AstroNuclPhysics ® Nuclear Physics - Astrophysics - Cosmology - Philosophy | Gravity, black holes and physics |
Chapter 5
GRAVITATION
AND THE GLOBAL STRUCTURE OF THE UNIVERSE :
RELATIVISTIC COSMOLOGY
5.1. Basic
starting points and principles of cosmology
5.2. Einstein's
and deSitter's universe. Cosmological
constant.
5.3. Fridman's dynamic models of the universe
5.4. Standard
cosmological model. Big
Bang.
5.5. Microphysics
and cosmology. Inflationary
universe.
5.6. The
future of the universe. time
arrow. Dark matter. Dark
energy
5.7. Anthropic
principle and existence of multiple universes 5.8.
Cosmology and physics
5.3. Fridman's dynamic models of the universe
Obviously, the real universe, at least at its current stage of development, cannot be described by any of the models based on the assumption of static, because in Einstein's model there is no redshift of light from distant galaxies, and in de Sitter's model space cannot contain any matter or radiation. To model the real universe, it is therefore necessary to abandon the assumption of static (which is incompatible with current astronomical knowledge) and create a more general time - dynamic cosmological model.
Metrics of
a non-stationary homogeneous isotropic universe
We will therefore consider a homogeneous isotropic universe,
which will generally not
be stationary . The metric of three-dimensional space (ie the spatial part of the interval) in such a case will again have a
homogeneous isotropic general shape (5.4), but the radius of
curvature a here will generally be a function of time
a(t) :
(5.21) |
Recall that originally in relation (5.4) we
marked the value of the curvature of the space "R" (R :
- >
a) as "a" . However, in an application to a dynamically
expanding universe, a (t) will become important as a scale
expansion factor . The length element (5.21) is sometimes
modified to a shape in Cartesian
coordinates ,
in which it is proportional to the corresponding Euclidean
expression (however, this is for
illustration only, it is not relevant for cosmological analysis) . This can be done by
introducing a new coordinate nice r
using the transformation R ®
r / (1 + r ^{2} / a ^{2} ). The spatial metric (5.21) then has the form
^{ }
. |
It is natural to choose a spatial reference system to reflect the isotropy of space as well as the distribution and motion of matter. Thus, the most suitable is a locally "parallel" reference system moving in each place of space together with the matter contained therein. The coordinates are "carried" along with the extension. The local velocity of matter in such a system is thus zero everywhere, the reference system is formed by the matter filling the universe itself. All motion of mass is expressed deformation reference system (cosmological "mollusc") . It is therefore appropriate to transform the spherically symmetric coordinate system by replacing the original radial coordinate r with a parallel coordinate: r (t) ® r (t) .a(t). Or, because the radius of curvature a can be used as a natural unit for distance measurement, it is advantageous to introduce new dimensionless coordinates r ® r /a, in which the length element has the shape
(5.21 ') |
The distance dl
between any nearby points is thus proportional to the a (t) - scale factor , so that increase or decrease and (t)
over time means increasing or decreasing all distances in the
system - expanding or shrinking all matter carried by space ( cf. discussion in § 5.4,
passage " What expands during the
expansion of the universe? (And what does not expand) " ) .
^{ }It is advisable to choose the
time coordinate so that the space metric is the same at all
points and in all directions at all times. To all directions were
equivalent components g_{o}_{a} of metric tensor must be in this
reference frame zero. The space-time metric will therefore have
the form ds^{2} = g _{oo} dx° ^{2} + dl ^{2} . The coefficient g _{oo} is a function of only x °, so g
_{oo} = -1 (= -c ^{2} ) can be achieved by a suitable
choice of the time coordinate . The time coordinate x °, which
we can denote by t , then gives its own time at
each point in space. The space-time interval here will have the
simple form ds ^{2} = -c ^{2} dt ^{2} + dl ^{2} .
^{ }The spacetime
geometry of a homogeneous isotropic universe
can therefore be generally written in the form of the so-called Robertson-Walker-Fridman metric
:^{ }
(5.22) |
where a(t) is the scale factor, k is the
space curvature type parameter and r is the radial
coordinate in the above-mentioned "parallel-concurent"
reference system, in spherical coordinates (r, J , j ) with the origin r = 0 at any point. All time dependence
- the evolution of the universe - is contained here only in the
scale factor a (t).
