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Watsisname
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Cosmology Discussion Thread

02 Nov 2018 01:16

~The Cosmology Discussion Thread~

Understanding the Large Scale Universe 

and how it evolves


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Introduction:


Cosmology is a fascinating and conceptually challenging topic.  After a good deal of discussion of the subject in the Science and Astronomy Questions Thread, I decided to create a dedicated thread, which I have modelled after FastFourierTransform's outstanding thread on Understanding Solar System Dynamics.


In this thread I will be primarily answering any questions you all may have (feel free to use this as a Q&A!), and promoting discussions.  I will also use this space to drop in larger, more detailed posts about specific subjects in cosmology.  I will use computer simulations to show the evolution of different kinds of universes and explore their behavior, discuss cosmological observations, what our universe is made of, how it has evolved, and how astronomers figured all this complicated stuff out.



Table of Contents:  (Will be updated over time)


  • A History of Cosmology:  How has our understanding of the universe changed with time?  From the Ancients to today.
  • Tools and Observations:  How do we measure the universe?
  • Contents of the Universe:  What is the universe made of, and how do we know?
  • The Dark Universe:  The two great mysteries.  What are dark matter and dark energy?
  • The Shape of the Universe:  Understanding the spatial curvature.  Density = Geometry.
  • Mathematical Background:  Applying physics to understand how the universe evolves.  The Friedmann Equations.
  • Simulations: Calculating the evolution of universes with different contents.
  • The Concordance Model:  The Lambda-CDM model of cosmology, our current best description of our universe.
  • The Beginning of the Universe:  Understanding the Big Bang.
  • A History of the Universe:  From the Big Bang to the present.
  • The Fate of the Universe:  Future evolution.  Heat death, Big Crunch, or Big Rip?  Density = Destiny.
  • Beyond the Concordance Model:  What other possibilities are there?  Before the Big Bang?  After Heat Death?
  • Other Kinds of Universes:  Hypothetical Cyclic "Bouncing" Model.
  • Multiverses?   ¯\_(ツ)_/¯


Miscellaneous:  Common Questions and Misconceptions:


Title Image:
Inspired by the classic 1888 "Flammarion" engraving.  A traveler reaches the edge of the known world and, peering past the firmament of stars, discovers the cosmos.  The background is a slice of a computer simulation of the universe on large scales, showing the "Cosmic Web", a gigantic structure of sheets and filaments of galaxies.
 
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FastFourierTransform
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Cosmology Discussion Thread

02 Nov 2018 02:55

This is so exciting!
I have little background in the topic and a huge amount of questions (from the silly to the philosophical). Going to take notes of each one of your amazing explanations. Thanks for the effort of making this (and an entire cosmological simulator) just for us. I'm going to learn and profit the max from it.
 
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Cosmology Discussion Thread

22 Nov 2018 11:45

I'm slowly compiling some links to more detailed posts on given subjects.  There is also a new addition:  The testability of Lambda-CDM, and its components dark matter and dark energy.  This should be a required reading for anyone unsure of how cosmologists are confident that these things are real or necessary to include in our understanding of the universe. :)
 
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Cosmology Discussion Thread

30 Nov 2018 21:34

New additions:  What is the universe's shape?, and What is it expanding into?, with the discussion of the balloon and other common analogies for expansion.
 
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Cosmology Discussion Thread

01 Dec 2018 00:20

As far as I know, the outside of a universe is not defined. It does not exist. The space within our universe bubble is increasing, but there is nothing outside of our universe that could somehow shrink.
JackDole's Universe 0.990: http://forum.spaceengine.org/viewtopic.php?f=3&t=546
JackDole's Archive: http://forum.spaceengine.org/viewtopic.php?f=3&t=419
JackDole: Mega structures ... http://old.spaceengine.org/forum/17-3252-1 (Old forum)
 
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Cosmology Discussion Thread

01 Dec 2018 02:38

Say if you were an observer that happen to be close enough to the outside that the observable universe radius overlapped the edge presuming that the universe is finite?   Would you see some sort of something that you could determine is the edge of the universe? or would it still look uniform as we observe now?

This thought has always plagued my mind as long as I can remember.
 
