same in all directions. But why should this be the case? Unless there were a
mechanism in place to create these seed fluctuations, and unless that mechanism
created fluctuations with the same properties in causally disconnected locations, this
is simply another initial condition that arises without motivation.
The third is the monopole problem. In practically all grand unified theories, as
well as many other extensions to the standard model, as the Universe cools through
certain temperatures and conditions, it undergoes a phase transition. These transitions can create ultra-heavy relic particles, such as magnetic monopoles. In most
scenarios of beyond-the-standard-model physics, these relics should not only be
created, but they ought to be stable, persisting to the present day. Motivated by these
calculations, there were a series of dedicated searches for isolated magnetic monopoles traveling through the Universe. Although there was a famously controversial
detection of one monopole candidate (Cabrera 1982), follow-up experiments failed
to confirm the existence of these particles. Their absence is a mystery that the hot Big
Bang, if it were to have reached arbitrarily high temperatures and energies, has no
explanation for.
The fourth puzzle is the flatness problem. Today, via a number of methods, we
can measure the spatial curvature of the Universe. Particularly by looking at the
patterns of fluctuations in the cosmic microwave background radiation (Planck
Collaboration 2015a), we have determined that the Universe is spatially flat to
approximately 1%. But flatness, rather than positive or negative curvature, which
would imply a closed (sphere-like) or open (saddle-like) Universe, tells us that the
balance between the expansion rate and the amount of matter and energy in the
Universe must have been tremendously precise. If we extrapolate back to a time
where the Universe was filled with a quark-gluon plasma, the energy density and
expansion rate must have been balanced to better than one part in 10
25 . If the
Universe were just the tiniest bit denser, it would have already recollapsed; if it
were the tiniest bit less dense, it would be more than double its present size. Without
a mechanism to explain why these independent quantities are balanced, this too must
be an initial condition that the Universe is simply born with.
Finally, the fifth puzzle can be called the small fluctuation problem. There are tiny
imperfections in temperature (and therefore, in density) in the cosmic microwave
background: on the order of Æ0.003%, compared to the mean value. This is an
independent problem from the horizon problem, which offers no explanation for
why the mean temperature value is the same everywhere. Even if you allow for that,
there’s no mechanism to generate temperature fluctuations that are not only the same
magnitude everywhere, but merely one-part-in-30,000 times the average value. Why
would these temperature fluctuations be so small, given that other out-of-equilibrium
thermal systems where causal contact between disparate regions is disallowed
display fluctuations that are thousands of times greater than what we seen in the
cosmic microwave background? Again, this must be an initial condition that’s added
without any driving motivation, other than it’s what we see when we examine the
Universe in detail.
In theory, there’s no reason why these puzzles couldn’t simply be resolved by
putting in the initial conditions we require. But if we solve these puzzles in that
90
E. R. Siegel
mechanism in place to create these seed fluctuations, and unless that mechanism
created fluctuations with the same properties in causally disconnected locations, this
is simply another initial condition that arises without motivation.
The third is the monopole problem. In practically all grand unified theories, as
well as many other extensions to the standard model, as the Universe cools through
certain temperatures and conditions, it undergoes a phase transition. These transitions can create ultra-heavy relic particles, such as magnetic monopoles. In most
scenarios of beyond-the-standard-model physics, these relics should not only be
created, but they ought to be stable, persisting to the present day. Motivated by these
calculations, there were a series of dedicated searches for isolated magnetic monopoles traveling through the Universe. Although there was a famously controversial
detection of one monopole candidate (Cabrera 1982), follow-up experiments failed
to confirm the existence of these particles. Their absence is a mystery that the hot Big
Bang, if it were to have reached arbitrarily high temperatures and energies, has no
explanation for.
The fourth puzzle is the flatness problem. Today, via a number of methods, we
can measure the spatial curvature of the Universe. Particularly by looking at the
patterns of fluctuations in the cosmic microwave background radiation (Planck
Collaboration 2015a), we have determined that the Universe is spatially flat to
approximately 1%. But flatness, rather than positive or negative curvature, which
would imply a closed (sphere-like) or open (saddle-like) Universe, tells us that the
balance between the expansion rate and the amount of matter and energy in the
Universe must have been tremendously precise. If we extrapolate back to a time
where the Universe was filled with a quark-gluon plasma, the energy density and
expansion rate must have been balanced to better than one part in 10
25 . If the
Universe were just the tiniest bit denser, it would have already recollapsed; if it
were the tiniest bit less dense, it would be more than double its present size. Without
a mechanism to explain why these independent quantities are balanced, this too must
be an initial condition that the Universe is simply born with.
Finally, the fifth puzzle can be called the small fluctuation problem. There are tiny
imperfections in temperature (and therefore, in density) in the cosmic microwave
background: on the order of Æ0.003%, compared to the mean value. This is an
independent problem from the horizon problem, which offers no explanation for
why the mean temperature value is the same everywhere. Even if you allow for that,
there’s no mechanism to generate temperature fluctuations that are not only the same
magnitude everywhere, but merely one-part-in-30,000 times the average value. Why
would these temperature fluctuations be so small, given that other out-of-equilibrium
thermal systems where causal contact between disparate regions is disallowed
display fluctuations that are thousands of times greater than what we seen in the
cosmic microwave background? Again, this must be an initial condition that’s added
without any driving motivation, other than it’s what we see when we examine the
Universe in detail.
In theory, there’s no reason why these puzzles couldn’t simply be resolved by
putting in the initial conditions we require. But if we solve these puzzles in that
90
E. R. Siegel