As early as the mid-1930s, H.P.Robertson
and A.G.Walker showed, that the expression of type (5.22) is the most
general metric that can describe an expanding homogeneous
and isotropic universe.
^{ }To determine the time evolution of a (t), we use
Einstein's equations, where we substitute the necessary
components of the Ricci curvature tensor R _{ik}^{ }and scalar
curvature R, which we calculate from metric (5.22) (using the methods from §2.4) .
The non-zero components here are :
R_{tt} = -3.ä/a ; R_{rr} = [(a.ä + 2.^{.}a^{2})/c^{2} + 2k]/(1-kr^{2}) ; R_{JJ} = r^{2}.[(a.ä + 2.^{.}a^{2})/c^{2}+ 2k] ; R_{jj} = r^{2}.[(a.ä + 2.^{.}a^{2})/c^{2}+ 2k].sin^{2}J ;
R = 6.[ä/(c^{2}.a) + ^{.}a^{2}/(c^{2}.a^{2}) + k/a^{2}] ,
where each dot above a represents a derivative according
to time t .
Fridman´s
space evolution equation
Equation evolution space resulting from general Einstein equation
(5.7) R_{ik} - ^{1}/_{2} g_{ik} R - L.g_{ik} = 8pT_{ik} , where substitute components of
the tensor of energy-momentum T _{ik} components metric tensor g _{ik} and the Ricci curvature tensor R
_{ik} for metric (5.22) (calculated
above) . The
energy-momentum tensor T ^{ik }of the
cosmological "gas" (3.3) in everywhere locally
quiescent reference system has non-zero components only T ^{o }_{o} = r. c ^{2} , T ^{1
}_{1} =
T ^{2 }_{2} = T ^{3
}_{3} =
-p, where in a homogeneous and isotropic universe the density r and the pressure p can only be functions
of time t . Einstein's equations (5.7) for the
metric (5.22) then lead to two differential equations after
adjustment - they are important
Fridman's equations : |
(5.23a) (5.23b) ^{ } |
which are the basis of relativistic cosmology . Together with the equation of state p = p ( r ) of a cosmological substance (gas, liquid, plasma, radiation), these equations make it possible to determine a, p, r as a function of time t , ie to determine the evolution of the universe . Each dot above a denotes the derivative by time. Both of these equations are related by identity
d ( r .c ^{2} and ^{3} ) / dt = - p. d (a ^{3} ) / dt, | (5.23c) |
which is an expression
of the local law of conservation of energy .
Orthographic note :
In the literature, the author's name is usually written " Friedmann
", but the St. Petersburg author of cosmological equations
was actually written " Fridman
" ( ).
^{ }Fridman's equations
(5.23) are an exact solution of Einstein's
equations of general relativity for a homogeneous and isotropic
universe with Robertson-Walker metric (5.22), filled with a
substance with density r , showing pressure p (according
to the corresponding equation of state) ,
with energy-momentum tensor (5.3).
In relativistic cosmology, instead of absolute values of time changes of the scale factor ^{. }a = da / dt, ä = d ^{2} and / dt ^{2} , relative quantities derived from them are often introduced, which can be (at least in principle) directly measured from astronomical observations. The Hubble constant H * is used as a measure of the relative rate of change of the radius of curvature, i.e., the rate of expansion (or compression ); a more apt name should be Hubble speed :
H =^{def} ^{.}a(t) / a(t). | (5.24) |
*) The quantity H is called a "constant" only in the sense that it is the same for all places (it does not depend on the coordinates); in general, however, it can be a function of time H (t). A more apt name is therefore the Hubble parameter . The current value of the Hubble constant H _{o} was estimated mostly in the range H _{o} » (50 ¸ 100) km s ^{-1} / megaparsec. Newer astronomical measurements give a more precise value:
H _{0} » 67 km s ^{-1} / megaparsec . ^{ } | (5.24´) |
In addition to the "classical" Hubble constant H , the so-called reduced dimensionless Hubble constant h , normalized to a speed of 100 km s ^{-1} /^{ }Mpc, is sometimes used :
h = ^{def} H _{0} / 100 km and ^{-1} / Mpc . | (5.24´´) |
The Hubble constant H was already
introduced in §5.1, relation (5.2), as a coefficient of
proportionality in Hubble's law of expansion, which
determines by how much the velocity [km / s] of a distant cosmic
object (galaxy) increases when its distance increases by 1
megaparsec. : v = H . r, where r is the
distance and v is the retraction speed. The velocity of
the distance is astronomically measured on the basis of the
Doppler effect using the redshift z = Dl /l - the relative
increase of the wavelength l
of radiation of this galaxy: for z <1 it
is approximately v ~ c . z; the exact
relationship between the retraction speed and the redshift is v =
c . [(z
+ 1) ^{2} -1]
/
[(z + 1) ^{2}
+1].