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Cosmology Discussion Thread

01 Dec 2018 06:57

Earth is finite yet there is no edge.
NIL DIFFICILE VOLENTI
 
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Cosmology Discussion Thread

01 Dec 2018 15:46

Yes, it's just like going around the world and coming back to where you started.  Except with one more dimension.  Pretty trippy to try to visualize that.

If the universe is curved in this way, and if you could actually make such a journey, then as you go off in a straight line you would simply encounter more space and more galaxies along the way, until you suddenly realize you're back at the Milky Way again.  But nowhere was there an edge or anything weird.  

Unfortunately we cannot actually make such a journey, because the cosmic expansion prevents us from being able to travel far enough quickly enough to wrap all the way around.


JackDole wrote:
Source of the post As far as I know, the outside of a universe is not defined. It does not exist. The space within our universe bubble is increasing, but there is nothing outside of our universe that could somehow shrink.

Exactly. :)  The universe is all of the space that is connected with the space we live in.  Any space that might exist that isn't connected is irrelevant to it.  So there is no sense in imagining an "outside" of the universe.  The expansion of the universe is an expansion of the space and requires no outside to exist at all.  It is not like a sphere expanding into space around it.  It is absolutely unlike any other thing you can imagine.
 
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Cosmology Discussion Thread

01 Dec 2018 20:21

Wat, do you think other universes exist?  I do for the simple reason that there is never one of anything, and perhaps in the future we could prove it.  Universes of different laws of physics and such- as a matter of fact, at the highest levels of technology (Kardashev level 3 or 4), it's been conjectured that species that attain that high level of technology could create their own universes with their own laws of physics.

https://en.wikipedia.org/wiki/Nikodem_Pop%C5%82awski

Black holes as doorways
Popławski's approach is based on the Einstein-Cartan theory of gravity which extends general relativity to matter with intrinsic angular momentum (spin). Spin in curved spacetime requires that the affine connection cannot be constrained to zero and its antisymmetric part, the torsion tensor, must be a variable in Hamilton's principle of stationary action which gives the field equations. Torsion gives the correct generalization of the conservation law for the total (orbital plus intrinsic) angular momentum to the presence of the gravitational field, but also modifies the Dirac equation for fermions.

Gravitational effects of torsion on fermionic matter are significant at extremely high densities which exist inside black holes and at the beginning of the Universe. Popławski theorizes that torsion manifests itself as a repulsive force which causes fermions to be spatially extended and prevents the formation of a gravitational singularity within the black hole's event horizon.[10] Because of torsion, the collapsing matter on the other side of the horizon reaches an enormous but finite density, explodes and rebounds, forming an Einstein-Rosen bridge (wormhole) to a new, closed, expanding universe.[11][12] Analogously, the Big Bang is replaced by the Big Bounce before which the Universe was the interior of a black hole.[13] This scenario explains why the present Universe at largest scales appears spatially flat, homogeneous and isotropic, providing a physical alternative to cosmic inflation. It may explain the arrow of time, solve the black hole information paradox, and explain the nature of dark matter.[14] Torsion may also be responsible for the observed asymmetry between matter and antimatter in the Universe.[15] The rotation of a black hole could influence the spacetime on the other side of its event horizon and result in a preferred direction in the new universe. Popławski suggests that the observed fluctuations in the cosmic microwave background might provide evidence for his hypothesis.[16]

https://en.wikipedia.org/wiki/Lee_Smoli ... _selection
Cosmological natural selection 
Smolin's hypothesis of cosmological natural selection, also called the fecund universes theory, suggests that a process analogous to biological natural selection applies at the grandest of scales. Smolin published the idea in 1992 and summarized it in a book aimed at a lay audience called The Life of the Cosmos.

Black holes have a role in natural selection. In fecund theory a collapsing[clarification needed] black hole causes the emergence of a new universe on the "other side", whose fundamental constant parameters (masses of elementary particles, Planck constant, elementary charge, and so forth) may differ slightly from those of the universe where the black hole collapsed. Each universe thus gives rise to as many new universes as it has black holes. The theory contains the evolutionary ideas of "reproduction" and "mutation" of universes, and so is formally analogous to models of population biology.

