General Relativity (GR) implies an expanding universe where the expansion rate is the Hubble constant. There are two different methods to measure the Hubble constant. The results of these two methods are two different values for the Hubble constant (see e.g.  ). Hence, the assumption that the universe expands is not correct and the universe doesn’t expand (see e.g. ). The expansion is a generally accepted assumption supported by GR. We can say that GR isn’t a correct description of gravitation. There are authors who ask for new physics (see ). Therefore, we will use the theory of gravitation in flat space-time (GFST) instead of GR which is studied by the author in the book and in several articles (see e.g. the articles    ). GFST gives non-expanding space for the universe. The metric is the pseudo-Euclidean geometry and the proper time is formally similar to the metric of GR. The source of the gravitation field is the total energy-momentum tensor including that of gravitation. This is in full agreement with Einstein who stated that matter is equal to energy and reverse. GR doesn’t satisfy this condition and in addition the energy-momentum of gravitation by GR is not a tensor. It is worth to mention that GFST was already studied in article  with application to non-singular cosmological models in . Surface data show evidence for a non-expanding universe . The possibility of non-expanding, cosmological models is already given in the article  by the use of GFST. Non-singular universes by GFST with matter creation and entropy production are also studied in .
The theory of GFST is shortly summarized. The metric is flat space-time given by
where is a symmetric tensor. Especially, pseudo-Euclidean geometry has the form
Here, are the Cartesian coordinates and . Let
The gravitational field is described by a symmetric tensor . Let be defined by
and put similar to (3)
The proper time is defined by
The Lagrangian of the gravitational field is given by
where the bar “/” denotes the covariant derivative relative to the flat space-time metric (1). The Lagrangian of dark energy (given by the cosmological constant ) has the form
where k is the gravitational constant. Then, the mixed energy-momentum tensor of gravitation, of dark energy and of matter of a perfect fluid is
Here, , p and denote density, pressure and four-velocity of matter. it holds by (6)
Define the covariant differential operator
of order two. Then, the field equations for the gravitational potentials have the form
Define the energy-momentum tensor
Then, the equations of motion in covariant form are
In addition to the field Equation (13) and the equations of motion (16) the conservation law of the total energy-momentum holds, i.e.
The results of this chapter may be found in the book  and in the subsequently appeared articles   . In article  the gravitation theories of GFST and GR and their results are compared with one another. Furthermore, the redshift formula for GFST is derived.
4. GFST and the Universe
GFST is defined in flat space-time metric, e.g. in the pseudo-Euclidean geometry which is used in the following to study homogeneous, isotropic, cosmological models. The matter tensor is given by a perfect fluid with velocity equal to zero. The total matter is given by the sum of density of matter and of radiation
with the corresponding pressure density of matter and of radiation . It holds for homogeneous, isotropic, cosmological models
The initial conditions at present time are
where is the Hubble constant and is an additional constant not appearing in GR. Relation (16) for i = 4 implies under the assumption that matter and radiation do not interact
It follows by the use of the field Equation (13)
The expression is the density of gravitation field. The conservation law of the total energy is
where is a constant of integration. Define the quantity
The field Equation (19) imply by the use of the conservation law (20) and the initial conditions the relation
It follows from (20) with the present time by the use of the initial conditions and the standard definitions of the density parameters of matter, radiation and of the energy given by the cosmological constant with the abbreviation
the differential equation
Here, , and are the density parameters of radiation, matter and the energy given by the cosmological constant. The initial condition for the differential Equation (23a) is
Relation (20) with gives by elementary calculations
implies that the solution of (23) is non-singular for all. It exists with, that is
for all. (26)
It follows from (23a)
The time must be long time before the present time implying, i.e.
Therefore, starts at a positive value at time equal to minus infinity, decreases to at and then increases for all t. The function can then be calculated from relation (21). Let us introduce the proper time instead of the time
The differential Equation (23a) can by the use of (21) be rewritten
This differential equation is for not too small functions nearly identical with that of GR for a flat homogeneous, isotropic universe by virtue of (25) and (27).
Then, the conditions (25) and (27) give
i.e. corresponds to the time of the big bang of GR with value very small but not zero. This result is received by GFST without any additional assumption or change of the theory.
There are two methods of measuring the Hubble constant of the universe: the cosmic distance ladder and looking at the signals originated from the beginning of the universe. Two different results for the Hubble constant are received. Therefore, the universe doesn’t expand because the methods use the expansion of the universe. It is worth to mention that GR implies expansion because the universe starts from a point singularity and the observed universe is very big. Furthermore, the universe must be inflationary expanding because the observed universe is flat. Summarizing, it follows that GR doesn’t correctly describe gravitation if two Hubble constants are measured.
A theory of gravitation in pseudo-Euclidean geometry has been given in article . Later on, it is studied more generally in flat space-time. The applications of this theory to homogeneous, isotropic, cosmological models are given in article  where non-singular solutions are received, i.e. big bang did not exist. It was proved that for weak gravitational fields the results of GFST and GR agree to measurable accuracy. The theory and the applications of GFST is studied in several articles and summarized in the book . Differences of the results of GFST and GR arise for cosmological models in the beginning of the universe. The metric of GFST is the pseudo-Euclidean geometry, i.e. space is not expanding. It is worth to mention that by virtue of the covariance of GFST an expansion of the universe would also be possible by a suitable transformation. But this is not realistic. A non-expanding universe is important because expansion of the universe implies two different Hubble constants. For cosmological models of GFST the source is the total energy-momentum tensor inclusive that of the gravitational field (as it should be by Einstein: matter is equal to energy and reverse) whereas the source is only the matter tensor and no gravitational energy-momentum for cosmological models of GR which is no tensor for GR. The redshift of distant objects follows by the energy of time-dependent gravitational fields which is converted to matter where the total energy is conserved and it doesn´t follow from velocities (expanding space).
 Teplitz, H., et al. (2018) The Universe’s Rate of Expansion Is in Dispute—And We May Need New Physics to Solve It.