^{ }Furthermore, the so-called deceleration parameter q is introduced
q = ^{def} a. ä / ^{. }a ^{2} | (5.25) |
characterizing the slowing or acceleration of expansion or contraction. Using the quantities H and q, Fridman's equations (5.23a, b) can be expressed in the form
k.c^{2}/a^{2} = 8pGr/3 - H^{2} + L.c^{2}/3 , k.c^{2}/a^{2} = (2 q - 1) H^{2} + L.c^{2} . | (5.23'a, b) |
Dynamics
and evolution of cosmological models
Using Fridman's equations can analyze the
dynamics of the expansion of a(t) of space, depending on the mass
density of matter r , the equation of state p ( r ) and the cosmological constant L . Let us first notice the simpler case L = 0 , without the cosmological constant :
^{ }From equation (5.23'a) it can be
seen, that the sign of 8pGr/3 - H^{2} decides which of the variants k
= 1, 0, -1 can be realized, ie the relationship between mass
density and expansion rate.
^{ }The case k
= 1
corresponding to a closed universe occurs when 8 p G^{ }r / 3> H ^{2} , ie when the mean mass density r in space is greater than a certain " critical density" r _{crit}
r_{ crit} = 3 H^{2} / 8p G . | (5.26) |
Based on currently
observed galaxy distance rates (Hubble constants), this critical
density is approximately 8.10 ^{-30} g / cm ^{3} , which corresponds to only
about 5 hydrogen atoms per 1 m ^{3} .
^{ }If r <
r _{crit} , k =
-1 - it is an open universe , in the boundary case r = r_{crit} we have k = 0 - it corresponds to the Euclidean universe , which is also open. Equation (5.23b)
shows that the equivalent criterion for the curvature character
of the Fridman universe is the value of the deceleration
parameter q:^{ } in a closed universe it is q>
1/2, in an open universe q <1/2 and in the Euclidean universe
it corresponds to q = 1/2. These variants will be analyzed in
more detail below.
In the extreme case r = r_{crit} will be k = 0 , the universe has an infinitely large radius of curvature - it is a model of an open universe with flat space (Euclidean). Spacetime metrics have a simple shape here
ds ^{2} = - c ^{2} dt ^{2} + a ^{2} (t) . (dx ^{2} + dy ^{2} + dz ^{2} ) , | (5.27) |
whereas the time-varying
coefficient a (t) does not express the curvature of space, but is
only a scale factor .
It should be noted
that although the Euclidean metric of three-dimensional space is
based on the r = r _{crit} , the whole four-dimensional
space-time is not flat here! Only certain special sections
(superficials) of spacetime are flat, corresponding to the same proper time
of all particles filling the universe.
^{ }Fridman's equations have an
accurate analytical solution in the case of a universe filled
with an " ideal fluid " with the equation of
state
^{ }
p = w. c ^{2} . r , | (5.28) |
where p is the pressure, r is the mass-energy density of the fluid (in a locally quiescent reference frame) and w is the state constant. The energy-momentum tensor T _{ik} was given in §5.1, relation (5.3). The time dependence of the scale function a (t) is then obtained by substituting p into Fridman's equation (5.23a). In our case k = 0 has a simple form
a (t) = a _{0} . t ^{2 }^{/ }^{[3 (w + 1)]} ,^{ } | (5.29) |
where a _{0} is the corresponding integration constant depending on
the initial conditions. We can distinguish two basic cases here :
^{ }w = 0 corresponds to incoherent dust that does not create any pressure, p = 0. The
universe is controlled by matter, where the pressure is
negligible due to the density of matter - it corresponds to the
late stages of evolution. Then it follows from the general
solution (5.29) that the distance between every two points increases with time according to the law
^{ }
a (t) = a _{1} . t ^{2/3} , a _{1} = ...... | (5.30) |
w = 1/3 - radiation dominant universe. In the early stages of the evolution of the universe, which is to be considered the maximum pressure p =r.c^{ 2} /3 of (5.29) for the expansion we get the time dependence of the shape
a (t) = a _{2} . t ^{1/2} . a _{2} = ...... | (5.31) |
In both cases space with dominant agent or radiation at r = r_{crit} dependence A = A (t) here has the shape of a parabola (Fig.5.3) - radius of curvature a increases monotonically from zero (a singularity) at t = 0 to infinity when t ®¥ .