Alternatively, black holes play a role in cosmological natural selection by reshuffling only some matter affecting the distribution of elementary quark universes. The resulting population of universes can be represented as a distribution of a landscape of parameters where the height of the landscape is proportional to the numbers of black holes that a universe with those parameters will have. Applying reasoning borrowed from the study of fitness landscapes in population biology, one can conclude that the population is dominated by universes whose parameters drive the production of black holes to a local peak in the landscape. This was the first use of the notion of a landscape of parameters in physics.

Leonard Susskind, who later promoted a similar string theory landscape, stated:

I'm not sure why Smolin's idea didn't attract much attention. I actually think it deserved far more than it got.[6]

However, Susskind also argued that, since Smolin's theory relies on information transfer from the parent universe to the baby universe through a black hole, it ultimately makes no sense as a theory of cosmological natural selection.[6] According to Susskind and many other physicists, the last decade of black hole physics has shown us that no information that goes into a black hole can be lost.[6] Even Stephen Hawking, who was the largest proponent of the idea that information is lost in a black hole, has reversed his position.[6] The implication is that information transfer from the parent universe into the baby universe through a black hole is not conceivable.[6]

Smolin has noted that the string theory landscape is not Popper-falsifiable if other universes are not observable.[citation needed] This is the subject of the Smolin–Susskind debate concerning Smolin's argument: "[The] Anthropic Principle cannot yield any falsifiable predictions, and therefore cannot be a part of science."[6] There are then only two ways out: traversable wormholes connecting the different parallel universes, and "signal nonlocality", as described by Antony Valentini, a scientist at the Perimeter Institute.[clarification needed]

In a critical review of The Life of the Cosmos, astrophysicist Joe Silk suggested that our universe falls short by about four orders of magnitude from being maximal for the production of black holes.[7] In his book Questions of Truth, particle physicist John Polkinghorne puts forward another difficulty with Smolin's thesis: one cannot impose the consistent multiversal time required to make the evolutionary dynamics work, since short-lived universes with few descendants would then dominate long-lived universes with many descendants.[8] Smolin responded to these criticisms in Life of the Cosmos, and later scientific papers.

When Smolin published the theory in 1992, he proposed as a prediction of his theory that no neutron star should exist with a mass of more than 1.6 times the mass of the sun.[citation needed] Later this figure was raised to two solar masses following more precise modeling of neutron star interiors by nuclear astrophysicists. If a more massive neutron star was ever observed, it would show that our universe's natural laws were not tuned for maximal black hole production, because the mass of the strange quark could be retuned to lower the mass threshold for production of a black hole. A 1.97-solar-mass pulsar was discovered in 2010.[9]

In 1992 Smolin also predicted that inflation, if true, must only be in its simplest form, governed by a single field and parameter. Both predictions have held up, and they demonstrate Smolin's main thesis: that the theory of cosmological natural selection is Popper falsifiable.
 
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Cosmology Discussion Thread

01 Dec 2018 20:23

Those are some fantastic ideas, I was wondering if there are any computer simulations out there that allow us to play with the laws of physics and the strengths of the four fundamental forces and create different types of universes.  Wat or anyone else, do you know of any software that does this?
 
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Cosmology Discussion Thread

01 Dec 2018 20:27

Watsisname wrote:
Yes, it's just like going around the world and coming back to where you started.  Except with one more dimension.  Pretty trippy to try to visualize that.

If the universe is curved in this way, and if you could actually make such a journey, then as you go off in a straight line you would simply encounter more space and more galaxies along the way, until you suddenly realize you're back at the Milky Way again.  But nowhere was there an edge or anything weird.  

Unfortunately we cannot actually make such a journey, because the cosmic expansion prevents us from being able to travel far enough quickly enough to wrap all the way around.


JackDole wrote:
Source of the post As far as I know, the outside of a universe is not defined. It does not exist. The space within our universe bubble is increasing, but there is nothing outside of our universe that could somehow shrink.

Exactly. :)  The universe is all of the space that is connected with the space we live in.  Any space that might exist that isn't connected is irrelevant to it.  So there is no sense in imagining an "outside" of the universe.  The expansion of the universe is an expansion of the space and requires no outside to exist at all.  It is not like a sphere expanding into space around it.  It is absolutely unlike any other thing you can imagine.

ah but whats on the outside of the universe might have some relevance to it, if gravity leaks into our universe from another brane and that would explain why unification between gravity and the other three fundamental forces has been so difficult.  There is also the concept of dark flow, which hasn't been well explained yet.
I find Poplawski's idea of black hole cosmology mentioned above interesting because it eloquently deals with a handful of unsolved problems in physics.


