  We show that the radial geodesic motion of a particle inside a black hole in
isotropic coordinates (the Einstein-Rosen bridge) is physically different from
the radial motion inside a Schwarzschild black hole. A particle enters the
interior region of an Einstein-Rosen black hole which is regular and physically
equivalent to the asymptotically flat exterior of a white hole, and the
particle's proper time extends to infinity. Because the motion across the
Einstein-Rosen bridge is unidirectional, and the surface of a black hole is the
event horizon for distant observers, an Einstein-Rosen black hole is
indistinguishable from a Schwarzschild black hole for such observers. Observers
inside an Einstein-Rosen black hole perceive its interior as a closed universe
that began when the black hole formed, with an initial radius equal to the
Schwarzschild radius of the black hole $r_g$, and with an initial accelerated
expansion. Therefore the model of a universe as a black hole in isotropic
coordinates explains the origin of cosmic inflation. We show that this kind of
inflation corresponds to the effective cosmological constant $\Lambda=3/r_g^2$,
which, for the smallest astrophysical black holes, is $~10^{-8}m^{-2}$. If we
assume that our Universe is the interior of an Einstein-Rosen black hole,
astronomical observations give the time of inflation $~10^{-3}s$ and the size
of the Universe at the end of the inflationary epoch $~10^{32}m$.