If r > r _{crit} , is k = 1 - this is the closed space . Equation (5.23a) for k = 1 , L = 0 has the form ^{.}a ^{2} + 1 = a ^{2} . 8 p G r / 3. If the universe is filled with incoherent dust, ie p = 0, it follows from equation (5.23c) r .a ^{3} = const.; since the volume of a closed universe is V = 2 p^{2} a^{3} , the sum of mass in the whole space is constant:
r . 2 p ^{2} a ^{3} = M = const. = 2 p ^{2} a _{o }^{3} r _{o} , | (5.32) |
where a _{o} and r
_{o} are the radius and density of
the mass of the universe at some fixed point in time t _{o} .
^{ }The function a (t) is often
expressed in parametric form. After introducing a new
"time" variable h, by substituting d h = a.dt, the solution of equation (5.23a)
can be written in parametric form
a = (4GM / 6 p c ^{2} ). (1 - cos h ), t = (4GM / 6 p c ^{2} ). ( h - sin h ). | (5.33) |
The graphical representation of the time dependence a = a (t) is thus a cycloid (Fig. 5.3a), which describes a fixed point on a circle of radius
a _{max} = 4GM / 3 p c ^{2} | (5.34) |
when rolling along a
straight line (time axis t ); the parameter h is the rolling angle. The density of the
mass by altering by law r = 3/a^{2}_{max}(1 - cos h)^{3} = 6H^{2}/8pG(1+cos h). In Friedman's model of a
closed universe filled with dust with density r > r _{krit} , evolution looks so (Fig.5.2)
that at the beginning t = 0 the universe is
based on an initial singular state a = 0 with zero volume and infinite mass
density, gradually expanding to to dimension a = a _{max} , and then shrinks again to the
point a = 0 - terminal
singularities .
According to the left part of Fig. 5.2, the evolution of the
universe is often modeled by an inflating balloon
(and then shrinking), on the surface of which galaxies or
clusters of galaxies are drawn. In such an inflation of the
balloon, all points of its surface move away
from each other at a speed proportional to their distance from
each other , in accordance with Hubble's law (5.2). A critical
assessment and refinement of this model will be discussed in the
following §5.4, passage " What is actually
expanding? "
Fig.5.2. Temporal evolution of a closed universe.
Left : The
closed Fridman universe can be imagined as a three-dimensional
sphere that gradually "inflates" from zero radius
(initial singularity at time t = 0) to a certain maximum radius,
and then shrinks back to a point (final singularity). All
distances D l between any objects (galaxies or
clusters of galaxies) increase or decrease in proportion to the
radius of curvature as the universe expands or contracts.
Middle : A
space-time diagram of a closed universe nested in a fictional
five-dimensional space.
Right : An
illustration of expanding and contracting matter during the
evolution of the universe.
However, in the stages a ® 0, ie at the beginning and at the end of evolution, the assumption of the equation of state of incoherent dust is not realistic. Conversely, a substance are inevitably becomes ultrarelativistic, so closer to reality is the state equation p = r.c^{2}/3 . Equation (5.23c) then gives r .a ^{4} = const. and the solution of equation (5.23a) is here
a = a ^{~} . sin h , t = a ^{~} . (1 - cos h ) / c |
(the graph is a semicircle ), where a ^{~} = Ö ( 8 p G. r a ^{4} / 3c ^{4} ) = const. = Ö ( 8 p G. r _{o} a _{o }^{4} / 3c ^{4} ) . The global nature of evolution will be the same as in the previous case - no pressure of matter filling the closed universe is able to prevent singular points a = 0.