JackDole wrote wrote:
As far as I know, the outside of a universe is not defined. It does not exist. The space within our universe bubble is increasing, but there is nothing outside of our universe that could somehow shrink.


I dont know if something actually has to shrink since each universe would occupy its own space, different universes would be hiding behind event horizons with no interaction between each (that we know of, unless gravity can leak between universes and create something like dark flow.)
However, the Two Time Theory that came out a few years ago is interesting, in that it conjectured a dual universe model with opposing arrows of time (but both forward relative to themselves) and while one universe expands the other would contract.  They'd have to consist of complementary dimensions (similar to complementary colors) so while the primary spatial dimensions of one universe might be RGB with time as a background of black, the complementary universe would have primary spatial dimensions of CMY with complementary time as a background of white.  These correspond to additive and subtractive primaries respectively.
 
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Cosmology Discussion Thread

01 Dec 2018 22:01

A-L-E-X wrote:
Source of the post Wat, do you think other universes exist?  I do for the simple reason that there is never one of anything

¯\_(ツ)_/¯

Your reason sounds simple, but it's quite an Aristotelian kind of argument.  How do you know that there is never one of anything?  You have to check the entire universe to show that it's true, and you cannot check the entire universe.  It also only takes one counterexample to falsify it, and the universe itself could be the counterexample.

As for myself, I do not have a view one way or the other on whether other universes exist.  For the time being I am the classic agnostic about it.  There are tons and tons of examples of theorists putting out ideas for how they could work.  Some are very clever and interesting (I always thought Poplawski's universes inside of black holes idea was real neat).  But I don't pick favorites just because I think they're clever or interesting or they appeal to my love of black holes. :)  I judge them by how well I think they flow naturally from known physics, then by how many unverified assumptions they need to make, and then how potentially falsifiable they are.

Ideas which claim to resolve outstanding problems in physics also abound.  I don't think claiming that they resolve issues makes them compelling.  That's their motivation, the whole reason they were thought of.  What makes them compelling is if they successfully predict new observations.

One of the ideas you mentioned a while back which did claim to be verified by observations was Penrose's Conformal Cyclic Cosmology.  It was argued that patterns in the CMB radiation agreed with his model.  The trouble was, other cosmologists checked his findings and could not verify them.  The patterns he claimed to detect are not statistically significantly different than what we expect to see anyway, and he had to use his own analysis to argue otherwise.  That's not good!  If you have to use your own model to show that the observation that supports your model even exists, then it's very circular.  The evidence should be apparent independently of the model.  I don't need to invoke general relativity to observe that clocks at different altitudes tick at different rates!


Probably the most simple way I know of for many universes to exist (in some sense) is if the universe is spatially infinite.  Then if the cosmological principle also holds, it is statistically guaranteed that multiple versions of you, the world, and even our entire observable universe exist out there somewhere.  And many many more which are nearly the same but have some differences.  In a sense, these could be called other universes.

Issues with this idea?  We don't know if the universe is truly spatially infinite or if the cosmological principle actually does hold for the whole thing.  And even if it is correct, then it's somewhat irrelevant.  All those other identical you's, Earth's, and so on are so far away that they lie beyond our cosmic event horizon.  Unless something crazy like faster than light travel is discovered, then we cannot ever reach or communicate with them.  So for our purposes they are truly "island universes".
 
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Cosmology Discussion Thread

01 Dec 2018 22:12

A-L-E-X wrote:
Source of the post I was wondering if there are any computer simulations out there that allow us to play with the laws of physics and the strengths of the four fundamental forces and create different types of universes.  Wat or anyone else, do you know of any software that does this?

Yes, at least to a certain extent!  It can be done with computer code, and in a sense the code that I wrote to show the Lambda-CDM model and other types of universes could already be considered a form of this.  With that code I can see how a universe evolves with different amounts of different types of matter and energy, and it would also be very easy to change things like the gravitational constant or the speed of light.

Changing some of the other constants or laws of physics (like the Planck constant, which would affect how matter and radiation itself behaves), or seeing how those changes affect the formation of structure and the types of objects that can exist in the universe, would be a lot harder.  It would require running something more similar to the Millenium Simulations, since we must then simulate the interactions between every particle.  It would not be possible on a home computer, and it would probably also not be easily generalizable to any arbitrary change in the laws of physics.
 
