (roughly 5% of the
→ critical density). This leads to the value of ~ 2 x 10^{-7}
for the number density of baryons.
Thus, the baryon/photon ratio is approximately equal to
η = n_{b}/n_{γ} =
2 x 10^{-7}/3.7 x 10^{2} ~ 5 x 10^{-10}. In other words,
for each baryon in the Universe there is 10^{10} photons. This estimate
is in agreement with the precise value of the baryon-photon ratio
6.14 x 10^{-10} derived with the → WMAP.
Since the photon number and the baryon number are conserved,
the baryon-photon ratio stays constant as the Universe expands.

→ Dark matter made up of → baryons
that are not luminous enough to produce any detectable radiation. It is
generally believed that most dark matter is → non-baryonic.
The baryonic dark matter could reside in a number of forms, including cold gas and
compact objects.

Matter that, unlike the ordinary matter, is not made of baryons
(including the neutrons and protons). It is proposed as a possible
constituent of dark matter.