If r < r _{crit} , then k = -1 - it is an open universe . If it is filled with dust, the solution of equation (5.23a) is
a = â. (cosh h - 1) , t = â. (sinh h - h ) / c , |
where â = 8 p G. r a ^{3} / 3c ^{2} = const. = 8 p G. r _{o} a _{o }^{3} / 3c ^{2} ) . The dependence a = a (t) here has the shape of a hyperbola (Fig. 5.3a) - the radius of curvature a grows monotonically from zero (singularity!) at t = 0 to infinity at t ®¥ . A similar picture is obtained when the effect of pressure is included; the extreme case p = r .c ^{2} /3 is a solution
a = a ^{~} . sinh h , t = a ^{~} . (cosh h - 1) / c. |
Thus, Friedman's open universe also has a singularity, but only one thing - the initial one .
Fig.5.3. Evolution of cosmological models (time course of radius a
of space) depending on the value of
the cosmological constant L and the density of mass distribution r .
(and _{E} and L
_{E} in the figure on the right denote the
values of the radius of the universe and the cosmological
constant corresponding to Einstein's cosmological model)
In addition, when a non-zero cosmological constant L is included, a certain additional force appears in the universe (repulsive for L > 0 and attractive at L <0), which accelerates or slows down
the expansion or contraction of the universe. This force does not
depend on the weight and increases with distance. From the point
of view of the global evolution of the universe, the effective energy of the vacuum , generated by the cosmological
member, has an important property (different from the material
form of matter) - it does not dilute or thicken during the expansion or
contraction of the universe, it maintains a constant value. The solution of equations
(5.23) is then at L ¹ 0 leads to the following
possibilities :
^{ }If L <0 , gravity always ultimately prevails and the evolution of the
universe has a course according to Fig. 5.3b at any mass density r . This variant probably does not apply in space . More varied possibilities of
space evolution arise at L
>
0 - are shown in Fig. 5.3c:
- If the cosmological constant L <
L _{E is} less than the Einstein value (5.15) L _{E} = 4 p G r / c ^{2} , for supercritical density r > r _{crit} evolution of the universe to
proceed roughly (qualitatively) as for L = 0.
- At L > L _{E} a(t) increases from zero to
infinity, but at a certain stage the expansion slows down significantly for a time - there is a
kind of "quasi-static the phase "during which the
attractive forces are balanced by the repulsive ("
indecisive "universe); later dominated by
the repulsive force
*). The duration T _{st }of this
quasi-static phase , during which the radius of curvature of
the universe is maintained approximately at the value of the
radius of the Einstein static model (5.16) a = a _{E} , is longer the smaller the
difference L^{ }-
L _{E} : T _{st} ~ ln [ L / ( L - L _{E} ) ] .
- With L®L _{E} , the universe enters the state
of Einstein's static universe mentioned in the previous
paragraph. However, this Einstein model is unstable because the slightest
perturbation of density will lead to expansion.
^{ }For r > r _{crit} and L = L _{E} there are two other solutions :
1. In the infinitely distant past t ®-¥ was a = a_{E} , in the
future unlimited expansion (unlikely
variant) ;
2. Space at time t = 0 and the
condition a(0) = 0, then expands and asymptotically
(at infinitely distant future t ®¥ )
reaches a radius a® a _{E}
.
For L > 0 exists, apart from m ínìných
special options, also solutions, according to which at t = - ¥ universe was infinite radius, then the running in hall
contraction to a minimum value a _{min} then occurs unbounded expansion (an unlikely scenario) .
^{ }The mentioned peculiarities of
cosmological models with non - zero cosmological constant are used (and
are used) from time to time in attempts to overcome the supposed
or real difficulties of relativistic cosmology (internal
difficulties and inconsistencies with the results of
observations) - cf. .......
Three basic dynamics of
the scale factor - summary
For the astrophysically plausible scenario of evolution of our
universe (according to the standard model of physical cosmology -
§5.4), three basic dynamics should be applied in individual
stages of evolution - three time dependences of the scale factor
a = a(t), resulting from solving Fridman equations (5.23) :
- The
era of radiation - radiation dominant plasma ,
when the material content of the universe is dominated by
radiation and relativistic particles, arousing pressure p=r.c^{2}/3. The time dependence of the scale function a(t) is then
a(t) ~ t^{ 1/2} .
This situation was in the early hot universe (after inflation expansion) and
lasted (in "pure form") for about 50,000 years.