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Cosmology Discussion Thread

01 Dec 2018 22:15

Watsisname wrote:
A-L-E-X wrote:
Source of the post I was wondering if there are any computer simulations out there that allow us to play with the laws of physics and the strengths of the four fundamental forces and create different types of universes.  Wat or anyone else, do you know of any software that does this?

Yes, at least to a certain extent!  It can be done with computer code, and in a sense my own code that I wrote to show the Lambda-CDM model and other types of universes could already be considered a form of this.  With that code I can see how a universe evolves with different amounts of different types of matter and energy, but it would also be very easy to change things like the gravitational constant or the speed of light.

Changing other constants or laws of physics (like the Planck constant, which would affect how matter and radiation itself behaves), or seeing how changing those things affects the formation of structure and the types of objects that can form in the universe, would be a lot harder, and require running something more similar to the Millenium Simulations, since we must then simulate the interactions between every particle.  It would not be possible on a home computer, and it would probably also not be easily generalizable to any odd change in the constants or laws of physics.

Wat have you ever read this book?
Ever since I read this since I was a child I have been thinking about other universes and all the different possibilities.
https://en.wikipedia.org/wiki/The_Gods_Themselves
Isaac Asimov was so ahead of his time, when I was in HS I used to read 100 of his books every year, basically I learned far more by reading his books (fiction and nonfiction) then I ever did in any class I attended.

Second part: ...The Gods Themselves...

The second part is set in the parallel universe where, because the nuclear force is stronger, stars are smaller and burn out faster than in our universe. It takes place on a world orbiting a sun that is dying. Because atoms behave differently in this universe, substances can move through each other and appear to occupy the same space. This gives the intelligent beings unique abilities. Time itself appears to flow differently in this universe: the events take place in an apparently short space of time in the lives of the inhabitants, while more than twenty years pass in our universe, and a long feeding break of one of the characters translates into a two-week gap on Lamont's side.
 
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Cosmology Discussion Thread

01 Dec 2018 22:28

https://en.wikipedia.org/wiki/Sean_M._Carroll

Thats another writer whose work fascinates me-he writes for Discovery

His most-cited work, "Is Cosmic Speed-Up Due To New Gravitational Physics?" was written with Vikram Duvvuri, Mark Trodden, and Michael Turner. With over 1,900 citations, it helped pioneer the study of f(R) gravity in cosmology.[7]

In 2010, Carroll was elected as a fellow of the American Physical Society, for "contributions to a wide variety of subjects in cosmology, relativity, and quantum field theory, especially ideas for cosmic acceleration, as well as contributions to undergraduate, graduate, and public science education".[8] In 2014 he was awarded the Andrew Gemant Award, a prize given by the American Institute of Physics for "significant contributions to the cultural, artistic or humanistic dimension of physics."[9] In 2015 he was awarded a Guggenheim Fellowship.[10]

Carroll has worked on a number of areas of theoretical cosmology, field theory and gravitation theory. His research papers include models of, and experimental constraints on, violations of Lorentz invariance; the appearance of closed timelike curves in general relativity; varieties of topological defects in field theory; and cosmological dynamics of extra spacetime dimensions. In recent years he has written extensively on models of dark energy and its interactions with ordinary matter and dark matter, as well as modifications of general relativity in cosmology.

Carroll has also worked on the arrow of time problem. He and Jennifer Chen posit that the Big Bang is not a unique occurrence as a result of all of the matter and energy in the universe originating in a singularity at the beginning of time, but rather one of many cosmic inflation events resulting from quantum fluctuations of vacuum energy in a cold de Sitter space. They claim that the universe is infinitely old, but never reaches thermodynamic equilibrium as entropy increases continuously without limit due to the decreasing matter and energy density attributable to recurrent cosmic inflation. They assert that the universe is "statistically time-symmetric" insofar as it contains equal progressions of time "both forward and backward".[12][13][14] Some of his work has been on violations of fundamental symmetries, the physics of dark energy, modifications of general relativity, and the arrow of time. Recently he started focusing on issues at the foundations of cosmology, statistical mechanics, quantum mechanics, and complexity.
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