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R. N. Mohapatra
the cosmic microwave background, i.e. temperature is almost the same in every
direction. This means that all parts of the universe must have been in contact
for a long time since the beginning. This is necessary to make the temperature
the same in all directions. This is hard to understand, except as an accident,
in the Hubble expansion picture. Given information about the size of the
universe now, we can extrapolate it to the earlier epochs of the universe, by
using the Hubble rate of expansion to get the size at an earlier age. If Hubble
expansion were the only type of expansion of the universe, the universe at an
earlier time (such as the time of Big Bang nucleosynthesis) would have a much
smaller size than the expected extrapolation of the current size and thus would
only occupy a small fraction of this extrapolated size. There would then have
to be many different pieces to the universe in the beginning. This make it hard
to understand how the whole universe could have the same temperature since
there would have been no “overlap” between different disconnected parts.
To resolve this problem, Alan Guth [58] suggested that there was a period
of extremely rapid expansion in the beginning stage of the universe, which
increased the size of the universe so rapidly that all parts of the universe
overlapped and got causally connected. Things happening in one part of the
universe influenced those in other parts, as we observe today from the isotropy
of the microwave background. If one segment of the universe is isotropic, all
other parts would also be isotropic.
1 This paper was followed up by Andre
Linde, Andreas Albrecht, and Paul Steinhardt [72], who proposed a newer and
more refined version of the model, explicitly discussing how inflation would
end.
A poor analogy to causal connection through inflation can be seen from the
following example. Imagine living in a small cubicle with walls all around it
in a big hall full of walled cubicles. In such a setting you do not know what
is going on in the other corner of the big hall if you are far away from it. On
the other hand if there is no wall around the cubicles, you can see everything
and know everything and react to everything. Inflation is like removing all the
walls and making a whole big hall (the universe) into one large open area.
Another major puzzle of the Big Bang theory is that the universe in the
beginning was extremely smooth and right now it is extremely non-smooth.
There are stars, galaxies and clusters with voids in between, making the
universe anything but smooth. How did that happen? The inflation theory
seems to provide a solution to this problem. During the period of inflation,
1 The Guth paper came in July, 1980. The basic idea of inflation was there in several earlier papers as well,
see [64] for a history.
R. N. Mohapatra
the cosmic microwave background, i.e. temperature is almost the same in every
direction. This means that all parts of the universe must have been in contact
for a long time since the beginning. This is necessary to make the temperature
the same in all directions. This is hard to understand, except as an accident,
in the Hubble expansion picture. Given information about the size of the
universe now, we can extrapolate it to the earlier epochs of the universe, by
using the Hubble rate of expansion to get the size at an earlier age. If Hubble
expansion were the only type of expansion of the universe, the universe at an
earlier time (such as the time of Big Bang nucleosynthesis) would have a much
smaller size than the expected extrapolation of the current size and thus would
only occupy a small fraction of this extrapolated size. There would then have
to be many different pieces to the universe in the beginning. This make it hard
to understand how the whole universe could have the same temperature since
there would have been no “overlap” between different disconnected parts.
To resolve this problem, Alan Guth [58] suggested that there was a period
of extremely rapid expansion in the beginning stage of the universe, which
increased the size of the universe so rapidly that all parts of the universe
overlapped and got causally connected. Things happening in one part of the
universe influenced those in other parts, as we observe today from the isotropy
of the microwave background. If one segment of the universe is isotropic, all
other parts would also be isotropic.
1 This paper was followed up by Andre
Linde, Andreas Albrecht, and Paul Steinhardt [72], who proposed a newer and
more refined version of the model, explicitly discussing how inflation would
end.
A poor analogy to causal connection through inflation can be seen from the
following example. Imagine living in a small cubicle with walls all around it
in a big hall full of walled cubicles. In such a setting you do not know what
is going on in the other corner of the big hall if you are far away from it. On
the other hand if there is no wall around the cubicles, you can see everything
and know everything and react to everything. Inflation is like removing all the
walls and making a whole big hall (the universe) into one large open area.
Another major puzzle of the Big Bang theory is that the universe in the
beginning was extremely smooth and right now it is extremely non-smooth.
There are stars, galaxies and clusters with voids in between, making the
universe anything but smooth. How did that happen? The inflation theory
seems to provide a solution to this problem. During the period of inflation,
1 The Guth paper came in July, 1980. The basic idea of inflation was there in several earlier papers as well,
see [64] for a history.