- The
era of matter ,
when the energy density of a substance (non-relativistic
particles, gas, dust) is greater than the
energy of radiation. The time dependence of the scale function
a(t) is here
a(t) ~ t^{ 2/3} .
Both of these dependences have a similar parabolic shape
according to Fig. 5.3a (k> = 0). Proportionality coefficients depend on the specific
substance content. The era of matter in the universe is still
going on now ...
- Vacuum
energy stage - cosmological constant
when the density of matter and radiation is negligible compared
to the vacuum energy generated by the cosmological term in the
gravitational equations. The time dependence of the scale
function a(t) is then exponential
a(t) ~ e ^{H . t} ~ e ^{Ö}^{(}^{L}^{/3) . t}
and in fact corresponds to the special case of de Sitter's
model. In §5.5 "Microphysics and Cosmology.
Inflation Universe." we
will see that this situation probably occurred in a very early
universe, just after the Big Bang in a time range aprox. 10^{-36} ÷ 10^{-32} sec. - inflationary
expansion of the early universe. And then it may happen
again in the very distant future - accelerated expansion
of the universe, when the density of matter and
radiation drops to very low concentrations due to the expansion
of the universe and "dark energy", generated
by the cosmological constant, begins to dominate (§5.6, section "Dark
energy and accelerated expansion of the universe ").?..
Relative Omega - parameterization of
cosmological models
Instead of absolute values of mass density and cosmological
constant, their dimensionless relative
ratios W with respect to their respective
significant (critical) values are often used to model the
evolution of the universe . The ratio of the actual value of the mass density r with respect to the critical density is
introduced :
W _{M} = ^{def} r / r _{crit} (= r . 8 p G / 3 H ^{2} ) | (5.36) |
and the ratio of the actual value of the cosmological constant L to the Einstein value :
W _{L} = ^{def} L / L _{E} (= L. c ^{2} / 3H ^{2} ) . | (5.37) |
Fridman's equation (5.23a) expressed by the parameters W is as follows :
kc ^{2} / H ^{2} and ^{2} = W _{M} + W _{L} - 1 . | (5.38) |
The deceleration parameter q , introduced by equation (5.25), is expressed by the parameters W :
q = W _{M} / 2 - W _{L} . | (5.39) |
For W _{L} = W _{M} / 2 the expansion takes place at a constant speed, at W _{L} < W _{M} / 2 the expansion
slows down, at W _{L} > W _{M} / 2 the expansion accelerates.
^{ }Using dimensionless W
values, it is easier to test the evolution
of a cosmological model. E.g. for a flat model, W _{M} + W _{L} = 1. It
is also possible to construct clear graphs of the behavior of
cosmological models in coordinates, on the axes of which the
values W _{M} and W _{L} are plotted .
^{ }Sometimes an even more detailed W- parameterization of the
cosmological model is introduced . The general mass density r ( r _{M} ) is divided into a substance formed by
non-relativistic particles r _{m}
(especially baryons - also denoted by r _{B} ) and relativistic particles and radiation
r _{rad} (in the early stages of
the universe it is high-energy gamma radiation, also denoted by r _{g}
) . With W -option further expresses
the curvature parameter k . The basic Fridman equation
(5.23a) for the rate of expansion of the universe is then written
in the form :
(5.40) |
where W _{xxx} are the contributions of the individual components of matter ~ energy to the dynamics of expansion: W _{rad} from relativistic particles and radiation, W _{m} from non-relativistic matter, W _{k} from the curvature of space and W _{L} from the cosmological constant - "vacuum energy". The parameter H _{0} » 67 km s ^{-1} / Mpcs is the current value of the Hubble constant.
--------------------------------------------------
---
Current note:
According to recent astronomical observations of distant
supernovae, there are some indications that the expansion
of the universe is currently accelerating
, that in addition to dark (non-radiant) matter, there is also
so-called dark energy in space , which shows
"antigravity". Thus, the evolution of the universe
seems to follow the curve in Fig. 5.3c, the case of L > L
_{E} (see §5.6 "The
Future of the Universe. The Arrow of Time.", The passage
" Dark
Energy and the Accelerated Expansion of the Universe
"), corresponds to W _{L} > W _{M} / 2 ...
5.2. Einstein's and deSitter's universe. Cosmological constant. | 5.4. Standard cosmological model. Big Bang. |
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