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\begin{document}

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\section*{observable universe}

\begin{quote}%
under construction

This entry is about the notion in [[physics]]. For a different (but at least roughly analogous) notion in [[mathematics]]/[[logic]] see at \emph{[[universe]]}.


\end{quote}

\vspace{.5em} \hrule \vspace{.5em}
\hypertarget{context}{}\subsubsection*{{Context}}\label{context}

\hypertarget{physics}{}\paragraph*{{Physics}}\label{physics}

[[!include physicscontents]]

\hypertarget{contents}{}\section*{{Contents}}\label{contents}

\noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak
\noindent\hyperlink{related_entries}{Related entries}\dotfill \pageref*{related_entries} \linebreak
\noindent\hyperlink{References}{References}\dotfill \pageref*{References} \linebreak


\hypertarget{idea}{}\subsection*{{Idea}}\label{idea}

On the very largest scales observable in astrophysical [[experiment]], the cosmos is well described by an [[FRW model]] with [[cosmological constant]] $\Lambda$ (but see at \emph{[[inhomogeneous cosmology]]}), with plenty of [[dark matter]] and with primordial [[cosmic inflation]] (called the $\Lambda$-CDM ``[[standard model of cosmology|concordance model of cosmology]]''). A [[horizon]] in there and everything on smaller scales is the \emph{observable universe}.

There has never been a reason to assume that beyond this \emph{cosmic horizon} visible to us today, the cosmos would not extend further. In fact in the simple [[FRW models]] with positive cosmological constant, the [[spacetime]] [[manifold]] is not a [[closed manifold]] and extends indefinitely beyond our observable horizon. This is directly analogous to the ancient mariner who would stand on the deck of his ship and see a few miles of ocean around him. That was the world within his horizon, but there was no telling what lay beyond.

Therefore it is important to distinguish the \emph{observable universe} from the \emph{universe} as such. Unfortunately, intellectual laziness tended to ignore this distinction and at some point some people who said ``universe'' to mean just the stretch of our cosmic horizon felt the need to have a new term for whatever may lie beyond. For better or worse, that new term has become wide-spread these days and is ``[[multiverse]]''.

The main scientific observation that goes with this is the observation that [[cosmic inflation]] -- for which there is by now excellent and ever increasing [[experiment|experimental]] evidence that indeed it happened -- does, at least by the simple method which is currently used to model it, naturally predict a large ambient space in which local regions stochastically undergo inflationary expansion. This scenario of [[chaotic inflation]] has in fact been singled out as the most likely one (within the standard model) by the [[bicep2]]-[[experiment]] in 2014.

In any case, as far as the \emph{observable} universe is concerned, two facts are worth noticing:

\begin{enumerate}%
\item the observable universe alone is already pretty darn large, see the various discussion of its scales \hyperlink{References}{below};


\item [[Giordano Bruno]]`s old intuition remains plausible even at the largest scales: possibly the full xyz-verse beyond our present horizon is vastly larger still\ldots{} and we may never know.



\end{enumerate}


\begin{quote}%
graphics taken from \hyperlink{PrimackCh6}{Primack Chapter 6}

(\href{http://viewfromthecenter.com/images/Cosmic_Uroboros.jpg}{color version})


\end{quote}
\begin{quote}%
Recent measurements (c. 2001) by a number of ground-based and balloon-based experiments, including MAT/TOCO, Boomerang, Maxima, and DASI, have shown that the brightest spots are about 1 degree across. Thus the universe was known to be flat to within about 15\% accuracy prior to the WMAP results. WMAP has confirmed this result with very high accuracy and precision. We now know that the universe is flat with only a 2\% margin of error


\end{quote}


[[energy]] $\,$ [[scales]]:

$\backslash$begin\{center\}  $\backslash$end\{center\}

\begin{quote}%
graphics grabbed from \href{http://sten.astronomycafe.net/the-particle-desert/}{here}


\end{quote}
\hypertarget{related_entries}{}\subsection*{{Related entries}}\label{related_entries}

\begin{itemize}%
\item [[experimental observation]]

\begin{itemize}%
\item [[hydrogen line]]

\end{itemize}

\item [[cosmology]],

\begin{itemize}%
\item [[standard model of cosmology]],

\begin{itemize}%
\item [[FRW model]]


\item [[big bang]]


\item [[dark matter]]



\end{itemize}

\item [[structure formation]]



\end{itemize}

\item [[quantum cosmology]]


\item [[particle physics]], [[standard model of particle physics]]



\end{itemize}
\hypertarget{References}{}\subsection*{{References}}\label{References}

\begin{itemize}%
\item Wikipedia \emph{\href{http://en.wikipedia.org/wiki/Observable_universe}{Observable universe}}

\end{itemize}
A visualization of the scales in the [[solar system]] are in

\begin{itemize}%
\item \href{http://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html}{A tediously accurate scale model of the solar system}

\end{itemize}
Accurate visualization of many astronomical phenomena is at

\begin{itemize}%
\item \hyperlink{http://en.spaceengine.org}{en.spaceengine.org}

\end{itemize}
Visualization of the [[speed of light]] in relation to distances in the solar system:

\begin{itemize}%
\item \href{https://www.techexplorist.com/3-animations-provided-nasa-scientist-suggests-the-speed-of-light-frustratingly-slow/20215/}{www.techexplorist.com/3-animations-provided-nasa-scientist-suggests-the-speed-of-light-frustratingly-slow/20215/}

\end{itemize}
A visualization of the big observable structures in the universe, drawn to scale, is in

\begin{itemize}%
\item American Museum of Natural History, \emph{The known universe} (\href{https://www.youtube.com/watch?v=17jymDn0W6U#t=374}{video})

\end{itemize}
Visualization of local gakaxy densities and voids

https://phys.org/news/2019-07-astronomers-vast-void-cosmic-neighborhood.amp

A simulation of the [[structure formation]] (see there for more) of these is at

\begin{itemize}%
\item \emph{\href{http://www.mpa-garching.mpg.de/galform/virgo/millennium/}{The Millennium Simulation Project}}

\end{itemize}
On scales

\begin{itemize}%
\item [[Joel Primack]], \emph{\href{http://viewfromthecenter.com/}{The View from the Center of the Universe}} Chapter 6: \emph{What Size is the Universe? The Cosmic Uroboros} (\href{http://physics.ucsc.edu/~joel/Chapter6.pdf}{pdf}, [[PrimackSizeOfUniverse.pdf:file]])

\end{itemize}
Pointers to the past and future timeline of the observable universe are collected in

\begin{itemize}%
\item [[John Baez]], \emph{\href{http://math.ucr.edu/home/baez/timeline.html}{A Brief History of the Universe}}

\end{itemize}
Survey and exposition of aspects of organizing the large-scale structure of the universe are at

\begin{itemize}%
\item Wikipedia, \emph{\href{http://en.wikipedia.org/wiki/Cosmic_distance_ladder}{The cosmic distance ladder}}


\item [[Terence Tao]], \emph{\href{http://terrytao.wordpress.com/2007/05/31/the-cosmic-distance-ladder/}{The cosmic distance ladder}}



\end{itemize}
More technical discussion of cosmic horizons contains

\begin{itemize}%
\item [[George Ellis]], T. Rothman: \emph{Lost Horizons}, 1993 (\href{http://de.scribd.com/doc/235971566/G-F-R-Ellis-T-Rothman-Lost-Horizons}{scribd})

\end{itemize}
A sketch of a comprehensive picture had been proposed in

\begin{itemize}%
\item [[Erich Jantsch]], \emph{[[The Self-Organizing Universe]]} (1980)

\end{itemize}
[[!redirects cosmic horizon]] [[!redirects cosmic horizons]]



\end{document}